Scipedia: Documents published in 2020

Experimental and numerical analysis of a sphere falling into a viscous fluid

María Jesús Samper — Wed, 01 Jul 2020 10:16:38 +0200

The experimental and numerical analysis of spheres falling into viscous flows is considered. The physical model is built using a set of silicone and glass spheres falling into oil and water. The rigid‐body trajectory of the sphere and the free surface evolution are obtained from videos. The numerical results are obtained using two different finite element codes. The first code uses a fractional step approach with adaptive meshes and time‐step sizes whereas the second code uses a monolithic fully coupled fixed‐mesh technique. The results exhibit a good comparison between both numerical techniques and with the experiments.

Semi‐automatic porting of a large‐scale CFD code to multi‐graphics processing unit clusters

María Jesús Samper — Wed, 01 Jul 2020 10:10:18 +0200

A typical large‐scale CFD code based on adaptive, edge‐based finite‐element formulations for the solution of compressible and incompressible flow is taken as a test bed to port such codes to graphics hardware (graphics processing units, GPUs) using semi‐automatic techniques. In previous work, a GPU version of this code was presented, in which, for many run configurations, all mesh‐sized loops required throughout time stepping were ported. This approach simultaneously achieves the fine‐grained parallelism required to fully exploit the capabilities of many‐core GPUs, completely avoids the crippling bottleneck of GPU–CPU data transfer, and uses a transposed memory layout to meet the distinct memory access requirements posed by GPUs. The present work describes the next step of this porting effort, namely to integrate GPU‐based, fine‐grained parallelism with Message‐Passing‐Interface‐based, coarse‐grained parallelism, in order to achieve a code capable of running on multi‐GPU clusters. This is carried out in a semi‐automated fashion: the existing Fortran–Message Passing Interface code is preserved, with the translator inserting data transfer calls as required. Performance benchmarks indicate up to a factor of 2 performance advantage of the NVIDIA Tesla M2050 GPU (Santa Clara, CA, USA) over the six‐core Intel Xeon X5670 CPU (Santa Clara, CA, USA), for certain run configurations. In addition, good scalability is observed when running across multiple GPUs. The approach should be of general interest, as how best to run on GPUs is being presently considered for many so‐called legacy codes.

Iterative solution applied to the Helmholtz equation: Complex deflation on unstructured grids

María Jesús Samper — Wed, 01 Jul 2020 09:58:38 +0200

Extensions of deflation techniques developed for the Poisson and Navier equations (Aubry et al., 2008; Mut et al., 2010; Löhner et al., 2011; Aubry et al., 2011) [1], [2], [3], [4] are presented for the Helmholtz equation. Numerous difficulties arise compared to the previous case. After discretization, the matrix is now indefinite without Sommerfeld boundary conditions, or complex with them. It is generally symmetric complex but not Hermitian, discarding optimal short recurrences from an iterative solver viewpoint (Saad, 2003) [5]. Furthermore, the kernel of the operator in an infinite space typically does not belong to the discrete space. The choice of the deflation space is discussed, as well as the relationship between dispersion error and solver convergence. Similarly to the symmetric definite positive (SPD) case, subdomain deflation accelerates convergence if the low frequency eigenmodes are well described. However, the analytic eigenvectors are well represented only if the dispersion error is low. CPU savings are therefore restricted to a low to mid frequency regime compared to the mesh size, which could be still relevant from an application viewpoint, given the ease of implementation.

Robust Boundary Layer Mesh Generation

María Jesús Samper — Wed, 01 Jul 2020 09:46:02 +0200

In this paper, we introduce a 3D local operator that automatically combines typical simpler operators as removal of vertices, collapse of edges or swap of faces and edges. This operator is inherited from incremental methods where the mesh HkHk is modified iteratively through sequences of insertion of a point P:

Hk+1=Hk−CP+BP,Hk+1=Hk−CP+BP,

where CPCP is the cavity of P and BPBP the ball of P. We derive two algorithms to compute CpCp. The first algorithm is tuned to be a fast point reprojection to the geometry even in the presence of a boundary layer mesh. The second one is tuned to generate boundary layer meshes for complex geometries. We show how quasi-structured elements can be enforced. In addition, enhancements as multi-normals can be incorporated in the process. Both operators can be used with surface and volume point while preserving a given geometry. They rely on the use on an existing initial volume mesh and always produce a valid 3D mesh on output.

Advances in FEFLO

María Jesús Samper — Wed, 01 Jul 2020 09:42:31 +0200

This paper summarizes the major improvements and developments that have taken place during the last year for FEFLO, a general-purpose CFD code based on adaptive, unstructured grids. All aspects of a comprehensive simulation pipeline: pre-processing, gridding, field solvers and post-processing saw important advances, and are treated. The advent of machines with millions of cores focused a lot of the developments on distributed memory aspects of field solvers and multiphysics modules. Parallel grid generation, particles and flow, parallel interpolation, parallel domain splitting and repartitioning for multiphysics were all considered. Furthermore, timings on Xeon and AMD chips led to the realization that memory transfers play a considerable role as compared to floating point operations, something previous chip generations did not exhibit. This led to a thorough analysis of numerical algorithms with subsequent re-writing. This in-depth analysis determined that at present achievable speeds are limited by the communication between processors (MPI). Even as the number of cores/domains can increase to several hundred thousand, due to communication overheads the wall clock time it takes to update a flowfield is bounded. We expect that as more users get access to hundreds of thousands of cores and the domain size per core starts shrinking, they will also encounter this 'minimum timestep barrier'.

Linear Sources for Mesh Generation

María Jesús Samper — Wed, 01 Jul 2020 09:32:13 +0200

Sources offer a convenient way to prescribe a size distribution in space. For each newly created mesh point, the mesh generator queries the local size distribution, either to create a new point or element, depending on the underlying mesh generation method, to smooth the mesh, or to get a local relevant length scale. Sources may have different shapes such as points, edges, triangles, or boxes. They provide the size distribution given some user defined parameters and the distance of a point location to the source. Traditionally, the source strength is considered as constant. In this work, extensions to linear sources in space are proposed. It is shown that in the case of curvature refined mesh generation, substantial savings may occur due to the much better approximation of the curvature variation for a simple modification of traditional sources. Even though curvature refinement is the main application of this work, improvements through linear sources are relevant to other contexts such as user defined sources. The technique is very general as it deals with a fundamental aspect of mesh generation and can be easily incorporated into an existing mesh generator with traditional sources. Thorough details of source approximations and source filtering are provided. Relations with lower envelopes are highlighted. Practical examples illustrate the accuracy and efficiency of the method.

Error and work estimates for high‐order elements

María Jesús Samper — Wed, 01 Jul 2020 09:20:03 +0200

Error and work estimates for high‐order elements are derived. The comparison of error and work estimates shows that for relative accuracy in the 1% range, which is typical of engineering interest, it may prove very difficult to improve on linear elements. As expected, the estimates also show that the optimal order of element in terms of work and storage demands depends on the desired relative accuracy.

Handling tens of thousands of cores with industrial/legacy codes: Approaches, implementation and timings

María Jesús Samper — Wed, 01 Jul 2020 09:15:56 +0200

The consequences that the recent stagnation in clockrates for CPUs has had on large-scale CFD runs are examined. At first sight, the conclusion is that only massive parallelism at the loop or domain decomposition level will lead to higher FLOP counts. However, the significant differences in advances for CPUs/GPUs versus RAM and interprocessor communication bandwidth lead to a so-called ‘limiting domain size’, below which communication dominates execution times and performance degrades drastically. The consequences of this ‘red-shift’ for the future of CFD are manifold: the time to advance the solution one timestep is limited, implying that even with unlimited number of processors/cores, LES, DES and DNS runs for realistic Reynolds-numbers will require days or weeks of execution.

Scaling Up Multiphysics

María Jesús Samper — Tue, 30 Jun 2020 16:15:24 +0200

The present paper summarizes trends in supercomputing and the consequences they will have on coupled problems in computational mechanics. The appearance of parallel machines with more than 106
cores implies that the prevalent ‘scalar-pre, simple parallel solve, scalar-post’ environment will have to give way to a completely scalable simulation pipeline. The grid size alone will force distributed parallelization of meshing, domain splitting, load balancing and post-processing. Possible ways of addressing parallel meshing and dynamic load balancing for multiphysics are treated, and examples shown.

Butterfly-Effect for Massively Separated Flows

María Jesús Samper — Tue, 30 Jun 2020 16:11:51 +0200

Purpose ‐ During a routine benchmarking and scalability study of CFD codes for typical large-scale wind engineering runs, it was observed that the resulting loads for buildings varied considerably with the number of parallel processors employed. The differences remained very small at the beginning of a typical run, and then grew progressively to a state of total dissimilitude. A "butterfly-effect" for such flows was suspected and later confirmed. The paper aims to discuss these issues. Design/methodology/approach ‐ A series of numerical experiments was conducted for massively separated flows. The same geometry ‐ a cube in front of an umbrella ‐ was used to obtain the flowfields using different grids, different numbers of domains/processors, slightly different inflow conditions and different codes. Findings ‐ In all of these cases the differences remained very small at the beginning of a typical run, they then grew progressively to a state of total dissimilitude. While the mean and maximum loads remained similar, the actual (deterministic) instantiations were completely different. The authors therefore suspect that for flows of this kind a "butterfly effect" is present, whereby even very small (roundoff) errors can have a pronounced effect on the actual deterministic instantiation of a flowfield. Research limitations/implications ‐ This implies that for flows of this kind the CFD runs have to be carried out to much larger times than formerly expected (and done) in order to obtain statistically relevant ensembles. Practical implications ‐ For practical calculations this implies running to much larger times in order to reach statistically relevant ensembles, with the associated much higher CPU time requirements. Originality/value ‐ This is the first time such a finding has been reported in the numerical wind engineering context.

On mesh-particle techniques

María Jesús Samper — Tue, 30 Jun 2020 16:01:30 +0200

The treatment of dilute solid (or liquid) phases via Lagrangian particles within mesh-based gas-dynamics (or hydrodynamic) codes is common in computational fluid dynamics. While these techniques work very well for a large spectrum of physical parameters, in some cases, notably for very light or very heavy particles, numerical instabilities appear. The present paper examines ways of mitigating these instabilities, and summarizes important implementational issues.

Recent Advances in Parallel Advancing Front Grid Generation

María Jesús Samper — Tue, 30 Jun 2020 15:53:55 +0200

The quest for scalable, parallel advancing front grid generation techniques now spans more than two decades. A recent innovation has been the use of a so-called domain-defining grid, which has led to a dramatic increase in robustness and speed. The domain-defining grid (DDG) has the same fine surface triangulation as the final mesh desired, but a much coarser interior mesh. The DDG renders the domain to be gridded uniquely defined and allows for a well balanced work distribution among the processors during all stages of grid generation and improvement. In this way, most of the shortcomings of previous techniques are overcome. Timings show that the approach is scalable and able to produce large grids of high quality in a modest amount of clocktime. These recent advances in parallel grid generation have enabled a completely scalable simulation pipeline (grid generation, solvers, post-processing), opening the way for truly large-scale computations using unstructured, body-fitted grids.

Modeling subway air flow using CFD

María Jesús Samper — Tue, 30 Jun 2020 15:45:21 +0200

A more realistic Computational Fluid Dynamics approach has been developed to model air flow in subway tunnels and stations. This modeling incorporates the effects of multi-car trains as they arrive and depart in a network of interconnected stations at realistic speeds. It also includes thermal sources within the stations and a coupling to the street level through inlets and outlets. This new approach is validated against experimental data, and numerical simulations are carried out in realistic subway station geometries. The air flow dynamics in these stations are quantified in terms of their spatial complexity and temporal stability. The air flow dynamics correlate to the dispersion and transport of pollutants though the stations.

On maximum achievable speeds for field solvers

María Jesús Samper — Tue, 30 Jun 2020 15:19:03 +0200

Purpose ‐ Prompted by the empirical evidence that achievable flow solver speeds for large problems are limited by what appears to be a time of the order of O(0.1) sec/timestep regardless of the number of cores used, the purpose of this paper is to identify why this phenomenon occurs. Design/methodology/approach ‐ A series of timing studies, as well as in-depth analysis of memory and inter-processors transfer requirements were carried out for a typical field solver. The results were analyzed and compared to the expected performance. Findings ‐ The analysis shows that at present flow speeds per core are already limited by the achievable transfer rate to RAM. For smaller domains/larger number of processors, the limiting speed of CFD solvers is given by the MPI communication network. Research limitations/implications ‐ This implies that at present, there is a "limiting useful size" for domains, and that there is a lower limit for the time it takes to update a flowfield. Practical implications ‐ For practical calculations this implies that the time required for running large-scale problems will not decrease markedly once these applications migrate to machines with hundreds of thousands of cores. Originality/value ‐ This is the first time such a finding has been reported in this context.

Strategy for modeling flow diverters in cerebral aneurysms as a porous medium

María Jesús Samper — Tue, 30 Jun 2020 15:13:15 +0200

Simulations using the patient‐specific geometry of the aneurysm may help in a better planning of the treatment and in a consequent reduction of the associated risks. We propose, evaluate, and implement a methodology for the simulation of flow diverter (FD) devices in intracranial aneurysms by using a porous medium method (PMM), which greatly reduces the computational cost of these simulations compared with immersed method (IMM) approaches used to model complex FDs. The method relies on parameters from an empirical correlation derived from experimental observations in wire screens, consistent with CFD simulations. The verification of our PMM strategy was carried out by comparing the results of simulations in different (patient‐specific) geometries and FDs, to those obtained under identical conditions by the IMM. Overall, both quantitative and qualitative results are consistent between IMM and PMM in cases where the local porosity remains roughly uniform throughout the neck, with differences in the reduction of the observables lower than 10%. This PMM strategy is up to 10 times faster than the IMM, which allows for a runtime of hours instead of days, bringing it closer for its application in the clinic.

Verification of a Pedestrian Simulation Tool Using the NIST Recommended Test Cases

María Jesús Samper — Tue, 30 Jun 2020 14:58:04 +0200

In an attempt to develop a verification and validation standard for building fire evacuation models, Ronchi et al. (2013) at the United States’ National Institute of Standards and Technology (NIST) recommended a set of seventeen verification tests. We found that the application of these verification tests allowed us to make rather significant improvements to our simulation code (PEDFLOW) for approximately half of the recommended tests (Table 1). In some cases, we added capabilities that did not exist before. In other cases, we found anomalous behaviors and adjusted the existing code to remove these unexplained behaviors. This paper summarizes the work on the verification tests, highlighting the lessons learned and modifications made. We also discuss some modifications we recommend to the NIST verification tests, as well as demonstrate how to make these tests suitable for all pedestrian flow models (not just building fire evacuation).

Validation of a Pedestrian Simulation Tool Using the NIST Stairwell Evacuation Data

María Jesús Samper — Tue, 30 Jun 2020 14:30:46 +0200

Researchers at the United States National Institute of Standards and Technology (NIST) collected occupant egress data in the stairwells of several high-rise buildings for potential use in quantitative and qualitative validation of evacuation simulation tools (Kuligowski and Peacock (2010)). We found this data suitable for establishing occupant initial locations, pre-evacuation time distributions, and other parametric inputs for our simulation code (PEDFLOW). With this data set, we were able to validate several core behavioral components of PEDFLOW by directly comparing actual versus predicted values for occupant speed on stairs and building total evacuation times. This paper summarizes our work on the stairwell data sets, highlighting the methodology behind the extraction of values for the parametric inputs, and demonstrating the results obtained for one specific 10-story high-rise building data-set.

On critical densities and velocities for pedestrians entering a crowd

María Jesús Samper — Tue, 30 Jun 2020 14:05:34 +0200

The problem of pedestrians trying to enter and/or advance against an incoming crowd is studied. The analysis shows that starting with a very limited set of assumptions (elliptical cross-section of pedestrians; constant ratio of forward to lateral separation) one is able to derive from purely kinematic considerations critical densities beyond which it is impossible for pedestrians to enter and/or advance into an incoming crowd. The results obtained indicate that for the common pedestrian size of a = 0.5 m, b = 0.3 m, the limit densities range from ρ = O(5.0 – 6.06) [p/m2], in good agreement with empirical observations.

On the Influence of columns in densely populated corridors

María Jesús Samper — Tue, 30 Jun 2020 14:00:42 +0200

An experimental and computational campaign was undertaken to measure the influence of columns in densely populated corridors. undertaken. A instrumented corridor of 5x12.54 m with two columns was enclosed by walls, and several hundred people were asked to walk repeatedly through the enclosed region. The maximum density achieved was of the order of 6 people/sqm, higher than what is deemed safe for many operating standards. The experiments were run with and without columns to see their influence. Remarkably, even at high densities, the columns had no noticeable influence on the pedestrian motion. Furthermore, the forces measured on the columns were very small (noise-level).

Quantitative Validation of PEDFLOW for Description of Unidirectional Pedestrian Dynamics

María Jesús Samper — Tue, 30 Jun 2020 13:54:07 +0200

The results of a systematic quantitative validation of PEDFLOW based on the experimental data from FZJ are presented. Uni- directional flow experiments, totaling 28 different combinations with varying entry, corridor and exit widths, were considered. The condition imposed on PEDFLOW was that all the cases should be run with the same input parameters. The exit times and fundamental diagrams for the measuring region were evaluated and compared. This validation process led to modifications and enhancements of the model underlying PEDFLOW. The preliminary conclusions indicate that the results agree well for densities smaller than 3 m-2 and a good agreement is observed even at high densities for the corridors with bcor = 2.4 m, and bcor = 3.0 m. For densities between 1 and 2 m-2 the specific flow and velocities are underpredicted by PEDFLOW.

Mechanisms Involved in the Formation of Biocompatible Lipid Polymeric Hollow Patchy Particles

María Jesús Samper — Tue, 30 Jun 2020 13:37:59 +0200

Patchy polymeric particles have anisotropic surface domains that can be remarkably useful in diverse medical and industrial fields because of their ability to simultaneously present two different surface chemistries on the same construct. In this article, we report the mechanisms involved in the formation of novel lipid–polymeric hollow patchy particles during their synthesis. By cross-sectioning the patchy particles, we found that a phase segregation phenomenon occurs between the core, shell, and patch. Importantly, we found that the shear stress that the polymer blend undergoes during the particle synthesis is the most important parameter for the formation of these patchy particles. In addition, we found that the interplay of solvent–solvent, polymer–solvent, and polymer–polymer–solvent interactions generates particles with different surface morphologies. Understanding the mechanisms involved in the formation of patchy particles allows us to have a better control on their physicochemical properties. Therefore, these fundamental studies are critical to achieve batch control and scalability, which are essential aspects that must be addressed in any type of particle synthesis to be safely used in medicine.

Recent advances in computational wind engineering and fluid-structure interaction

María Jesús Samper — Tue, 30 Jun 2020 13:29:43 +0200

Recent developments that are pertinent to the particular field of computing lightweight structures exposed to windloads are described. The topics covered include computational fluid dynamics (CFD), computational structural dynamics (CSD) and fluid–structure interaction (FSI) for wind vs. aerospace engineering, recent hardware and software trends, butterfly effects, rogue loads, and adjoint estimation of boundary conditions.

Real-time micro-modelling of a million pedestrians

María Jesús Samper — Tue, 30 Jun 2020 13:18:47 +0200

Purpose - The purpose of this paper is to develop a first-principles model for the simulation of pedestrian flows and crowd dynamics capable of computing the movement of a million pedestrians in real-time in order to assess the potential safety hazards and operational performance at events where many individuals are gathered. Examples of such situations are sport and music events, cinemas and theatres, museums, conference centres, places of pilgrimage and worship, street demonstrations, emergency evacuation during natural disasters. Design/methodology/approach - The model is based on a series of forces, such as: will forces (the desire to reach a place at a certain time), pedestrian collision avoidance forces, obstacle/wall avoidance forces; pedestrian contact forces, and obstacle/wall contact forces. In order to allow for general geometries a so-called background triangulation is used to carry all geographic information. At any given time the location of any given pedestrian is updated on this mesh. The model has been validated qualitatively and quantitavely on repeated occasions. The code has been ported to shared and distributed memory parallel machines. Findings - The results obtained show that the stated aim of computing the movement of a million pedestrians in real-time has been achieved. This is an important milestone, as it enables faster-thanreal- time simulations of large crowds (stadiums, airports, train and bus stations, concerts) as well as evacuation simulations for whole cities. Research limitations/implications - All models are wrong, but some are useful. The same applies to any modelling of pedestrians. Pedestrians are not machines, so stochastic runs will be required in the future in order to obtain statistically relevant ensembles. Practical implications - This opens the way to link real-time data gathering of crowds (i.e. via cameras) with predictive calculations done faster than real-time, so that security personnel can be alerted to potential future problems during large-scale events. Social implications - This will allow much better predictions for large-scale events, improving security and comfort. Originality/value - This is the first time such speeds have been achieved for a micro-modelling code for pedestrians.

The simulation of dust effects from fragmenting charges

María Jesús Samper — Tue, 30 Jun 2020 13:11:16 +0200

Purpose - The purpose of this paper is to determine the reason for the discrepancy in estimated and observed damage caused by fragmenting charges in closed environments. Design/methodology/approach - A series of carefully conducted physical and numerical experiments was conducted. The results were analyzed and compared. Findings - The analysis shows that for fragmenting charges in closed environments, dust plays a far larger role than previously thought, leading to much lower pressures and damage. Research limitations/implications - In light of these findings, many assumptions and results for fragmenting charges in closed environments need to be reconsidered. Practical implications - This implies that for a far larger class of problems than previously estimated it is imperative to take into consideration dust production and its effect on the resulting pressures. Originality/value - This is the first time such a finding has been reported in this context.

Mechanistic studies on the self-assembly of PLGA patchy particles and their potential applications in biomedical imaging

María Jesús Samper — Tue, 30 Jun 2020 13:07:03 +0200

Currently, several challenges prevent poly(lactic-co-glycolic acid) (PLGA) particles from reaching clinical settings. Among these is a lack of understanding of the molecular mechanisms involved in the formation of these particles. We have been studying in depth the formation of patchy polymeric particles. These particles are made of PLGA and lipid–polymer functional groups. They have unique patch–core–shell structural features: hollow or solid hydrophobic cores and a patchy surface. Previously, we identified the shear stress as the most important parameter in a patchy particle’s formation. Here, we investigated in detail the role of shear stress in the patchy particle’s internal and external structure using an integrative experimental and computational approach. By cross-sectioning the multipatch particles, we found lipid-based structures embedded in the entire PLGA matrix, which represents a unique finding in the PLGA field. By developing novel computational fluid dynamics and molecular dynamics simulations, we found that the shear stress determines the internal structure of the patchy particles. Equally important, we discovered that these particles emit a photoacoustic (PA) signal in the optical clinical imaging window. Our results show that particles with multiple patches emit a higher PA signal than single-patch particles. This phenomenon most likely is due to the fact that multipatchy particles absorb more heat than single-patchy particles as shown by differential scanning calorimetry analysis. Furthermore, we demonstrated the use of patchy polymeric particles as photoacoustic molecular probes both in vitro and in vivo studies. The fundamental studies described here will help us to design more effective PLGA carriers for a number of medical applications as well as to accelerate their medical translation.

Running large‐scale CFD applications on Intel‐KNL–based clusters

María Jesús Samper — Tue, 30 Jun 2020 12:13:37 +0200

Intel's latest Xeon Phi processor, Knights Landing (KNL), has the potential to provide over 2.6 TFLOPS. However, to obtain maximum performance on the KNL, significant refactoring and optimization of application codes are still required to exploit key architectural innovations that KNL features—wide vector units, many‐core node design, and deep memory hierarchy. The experience and insights gained in porting and running FEFLO (a typical edge‐based finite element code for the solution of compressible and incompressible flows) on the KNL platform are described in this paper. In particular, optimizations used to extract on‐node parallelism via vectorization and multithreading and improve internode communication are considered. These optimizations resulted in a 2.3× performance gain on a 16 node runs of FEFLO, with the potential for larger performance gains as the code is scaled beyond 16 nodes. The impact of the different configurations of KNL's on‐package MCDRAM (Multi‐Channel DRAM) memory on FEFLO's performance is also explored. Finally, the performance of the optimized versions of FEFLO for KNL and Haswell (Intel Xeon) is compared.

Load balancing for chemically reacting flows

María Jesús Samper — Tue, 30 Jun 2020 11:58:13 +0200

Purpose A common observation made when computing chemically reacting flows is how CPU intensive these are in comparison to cold flow cases. The update of tens or hundreds of species with hundreds or thousands of reactions can easily consume more then 95\% of the total CPU time. In many cases the region where reactions (combustion) are actually taking place comprises only a very small percentage of the volume. Typical examples are flame fronts propagating through a domain. In such cases only a small fraction of points/cells needs a full chemistry update. This leads to extreme load imbalances on parallel machines. Design/methodology/approach Points that require a full chemistry update are identified, gathered, and distributed across the network so that work is evenly distributed. Once the chemistry has been updated, the unknowns are gathered back. Findings The procedure has been found to work extremely well, leading to optimal load balance with insignificant communication overheads. Research limitations/implications In many production runs, the procedure leads to a reduction in CPU requirements of more than an order of magnitude. This allows much larger and longer runs, improving accuracy and statistics. Practical implications The procedure has allowed the calculation of chemically reacting flow cases that were hitherto not possible. Originality/value As far as we know, this type of load balancing has not been published before.

Perspectives on Sharing Models and Related Resources in Computational Biomechanics Research

María Jesús Samper — Tue, 30 Jun 2020 11:47:45 +0200

The role of computational modeling for biomechanics will be increasingly prominent. In computational biomechanics, model sharing can facilitate assessment of reproducibility, and can provide an opportunity for repurposing and reuse, and a venue for medical training. The community's desire to investigate biological and biomechanical phenomena crossing multiple systems, scales, and physical domains, also motivates sharing of modeling resources as blending of models developed by domain experts is anticipated. The goal of this article is to understand current perspectives in the biomechanics community for the sharing of computational models and related resources. Opinions on opportunities, challenges, and pathways to model sharing, particularly as part of the scholarly publishing workflow, were sought. A synthesis of these opinion pieces indicates that the community recognizes the necessity and usefulness of model sharing. There is a strong will to facilitate model sharing and there are corresponding initiatives by the scientific journals. Outside the publishing enterprise, infrastructure to facilitate model sharing in biomechanics exists and simulation software developers are interested in accommodating the community's needs for sharing of modeling resources. Encouragement for the use of standardized markups, concerns related to quality assurance, acknowledgement of increased burden, and importance of stewardship of resources are noted. In the short-term, it is advisable that the community builds upon recent strategies and experiments with new pathways for continued demonstration of model sharing, its promotion, and its utility. Nonetheless, the need for a long term strategy to unify approaches in sharing computational models and related resources is acknowledged.

Fundamental Diagrams for Specific Very High Density Crowds

María Jesús Samper — Tue, 30 Jun 2020 11:19:32 +0200

An experimental campaign was undertaken to measure the pedestrian flow in the region close to the Kaaba during the Hajj pilgrimages of 2014 and 2015. High resolution video and photographs were used. The space was divided into areas of 10 sqm. The pedestrians were counted, and the velocity measured from video clips. The results were surprising: the velocity in the very high density region increases, which implies also an increase of the flux. The flux in this region (with more than 8 p/sqm) reaches values that exceed 3.5 p/m/sec, much higher than previously recorded under more `standard conditions' in corridors and passages.

Practical applicability of optimizations and performance models to complex stencil-based loop kernels in CFD

María Jesús Samper — Tue, 30 Jun 2020 11:16:11 +0200

This work investigates the application and interaction of optimization techniques and performance models in a computational fluid dynamics (CFD) approach employing an OpenMP parallelized, explicit, weakly compressible, finite difference–based solver for the incompressible Navier–Stokes equations using a five-point wide stencil. The presented loop and stencil optimizations lead to a 6.8× increase in per core throughput. In order to verify optimal CPU utilization, performance models are applied to the tuned code. Three different performance models are considered: a roofline-based model, utilizing purely theoretical figures, one which is enhanced by measurements, and the execution cache memory model. It is shown that the models provide reliable estimates for simple benchmarks, such as seven-point stencils for scalar Laplacians, but the estimate quality is significantly worse for the complex and tuned stencil. While it is possible to include even more details in the model, it eventually leads to a state in which it purely reproduces the benchmarks from which it was derived. Thus, the applied general-purpose performance models are found to inaccurately predict the actual performance. They overestimate the achievable performance by more than about 97% for highly tuned code. Through further code tuning, 66% of the predicted performance could be achieved.

Post processing-based interpolation schemes for nested Cartesian finite difference grids of different size

María Jesús Samper — Tue, 30 Jun 2020 11:10:15 +0200

Nested Cartesian grid systems by design require interpolation of solution fields from coarser to finer grid systems. While several choices are available, preserving accuracy, stability, and efficiency at the same time require careful design of the interpolation schemes. Given this context, a series of interpolation algorithms based on postprocessing halo information on nested Cartesian finite difference grids of different size were developed and tested. The results obtained indicate that most of these do not yield the expected improvement, and some even tend to make the solver unstable. However, some third‐ and fourth‐order interpolation functions do yield considerable accuracy improvement, are stable, and are worth implementing.

Using ensemble Kalman filter to determine parameters for computational crowd dynamics simulations

María Jesús Samper — Tue, 30 Jun 2020 10:50:42 +0200

It is of paramount importance to ensure safe and fast evacuation routes in cities in case of natural disasters, environmental accidents or acts of terrorism. The same applies to large-scale events such as concerts, sport events and religious pilgrimages as airports and to traffic hubs such as airports and train stations. The prediction of pedestrian is notoriously difficult because it varies depending on circumstances (age group, cultural characteristics, etc.). In this study, the Ensemble Kalman Filter (EnKF) data assimilation technique, which uses the updated observation data to improve the accuracy of the simulation, was applied to improve the accuracy of numerical simulations of pedestrian flow.

Detailed simulation of viral propagation in the built environment (preprint)

María Jesús Samper — Tue, 30 Jun 2020 10:50:06 +0200

A summary is given of the mechanical characteristics of virus contaminants and the transmission via droplets and aerosols. The ordinary and partial differential equations describing the physics of these processes with high fidelity are presented, as well as appropriate numerical schemes to solve them. Several examples taken from recent evaluations of the built environment are shown, as well as the optimal placement of sensors.

An Investigation of Stationary and Moving Cased Charge Detonations in Stone Lined Pipes

María Jesús Samper — Tue, 30 Jun 2020 10:40:02 +0200

This paper describes the application of a coupled CFD and CSD methodology to the simulation of a thick-cased explosive-filled cylinder placed inside a pipe composed of either steel or various strength stones. The objective is to better understand the energy exchange mechanisms between the impacting case debris produced by either stationary or moving charges on the surrounding pipe material and its affect on the energy propagation in the pipe.

An embedded Finite Element framework for the resolution of strongly coupled Fluid–Structure Interaction problems. Application to volumetric and membrane-like structures

María Jesús Samper — Tue, 30 Jun 2020 10:38:45 +0200

This work presents a Fluid–Structure Interaction framework for the robust and efficient simulation of strongly coupled problems involving arbitrary large displacements and rotations. We focus on the application of the proposed tool to lightweight membrane-like structures. Nonetheless, all the techniques we present in this work can be applied to both volumetric and volumeless bodies. To achieve this, we rely on the use of embedded mesh methods in the fluid solver to conveniently handle the extremely large deflections and eventual topology changes of the structure. The coupling between the embedded fluid and mechanical solvers is based on an interface residual black-box strategy. We validate our proposal by solving reference benchmarking examples that consider both volumetric and volumeless geometries. Whenever it is possible, we also compare the embedded solution with the one obtained with our reference body fitted solver. Finally we present a real-life application of the presented embedded Fluid–Structure Interaction solver.

PedPIV: Pedestrian Velocity Extraction From Particle Image Velocimetry

María Jesús Samper — Tue, 30 Jun 2020 10:31:25 +0200

The analysis of velocities from high-density pedestrian events may provide more information on pedestrian flow dynamics. A framework based on particle image velocimetry (PIV)-a technique commonly used in experimental fluid dynamics-has been developed to evaluate the pedestrian velocities from high-density pedestrian events. The framework takes a sequence of two or more images from a regular closed-circuit television camera and obtains the flow properties (speed, direction) of the pedestrians. A detailed analysis has been done in both the qualitative and quantitative aspects of adapting PIV to pedestrian flow. An investigation through the use of the fundamental diagram and real-time extraction of crowd velocities is also presented and discussed. The proposed PIV-based framework enables on-the-spot analysis of pedestrian flow via velocity extraction in a reliable, automated manner.

A Study of Dispersion, Vaporization, and Combustion of Burnable Liquids Surrounding Charges

María Jesús Samper — Tue, 30 Jun 2020 10:25:25 +0200

A complex numerical simulation where blast waves interacted with a liquid, burnable simulant inside a chamber was conducted. The liquid simulant was dispersed by a small high explosive (HE) charge and then impacted by the blast wave produced by a significantly larger HE charge. The physics of the problem required numerical tools that allowed for (1) coupling of compressible and near-incompressible (+VOF) solvers, (2) a dropletization model by liquid bulk dispersion, (3) droplet breakup and vaporization, (4) a chemical package for combustion, and (5) a particle update technique. The resulting numerical code successfully simulated the scenario, and the predicted pressure agreed excellently with the measured data.

Towards overcoming the LES crisis

María Jesús Samper — Tue, 30 Jun 2020 10:18:28 +0200

Industrial large-eddy simulation (LES), i.e. overnight runs with O(109) degrees of freedom (DOFs) and O(107) timesteps, has been one of the top priorities of CFD research over the last two decades. Current network and solver timings indicate that the required target of 5 ms/timestep is within reach: some cumulant Lattice Boltzmann Method (LBM) codes are reporting speeds in this range using multi-GPU systems, while some Finite Difference Method (FDM) codes running on traditional CPUs are achieving speeds that are only a factor of 2–4 slower. This implies that the long wait may be coming to a close. During the last three decades, a number of promising approaches have been tried. Given that the majority of these were promoted as the ‘solution to LES’ or the ‘solution to turbulence’, the paper lists them under the label of ‘the false prophecies’. Furthermore, some of the assumptions that are always part of the scientific discovery process turned out to be incorrect. These are listed under the term 'the false assumptions'. From an informal survey conducted in January of 2018, it appears that simple Cartesian Finite Difference codes or, equivalently, Lattice Boltzmann codes may be the first to achieve industrial LES.

A Novel Framework for Automated Monitoring and Analysis of High Density Pedestrian Flow

María Jesús Samper — Tue, 30 Jun 2020 10:08:02 +0200

Pedestrian traffic is an important subject of surveillance to ensure public safety and traffic management, which may benefit from intelligent and continuous analysis of pedestrian videos. State-of-the-art methods for intelligent pedestrian surveillance have a number of limitations in automating and deriving useful information of high-density pedestrian traffic (HDPT) using closed circuit television (CCTV) images. This work introduces an automatic and improved HDPT surveillance system by integrating and optimizing multiple computational steps to predict pedestrian distribution from input video frames. A fast and efficient particle image velocimetry (PIV) technique is proposed to yield pedestrian velocities. A machine learning regressor model, boosted Ferns, is used to improve pedestrian count and density estimation: an essential metric for HDPT analysis. A camera perspective model is proposed to improve the speed and position estimates of HDPT by projecting 2D image pixels to 3D world-coordinate data. All these functional improvements in HDPT velocity and displacement estimations are used as inputs to a sophisticated pedestrian flow evolution model, PEDFLOW to predict HDPT distribution at a future time point, which is a crucial information for pedestrian traffic management. The predicted and simulated HDPT properties (density, velocity) obtained using the proposed framework show low errors when compared to the ground truth data. The proposed framework is computationally efficient, suitable for multiple camera feeds with HDPT videos, and capable of rapidly analyzing and predicting flows of thousands of pedestrians. The paper shows one of the first steps towards fully integrated CCTV-based automated HDPT management system.

Empirical Evaluation of Crowds Using Automated Methods

María Jesús Samper — Tue, 30 Jun 2020 09:58:07 +0200

This work presents a novel framework for automated monitoring of high density crowds from closed circuit television (CCTV) image data. The framework obtains pedestrian velocities from particle image velocimetry (PIV) technique and densities from a boosted ferns machine learning model. A pinhole camera based perspective correction scheme is employed to convert the 2D pixel coordinates into 3D metric coordinates. The framework is trained with and tested against real-world event data from the Hajj.

Overnight Industrial LES for External Aerodynamics

María Jesús Samper — Tue, 30 Jun 2020 09:47:53 +0200

This paper summarizes the efforts carried out over the last year to achieve overnight industrial Large-Eddy Simulation (LES) runs for external car aerodynamics. The solver employed is based on adaptive cartesian blocks, uses explicit timestepping to advance the Navier-Stokes equations describing mildly compressible flows, and scales well to tens of thousands of cores. The capabilities developed to date are tested on a Volkswagen Golf.

Towards real-time monitoring of the Hajj

María Jesús Samper — Tue, 30 Jun 2020 09:41:54 +0200

An automated approach to explore the fundamental properties of high-density pedestrian traffic is outlined. The framework operates on video or time lapse images captured from surveillance cameras. For pedestrian velocity extraction, the framework incorporates cross-correlation based Particle Image Velocimetry (PIV) techniques. For pedestrian density estimation, the framework relies on the Machine Learning technique of the Boosted Regression Trees. The information collected from images in pixel coordinates are transformed to world coordinates with a pin-hole camera based projective transformation technique. The framework has been tested with high density crowd images acquired during the Muslim religious event, the Hajj. Accuracy and performance of the framework are reported.

Simple fault-tolerant computing for field solvers

María Jesús Samper — Tue, 30 Jun 2020 09:31:56 +0200

Fault-tolerant computing options based on the use of restart information stored on and off node and the use of reserve processes have been developed, implemented and tested in a large-scale, production field solver taken from the domain of computational fluid dynamics. The tests conducted to date have shown good results, with recovery rates approaching 100% under realistic node failure scenarios. Even though the computational overhead of the field solver is very low (explicit time-marching and finite differences), the fault-tolerant implementation adds a run-time penalty that is only in the range of 6–12%, depending on the spatial and temporal approximation used. The procedures developed are generally applicable, and could easily be ported to other code.

IberWQ: new simulation tool for 2D water quality modelling in rivers and shallow estuaries

María Jesús Samper — Mon, 29 Jun 2020 15:52:20 +0200

The model computes the spatial and temporal evolution of several species and variables which are relevant for the evaluation of the environmental status of rivers and estuaries, including: Escherichia coli, dissolved oxygen, carbonaceous biochemical oxygen demand, organic nitrogen, ammoniacal nitrogen, nitrate–nitrite nitrogen, water temperature and salinity. A depth-averaged transport equation is solved for each variable with a mass conservative unstructured finite volume solver. IberWQ is fully coupled to the hydrodynamic module of the software Iber, a freeware simulation tool for solving the 2D shallow water equations. Both models are integrated in the same windows graphical environment, including several tools which allow the user to generate unstructured meshes adapted to the site topography, define spatially variable input parameters and visualize model outputs. We present four application examples to illustrate the possibilities of the software for the dynamic simulation of water quality in rivers and estuaries.

A simple and efficient unstructured finite volume scheme for solving the shallow water equations in overland flow applications

María Jesús Samper — Mon, 29 Jun 2020 15:44:20 +0200

This paper presents the Decoupled Hydrological Discretisation (DHD) scheme for solving the shallow water equations in hydrological applications involving surface runoff in rural and urban basins. The name of the scheme is motivated by the fact that the three equations which form the two-dimensional shallow water system are discretised independently from each other and thus, the numerical scheme is decoupled in a mathematical sense. Its main advantages compared to other classic finite volume schemes for the shallow water equations are its simplicity to code and the lower computational cost per time step. The validation of the scheme is presented in five test cases involving overland flow and rainfall-runoff transformation over topographies of different complexity. The scheme is compared to the finite volume scheme ofRoe [1986], to the simple inertia formulation [Bates et al., 2010], and to the diffusive wave model. The test cases show that the DHD scheme is able to compute subcritical and supercritical flows in rural and urban environments, and that in overland flow applications it gives similar results to the second order scheme of Roe with a lower computational cost. The results obtained with the simple inertia and diffusive wave models are very similar to those obtained with the DHD scheme in rural basins in which the bed friction and topography dominate the flow hydrodynamics but they deteriorate in typical urban configurations in which the presence of supercritical flow conditions and small scale patterns boost the relevance of the inertial terms in the momentum equations

Avances en el análisis del material leñoso en ríos: incorporación, transporte e influencia en el riesgo por inundaciones

María Jesús Samper — Mon, 29 Jun 2020 15:21:21 +0200

El transporte y depósito de restos de vegetación en los ríos (troncos o árboles completos, ramas y raíces, material leñoso en general) puede agravar las consecuencias de los eventos de avenida e inundación e incrementar su peligrosidad potencial. Este material puede afectar especialmente a infraestructuras como drenajes, puentes, etc., que intersectan ríos en cuencas montañosas forestadas. Una práctica muy extendida y aceptada hasta hace pocos años ha sido la extracción sistemática de este material leñoso de los cauces como medida de prevención. Sin embargo, se ha demostrado que esta práctica puede resultar inútil (el material es transportado y depositado tras cada inundación), e incluso no beneficiosa para el equilibrio natural del sistema fluvial a largo plazo. Por lo tanto, es necesario gestionar la presencia del material leñoso en los ríos, e integrar su estudio en los análisis de peligrosidad y riesgo por inundación. Sin embargo, en España existen escasos trabajos en los que el material leñoso haya sido objetivo fundamental de estudio y, por tanto, tampoco existe demasiada bibliografía relacionada con esta temática. En este trabajo, se presenta una exhaustiva revisión de los principales métodos empleados para evaluar la peligrosidad de las avenidas e inundaciones que incorporan material leñoso. Asimismo, se propone una metodología integral para estudiar esta carga de detritos leñosos en ríos. En primer lugar, hay que caracterizar la dinámica de incorporación de este material a los cauces. Para ello, es necesario establecer las áreas contribuyentes que pueden aportar el material a los cursos de agua y los procesos que lo incorporan. De este modo, es posible estimar el volumen de madera potencialmente disponible, para lo cual se presenta un modelo numérico que permite la simulación del transporte de carga leñosa junto con la hidrodinámica. Asimismo, se analiza la incidencia de la carga flotante en la peligrosidad y el riesgo por inundaciones. Esta propuesta metodológica se ha puesto en práctica en diversos ríos de la Sierra de Gredos (Sistema Central), presentándose una síntesis de algunos de los resultados principales obtenidos. Como conclusión fundamental, se puede afirmar que una correcta gestión de los ríos requiere a su vez de una apropiada gestión de la vegetación de ribera y el material leñoso transportado y depositado en los mismos; y esta gestión debe encontrar el compromiso para mantener las buenas condiciones ecológicas, reduciendo en lo posible los potenciales riesgos

Large wood transport as significant influence on flood risk in a mountain village

María Jesús Samper — Mon, 29 Jun 2020 15:13:23 +0200

An important issue that is not considered in most flood risk assessments in mountain villages in Spain is the transport of solids associated with the flood flow, in this case, large wood transport. The transport and deposition of this wood in urban areas may be a potentially worse hazard than the flood flow itself. Despite its importance, large wood is a key ecological element in rivers, so removing it could be an unsuccessful approach. Therefore, efforts are needed in the better understanding of wood transport and deposition in streams. To analyse this process, scenario-based 2D hydrodynamic flood modelling was carried out. Since flood risk assessment has considerable intrinsic uncertainty, probabilistic thinking was complemented by possibilistic thinking, considering worst-case scenarios. This procedure obtained a probabilistic flood map for a 500-year return period. Then, a series of scenarios was built based on wood budget to simulate wood transport and deposition. Results allowed us to identify the main infrastructures sensitive to the passing of large wood and simulate the consequences of their blockage due to wood. The potential damage was estimated as well as the preliminary social vulnerability for all scenarios (with and without wood transport). This work shows that wood transport and deposition during flooding may increase potential damage at critical stream configurations (bridges) by up to 50 % and the number of potentially exposed people nearby these areas by up to 35 %.

Modelización numérica de zonas fluviales inundables

María Jesús Samper — Mon, 29 Jun 2020 15:00:02 +0200

La modelización numérica del flujo de agua en ríos es una herramienta que ayuda a dar respuesta a la legislación vigente europea y española referida a riesgo de inundación. Los modelos numéricos disponibles actualmente se encuentran en constante evolución. Mientras hace una década la modelización unidimensional era prácticamente la única alternativa, en el presente se han generalizado los modelos bidimensionales y se empiezan a utilizar los tridimensionales. Gracias a las actuales herramientas SIG, los resultados de la modelización numérica se pueden combinar con datos georreferenciados para realizar una cuantificación sistemática del riesgo de inundación. En este proceso existen aún una serie de desafíos como la consideración de los caudales sólidos y sus efectos en la morfología del cauce, una correcta descripción del flujo en puentes y estructuras, la integración de modelos hidrológicos con los modelos hidráulicos para una mejor consideración de las condiciones de contorno, y finalmente la optimización de las herramientas para disminuir los tiempos de cálculo actuales.

Two dimensional modelling of large wood transport during flash floods

María Jesús Samper — Mon, 29 Jun 2020 14:34:40 +0200

Large woody material (LW) transported by rivers may be entrapped at critical stream geometry configurations (e.g. bridges) and therefore dramatically increase the destructive power of floods. This was the case in a Spanish mountain river where a flood event with a high degree of LW transport took place in 1997. The aim of this study was to simulate a bridge clogging process and reconstruct the wood deposit patterns, modelling individual pieces of wood moving with the water flow and interacting among them and with the bridge. A two dimensional numerical model was developed to simulate the transport of LW and its effect on hydrodynamics. Different scenarios for the wood transport rate allowed us to study the influence of inlet boundary conditions on bridge clogging. For the studied event, the scenario which best reproduced the bridge clogging effect and flood characteristics was one in which 60% of the total wood entered before the peak discharge. This dropped to 30% at the peak itself, and finally fell to 10% during the recession curve. In addition, the accumulation patterns of LW along the reach were computed and compared with post-event field photographs, showing that the model succeeded in predicting the deposition patterns of wood and those areas prone to form wood jams.

Robust Modeling and Simulation State Space Model Based BLDC Motor Fed Universal Actuation System

Murali M — Mon, 29 Jun 2020 14:20:00 +0200

This paper deals with mathematical modeling of Permanent magnet brushless DC (BLDC) motor in MATLAB-SIMULINK environment. Modeling of BLDC motor carried in transfer function, transfer equations and state space model to verify the performance as actuators. Mathematical switches to control electronic commutation of BLDC motor based on signals of Hall Effect position sensor using threephase inverter drive. Performance of the simplified mathematical inverter fed BLDC motor under steady state and dynamic conditions analyzed. Due to the switching losses during PWM generation generates low ripple content in torque of BLDC motor which described and eliminated through state space model. Comparison made of proposed modeling of BLDC motor with motor parameters like back-EMF, stator current and speed of BLDC motor, proposed work suggests the state space modeling holds a superior method for design of BLDC motor during high dynamic load performance and operating ranges

Two-dimensional numerical modeling of wood transport

María Jesús Samper — Mon, 29 Jun 2020 14:07:10 +0200

The transport of wood material in rivers has been the subject of various studies in recent years. Most research has focused on the ecological and geomorphologic role of wood, its recruitment processes and spatial distribution in streams. In this study, we focused on wood transport dynamics, and we have developed a numerical model to simulate wood transport coupled with a two-dimensional (2D) hydrodynamic model. For this purpose, wood drag forces were incorporated as additional source terms into the shallow water equations, which are solved together with wood transport by using the finite volume method. This new tool has been implemented as a computational module into ‘Iber’, a 2D hydraulic simulation software. The new module analyzes the initial motion threshold of wood based on the balance of forces involved in the wood's movement, and computes the position and velocity of differently shaped logs using a kinematic approach. The method also considers the interaction between the logs themselves and between the logs and the channel walls or boundaries. Flume experiments were used in a straight channel with obstructions to validate the model's capacity to accurately reproduce the movement of floating logs.

Large wood in rivers and its Influence on flood hazard

María Jesús Samper — Mon, 29 Jun 2020 14:01:40 +0200

In terms of flood hazard, the presence of large wood (logs, trees, branches and roots) in rivers may aggravate the consequences of flood events. This material may affect infrastructures such as bridges, weirs, etc., especially those intersecting forested mountain rivers. Until recently, a widely accepted practice was to systematically remove wood debris from river channels as a preventive measure. However, studies have shown that this practice may be useless as the material is transported and deposited after each flood and may even not benefit the long term natural balance of the river ecosystem. Therefore, the presence of this woody material in rivers must be managed and included in flood hazard and risk analysis. In this paper we present a comprehensive methodological approach to study the role of large wood in rivers, with a focus on flood hazard. First, to understand the dynamics of wood recruitment, the contributing areas delivering wood to the streams have to be delineated and the recruitment mechanisms studied. Thus, an estimate can be obtained of the potential volume of deliverable wood. To analyse wood transport we present a numerical model, which allows simulates the behaviour of individual pieces of wood together with hydrodynamics. Finally, we analyse the impact of wood on the magnitude of flood events (in terms of water level, flow velocity or flooded areas), using as an example a flood which occurred in December 1997 in the Sierra de Gredos. The results allowed us to reproduce the wood deposit patterns during the event and to reconstruct the bridge blockage. This caused the upstream water level to rise by up to 2 meters and reduced the flow velocity, which favoured debris and sediment deposits. Consequently, the effects of flooding were equivalent to those of a greater magnitude event. This increase in the flood hazard has been numerically quantified.

“Enfoque ráster” del problema hidrodinámico del flujo en lámina libre en 2D

María Jesús Samper — Mon, 29 Jun 2020 13:57:08 +0200

Geographical information systems traditionally provide information using two formats–raster and vector. Each one of these formats is handled according to their independent algorithms. Consequently, any type of hydraulic study performed in a territory contains information that can be classified by these two formats. LiDAR is the most precise technology used to generate Digital Elevation Models (DEM) for large areas, with a spatial resolution of 1 to 2 m and a minimum precision in height. It gives the cartographic information its raster characteristic, with the cell as its basic unit of information. Consequently, this kind of information is very suitable to the generation of structured quadrilateral meshes to numerically model free surface flow in two dimensions. The main objective of this work was to present an adaptation of the classic explicit first order finite volumes scheme to a finite square volumes (FSV) mesh which, directly and without interpolations, makes each cell in the DEM (or raster) equal to the corresponding FSV. Consequently, the dimensions of the FSV are those of the base DEM cell. This is presented herein as a raster approach to the hydrodynamic problem of free surface flow in two dimensions. With the raster approach, the database for the mesh topology does not need to be consulted for each instant of time and, therefore, the efficiency of the numeric process is greatly improved. Consequently, larger-sized problems can be addressed. An additional objective was to compare both approaches by solving an illustrative example.

Correlación de parámetros geotécnicos con parámetros de un modelo hidrológico superficial

María Jesús Samper — Mon, 29 Jun 2020 13:52:49 +0200

Este artículo presenta la relación entre parámetros geotécnicos básicos y parámetros del modelo hidrológico de Thomas para balance hídrico de cuencas hidrográficas rurales. Esta relación funciona como una aplicación metodológica de calibración, herramienta útil en modelos distribuidos como es el caso de IBER, que representa un sistema distribuido de simulación numérica para flujo no permanente en lámina libre. Se realizó una exploración en 25 puntos (en 15 subcuencas) de la cuenca de Rio de Piedras (Boyacá), obteniendo muestras de suelo a las cuales se les realizó caracterización geotécnica mediante ensayos de laboratorio.

Modelo de transferencia vertical dentro de un modelo hidrológico continuo en un esquema de volúmenes finitos

María Jesús Samper — Mon, 29 Jun 2020 13:29:14 +0200

Tomando como base modelos actuales en hidrología, y de acuerdo a las características y a la base del modelo IBER (modelo matemático bidimensional para la simulación de flujos en ríos y estuarios), se ha ajustado el esquema preliminar de lo que será el componente HMC (Modelo Hidrológico Continuo) del modelo IBER, adaptando condiciones de subsuelo y zona subterránea de manera continua al modelo hidrológico basado en esquemas de volúmenes finitos. El modelo escribe el paso del agua desde su estado como precipitación, pasando por el encharcamiento y flujo superficial, hasta el componente que viaja por infiltración y vía subterránea a través de los acuíferos y zonas de retorno de flujo a zonas superficiales, todo mediante una modelización que tiene como base la solución explícita de ecuaciones que trabajan celda a celda en un esquema de volúmenes finitos

Simulación de la onda de avenida por ruptura de tanque de enfriamiento

María Jesús Samper — Mon, 29 Jun 2020 13:16:23 +0200

Se llevó a cabo la simulación de la ruptura del borde de un tanque de enfriamiento usando dos métodos: Uno de volúmenes finitos denominado CARPA y un algoritmo en diferencias finitas centradas. El algoritmo en volúmenes finitos CARPA utilizado con el pre y post procesador GiD mostró ser una herramienta muy poderosa en la animación de planicies de inundación, útiles en la interpretación de resultados tanto en el espacio como en el tiempo, para la protección civil, así como para definir las posibles zonas afectadas debido a fenómenos como el de la ruptura de bordos

Dam wall break simulation by means of a high resolution algorithm applied to the La Parota dam, Guerrero, Mexico.

María Jesús Samper — Mon, 29 Jun 2020 13:11:43 +0200

This document presents a dam wall break simulation of the La Parota hydropower project in Guerrero, Mexico, by means of the CARPA system, which is based in a 2D WAF-TVD (Weight Averaged Flux-Total Variation Diminishing) high resolution finite volume numerical scheme. With the results, it is possible to produce maps containing water depth, flow velocity, and affected zones, which are useful in the civil protection field

Análisis del riesgo del transporte fluvial de los lodos mineros residuales del vertido de Aznalcóllar en el valle medio-bajo del río Guadiamar

María Jesús Samper — Mon, 29 Jun 2020 12:07:05 +0200

After de Aznalcóllar pyrite mine disaster (Seville, Spain) which caused the spilling of some 4.5 hm3 of acid water, the floodplains of the rivers Agrio and Guadiamar were rapidly cleaned of waste sludge.

Hydrological and geomorphological criteria to evaluate the dispersion risk of waste sludge generated by the Aznalcollar mine spill (SW Spain)

María Jesús Samper — Mon, 29 Jun 2020 11:43:14 +0200

Following the Aznalcóllar pyrite mine disaster (Seville, Spain) which caused the spilling of some 4.5 hm3 of acid water, the floodplains of the rivers Agrio and Guadiamar were rapidly cleaned of waste sludge. However, despite the efficiency of cleaning activities, there is still evidence of a fine superficial layer of sludge and some soil contamination, with the consequent risk of remobilisation of the pollutants by water erosion. There is much concern that these contaminated sediments may affect the precious ecosystems of the Doñana National Park and the Guadalquivir marshlands. This report describes the evaluation of the risk of mobilisation of the waste sludge through (1) detailed geomorphological analysis, indicating potential areas of erosion-sedimentation on the floodplains of the rivers Agrio and Guadiamar, and predicted dynamics of the waste sludge, and (2) evaluation of the potential dispersion of the waste sludge provoked by future flood events, including hydraulic calculations to model channel flow and the analysis of the texture of the sludge to obtain critical transport and sedimentation values. Findings suggest that the waste sludge is likely to be transported and deposited within the Doñana National Park during future flood events

Dams and Reservoirs in the Lower Ebro River and Its Effects on the River Thermal Cycle

María Jesús Samper — Mon, 29 Jun 2020 11:21:44 +0200

River regulation can cause direct and indirect alterations in the river thermal cycle, which in turn may affect biological processes. In the lower Ebro River, two weirs (Ascó and Xerta) and a system of three dams (Mequinensa, Riba-roja, Flix) can be found. The weir of Ascó is used for the derivation of water to be used in the cooling system of a nuclear plant, which is returned later to the river and causing an increase of 3°C in the river water temperature. Instead, the weir of Xerta is used for the derivation of water for irrigation which most probably results in a diminution of the thermal inertia of the flowing water mass. On the other side, the system of reservoirs of Mequinensa, Riba-roja and Flix have a seasonal effect on water temperature. Water exiting the system of reservoirs is cooler than the river water entering them in spring and summer and is warmer in autumn and winter. Also, water temperature variability is reduced both in the daily and annual timescales. The reservoir of Riba-roja, receiving the contribution of two main affluents (Ebro and Segre), presents a most interesting hydrodynamic behaviour, that neither is typical lacustrine nor is that of a river. It has a variable pattern of circulation: in winter and early spring, the water column is mixed; in late spring and most of summer, the Segre River water flows above the Ebro River water; and in the rest of summer and most of autumn, the circulation pattern is inverse to the previous one, with the Ebro River water flowing above the Segre River water.

Incidencia de los embalses en el comportamiento térmico del rio

María Jesús Samper — Mon, 29 Jun 2020 11:12:48 +0200

The river water maintains a heat exchange with the atmosphere and with the bottom; this process can reach its equilibrium along a stretch of the river with similar geologic and climatic characteristics. Its behavior can be modified by artificial effects, such as the use of the river water for the cooling of thermal power stations or by the presence of reservoirs. In the case of regions with Mediterranean climate, where the studied reservoirs are located, the effects of changes in the natural thermal regime caused by them produce a trend towards a constant seasonal thermal condition, an increase in daily uniformity, an increase in winter temperatures, and a decrease in summer temperatures. Water temperature is an important factor in the river's ecology, because it greatly conditions the life of fluvial communities. Downstream from power stations or dams, the water temperature evolves in such a way that tends to achieve equilibrium with the environment

Thermal response of Sanabria lake to global change (NW Spain)

María Jesús Samper — Mon, 29 Jun 2020 10:45:59 +0200

Are large water bodies able to act as sensors of global change? As accumulators of water and heat, some of their thermal characteristics might be altered by long term (decadal) hydrometeorological changes and thus may be used as indicators of the effects of global change on fluvial ecosystems. This work focuses on the effect of global change (climate change plus water quantity and land use changes) in the internal organization of Sanabria Lake, specifically in its thermal annual cycle. The existence of temporal trends in the thermal behaviour of the lake was investigated based on the water temperature profiles are available since 1986. Data analyses include the non-parametric Mann-Kendall trend test and the Sen slope estimate to evaluate long term and seasonal patterns of hydrometeorological and in-lake thermal variables. The main results surprisingly point to a net lake cooling that could be explained by a stronger thermocline and a weaker vertical transfer of heat to the hypolimnion during the stratification period. These results contribute to understand and quantify the effects of global change on Iberian freshwater bodies

Modelling flood propagation in the service galleries of a nuclear power plant

María Jesús Samper — Mon, 29 Jun 2020 10:38:26 +0200

In the context of the stress tests that have to be applied to nuclear power plants, this work presents the study of the flooding processes of the service galleries of a nuclear plant caused by a hypothetical failure of some of the pipes of the Essential Services Water System (ESWS). To assess the flood propagation along the galleries, two-dimensional hydraulic modelling tools, based on the solution of the shallow water equations with the finite volume method, were used. Due to the complexity and special features of the geometry and hydraulic processes, when compared with more standard urban flood assessment works, several specific modules were developed. A relevant one is a new module to consider the effect of the occupation of the galleries on the advance of the waterfront. This module was developed and verified prior to being applied to a case study. The results show the suitability of the proposed methodology to be used as part of the stress tests to ensure high security standards of nuclear power plants.

Computational modelling of fine sediment relocation within a dam reservoir by means of artificial flood generation in a reservoir cascade

María Jesús Samper — Mon, 29 Jun 2020 10:28:29 +0200

Sediment relocation is a process where water turbulence moves deposits from shallower to deeper zones of a lake or reservoir. Additionally, in the context of a reservoir cascade relocation is a one of the possible sediment management strategies used to maintain the operational capacity of reservoirs. Numerical modelling tools applied to sediment dynamics can help to better understand the reservoir sedimentary processes and enhance the design of management strategies. This paper describes the detailed analysis of the cohesive sediment dynamics within a reservoir and a methodology for the calibration of numerical models for the management of sediment deposits through dam operation and upstream generation of artificial floods. The calibration process was based on an Accumulated Sediment Volume (ASV) curve and the usage of simplified models for an initial parameter estimation. The calibrated model has been applied to analyse different possibilities of sediment relocation strategies.

Balances de agua y calor en la marisma de Doñana

María Jesús Samper — Mon, 29 Jun 2020 10:23:24 +0200

Este trabajo expone los principales resultados del estudio de los balances hídrico y térmico de la marisma de Doñana. El estudio se ha basado en una amplia base de datos hidrometeorológicos tomados a intervalos de 10 minutos entre los años 2006 y 2011 por una red de seis estaciones de medida localizadas en áreas no vegetadas de la marisma. Esta información ha permitido caracterizar, a diferente escala temporal, el comportamiento térmico de la marisma mediante la modelización y análisis de los flujos de calor entre el agua y el suelo inundado, así como los procesos de transferencia de calor entre la superficie libre del agua y la atmósfera. Se ha puesto especial atención en la cuantificación de la evaporación, de la que depende en gran medida el proceso de vaciado de la marisma.

Physical modeling of a stepped spillway without sidewalls

María Jesús Samper — Mon, 29 Jun 2020 10:13:58 +0200

The interest of a consulting company in designing stepped spillways in roller compacted concrete (RCC) dams led us to propose the possibility of building this type of spillway without sidewalls. Previous research on stepped spillways has focused on characterizing the complex hydraulic behavior of flow on these structures, as well as design criteria. Such studies have usually been conducted on stepped spillways with a constant width along the spillway, that is, with sidewalls. In the present work, we report the results of the physical modeling of a generic stepped spillway without sidewalls (slope 1v:0.8h). In general terms, the lack of sidewalls produces a lateral expansion of water and therefore a non-uniform longitudinal and transversal discharge distribution. Consequently, the flow type, characteristic water depth, velocity, air concentration, and pressure fields change along and across the spillway. The resulting data demonstrate that the distribution of the different variables studied depend on the specific discharge at the entrance and the spillway height.

Local isotropy indicator for SAR image filtering: application to Envisat/ASAR images of the Doñana Wetland

María Jesús Samper — Mon, 29 Jun 2020 10:04:01 +0200

This paper explores a geometrical and computationally simple operator, named Ds, for local isotropy assessment on SAR images. It is assumed that isotropic intensity distributions in natural areas, either textured or nontextured, correspond to a single cover class. Ds is used to measure isotropy in processing neighborhoods and decide if they can be considered as belonging to a unique cover class. The speckle statistical properties are used to determine suitable Ds thresholds for discriminating heterogeneous targets from isotropic cover types at different window sizes. An assessment of Ds as an edge detector showed sensitivities similar to those of the ratio edge operator for straight, sharp boundaries, centered in the processing window, but significantly better sensitivity for detecting heterogeneities during the window expansion in multiresolution filtering. Furthermore, Ds presents the advantage versus the ratio edge coefficient of being rotationally invariant, and its computation indicates the direction of the main intensity gradient in the processing window. The Ds operator is used in a multiresolution fashion for filtering ASAR scenes of the Doñana wetland. The intensities in isotropic areas are averaged in order to flatten fluctuations within cover types and facilitate a subsequent land cover classification. The results show high degree of smoothing within textured cover classes, plus effective spatial adaptation to gradients and irregular boundaries, substantiating the usefulness of this operator for filtering SAR data of natural areas with the purpose of classification.

Mixing dynamics at the confluence of two large rivers undergoing weak density variations

María Jesús Samper — Mon, 29 Jun 2020 09:21:14 +0200

Simulations of tracer experiments conducted with a three‐dimensional primitive‐equation hydrodynamic and transport model are used to understand the processes controlling the rate of mixing between two rivers (Ebro and Segre), with distinct physical and chemical properties, at their confluence, upstream of a meandering reservoir (Ribarroja reservoir). Mixing rates downstream of the confluence are subject to hourly scale oscillations, driven partly by changes in inflow densities and also as a result of turbulent eddies that develop within the shear layer between the confluent rivers and near a dead zone located downstream of the confluence. Even though density contrasts are low—at most O(10−1) kg m−3 difference among sources—and almost negligible from a dynamic point of view—compared with inertial forces—they are important for mixing. Mixing rates between the confluent streams under weakly buoyant conditions can be of up to 40% larger than those occurring under neutrally buoyant conditions. The buoyancy effects on mixing rates are interpreted as the result of changes in the contact area available for mixing (distortion of the mixing layer). For strong density contrasts, though, when the contact area between the streams becomes nearly horizontal, larger density differences between streams will lead to weaker mixing rates, as a result of the stabilizing effect of vertical density gradients.

Envisat/ASAR Images for the Calibration of Wind Drag Action in the Donana Wetlands 2D Hydrodynamic Model

María Jesús Samper — Mon, 29 Jun 2020 09:07:23 +0200

Donana National Park wetlands are located in southwest Spain, on the right bank of the Guadalquivir River, near the Atlantic Ocean coast. The wetlands dry out completely every summer and progressively flood again throughout the fall and winter seasons. Given the flatness of Donana's topography, the wind drag action can induce the flooding or emergence of extensive areas, detectable in remote sensing images. Envisat/ASAR scenes acquired before and during strong and persistent wind episodes enabled the spatial delineation of the wind-induced water displacement. A two-dimensional hydrodynamic model of Donana wetlands was built in 2006 with the aim to predict the effect of proposed hydrologic restoration actions within Donana's basin. In this work, on-site wind records and concurrent ASAR scenes are used for the calibration of the wind-drag modeling by assessing different formulations. Results show a good adjustment between the modeled and observed wind drag effect. Displacements of up to 2 km in the wind direction are satisfactorily reproduced by the hydrodynamic model, while including an atmospheric stability parameter led to no significant improvement of the results. Such evidence will contribute to a more accurate simulation of hypothetic or design scenarios, when no information is available for the atmospheric stability assessment. Doñana National Park wetlands are located in southwest Spain, on the right bank of the Guadalquivir River, near the Atlantic Ocean coast. The wetlands dry out completely every summer and progressively flood again throughout the fall and winter seasons. Given the flatness of Doñana’s topography, the wind drag action can induce the flooding or emergence of extensive areas, detectable in remote sensing images. Envisat/ASAR scenes acquired before and during strong and persistent wind episodes enabled the spatial delineation of the wind-induced water displacement. A two-dimensional hydrodynamic model of Doñana wetlands was built in 2006 with the aim to predict the effect of proposed hydrologic restoration actions within Doñana’s basin. In this work, on-site wind records and concurrent ASAR scenes are used for the calibration of the wind-drag modeling by assessing different formulations. Results show a good adjustment between the modeled and observed wind drag effect. Displacements of up to 2 km in the wind direction are satisfactorily reproduced by the hydrodynamic model, while including an atmospheric stability parameter led to no significant improvement of the results. Such evidence will contribute to a more accurate simulation of hypothetic or design scenarios, when no information is available for the atmospheric stability assessment

Mixing and circulation at the confluence of two rivers entering a meandering reservoir

María Jesús Samper — Mon, 29 Jun 2020 08:41:53 +0200

A field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows (Ebro and Segre) with distinct physical‐chemical characteristics as they join at the upstream end of Ribarroja reservoir in northern Spain. Given the short average residence time of water in the reservoir, the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam. In winter, inflows have similar densities—Δρ/ρ 0 ≈ O (10−5)—and their spatial distribution is largely determined by inertial forces, and in particular, by the discharge ratio. Downstream of the confluence, both rivers flow side by side and largely unmixed over long distances. In summer, with Δρ/ρ 0 of O (10−3), the flow fields at the confluence are largely controlled by buoyancy forces. Atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms, leading to localized high mixing rates. Mixing, in general, though is weak at this time of the year. In fall and early winter, density differences are largely controlled by conductivity differences between the incoming flows. The warmer Ebro water, with larger thermal inertia, flows beneath the colder Segre water. The spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong, likely as a result of mixed water being denser than either of the incoming flows.

Wetland inundation monitoring by the synergistic use of ENVISAT/ASAR imagery and ancilliary spatial data

María Jesús Samper — Mon, 29 Jun 2020 08:32:51 +0200

Wetlands are among the most ecologically important ecosystems on Earth and their sustainability depends critically on the water resources. In a scenario of climate change and increased anthropogenic pressure, detailed monitoring of the water resources provides a fundamental tool to assess the ecosystem health and identify potential threats.

Doñana wetlands, in Southwest Spain, dry out every summer and progressively flood in fall and winter to a maximum extent of 30,000 ha. The wetland filling up process was monitored in detail during the 2006–2007 hydrologic cycle by means of twenty-one Envisat/ASAR scenes, acquired at different incidence angles in order to maximize the observation frequency. Flood mapping from the two uncorrelated ASAR channel data alone was proved unfeasible due to the complex casuistic of Doñana cover backscattering. This study addresses the synergistic utilization of the ASAR data together with Doñana's digital elevation model and vegetation map in order to achieve flood mapping.

Filtering and clustering algorithms were developed for the automated generation of Doñana flood maps from the ASAR images. The use of irregular filtering neighborhoods adapted to the elevation contours drastically improved the ASAR image filtering. Edge preservation was excellent, since natural edges closely follow terrain contours. Isotropic neighborhoods were assumed of a single class and their intensities were averaged. As a result, intensity fluctuations due to speckle and texture over areas of the same cover type were smoothed remarkably.

The clustering and classification algorithm operate on individual sub-basins, as the pixel elevation is more accurately related to the cover classes within them. Vegetation and elevation maps plus knowledge of Doñana backscattering characteristics from preceding studies were initially used to select seed pixels with high confidence on their class membership. Next, a region growing algorithm extends the seed regions with new pixels based on their planimmetric adjacency and backscattering Mahalanobis distance to the seeds.

During the seed region growth, new pixels' possible classes are not constrained to their cover type according to the vegetation map, so the algorithm is able to capture temporal changes in the vegetation spatial distribution. Comparison of the resultant classification and concurrent ground truth yielded 92% of flood mapping accuracy. The flood mapping method is applicable to the available ASAR images of Doñana from six other hydrologic cycles.

Physical and chemical characterisation of superficial sediment of the Ribarroja Reservoir (River Ebro, NE Spain)

María Jesús Samper — Fri, 26 Jun 2020 14:42:11 +0200

Con el objetivo de determinar la heterogeneidad especial de las características físicas y químicas de los sedimentos superficiales del embalse de Ribarroja, se ha analizado la distribución de tamaños de las partículas y las concentraciones de los elementos mayoritarios (Al, Fe, Si, K, Ca, Mg, Ti, Mn, P) y minoritarios (Mo, Nb, Zr, Y, Sr, Rb, Th, Pb, Sn, Ce, Ga, Zn, W, Cu, Co, Ni y V) en dieciséis puntos distribuidos a lo largo del embalse. Globalmente, el sedimento era bastante homogéneo:

el tamaño del grano estaba dominado por limo y arcilla, y los componentes de mayor tamaño se relacionan con una mezcla de carbonatos y silicatos meteorizados, con niveles moderados de nitrógeno y fósforo. Las concentraciones de los elementos minoritarios no sobrepasaron los niveles de Nivel de Toxicidad Probable, aunque el níquel y el cromo sobrepasaron el Nivel Umbral de Toxicidad. Un análisis de Componentes Principales realizado con los valores normalizados de los elementos minoritarios ha mostrado cuales son los principales procesos que determinan la variabilidad en la composición de los sedimentos, el grado de alteración/meteorización de los materiales y la asociación de algunos metales a los agregados orgánicos/arcilla.

Integration of 1D and 2D finite volume schemes for computations of water flow in natural channels

María Jesús Samper — Fri, 26 Jun 2020 14:09:40 +0200

A wide variety of flood simulation models are available nowadays. Some of them use a 1D approach and others a 2D one, but there are also some which allow the performance of integrated 1D–2D simulations. These latter models, which have important advantages in optimizing computational costs, commonly use the 1D approach in river channels and the 2D one in floodplains. The coupling of 1D and 2D flows usually ensures mass conservation and makes use of simplified weir-type or friction slope equations, but neglects momentum transfer between the two domains. This paper presents a fully conservative method for the coupling of 1D and 2D domains to be used in numerical schemes based on finite volumes. The method, based on a discretization of the numerical fluxes which ensures the conservation of mass and momentum, is verified with simple test cases. The proposed scheme is compared with the standard method based on the source term of the equations and is applied to the hydrodynamic characterization of a river-reservoir system situated in the River Ebro in Spain.

Water temperature modeling in the lower Ebro river (Spain) : heat fluxes, equilibrium temperature, and magnitude of alteration caused by reservoirs and thermal effluent

María Jesús Samper — Fri, 26 Jun 2020 13:56:34 +0200

The lower Ebro River (Spain) is subject to the thermal and hydrological alterations caused by the system of reservoirs of Mequinenza, Riba‐roja, and Flix and to the thermal effluent of the nuclear power plant of Ascó, located 5 km downstream from the three reservoirs. In this paper, a modeling approach is used to determine the equilibrium temperature and recuperation distance at the study reach for different seasons and hydrological years. The intensity of the alteration caused by the reservoirs and nuclear power plant effluent is studied in reference to the equilibrium temperature and discharge rate. Mean daily water temperature downstream from the reservoirs is higher than mean daily equilibrium temperature in the fall and is lower during the rest of the year. Changes in the heat fluxes induced by thermal alteration are also addressed, showing important variations in evaporation and conduction to the atmosphere.

Comparison of Models for Calculation of Diel Sediment-Water Heat Flux from Water Temperatures

María Jesús Samper — Fri, 26 Jun 2020 13:44:07 +0200

We investigated sediment-water heat flux estimation in relation to applications (hydrodynamics simulation, evaporation studies, or global change effects assessment) in which sediment temperatures are not available because of technical complications (difficulties in installing sensors, for example) or because of methodology used (remote sensing, for example). We used field sediment temperature data measured every 10 min to 1 m depth at Doñana National Park marshland to obtain sediment thermal properties and to calculate diel sediment-water heat exchange through Beck’s sequential function specification method. We compare four models for the simulation of sediment-water heat flux by using surface temperatures. Two models need initial estimated inside sediment temperatures; the other two do not. Influence of estimated initial temperature profiles depends on the temperature distribution assumption and is significant for three days or fewer at the daily timescale. A model that does not use initial sediment temperatures provides accurate estimations with low computation time.

Ecuación de Jacobi. Puntos conjugados.

Antonio Lechuga — Fri, 26 Jun 2020 13:39:02 +0200

Una revisión de algunos problemas ingenieriles
usando la ecuación de Jacobi.

Experimental study of subcritical dividing flow in an equal-width four-branch junction

María Jesús Samper — Fri, 26 Jun 2020 13:28:00 +0200

An experimental study of the subcritical dividing flow in an equal-width, four-branch junction with two inflows and two outflows is presented. A brief description of the flow characteristics is given. From the analysis of test data, a linear relationship among five nondimensional parameters, including inflow ratio, outflow ratio, Froude number of inflow in one direction, outflow depth ratio, and outflow aspect ratio, is proposed and proven to successfully predict the flow distribution in the junction. It can be used to solve the dividing flow problem in a street junction, and it can be included as a part of a numerical model of a street network involving subcritical flows.

Experimental and numerical modelling of symmetrical four-branch supercritical cross junction flow

María Jesús Samper — Fri, 26 Jun 2020 13:14:29 +0200

The discussers congratulate the authors for their work and would like to raise some questions and comments about their results ans statements. The papers compares the computed with the measured flow depths for five flow configurations and the computed with the measured discharges for more than 200 configurations. The authors found discrepancies concerning the prediction of the location and the thickness of the oblique jumps mainly because these are set on one cell in the numerical model.

ASAR polarimetric, multi-incidence angle and multitemporal characterization of Doñana wetlands for flood extent monitoring

María Jesús Samper — Fri, 26 Jun 2020 12:56:14 +0200

Doñana National Park wetlands, in South West Spain, undergo yearly cycles of inundation and drying out. During the hydrological year 2006–2007, 43 ASAR/Envisat images of Doñana, mostly in HH and VV polarizations, were acquired with the aim to monitor the flood extent evolution during an entire flooding cycle. The images were ordered in the seven ASAR incidence angles, also referred to as swaths, to achieve high observation frequency.

In this study, backscattering temporal signatures of the main land cover types in Doñana were obtained for the different incidence angles and polarizations. Plots showing the σ0HH/σ0VV ratio behavior were also produced. The signatures were analyzed with the aid of miscellaneous site data in order to identify the effect of the flooding on the backscattering. Conclusions on the feasibility to discriminate emerged versus flooded land are derived for the different incidence angles, land cover types and phenological stages: intermediate incidence angles (ASAR IS3 and IS4) came up as the most appropriate single swaths to discriminate open water surface from smooth bare soil in the marshland deepest areas. Flood mapping in pasture lands, the most elevated regions, is feasible at steep to mid incidence angles (ASAR IS1 to IS4). In the medium elevation zones, colonized by large helophytes, shallow incidence angles (ASAR IS6 and IS7) enable more accurate flood delineation during the vegetation growing phase.

Since Doñana land covers require different observation swaths for flood detection, the composition of different incidence angle images close in time provides the optimum flood mapping. Such composition is possible four times per ASAR 35-day orbit cycle, using pairs of 12-h apart IS1/IS6 and IS2/IS5 Doñana images.

Temporal variability in the thermal regime of the lower Ebro River (Spain) and alteration due to anthropogenic factors

María Jesús Samper — Fri, 26 Jun 2020 12:43:32 +0200

The Ebro River is one of the longest rivers in Spain and it also has the greatest discharge. Its lower part is highly regulated and includes a system of three reservoirs (Mequinensa, Riba-roja and Flix). The water temperature is altered because of the release of hypolimnetic water and the use of water for cooling at the Ascó nuclear power plant. The thermal regime of the lower Ebro River on different time scales and the changes caused by anthropogenic factors, especially the system of reservoirs and the thermal effluent of the nuclear power plant, have been studied by installing a net of water temperature measuring stations and by using historical water temperature data provided by the thermal power plant at Escatrón. An increase of 2.3ºC in the mean annual water temperature could be demonstrated in the period 1955–2000 at this site. The effects of the system of reservoirs and of the nuclear power plant were the usual for this kind of structures and could be detected many kilometres downstream. In the summer, the cooling effect of the reservoirs and the warming effect of the nuclear power plant compensated each other. In winter, the warming effect of both summed up

Developing Flow Region and Pressure Fluctuations on Steeply Sloping Stepped Spillways

María Jesús Samper — Fri, 26 Jun 2020 12:31:20 +0200

The hydrodynamic pressure field is important for the design and safety of steeply sloping stepped spillways, which are typically designed for considerably lower maximum specific discharges than smooth spillways. The hydraulic performance of stepped spillways at high velocities may compromise its use due to major concern with safety against cavitation damage. Hydraulic model investigations were conducted in different large-size stepped chutes to characterize the nonaerated flow region which is potentially prone to cavitation damage and the pressure field acting on the step faces. The clear water depths and energy dissipation in the developing flow region are described in terms of integral measures of the turbulent boundary layer. Expressions for the location of and the flow depth at the inception point of air entrainment are derived. Pressure distributions on the horizontal and vertical faces of the step along the spillway are presented. Measurements indicated a different behavior of the pressure field in the aerated and nonaerated flow region. The mean and fluctuating pressure coefficients along the spillway are approximated by a regression function. The vertical face near the outer step edge close to the inception point of air entrainment is identified as a critical region for predicting cavitation inception in flow over stepped spillways. From the analysis of the pressure fluctuations in that region a maximum velocity of 15 m/s is proposed as a criterion to avoid extreme negative pressures in typical prototype steeply sloping stepped spillways, eventually leading to the occurrence of cavitation in the nonaerated flow.

Variabilidad temporal en el comportamiento hidráulico del curso inferior del río Ebro

María Jesús Samper — Fri, 26 Jun 2020 11:50:25 +0200

La presencia de los embalses de Mequinenza, Riba-Roja y Flix en el tramo inferior del río Ebro ha dado lugar a una serie de alteraciones (reducción de la entidad y frecuencia de las avenidas, estabilización de caudales, etc.) que han favorecido el desarrollo de macrófitos sumergidos. La viabilidad diaria y anual del régimen de caudales del tramo inferior del río Ebro como consecuencia de la regulación ejercida por los embalses se analiza en este trabajo. Los principales efectos son: importantes variaciones diarias de caudal, un menor caudal los fines de semana y la estabilización de los caudales mínimos. Asimismo, se analiza la influencia de la proliferación de macrófitos en el comportamiento hidráulico del río. En el período estudiado (200-2006) el río presentó dos tipos de comportamiento hidráulico: un comportamiento que puede considerarse como ordinario, con el cauce limpio, y otro más lento, caracterizado por menores velocidades y mayores calados para un mismo caudal. Este segundo comportamiento coincidió con épocas de sequía, con caudales bajos durante gran parte del año y proliferación de macrófitos. El paso del comportamiento lento al comportamiento ordinario se produce con avenidas importantes, que tienen el poder suficiente para arrastrar a los macrófitos.

Dam wall break simulation by means of a high resolution algorithm applied to the La Parota dam, Guerrero, Mexico

María Jesús Samper — Fri, 26 Jun 2020 11:40:08 +0200

L'abastament d'aigua a Catalunya i la seva garantia

María Jesús Samper — Fri, 26 Jun 2020 11:25:20 +0200

Genetic programming and standardization in water temperature modelling

María Jesús Samper — Fri, 26 Jun 2020 10:51:55 +0200

An application of Genetic Programming (an evolutionary computational tool) without and with standardization data is presented with the aim of modeling the behavior of the water temperature in a river in terms of meteorological variables that are easily measured, to explore their explanatory power and to emphasize the utility of the standardization of variables in order to reduce the effect of those with large variance. Recorded data corresponding to the water temperature behavior at the Ebro River, Spain, are used as analysis case, showing a performance improvement on the developed model when data are standardized. This improvement is reflected in a reduction of the mean square error. Finally, the models obtained in this document were applied to estimate the water temperature in 2004, in order to provide evidence about their applicability to forecasting purposes.

Preserving steady-state in one-dimensional finite-volume computations of river flow

María Jesús Samper — Fri, 26 Jun 2020 10:44:00 +0200

When using finite-volume methods and the conservative form of the Saint Venant equations in one-dimensional flow computations, it is important to establish the correct balance between the discretized flux vector and the geometric source terms. Over the last few years various improvements to numerical schemes have been presented to achieve this correct balance, focusing on the capability to simulate water at rest on irregular geometries (C-property). In this paper it is shown that common schemes can lead to energy-violating solutions in the case of steady flow. We present developments based on the Roe TVD finite-volume scheme for one-dimensional Saint Venant equations, which results in a method that not only satisfies the C-property, but also preserves the correct steady flow when stationary boundary conditions are used. We also present a totally irregular channel test case for the verification of the method.

Hydraulic management drives heat budgets and temperature trends in a Mediterranean reservoir

María Jesús Samper — Fri, 26 Jun 2020 10:31:02 +0200

By contrast to the more regular and predictable temperate lakes, heat budgets and temperature dynamics in Mediterranean reservoirs are characterized by a marked interannual variability. In the present paper, the heat content, annual Birgean heat budget (ABHB), and thermal structure of Sau Reservoir were examined during a period of hypolimnetic withdrawal between 1980 and 1985, and during a period of withdrawal at intermediate depths between 1996 and 2003. The two study periods were also characterized by a wide range of stored water volume fluctuations. Results were used to develop and validate an empirical model to predict annual and monthly heat dynamics statistics and mixed layer depth as a function of hydraulic management parameters such as water volume and selective withdrawal depth. During the hypolimnetic withdrawal period elevated ABHB and deep mixed layer depths were recorded in the reservoir, which behaved as a heat trap. By contrast, intermediate depth withdrawal promoted a shallower and more stable thermocline, thus increasing the cold hypolimnetic water volume and decreasing heat content and ABHB. The study reveals that hydraulic management constitutes the main driver of the heat and thermal dynamics in reservoirs with multiple withdrawal outlets. By contrast with the increasing temperature trends recorded in many natural lakes, the hydraulic management in Sau Reservoir induced a progressive reduction in water temperature and heat content in the system, thus partially counteracting the possible deleterious effects of global warming. Our intensive study in a single, highly‐dynamic ecosystem constitutes a new approximation to the study of thermal structure and heat dynamics in water bodies.

Analysis of pressures on a stepped spillway

María Jesús Samper — Fri, 26 Jun 2020 10:16:05 +0200

The pressure profiles on the horizontal and vertical step faces of a stepped spillway in the fully developed skimming flow region were measured. These observations allow to provide generalized equations that enable designers to analyze the amount and the location of negative pressures along the steps. The step locations subjected to negative pressure are also discussed including an analysis of the duration of the negative pressures. On the horizontal step faces, negative pressure were only observed on their upstream half for dimensionless discharges larger than dc/h = 1.3. On the vertical step faces, only the region close to the adjacent horizontal face was not subjected to negative pressure

Dynamic thermal simulation of a solar chimney with PV modules

María Jesús Samper — Fri, 26 Jun 2020 09:52:27 +0200

The issue of this paper is to present theoretical results for a solar chimney with thermal mass, where the glass surface is replaced by photo-voltaic (PV) modules. A portion of the heat ab-sorbed by the PV modules is dissipated to the air channel in convective form, and it exchanges radiation heat with the concrete wall. These cooling phenomena for the PV modules im-prove their efficiency with a lower working temperature. Both phenomena are heating proc-ess to the air and the concrete wall, that produce natural ventilation. The solar chimney is sup-posed to be isolated from any building. The re-sults obtained are an average air mass flow rate around 0.02 kg/s along the day and night, and a maximum PV temperature of 321 K.

Dynamic physical model for a solar chimney

María Jesús Samper — Fri, 26 Jun 2020 09:37:08 +0200

The aim of this research is to investigate the theoretical usefulness of a solar chimney with thermal inertia applied to the Mediterranean climates, offering nocturnal ventilation benefits. A mathematical dynamical model is proposed to evaluate the energy performance of a solar chimney with 24 cm concrete wall as storage surface for solar radiation. The results obtained with the proposed model are coherent with several models response and experiments reported on solar chimneys. As well, the difference of the proposed model to others is the incorporation of an unsteady state and the inclusion of thermal inertia. The results show that for a 2 m height and width of air channel of 14.5 cm, 0.011 kg/s air mass flow rate is obtained for 450 W/m2. The 24 cm thickness concrete wall, reaches its greater temperature 2 h later with respect to the maximum ambient temperature, maintaining its temperature over the beginning of the night, so nocturnal ventilation is achieved. The model shows the interest in continuing investigating on this cooling techniques and to built a solar chimney with thermal inertia for future experimental research.

Transfer of low-cost plastic biodigester technology at household level in Bolivia

María Jesús Samper — Fri, 26 Jun 2020 09:07:19 +0200

The familiar low cost biodigestores are an important tool for urban and rural development. These natural systems fulfill the criteria of appropriate technology, producing biogás and improved natural fertilizer, through the anaerobic digestion of organic remainders. Also the health and hygiene of the families, as well as of the cattle, affect positively, being a sustainable handling environmentally of remainders that otherwise are polluting. The potential of this technology in Bolivia is very high, and once it has already been developed to cover the three different regions (Altiplane, valley and jungle), validated like development tool and accepted by the families, it is precise `to popularize the technology', so to make it accessible to the greater number of people. In this article a review to the development of the biodigestores in Bolivia, its adaptation to high altitudes, as well as the learned lessons is made, to in the end comment the first steps that already are occurring to popularize this technology.

Reduced hydraulic retention times in low-cost tubular digesters: Two issues

María Jesús Samper — Fri, 26 Jun 2020 08:50:56 +0200

Low-cost tubular digesters have been implemented in several developing countries. One of the problems reported from field surveys is that biogas generation that does not meet the user’s expectations. This report provides two reasons for the discrepancy between the biogas generation rate estimated in the design phase and the actual rate measured after construction, due to a lower final hydraulic retention time (HRT). The hydraulic retention time is normally determined from the liquid volume calculated from the cylindrical shape of the bag and not from the trench dimensions. The result is a reduction in HRT of 6%–51%, depending on the dimensions of the trench recommended by various authors. Another factor that is not normally considered is the influence of the biogas pressure on the liquid level inside the digester which negatively affects the liquid volume of the digester, reducing HRT by as much as 15%.

Towards thermal design optimization of tubular digesters in cold climates: A heat transfer model

María Jesús Samper — Fri, 26 Jun 2020 08:30:58 +0200

A cold climate, low cost, tubular digester is monitored and temperatures from different parts of the slurry, greenhouse, and adobe walls are presented, discussing the thermal performance of the digester. The slurry exhibits a vertical gradient of 6 °C, with a mean value of 24.5 °C, while the ambient temperature varies from 10 °C to 30 °C, showing the efficiency of the system as a solar heat collector with thermal inertia. A simple time-dependent thermal model is developed using inputs of solar radiation, wind velocity, ambient temperature, and digester geometry. The model outputs include temperatures of the slurry, the biogas, its holding membrane and the greenhouse air, wall and cover. Radiative, convective and conductive heat transfer phenomena are considered between all system elements. The model has 0.47 °C (2%) standard error for the average slurry temperature. This model can be used to predict the influence of geometry and materials on the performance of the digester.

Scour of rock due to the impact of plunging high velocity jets Part I: A state-of-the-art review

María Jesús Samper — Thu, 25 Jun 2020 17:10:18 +0200

This paper presents the state-of-the-art on methods to estimate rock scour due to the impingement of plunging high velocity water jets. The following topics are addressed: empirical formulae, semi-empirical and analytical approaches, determination of extreme pressure fluctuations at plunge pool bottoms and, finally, the transfer of these pressure fluctuations in joints underneath concrete slabs or rock blocks. Available methods on rock scour have been thoroughly investigated on their ability to represent the main physical-mechanical processes that govern scour. This reveals lack of knowledge on turbulence and aeration effects, as well as on transient pressure flow conditions in rock joints. These aspects may significantly influence the destruction of the rock mass and should be accounted for in scour evaluation methods. Their relevance has been experimentally investigated by dynamic pressure measurements at modeled plunge pool bottoms and inside underlying one-and two-dimensional rock joints. Test results are described and discussed in Part II of this paper.

A methodological approach to the reconstruction of the 1949-2000 water temperature series in the Ebro River at Escatrón

María Jesús Samper — Thu, 25 Jun 2020 16:50:55 +0200

Water temperature is one of the main variables influencing freshwater organisms. In recent years, increasing water temperature trends have been observed in freshwater ecosystems worldwide. To assess the importance of such increases, they need to be put into context by studying long-term water temperature variability. However, it is often difficult to obtain a sufficiently long data series. This problem may be overcome by using data collected by different entities in different periods of time. However, this can lead to data comparability problems. In this paper, we present the process of reconstructing the annual mean water temperature series for the Ebro River at Escatron using data measured by the Escatr ´ on power station, the Ebro Hydrographic ´ Confederation and the Flumen research group. The data sources used different measurement methodologies and frequencies. Therefore, particular attention was paid to analysing the bias of the annual mean water temperature estimations calculated from the available historical data. A resampling methodology applied to 10-minute water temperature data was performed to assess the quality of the different estimators used to calculate the annual mean water temperature. Only one of the three estimators used could be considered to be unbiased. Another estimator could be corrected by estimating its bias. As a result, it was possible to reconstruct a water temperature series from 1949 to 2000, with a gap in the period 1978-1995. The series shows that there was an increase of 2.3◦C in the period 1955-2000.

Pressures on a stepped spillway

María Jesús Samper — Thu, 25 Jun 2020 16:19:31 +0200

This paper presents an analysis of the pressure field on the steps of a stepped spillway on an RCC dam. The fundamental component of this study is a series of pressure measurements obtained in the fully-developed zone of skimming flow regime along a reduced-size stepped spillway model constructed at the Department of Hydraulic, Maritime and Environmental Engineering Laboratory of the Technical University of Catalonia (UPC). A mathematical model was fitted to the pressure profiles on the horizontal and vertical faces of the steps. This equation allows designers to estimate the possible negative values of pressure on the vertical and horizontal faces of the steps in the fully-developed zone of skimming flow regime.

Surface stormwater hazard assessment in steep urban areas. case of the city of mendoza, argentina

María Jesús Samper — Thu, 25 Jun 2020 16:12:44 +0200

The current paper focusses on the hazard assessment associated with urban runoff on streets. A review of the existing criteria to evaluate such a hazard is made. Two new criteria based on theoretical analysis of the water's force acting on a static pedestrian are presented: no slipping criterion and stability to tilt criterion. According to these criteria, either a maximum depth, or a maximum velocity or some relation between depths and velocities should be fulfilled in order to guarantee the pedestrian's and driver's safety in the case of medium to large storms. A one-dimensional numerical model is used to solve the urban storm runoff within a street network. This model is applied to an urban watershed of the city of Mendoza (Argentina), obtaining the runoff values belonging to return periods of 5, 10 and 25 years. The results are evaluated bearing in mind four hazard criteria. In conclusion, the numerical model is shown to be a useful tool in relation to the application of the hazards criteria. It is also conclude that an effort should be made to determine specific hazard criteria based on experimental data.

M.D.T. de precisión de la marisma del P.N. de Doñana mediante Láser Escáner Aerotransportado (LiDAR)

María Jesús Samper — Thu, 25 Jun 2020 16:07:44 +0200

Desde su implementación hace más de diez años , la adquisición de datos mediante Láser-Escáner Aerotransportado (ALS-LIDAR) se ha ido revelando como una técnica geomática precisa y de alta productividad. Las múltiples aplicaciones en las cuales se ha probado dicha tecnología junto con su precisión y resolución son su punto fuerte frente a otras técnicas (fotogrametría-teledetección).

Dentro del Proyecto Doñana 2005, la Confederación Hidrográfica del Guadalquivir en el año 2002 ejecutó un vuelo LIDAR que cubrió la marisma y áreas inundables del Parque Nacional de Doñana cubriendo un total de 55455 Ha.

Se ha procedido a su análisi, validación, calibración y mejora. El volumen total de puntos es de varios centenares de millones. Lo masivo de la información disponible dificulta el análisis desde la propia visualización hasta las comparaciones entre modelos o con información georeferenciada independiente. Por lo que se han tenido que elaborar programas de visualización y estrategias específicas. La calibración interna y externa de los resultados (control de la calidad) ha consistido en verificar su precisión altimétrica y planimétrica y su distribución o densidad espacial. También se ha comprobado si la campaña ha cubierto la zona deseada y el mayor o menos éxito de los filtros y la clasificación en función de la penetración del láser en algunas zonas con vegetación. La riqueza de la información geométrica disponible (1 punto cada 3 m2 en media) conduce a un cambio en la filosofía de las comprobaciones : en lugar de concentrarnos en la comparación de las coordenadas de un número reducido de puntos se pasa a comparar los valores generales de algunas superficies o líneas, en términos estadísticos. Se han podido comparar de esta manera las coordenadas de cerca de 18000 puntos con los valores del vuelo LIDAR en su entorno. En conjunto las disparidades se enmarcan perfectamente en las precisiones esperadas para la técnica.

Además de las comprobaciones cuantitativas fruto del extenso conocimiento de la zona y de su topografía extremadamente plana, se han podido llevar a cabo validaciones de tipo cualitativo y de tipo mixto. Enlazando con el empleo futuro del MDT en aplicaciones de hidrología superficial se han usado técnicas de inyección de agua por puntos bajos interiores e inundación progresiva del entorno para visualizar posibles errores en los contornos. También se han obtenido las curvas de capacidad de embalse de la marisma en función de una cota ficticia de inundación común en todo el área. El conjunto de las verificaciones ha significado la introducción de diversas mejoras en áreas concretas del MDT.

Diseño hidráulico de aliviaderos escalonados en presas de HCR

María Jesús Samper — Thu, 25 Jun 2020 15:56:02 +0200

En el presente artículo se presenta un conjunto de criterios para el diseño de aliviaderos escalonados con pendientes típicas de presas de hormigón compactado con rodillo. El texto se basa en Información recogida en la bibliografía e incorpora los resultados del trabajo de Investigación desarrollado en Amador (2005). Se aborda el diseño de la cresta, la selección del ancho del aliviadero y la geometría de los peldaños. Se proponen criterios para determinar el tipo de flujo existente sobre la estructura y, en el caso del flujo rasante, se presentan expresiones que permiten obtener las principales características del flujo a lo largo de la rápida. Con ha.se en los resultados del análisis del campo de presiones se proponen un conjunto de ecuaciones para estimar las solicitaciones que el vertido ejerce sobre los peldaños a lo largo del aliviadero. Finalmente se hacen algunas consideraciones sobre los aspectos constructivos de los aliviaderos escalonados.

Characterization of the nonaerated flow región in a stepped spillway by PIV

María Jesús Samper — Thu, 25 Jun 2020 15:45:58 +0200

The development of the roller-compacted concrete (RCC) as a technique of constructing dams and the stepped surface that results from the construction procedure opened a renewed interest in stepped spillways. Previous research has focused on studying the air-water flow down the stepped chute with the objective of obtaining better design guidelines. The nonaerated flow region enlarges as the flow rate increases, and there is a lack of knowledge on the hydraulic performance of stepped spillways at high velocities that undermines its use in fear of cavitation damage. In the present, study the developing flow region in a stepped channel with a slope 1v:0.8h is characterized using a particle image velocimetry technique. An expression for the growth of the boundary layer thickness is proposed based on the streamwise distance from the channel crest and the roughness height. The local flow resistance coefficient is calculated by application of the von Kármán integral momentum equation. The shear strain, vorticity, and swirling strength maps obtained from the mean velocity gradient tensor are presented. Also, the fluctuating velocity field is assessed. The turbulent kinetic energy map indicates the region near the pseudobottom (imaginary line joining two adjacent step edges) as the most active in terms of Reynolds stresses. The turbulence was found to be very intense with maximum levels of turbulence intensity from 0.40 to 0.65 measured near the pseudobottom. Finally, the quadrant analysis of the velocity fluctuations suggests the presence of strong outflows of fluid from the cavities as well as inflows into the cavities. It is conjectured that the mass transfer/exchange between cavities and main stream, play an important role in the high levels of turbulent energy observed.