Scipedia: Documents published in 2020

Análisis de la peligrosidad de la escorrentía pluvial en zona urbana utilizando un enfoque numérico-experimental

María Jesús Samper — Thu, 25 Jun 2020 15:30:47 +0200

La escorrentía en las calles a causa de la lluvia es un hecho frecuente en ciudades con sistemas de drenaje pluvial tanto superficial como subterráneo. Los sistemas superficiales se utilizan para transportar las aguas pluviales hasta ejes drenantes de mayor importancia y los subterráneos llevan el agua pluvial por un camino superficial antes de llegar a los imbornales o bocas de tormenta. En el presente artículo se describen las características de la escorrentía en calles y se presenta un modelo numérico creado para analizar el flujo en redes de calles, utilizando los resultados experimentales obtenidos en cruces de calles con relaciones de anchos 1:1, 1:2 y 2:3, e incluyendo la simulación numérica de la captación de caudales a través de imbornales. Se describen los criterios existentes para evaluar el riesgo del flujo en las calles, dos de los cuales son de desarrollo propio. Como ejemplo de aplicación se presenta el análisis del flujo en una red de calles de la ciudad de Mendoza, Argentina, con el modelo numérico descrito. Se analizan los resultados en función de cuatro criterios de riesgo diferentes. Se concluye que el modelo presentado es una herramienta sumamente útil para el análisis del flujo en redes de calles, la aplicación de los criterios de riesgo existentes y para estudiar la influencia del número de imbornales en el cumplimiento de dichos criterios.

Alteración que presenta la temperatura del agua por la existencia de embalses

María Jesús Samper — Thu, 25 Jun 2020 15:06:45 +0200

El agua de un río mantiene un intercambio de calor con la atmósfera y con el fondo. Este proceso puede alcanzar su equilibrio siempre y cuando las características geológicas y climáticas a lo largo de un tramo del río sean similares. Sin embargo, el comportamiento térmico del río puede ser alterado por los diversos usos del agua, como la refrigeración de centrales termoeléctricas, o por el almacenamiento del agua debido a la existencia de una presa. En el caso de las regiones con clima mediterráneo, donde se encuentran los embalses aquí estudiados, los efectos de las alteraciones en el régimen térmico, resultado a su vez de dichos embalses, provocan una tendencia a la constancia térmica estacional (elevación de las temperaturas invernales y disminución de las temperaturas estivales) y también un aumento de la uniformidad diaria. Aguas abajo de las termoeléctricas o presas, la temperatura del agua evoluciona de tal forma que tiende a lograr el equilibrio con el medio ambiente.

Plunge pool pressures due to a falling rectangular jet

María Jesús Samper — Thu, 25 Jun 2020 14:58:43 +0200

This Technical Note presents the results of a large set of laboratory tests and tries to determine the conditions that ensure the existence of a water cushion that is not expelled by the impingement of a nappe (effective cushion). Second, values of mean dynamic pressures at the stagnation point of nonsymmetric plunge pools downstream of arch dams are assessed.

Experimental study of transition and skimming flows on stepped spillways in rcc dams: qualitative analysis and pressure measurements

María Jesús Samper — Thu, 25 Jun 2020 14:53:14 +0200

This paper is a presentation of an experimental facility used for analyzing the flow over the stepped spillway of an RCC dam. The technique of digitizing video sequences is applied in order to obtain a qualitative analysis of the hydraulic behavior of these chutes. The onset of skimming flow is studied using two different criteria: a visual criterion and another that makes use of the pressure pattern on the horizontal faces of the steps. The evolution along the length of the spillway of pressures measured on the centers of symmetry of the horizontal faces of the steps is also analyzed to obtain further conclusions regarding hydraulic behavior during a skimming flow regime.

Experimental study of the dividing flow in steep street crossings

María Jesús Samper — Thu, 25 Jun 2020 14:39:22 +0200

A first approach to the experimental study of the dividing flow in steep street crossings is presented. A scale model of a street crossing has been built and a series of tests has been conducted for studying the flow pattern in the crossing when the flow in the streets is supercritical. It has been found that the inflow power ratio at the entrance of the crossing can be used as a non-dimensional parameter for predicting the flow distribution as well as the flow pattern. On the basis of these relationships a one-dimensional formulation can be developed in order to predict the dividing flow in crossings.

Incidencia de los embalses en el régimen térmico del río. El caso del tramo final del río Ebro

María Jesús Samper — Thu, 25 Jun 2020 14:27:50 +0200

El agua del río mantiene un intercambio de calor con la atmósfera y con el fondo; este proceso puede lograr su equilibrio a lo largo de un tramo de río con características geológicas y climáticas similares. Su comportamiento puede ser modificado por efectos artificiales; tales como el uso del agua del río para refrigeración de centrales termoeléctricas o por la existencia de embalses. En el caso de regiones con clima mediterráneo, donde se encuentran los embalses aquí estudiados, los efectos de las alteraciones en el régimen térmico producido por éstos, provocan una tendencia a la constancia térmica estacional, un aumento de la uniformidad diaria, una elevación de las temperaturas invernales y la disminución de las temperaturas estivales. La temperatura del agua es un factor importante en la ecología del río, ya que condiciona en gran medida la vida de las comunidades fluviales. Aguas abajo de las termoeléctricas o presas, la temperatura del agua evoluciona de tal forma que tiende a lograr el equilibrio con el medio ambiente.

Hydraulic behaviour of the diversion conduit gates at Caruachi

María Jesús Samper — Thu, 25 Jun 2020 14:23:13 +0200

Discharge and aeration capacity of the diversion conduits, the pressures in the ducts and vibration and downpull forces on the gates were analyzed. Model studies were carried out to study the hydraulic behavior of the gates used for closing operation. The model was built to a scale of 1:20 and was based on Froude's similarity criterion. The tests consisted of picking up the vibratory vertical response to impulsive stimulations, using accelerometers. Results indicated that on the current model, no interference was caused by the uncertainties related to the modelled areas up- and downstream

Presa de Caruachi. Estudio del comportmiento hidráulico de las compuertas de los conductos de desvío

María Jesús Samper — Thu, 25 Jun 2020 14:16:18 +0200

Se presenta el estudio desarrollado en el Laboratorio de Modelos Reducidos del Dpto. de Ing. Hidráulica, Marítima y Ambiental de la UPC, para el análisis del comportamiento hidráulico de las compuertas a utilizar en la operación de cierre de la presa de Caruachi en el río Caroní (Venezuela). Se han analizado la capacidad de desagüe, la aireación de los conductos de desvío, las presiones en los mismos durante la operación, así como las vibraciones y fuerza vertical descendente (down-pull) sobre las compuertas. The present article describes the study carried out at the Reduced Model Laboratory (Hydraulic, Maritime and Environmental Engineering Department, UPC) into the hydraulic behaviour of the gates employed in the closing operation of the Caruachi dam on the River Caroní (Venezuela). An analysis is made of the discharge and aeration capacity of the diversión conduits, the operation pressures in the same and the vibration and down-pull on the gates.

El episodio pluviométrico del 10 de junio en Cataluña: un primer estudio hidrometeorológico

María Jesús Samper — Thu, 25 Jun 2020 14:06:49 +0200

En este artículo se estudia el episodio pluviométrico registrado el día 10 de junio de 2000 en Cataluña. En particular se analiza la lluvia caída en la cuenca de 97 km2 correspondiente a la riera de Magarola en su cruce con la autovía N-II en Esparreguera (Barcelona). Para ello se utiliza información de campo obtenida a partir de radar meteorológico, pluviómetros y pluviógrafos. Sobre la mayor parte de la cuenca se recogieron más de 150 mm de lluvia durante el episodio. También prácticamente sobre toda la cuenca cayeron más de 100 mm en dos horas. El cálculo del período de retorno asociado a la lluvia de 24 horas evidencia la naturaleza excepcional del fenómeno. No obstante la información histórica disponible es insuficiente y solo permite realizar estimaciones con una gran incertidumbre.

Las aguas superficiales y la marisma del Parque Nacional de Doñana

María Jesús Samper — Thu, 25 Jun 2020 14:01:51 +0200

La Marisma del Parque Nacional de Doñana ocupa aproximadamente 27.000 Ha. y constituye uno de los tres grandes ecosistemas existentes en el parque. En el presente siglo se llevaron a cabo actuaciones que redujeron notablemente su extensión y alteraron sustancialmente sus características hidrológicas. A lo largo de las últimas décadas se han tomado decisiones y realizado obras tendentes a la recuperación de la funcionalidad hidrológica original de la marisma y de este modo poder asegurar su conservación. En el presente artículo se analiza el origen, evolución histórica y estado actual de la hidrología de la marisma. También se describen las obras realizadas para su regeneración hídrica y finalmente se hacen unas consideraciones respecto la necesidad de mejorar el conocimiento cuantitativo del funcionamiento hidráulico e hidrológico de este espacio de características que, sin exageración alguna, pueden calificarse de únicas. The marsh area of Doñana, one of the three large eco-systems of the Park, covers some 65.000 acres. It was considerably reduced in size during this century and its hydrological capacity gravely affected. Work has been carried in recent years to restore the original hydrological function of the marshland and to ensure its survival. The origin, the history and the present state of the marshland are examined in this paper, and an account is given of the work of regeneration. Attention is drawn to the need for more information about the quantitative hydraulic and hydrological features of this unique area.

Consecuencias hidrologicas de los incendios forestales

María Jesús Samper — Thu, 25 Jun 2020 13:56:29 +0200

La alteración del terreno y la destrucción de la vegetación debida a los incendios provoca cambios importantes en las características hidrológicas de las cuencas afectadas. El presente estudio aborda una cuantificación de los efectos inducidos por el fuego en los caudales punta y en la modificación de la respuesta hidrológica de una pequeña cuenca mediterránea francesa gracias a la disponibilidad de una serie de 23 años de datos antes del incendio y de 3 años después. La relevancia de las consecuencias puestas de manifiesto resulta especialmente crítica en la región mediterránea, donde una climatología propicia convierte a la secuencia fuego-inundaciones-erosión en el principal agente del proceso de desertificación.

Problematica del drenaje de aguas pluviales en zonas urbanas y del estudio hidraulico de las redes de colectores

María Jesús Samper — Thu, 25 Jun 2020 13:46:19 +0200

Se analiza la problemática del drenaje de aguas pluviales en zonas urbanas, en particular en áreas de rápido y reciente desarrollo urbano como es el caso del litoral mediterráneo español. Se estudia la repercusión que tiene sobre el drenaje un proceso urbanizador no respetuoso con la hidrología de las cuencas naturales preexistentes. Asimismo se analiza de una forma conceptual la problemática que presenta la modelación numérica de los diferentes procesos involucrados en el drenaje urbano, especialmente el comportamiento hidráulico de las redes de colectores.

Erosion fluvial y proteccion en pilas de puente: modelo reducido del puente nuevo de mora del Ebro

María Jesús Samper — Thu, 25 Jun 2020 13:40:25 +0200

Este trabajo se propone llamar la atención sobre la acción del agua en las pilas de los puentes, origen del fenómeno de la socavación local, responsable de un gran número de fallos de puentes. En primer lugar, se presenta un breve resumen del conocimiento actual sobre la erosión local y las medidas de protección con mantos de escollera. El núcleo del artículo es el caso del puente nuevo de Mora sobre el río Ebro, estudiado en modelo reducido debido a la complejidad geométrica de sus pilas y estribos, con el objetivo de dimensionar unas protecciones de escollera. Se trata de un modelo a escala 65, localmente erosionable, ensayado con aguas claras y con semejanza de Froude. Finalmente se comparan los resultados del modelo con las predicciones sobre erosión local y tamaño de escollera por algunas de las fórmulas más conocidas, quedando de manifiesto el carácter aproximado de estas fórmulas.

Kinematics of the moving hydraulic jump

María Jesús Samper — Thu, 25 Jun 2020 13:33:49 +0200

The moving hydraulic jump, considered as an example of rapidly varied unsteady flow, is theoretically revised. Experimental work is conducted to study the kinematic characteristics of the jump propagation. The position, velocity and conjugate depths of the jump during its propagation either upstream or downstream are investigated and some semi-empirical equations are derived. From this work, some differences are observed between the moving hydraulic jump propagating upstream and downstream. A phenomenon called extinction of the moving hydraulic jump is reported in the latter case. This phenomenon is explained on the grounds of previous theoretical analysis.

R-adaptivity in limit analysis

María Jesús Samper — Thu, 25 Jun 2020 13:01:56 +0200

Direct methods aim to find the maximum load factor that a domain made of a plastic material can sustain before undergoing full collapse. Its analytical solution may be posed as a constrained maximisation problem, which is computationally solved by resorting to appropriate discretisation of the relevant fields such as the stress or velocity fields. The actual discrete solution is though strongly dependent on such discretisation, which is defined by a set of nodes, elements, and the type of interpolation. We here resort to an adaptive strategy that aims to perturb the positions of the nodes in order to improve the solution of the discrete maximisation problem. When the positions of the nodes are taken into account, the optimisation problem becomes highly non-linear. We approximate this problem as two staggered linear problems, one written in terms of the stress variable (lower bound problem) or velocity variables (upper bound problem), and another with respect to the nodal positions. In this manner, we show that for some simple problems, the computed load factor may be further improved while keeping a constant number of elements.

Computation of bounds for anchor problems in limit analysis and decomposition techniques

María Jesús Samper — Thu, 25 Jun 2020 12:58:01 +0200

Numerical techniques for the computation of strict bounds in limit analyses have been developed for more than thirty years. The efficiency of these techniques have been substantially improved in the last ten years, and have been successfully applied to academic problems, foundations and excavations. We here extend the theoretical background to problems with anchors, interface conditions, and joints. Those extensions are relevant for the analysis of retaining and anchored walls, which we study in this work. The analysis of three-dimensional domains remains as yet very scarce. From the computational standpoint, the memory requirements and CPU time are exceedingly prohibitive when mesh adaptivity is employed. For this reason, we also present here the application of decomposition techniques to the optimisation problem of limit analysis. We discuss the performance of different methodologies adopted in the literature for general optimisation problems, such as primal and dual decomposition, and suggest some strategies that are suitable for the parallelisation of large three-dimensional problems. The propo sed decomposition techniques are tested against representative problems.

Stress dependent morphogenesis continuum mechanics and system of trusses

María Jesús Samper — Thu, 25 Jun 2020 12:55:27 +0200

The mechanical analysis of soft tissues in biomechanics has undergone an increasing progress during the last decade. Part of this success is due to the development and application of some techniques of continuum mechanics, in particular, the decomposition of the deformation gradient, and the introduction of mass, density or volume changes in the reference configuration. Resorting to the common terminology employed in the literature, the changes in biomechanical processes may be classified as growth (change of mass), remodelling (change of density or other material properties such as fibre orientation) or morphogenesis (change of shape). Although the use of those concepts in bone and cardiovascular analysis is well extended, their use in morphogenesis during embryo development has been far less studied. The reasons of this fact may be found in the large shape changes encountered during this process, or the complexity of the material changes involved. In this chapter we develop a general framework for the modelling of morphogenesis by introducing a growth process in the structural elements of the cell, which in turn depends on the stress state of the tissue. Some experimental observations suggest this feedback mechanism during embryo development, and only very recently this behaviour has started to be simulated. We here derive the necessary equilibrium equations of a stress controlled growth mechanism in the context of continuum mechanics. In these derivations we assume a free energy source which is responsible of the active forces during the elongation process, and a passive hyperelastic response of the material. In addition, we write the necessary conditions that the active elongation law must satisfy in order to be thermodynamically consistent. We particularise these equations and conditions for the relevant elements of the cytoskeleton, namely, microfilaments and microtubules. We apply themodel to simulate the shape changes observed during embryomorphogenesis in truss element. As a salient result, themodel reveals that by imposing boundary stress conditions, unbounded elongation would be obtained. Therefore, either prescribed displacements or cross-links between fibres are necessary to reach a homeostatic state.

Planar differential growth rates initiate precise fold positions in complex epithelia

María Jesús Samper — Thu, 25 Jun 2020 12:10:20 +0200

Tissue folding is a fundamental process that shapes epithelia into complex 3D organs. The initial positioning of folds is the foundation for the emergence of correct tissue morphology. Mechanisms forming individual folds have been studied, but the precise positioning of folds in complex, multi-folded epithelia is less well-understood. We present a computational model of morphogenesis, encompassing local differential growth and tissue mechanics, to investigate tissue fold positioning. We use the Drosophila wing disc as our model system and show that there is spatial-temporal heterogeneity in its planar growth rates. This differential growth, especially at the early stages of development, is the main driver for fold positioning. Increased apical layer stiffness and confinement by the basement membrane drive fold formation but influence positioning to a lesser degree. The model successfully predicts the in vivo morphology of overgrowth clones and wingless mutants via perturbations solely on planar differential growth in silico.

Computational modeling of epithelial wound healing: Short and long term chemo-mechanical mechanisms

María Jesús Samper — Thu, 25 Jun 2020 12:06:58 +0200

During the lifetime of all living multicellular organisms, wounds in their tissues are frequently observed. The capability of closing those gaps is fundamental for a healthy development. If done deficiently, many diseases may occur from simple inflammation to tumor formation. The wound healing process in epithelial tissue occurs in three different stages. The first one is the assembly of a supra-cellular actomyosin cable and its migration towards the wound edge, triggered by biochemical processes in which calcium plays a distinctive role. How this process is orchestrated following damage remains unclear. Later, after its positioning, the cable contracts driving the tissue towards the gap and reducing the wound area. Finally, cell migration towards the interior of the wound ends up sealing the tissue. In this work, we make use of a mechanical continuum model for the first two stages in order to developed and 2D finite element simulations within a monolithically fully implicit implementation. The model for the actomyosin cable formation involves the coupling of transient calcium ions transport, with actin fibers and myosin motors recruitment and non-linear mechanical response of the tissue. The contraction stage, the active deformation of the previously formed actomyosin cable is taken into account. The relative motion of the myosin motors over the actin filaments is modeled so there exists an active tissue contraction in the direction of those fibers. Upon implementation, the model is capable of performing a wide range of biophysical situations reported experimentally, as we demonstrate in our numerical results. We have been able to rationalize through computational mechanics the firing of calcium in the wound right after damage infliction as well as the consequent formation of actin ring, reproducing nicely what has been reported in biological literature. Thereafter, the numerical model of acto-myosin contraction, fully integrated with the non-linear mechanics of the problem, correlates with the mechanics of wound closure at the actin-ring contraction stage. More importantly, the approach is the first of its kind in the modeling of epithelial and embryonic cell layers, where a wide number of complex mechanics has been integrated and solved though computational methods in engineering. We believe that the simulations will help to unravel new insights in open questions of developmental biology.

Stress relaxation in epithelial monolayers is controlled by the actomyosin cortex

María Jesús Samper — Thu, 25 Jun 2020 12:02:26 +0200

Epithelial monolayers are one-cell-thick tissue sheets that line most of the body surfaces, separating internal and external environments. As part of their function, they must withstand extrinsic mechanical stresses applied at high strain rates. However, little is known about how monolayers respond to mechanical deformations. Here, by subjecting suspended epithelial monolayers to stretch, we find that they dissipate stresses on a minute timescale and that relaxation can be described by a power law with an exponential cut-off at timescales larger than about 10 s. This process involves an increase in monolayer length, pointing to active remodelling of cellular biopolymers at the molecular scale during relaxation. Strikingly, monolayers consisting of tens of thousands of cells relax stress with similar dynamics to single rounded cells, and both respond similarly to perturbations of the actomyosin cytoskeleton. By contrast, cell–cell junctional complexes and intermediate filaments do not relax tissue stress, but form stable connections between cells, allowing monolayers to behave rheologically as single cells. Taken together, our data show that actomyosin dynamics governs the rheological properties of epithelial monolayers, dissipating applied stresses and enabling changes in monolayer length.

Mechanical oscillations in biological tissues as a result of delayed rest-length changes

María Jesús Samper — Thu, 25 Jun 2020 11:56:01 +0200

Oscillatory behavior in tissue biology is ubiquitous and may be observed in the form of either chemical or mechanical signals. We present here and solve a mechanical cell model that exhibits oscillations emerging from delayed viscoelastic rheological laws. These include a time delay between the mechanical response and the rest-length changes, which evolve proportionally to the delayed cell deformation and use a remodeling rate parameter. We show that different regimes (no oscillatory response, sustained oscillations, and unstable oscillations) are obtained for different values of the delay or the remodeling rate. The results are analytically demonstrated in a one-dimensional problem with one and two cells that are represented by simple line elements. Oscillations of the cell deformations are obtained whenever different delays coexist, or when the delay is size-dependent. We also extend our results to a multicellular two-dimensional vertex model that includes the same rheological law, and which inherits the presence of critical values of the delay or remodeling rate. We numerically show that indeed the size-dependent rest-length changes induce oscillations in the cell shapes and areas.

Computation of forces from deformed visco-elastic biological tissues

María Jesús Samper — Thu, 25 Jun 2020 11:51:57 +0200

We present a least-squares based inverse analysis of visco-elastic biological tissues. The proposed method computes the set of contractile forces (dipoles) at the cell boundaries that induce the observed and quantified deformations. We show that the computation of these forces requires the regularisation of the problem functional for some load configurations that we study here. The functional measures the error of the dynamic problem being discretised in time with a second-order implicit time-stepping and in space with standard finite elements. We analyse the uniqueness of the inverse problem and estimate the regularisation parameter by means of an L-curved criterion. We apply the methodology to a simple toy problem and to an in vivo set of morphogenetic deformations of the Drosophila embryo.

Hybrid cell-centred/vertex model for multicellular systems with equilibrium-preserving remodelling

María Jesús Samper — Thu, 25 Jun 2020 11:47:36 +0200

We present a hybrid cell‐centred/vertex model for mechanically simulating planar cellular monolayers undergoing cell reorganisation. Cell centres are represented by a triangular nodal network, while the cell boundaries are formed by an associated vertex network. The two networks are coupled through a kinematic constraint which we allow to relax progressively. Special attention is paid to the change of cell‐cell connectivity due to cell reorganisation or remodelling events. We handle these situations by using a variable resting length and applying an Equilibrium‐Preserving Mapping on the new connectivity, which computes a new set of resting lengths that preserve nodal and vertex equilibrium. We illustrate the properties of the model by simulating monolayers subjected to imposed extension and during a wound healing process. The evolution of forces and the Equilibrium‐Preserving Mapping are analysed during the remodelling events. As a by‐product, the proposed technique enables to recover fully vertex or fully cell‐centred models in a seamless manner by modifying a numerical parameter of the model.

Preface

María Jesús Samper — Thu, 25 Jun 2020 11:41:16 +0200

Porous-based rheological model for tissue fluidisation

María Jesús Samper — Thu, 25 Jun 2020 11:34:05 +0200

It has been experimentally observed that cells exhibit a fluidisation process when subjected to a transient stretch, with an eventual recovery of the mechanical properties upon removal of the applied deformation. This fluidisation process is characterised by a decrease of the storage modulus and an increase of the phase angle. We propose a rheological model which is able to reproduce this combined mechanical response. The model is described in the context of continua and adapted to a cell-centred particle system that simulates cell–cell interactions. Mechanical equilibrium is coupled with two evolution laws: (i) one for the reference configuration, and (ii) another for the porosity or polymer density. The first law depends on the actual strain of the tissue, while the second assumes different remodelling rates during porosity increase and decrease. The theory is implemented on a particle based model and tested on a stretching experiment. The numerical results agree with the experimental measurements for different stretching magnitudes

Collective cell durotaxis emerges from long-range intercellular force transmission

María Jesús Samper — Thu, 25 Jun 2020 11:30:25 +0200

The ability of cells to follow gradients of extracellular matrix stiffness—durotaxis—has been implicated in development, fibrosis, and cancer. Here, we found multicellular clusters that exhibited durotaxis even if isolated constituent cells did not. This emergent mode of directed collective cell migration applied to a variety of epithelial cell types, required the action of myosin motors, and originated from supracellular transmission of contractile physical forces. To explain the observed phenomenology, we developed a generalized clutch model in which local stick-slip dynamics of cell-matrix adhesions was integrated to the tissue level through cell-cell junctions. Collective durotaxis is far more efficient than single-cell durotaxis; it thus emerges as a robust mechanism to direct cell migration during development, wound healing, and collective cancer cell invasion.

Non-regularised inverse finite element analysis for 3D traction force microscopy

María Jesús Samper — Thu, 25 Jun 2020 11:25:23 +0200

The tractions that cells exert on a gel substrate from the observed displacements is an increasingly attractive and valuable information in biomedical experiments. The computation of these tractions requires in general the solution of an inverse problem. Here, we resort to the discretisation with finite elements of the associated direct variational formulation, and solve the inverse analysis using a least square approach. This strategy requires the minimisation of an error functional, which is usually regularised in order to obtain a stable system of equations with a unique solution. In this paper we show that for many common three-dimensional geometries, meshes and loading conditions, this regularisation is unnecessary. In these cases, the computational cost of the inverse problem becomes equivalent to a direct finite element problem. For the non-regularised functional, we deduce the necessary and sufficient conditions that the dimensions of the interpolated displacement and traction fields must preserve in order to exactly satisfy or yield a unique solution of the discrete equilibrium equations. We apply the theoretical results to some illustrative examples and to real experimental data. Due to the relevance of the results for biologists and modellers, the article concludes with some practical rules that the finite element discretisation must satisfy.

Control of cell-cell forces and collective cell dynamics by the intercellular adhesome

María Jesús Samper — Thu, 25 Jun 2020 11:18:19 +0200

Dynamics of epithelial tissues determine key processes in development, tissue healing and cancer invasion. These processes are critically influenced by cell–cell adhesion forces. However, the identity of the proteins that resist and transmit forces at cell–cell junctions remains unclear, and how these proteins control tissue dynamics is largely unknown. Here we provide a systematic study of the interplay between cell–cell adhesion proteins, intercellular forces and epithelial tissue dynamics. We show that collective cellular responses to selective perturbations of the intercellular adhesome conform to three mechanical phenotypes. These phenotypes are controlled by different molecular modules and characterized by distinct relationships between cellular kinematics and intercellular forces. We show that these forces and their rates can be predicted by the concentrations of cadherins and catenins. Unexpectedly, we identified different mechanical roles for P-cadherin and E-cadherin; whereas P-cadherin predicts levels of intercellular force, E-cadherin predicts the rate at which intercellular force builds up.

Mapping forces and kinematics during collective cell migration

María Jesús Samper — Thu, 25 Jun 2020 11:13:07 +0200

Fundamental biological processes including morphogenesis and tissue repair require cells to migrate collectively. In these processes, epithelial or endothelial cells move in a cooperative manner coupled by intercellular junctions. Ultimately, the movement of these multicellular systems occurs through the generation of cellular forces, exerted either on the substrate via focal adhesions (cell–substrate forces) or on neighboring cells through cell–cell junctions (cell–cell forces). Quantitative measurements of multicellular forces and kinematics with cellular or subcellular resolution have become possible only in recent years. In this chapter, we describe some of these techniques, which include particle image velocimetry to map cell velocities, traction force microscopy to map forces exerted by cells on the substrate, and monolayer stress microscopy to map forces within and between cells. We also describe experimental protocols to perform these measurements. The combination of these techniques with high-resolution imaging tools and molecular perturbations will lead to a better understanding of the mechanisms underlying collective cell migration in health and disease.

AAR-based decomposition method for lower-bound limit analysis

María Jesús Samper — Thu, 25 Jun 2020 11:04:54 +0200

Despite the recent progress in optimisation techniques, finite-element stability analysis of realistic three-dimensional problems is still hampered by the size of the resulting optimisation problem. Current solvers may take a prohibitive computational time, if they give a solution at all. The possible remedies to this are the design of adaptive de-remeshing techniques, decomposition of the system of equations or of the optimisation problem. This paper concentrates on the last approach, and presents an algorithm especially suited for limit analysis. Optimisation problems in limit analysis are in general convex but non-linear. This fact renders the design of decomposition techniques specially challenging. The efficiency of general approaches such as Benders or Dantzig–Wolfe is not always satisfactory, and strongly depends on the structure of the optimisation problem. This work presents a new method that is based on rewriting the feasibility region of the global optimisation problem as the intersection of two subsets. By resorting to the averaged alternating reflections (AAR) method in order to find the distance between the sets, the optimisation problem is successfully solved in a decomposed manner. Some representative examples illustrate the application of the method and its efficiency with respect to other well-known decomposition algorithms

Cell-centred model for the simulation of curved cellular monolayers

María Jesús Samper — Thu, 25 Jun 2020 10:59:05 +0200

This paper presents a cell-centred model for the simulation of planar and curved multicellular soft tissues. We propose a computational model that includes stress relaxation due to cell reorganisation (intercellular connectivity changes) and cytoskeleton remodelling (intracellular changes). Cells are represented by their cell centres, and their mechanical interaction is modelled through active non-linear elastic laws with a dynamically changing resting length. Special attention is paid to the handling of connectivity changes between cells, and the relaxation that the tissues exhibit under these topological changes. Cell–cell connectivity is computed by resorting to a Delaunay triangulation, which is combined with a mapping technique in order to obtain triangulations on curved manifolds. Our numerical results show that even a linear elastic cell–cell interaction model may induce a global non-linear response due to the reorganisation of the cell connectivity. This plastic-like behaviour is combined with a non-linear rheological law where the resting length depends on the elastic strain, mimicking the global visco-elastic response of tissues. The model is applied to simulate the elongation of planar and curved monolayers.

Forces driving epithelial wound healing

María Jesús Samper — Thu, 25 Jun 2020 10:53:12 +0200

A fundamental feature of multicellular organisms is their ability to self-repair wounds through the movement of epithelial cells into the damaged area. This collective cellular movement is commonly attributed to a combination of cell crawling and ‘purse-string’ contraction of a supracellular actomyosin ring. Here we show by direct experimental measurement that these two mechanisms are insufficient to explain force patterns observed during wound closure. At early stages of the process, leading actin protrusions generate traction forces that point away from the wound, showing that wound closure is initially driven by cell crawling. At later stages, we observed unanticipated patterns of traction forces pointing towards the wound. Such patterns have strong force components that are both radial and tangential to the wound. We show that these force components arise from tensions transmitted by a heterogeneous actomyosin ring to the underlying substrate through focal adhesions. The structural and mechanical organization reported here provides cells with a mechanism to close the wound by cooperatively compressing the underlying substrate.

A truss element for modelling reversible softening in living tissues

María Jesús Samper — Thu, 25 Jun 2020 10:42:33 +0200

We resort to non-linear viscoelasticity to develop a truss element able to model reversible softening in lung epithelial tissues undergoing transient stretch. Such a Maxwell truss element is built by resorting to a three-noded element whose mid-node is kinematically constrained to remain on the line connecting the end-nodes. The whole mechanical system undergoes an additive decomposition of the strains along the truss direction where the total contribution of the mid-node is accounted for by using a null-space projection and static condensation techniques. Assembling of such line-elements in 3D networks allows us to model extended regions of living tissues as well as their anisotropies.

Modelling of mixed damage on fibre reinforced composite laminates subjected to low velocity impact

María Jesús Samper — Thu, 25 Jun 2020 10:32:38 +0200

This article presents a numerical technique for the computation of damage in fibre-reinforced laminated composites which is embedded into an explicit finite element method. The technique is composed of two main components. First, a novel characterisation of the directions in which the various modes of damage propagate, which produces an effective damage localisation. This is conducted taking into consideration the strain-rate dependence. Second, a new mapping between the strain and stress spaces for the computation of the damage surfaces whereby time-stepping convergence is enhanced. Additionally, new damage initiation criteria in terms of strain damage surfaces are presented. Details of the in-house code developed are presented as well as the programming features. The capabilities of the technique are shown by means of tests on single fibre-reinforced element and low velocity impact on the laminate. It is shown that delamination is located in the expected regions by gradual progression of internal damage variables.

Physiology-based model of cell viscoelasticity

María Jesús Samper — Thu, 25 Jun 2020 10:19:18 +0200

The measured viscoelastic properties of biological tissues is the result of the passive and active response of the cells. We propose an evolution law of the remodeling process in the cytoskeleton which is able to mimic the viscous properties of biological cellular tissues. Our model is based on dynamical changes of the resting length. We show that under the small strain regime, the linear rheology models are recovered, with the relaxation time being replaced by the cell resistance to remodel. We implement the one-dimensional model into network systems of two and three dimensions, and show that the same conclusions may be drawn for those systems.

A biomechanical analysis of ventral furrow formation in the Drosophila melanogaster embryo

María Jesús Samper — Thu, 25 Jun 2020 09:58:03 +0200

The article provides a biomechanical analysis of ventral furrow formation in the Drosophila melanogaster embryo. Ventral furrow formation is the first large-scale morphogenetic movement in the fly embryo. It involves deformation of a uniform cellular monolayer formed following cellularisation, and has therefore long been used as a simple system in which to explore the role of mechanics in force generation. Here we use a quantitative framework to carry out a systematic perturbation analysis to determine the role of each of the active forces observed. The analysis confirms that ventral furrow invagination arises from a combination of apical constriction and apical–basal shortening forces in the mesoderm, together with a combination of ectodermal forces. We show that the mesodermal forces are crucial for invagination: the loss of apical constriction leads to a loss of the furrow, while the mesodermal radial shortening forces are the primary cause of the internalisation of the future mesoderm as the furrow rises. Ectodermal forces play a minor but significant role in furrow formation: without ectodermal forces the furrow is slower to form, does not close properly and has an aberrant morphology. Nevertheless, despite changes in the active mesodermal and ectodermal forces lead to changes in the timing and extent of furrow, invagination is eventually achieved in most cases, implying that the system is robust to perturbation and therefore over-determined.

Diffusion-reaction model for Drosophila embryo development

María Jesús Samper — Thu, 25 Jun 2020 09:20:53 +0200

During the early stages of gastrulation in Drosophila embryo, the epithelial cells composing the single tissue layer of the egg undergo large strains and displacements. These movements have been usually modelled by decomposing the total deformation gradient in an (imposed or strain/stress dependent) active part and a passive response. Although the influence of the chemical and genetic activity in the mechanical response of the cell has been experimentally observed, the effects of the mechanical deformation on the latter have been far less studied, and much less modelled. Here, we propose a model that couples morphogen transport and the cell mechanics during embryogenesis. A diffusion-reaction equation is introduced as an additional mechanical regulator of morphogenesis. Consequently, the active deformations are not directly imposed in the analytical formulation, but they rather depend on the morphogen concentration, which is introduced as a new variable. In this study, we show that strain patterns similar to those observed during biological experiments can be reproduced by properly combining the two phenomena. In addition, we use a novel technique to parameterise the embryo geometry by solving two Laplace problems with specific boundary conditions. We apply the method to two morphogenetic movements: ventral furrow invagination and germ band extension. The matching between our results and the observed experimental deformations confirms that diffusion-reaction of morphogens can actually be controlling large morphogenetic movements.

Stress-dependent morphogenesis: continuum mechanics and truss systems

María Jesús Samper — Thu, 25 Jun 2020 09:12:07 +0200

A set of equilibrium equations is derived for the stress-controlled shape change of cells due to the remodelling and growth of their internal architecture. The approach involves the decomposition of the deformation gradient into an active and a passive component; the former is allowed to include a growth process, while the latter is assumed to be hyperelastic and mass-preserving. The two components are coupled with a control function that provides the required feedback mechanism. The balance equations for general continua are derived and, using a variational approach, we deduce the equilibrium equations and study the effects of the control function on these equations. The results are applied to a truss system whose function is to simulate the cytoskeletal network constituted by myosin microfilaments and microtubules, which are found experimentally to control shape change in cells. Special attention is paid to the conditions that a thermodynamically consistent formulation should satisfy. The model is used to simulate the multicellular shape changes observed during ventral furrow invagination of the Drosophila melanogaster embryo. The results confirm that ventral furrow invagination can be achieved through stress control alone, without the need for other regulatory or signalling mechanisms. The model also reveals that the yolk plays a distinct role in the process, which is different to its role during invagination with externally imposed strains. In stress control, the incompressibility constraint of the yolk leads, via feedback, to the generation of a pressure in the ventral zone of the epithelium that eventually eases its rise and internalisation.

− 2gy) (1 + y )dx La soluc´íon es, y = tag(α)x −g 2u (1 + tag (α))x Esta es la solución que parte del origen de coordenadas con una velocidad , u , yángulo α y pasa por un punto,B(x

Antonio Lechuga — Wed, 24 Jun 2020 13:21:02 +0200

Robust mechanisms of ventral furrow invagination require the combination of cellular shape changes

María Jesús Samper — Tue, 23 Jun 2020 16:22:18 +0200

Ventral furrow formation in Drosophila is the first large-scale morphogenetic movement during the life of the embryo, and is driven by co-ordinated changes in the shape of individual epithelial cells within the cellular blastoderm. Although many of the genes involved have been identified, the details of the mechanical processes that convert local changes in gene expression into whole-scale changes in embryonic form remain to be fully understood. Biologists have identified two main cell deformation modes responsible for ventral furrow invagination: constriction of the apical ends of the cells (apical wedging) and deformation along their apical–basal axes (radial lengthening/shortening). In this work, we used a computer 2D finite element model of ventral furrow formation to investigate the ability of different combinations of three plausible elementary active cell shape changes to bring about epithelial invagination: ectodermal apical–basal shortening, mesodermal apical–basal lengthening/shortening and mesodermal apical constriction. We undertook a systems analysis of the biomechanical system, which revealed many different combinations of active forces (invagination mechanisms) were able to generate a ventral furrow. Two important general features were revealed. First that combinations of shape changes are the most robust to environmental and mutational perturbation, in particular those combining ectodermal pushing and mesodermal wedging. Second, that ectodermal pushing plays a big part in all of the robust mechanisms (mesodermal forces alone do not close the furrow), and this provides evidence that it may be an important element in the mechanics of invagination in Drosophila.

A 3D finite element model of ventral furrow invagination in the Drosophila melanogaster embryo

María Jesús Samper — Tue, 23 Jun 2020 16:09:15 +0200

The paper describes a mechanical model of epithelial tissue development in Drosophila embryos to investigate a buckling phenomenon called invagination. The finite element method is used to model this ventral furrow formation in 3D by decomposing the total deformation into two parts: an imposed active deformation, and an elastic passive deformation superimposed onto the latter. The model imposes as boundary conditions (i) a constant yolk volume and (ii) a sliding contact condition of the cells against the vitelline membrane, which is interpolated as a B-Spline surface. The active deformation simulates the effects of apical constriction and apico-basal elongation of cells. This set of local cellular mechanisms leads to global shape changes of the embryo which are associated with known gene expressions. Using the model we have tested different plausible hypotheses postulated to account for the mechanical behaviour of epithelial tissues. In particular, we conclude that only certain combinations of local cell shape change can successfully reproduce the invagination process. We have quantitatively compared the model with a 2D model and shown that it exhibits a more robust invagination phenomenon. The 3D model has also revealed that invagination causes a yolk flow from the central region to the anterior and posterior ends of the embryo, causing an accordion-like global compression and expansion wave to move through the embryo. Such a phenomenon cannot be described by 2D models.

On the modelling of incompressibility in linear and non-linear elasticity with the master-slave approach

María Jesús Samper — Tue, 23 Jun 2020 15:56:51 +0200

The master–slave approach is adapted to model the kinematic constraints encountered in incompressibility. The method presented here allows us to obtain discrete displacement and pressure fields for arbitrary finite element formulations that have discontinuous pressure interpolations. The resulting displacements satisfy exactly the incompressibility constraints in a weak sense, and are obtained by solving a system of equations with the minimum (independent) degrees of freedom. In linear analysis, the method reproduces the well‐known stability results for inf–sup compliant elements, and permits to compute the pressure modes (physical or spurious) when they exist. By rewriting the equilibrium equations of a hyperelastic material, the method is extended to non‐linear elasticity, while retaining the exact fulfilment of the incompressibility constraints in a weak sense. Problems with analytical solution in two and three dimensions are tested and compared with other solution methods.

Modelling unilateral frictionless contact using the null-space method and cubic B-Spline interpolation

María Jesús Samper — Tue, 23 Jun 2020 15:44:59 +0200

The analysis of unilateral sliding contact in elasticity is equivalent to a minimisation problem subjected to a set of inequality constraints. However, the presence of boundary discontinuities, such as those stemming from the spatial discretisation, appears as a major problem to determine the set of active constraints. This work introduces a smoothing technique of the master surface resorting to cubic B-Spline interpolation, which is C1 continuous in contact situations between elastic and rigid bodies, and G1 continuous in elastic–elastic contact problems. The technique is applied in conjunction with the null-space method, where the solution is searched in an unconstrained manifold. The resulting formulation eases the contact transition along the master surface, and recovers the quadratic convergence of the iterative Newton–Raphson process. The robustness of the method is demonstrated using 2D and 3D examples.

A deformation gradient decomposition method for the analysis of the mechanics of morphogenesis

María Jesús Samper — Tue, 23 Jun 2020 15:40:34 +0200

A new finite element model is proposed for the analysis of the mechanical aspects of morphogenesis and tested on the biologically well studied gastrulation phenomenon, in particular ventral furrow invagination of the Drosophila melanogaster embryo. A set of mechanisms are introduced in the numerical model, which lead to the observed deformed shapes. We split the total deformation into two parts: an imposed active deformation, and an elastic deformation superimposed onto the latter. The active deformation simulates the effects of apical constriction and apico-basal elongation. These mechanisms are associated with known gene expressions and so in this way we attempt to bridge the well explored signalling pathways, and their associated phenotypes in a mechanical model. While the former have been studied in depth, much less can be said about the forces they produce and the mechanisms involved. From the numerical results, we are able to test different plausible mechanical hypotheses that generate the necessary folding observed in the invagination process. In particular, we conclude that only certain ratios between both modes (apical constriction and apico-basal elongation) can successfully reproduce the invagination process. The model also supports the idea that this invagination requires the contribution of several mechanisms, and that their redundancy provides the necessary robustness.

On the numerical simulation of fatigue driven delamination with interface elements

María Jesús Samper — Tue, 23 Jun 2020 15:31:10 +0200

This paper investigates robustness and efficiency of a method for the numerical simulation of fatigue driven delamination growth in laminated composites. The method is based on the fatigue degradation of a bilinear interface element and the paper studies the dependence of the predicted behaviour on the interface element size, Δl, and the number of cycles per increment, ΔN. It is concluded that the value of ΔN must be small enough to ensure that at least two increments are required to advance the crack by Δl. It is also shown that Δl must be sufficiently small so that at least two interface elements lie in the cohesive zone ahead of the crack tip. The paper also proposes extensions of the model to include a non-zero minimum cyclic load value and an improved algorithm for mixed-mode fatigue driven delamination growth.

Sliding joints in 3D beams: conserving algorithms using the master-slave approach

María Jesús Samper — Tue, 23 Jun 2020 15:25:38 +0200

This paper proposes two time-integration algorithms for motion of geometrically exact 3D beams under sliding contact conditions. The algorithms are derived using the so-called master–slave approach, in which constraint equations and the related time-integration of a system of differential and algebraic equations are eliminated by design. Specifically, we study conservation of energy and momenta when the sliding conditions on beams are imposed and discuss their algorithmic viability. Situations where the contact jumps to adjacent finite elements are analysed in detail and the results are tested on two representative numerical examples. It is concluded that an algorithmic preservation of kinematic constraint conditions is of utmost importance.

Sliding contact conditions using the master–slave approach with application on geometrically non-linear beams

María Jesús Samper — Tue, 23 Jun 2020 15:15:35 +0200

Frictionless sliding conditions between two bodies are usually defined using either the method of Lagrangian multipliers or by prescribing an artificial (penalty) stiffness which resists the penetration at the contact point. Both of these methods impose the condition that the contact force should be normal to the contact surface, with the Lagrangian multiplier or the penalty parameter serving as a measure of this force. In this work, an alternative approach is undertaken: the frictionless sliding condition is defined through a relationship between nodal parameters of the virtual displacements of a discretised principle of virtual work. This method, which does not involve additional force parameters or degrees of freedom, is known as the master–slave or the minimum-set method and is particularly convenient for displacement-based finite-element implementation. The method is analysed in detail in context of bilateral sliding constraints characteristic of prismatic and cylindrical joints in flexible beam assemblies undergoing large overall motion. Two numerical examples are presented and assessed against the results in the literature.

Master-slave approach for the modelling of joints with dependent degrees of freedom in flexible mechanisms

María Jesús Samper — Tue, 23 Jun 2020 14:55:49 +0200

The analysis of multibody systems requires an exact description of the kinematics of the joints involved. In the present work the master–slave approach is employed and endowed with the possibility of including several more complex types of joints. We present the formulation for joints where some relation between the different released degrees of freedom exists such as the screw joint, the rack‐and‐pinion joint or the cam joint. These joints are implemented in conjunction with geometrically exact beams and an energy‐momentum conserving time‐stepping algorithm.

An effective aggregate signature scheme suitable for VANETs and its implementation using OPNET IT Guru

Daxing Wang — Sun, 21 Jun 2020 03:04:18 +0200

Vehicular Ad hoc NETwork (VANET) is a multi-hop and high-speed mobile wireless communication network. In order to realize the security authentication of the information transmission between vehicle nodes in vehicular ad hoc networks, a certificateless aggregate signature scheme is designed. The proposed scheme uses certificateless cryptography, which eliminates the complex maintenance cost of certificate and solves the problem of key escrow. Communicating through pseudonyms and nodes around the roadside units generated, the conditional privacy protection is achieved for vehicle users. Then, the efficiency of the scheme is analyzed, and the relationship between the traffic density in VANETs environment and the time delay of message verification is simulated. The results show that the scheme satisfies the message authentication, anonymity, unforgeability and traceability, as well as the higher communication efficiency and the shorter delay of message verification, which is more suitable for dynamic vehicular ad hoc network environment.

Thermic Model to Predict Biogas Production in Unheated Fixed-Dome Digesters Buried in the Ground

María Jesús Samper — Fri, 19 Jun 2020 14:55:25 +0200

In many developing countries simple biogas digesters are used to produce energy for domestic purposes from anaerobic digestion of animal manure. We developed a simple, one-dimensional (1-D), thermal model with easily-available input data for unheated, unstirred, uninsulated, fixed-dome digesters buried in the soil to study heat transfer between biogas digester and its surroundings. The predicted temperatures in the dome, biogas and slurry inside the digester and the resulting biogas production are presented and validated. The model was well able to estimate digester temperature (linear slope nearly 1, R2 0.96). The results show that a suitable soil temperature profile renders the use of a full thermal model superfluous for buried, unheated, unstirred and uninsulated digesters. Model validation for methane production gave root-mean-square (RMSE) and relative-root-mean-square errors (rRMSEP(%)) of 54.4 L CH4 digester-1 day-1 and 35.4%, respectively. The validation result was considerably improved if only using winter data (RMSE = 26.1 L CH4 digester-1 day-1; rRMSEP(%) = 17.7%). The model performed satisfactorily in light of the uncertainties attached to it. Since unheated digesters suffer critically low methane production during the winter, the model could be particularly useful for assessing methane production and for improving the ability of unheated digesters to provide sufficient energy during cold periods.

Low cost tubular digesters as appropriate technology for widespread application: Results and lessons learned from Bolivia

María Jesús Samper — Fri, 19 Jun 2020 14:34:11 +0200

This paper presents the results and lessons learned from four and a half years of implementing low cost tubular digesters in Bolivia. The selection of this technology is justified in comparison with other popular technologies such as fixed dome or floating drum digesters. The highlighted weakness of the tubular model (its short life expectancy), is transformed into a strength, making the low cost tubular digester an appropriate technology for widespread application. The experiences in Bolivia show that the success of biogas programs depend more on socio-economic factors than on the validated technology selected, suggesting that local circumstances are a critical, and often underestimated, factor to be taken into consideration in the praxis. Finally, some testimonies of the use of biol (bio-slurry or effluent) are reported, identifying the high potential of this anaerobic digestion product that provides a food sovereignty approach, reduced expansion of the agricultural frontier, increased agricultural productivity and hence family income, that other household energizing systems do not have. A brief report of lessons learned is also included

Improvement through low cost biofilm carrier in anaerobic tubular digestion in cold climate regions

María Jesús Samper — Fri, 19 Jun 2020 14:20:27 +0200

The aim of this research is to evaluate the increase of biogas production with low cost tubular digesters in cold climates using PET rings inside the reactor. Two similar digesters have been operated and monitored in cold weather conditions and have been fed with cow manure. Digester 1 was filled with PET – rings as a biofilm carrier, Digester 2 was kept as a reference. Through the PET – rings the functional surface could be increased by a factor 4.2. The results show that 44% more biogas per KgSV has been produced with the biofilm carrier in use (0.33 m3/kgSV) (reference digester −0.23 m3/kgSV), at an organic load rate of 0.26 kgSV/m3/d. The thermal performance shows that with an adaptation of the low cost tubular digester the slurry temperature can be raised up to 16.6 °C (average) by surrounding temperature of 6.1 °C (average) without using any active heating system.

Cow, sheep and llama manure at psychrophilic anaerobic co-digestion with low cost tubular digesters in cold climate and high altitude

María Jesús Samper — Fri, 19 Jun 2020 14:11:22 +0200

The aim of this research is to evaluate the co-digestion of cow and llama manure combined with sheep manure, in psychrophilic conditions and real field low cost tubular digesters adapted to cold climate. Four digesters were monitored in cold climate conditions; one fed with cow manure, a second one with llama manure, the third one with co-digestion of cow–sheep manure and the fourth one was fed with llama–sheep manure. The slurry had a mean temperature of 16.6 °C, the organic load rate was 0.44 kgvs m−3 d−1 and the hydraulic retention time was 80 days. After one hundred days biogas production was stable, as was the methane content and the pH of the effluent. The co-digestion of cow–sheep manure results in a biogas production increase of 100% compared to the mono-digestion of cow manure, while co-digestion of llama–sheep manure results in a decrease of 50% in biogas production with respect to mono-digestion of llama manure

The influence of users' behavior on biogas production from low cost tubular digesters: A technical and socio-cultural field analysis

María Jesús Samper — Fri, 19 Jun 2020 14:06:13 +0200

The aim of this paper is to understand the influence of the user behavior on tubular digesters performance, through a technical and a social approach in the Bolivian context. Fifteen domestic digesters were evaluated, from which 6 were installed in the Altiplano and other 6 in the Andean Valleys. Data about slurry temperature, feedstock and biogas quality were collected from these 12 digesters, while daily biogas production and feeding pattern were also monitored from further three digesters in the valleys. Because of changes in user behavior along the monitoring period and particular characteristics of the digesters monitored, 5 complete patterns of biogas production and digester management were established. Furthermore, the results of a socio-cultural study with Andean families about the perception of poverty, their needs and the role played by digesters in their expectations in improving life quality, are correlated to the obtained technical data. The technical evaluation shows how the digester management seems to have a seasonal performance throughout the year according to the agricultural calendar. This means that families are more interested in using bioslurry in crops and agricultural improvements than in the use of biogas. The Bolivian government subsidy on liquefied gas seems to be one of the key issues to understand these results. Finally, data also reveals how the thermal behavior of tubular digesters adapted to cold climate that use a passive solar design, is similar to the thermal behavior of valley digesters, and therefore intends to add the same passive heating techniques for warm and tropical climates, to increase slurry temperatures and achieve a higher biogas production.

How to report biogas production when monitoring small-scale digesters in field

María Jesús Samper — Fri, 19 Jun 2020 13:35:21 +0200

The aim of this research was to evaluate the error originated when biogas production from field monitoring digesters, influenced by the diurnal temperature cycle, was normalized to standard conditions for pressure and temperature (273.15 K and 100 kPa) from local conditions. The biogas production data is often reported without indicating if done under local conditions, whether these conditions have been standardized and, if they have actually been standardized, the standard temperature and pressure is not indicated. In this research ambient and biogas temperature, as well as biogas production were monitored with a 30 min frequency during three consecutive days, in three different tubular digesters. Normalization was realized using the high frequency data collected as reference values, and also using daily biogas production with mean daily biogas, ambient and nearby meteorological station temperatures. The outcome of this research shows that normalization of biogas production can be obtained using daily biogas production and the daily mean ambient temperature with an overestimation by no more than 1.5%, in comparison to the normalization achieved by using high frequency data from biogas temperature and production. Using mean daily ambient temperature or mean daily biogas temperature results in the same overestimation, while using mean daily ambient temperature from a nearby airport weather station pushes the overestimation up to 2.7%. So, if ambient temperature and altitude is identified, biogas production reported in local conditions can be normalized.

Household anaerobic digesters for biogas production in Latin America: A review

María Jesús Samper — Fri, 19 Jun 2020 13:28:23 +0200

This review aims to provide an overview of household biogas digester implementation in rural areas of Latin America. It considers the history of household digesters in Latin America, including technical, environmental, social and economic aspects. Several successful experiences have been promoted during the last decade, including the creation of the Network for Biodigesters in Latin America and the Caribbean (RedBioLAC) that provides a forum to coordinate implementation and research programmes throughout the continent. Although the potential of this technology is well demonstrated, some barriers are identified, such as the need for technical improvements, lack of social acceptance and high investment costs. Thus, further efforts should be undertaken to overcome these barriers and improve the technical performance, social acceptance, economic benefits and environmental impact in order to enhance its wide-spread dissemination in energy poor communities.

Analysis of energy, CO2 emissions and economy of the technological migration for clean cooking in Ecuador

María Jesús Samper — Fri, 19 Jun 2020 12:58:32 +0200

The objective of this study is to analyze the CO2 emissions and economic impacts of the implementation of the National Efficient Cooking Program (NECP) in Ecuador, which aims to migrate the population from Liquefied Petroleum Gas (LPG)-based stoves to electric induction stoves. This program is rooted in the current effort to change Ecuador's energy balance, with hydroelectric power expected to generate 83.61% of national electricity by 2022, ending the need for subsidized LPG. For this analysis, the 2014 baseline situation has been compared with two future scenarios for 2022: a business-as-usual scenario and an NECP-success scenario. This study demonstrates the viability of migration from imported fossil fuels to locally-produced renewable energy as the basis for an efficient cooking facility. The new policies scenario would save US$ 1.162 billion in annual government expenditure on cooking subsidies, and reducing CO2 emissions associated to energy for cooking in 1.8 tCO2/y.

Monitoreo a escala de real de un digestor anaeróbico de bajo costo instalado en una planta de sacrificio animal

María Jesús Samper — Fri, 19 Jun 2020 12:42:59 +0200

Slaughterhouses generate an average 6.5 kg of manure/animal. Day. The animals has a weight between 350 and 400 kg. Manure causes economic, environmental and legal problems for the slaughter companies. The aim of this work, was to stabilize the 5,5 % of cattle manure of FOGASA Slaughterhouse by means of low cost digester. The results show organic matter reduce of 60% volatile solid and an average biogas production of 0,9 Nm/d. During anaerobic digestion process efficiencies were reach in terms of biogas production rate, specific biogas production and reduction of the pathogen content of 0,21 m3biogás/m3digestor d y 0,20 m3biogás/KgSV y 90% respectively. The digestate contained nitrogen, phosphorous and potassium, which is an important feature of fertilizers

Evaluation of the low technology tubular digesters in the production of biogas from slaughterhouse wastewater treatment: Low cost tubular digesters

María Jesús Samper — Fri, 19 Jun 2020 11:48:17 +0200

The aim of this research is to identify the operational conditions and performance of low cost tubular digesters producing biogas from the slaughterhouse wastewater treatment. Three full scale sequenced tubular digesters (of more than 8 m3 each), operating in a slaughterhouse in Cochabamba (Bolivia), have been monitored during one year. The Organic Load Rate (OLR) varied from 0.04 to 1.13 kgSV/m3d (0.04–1.07 kgCOD/m3d), and the Hydraulic Retention Time (HRT) from 3.2 to 87.4 days. The passive solar heating design considered drove the slurry temperature to 27 °C, 8 °C more than mean ambient temperature. The first digester has been filled with soda bottle plastic rings in order to increase the internal effective surface and evaluate, obtaining that an increment in 67% of the internal effective surface, more than doubles the biogas production and enables higher OLRs. A Biogas Production Rate (BPR) peak of 0.2 m3/m3d was observed for OLR = 0.37 kgSV/m3d, HRT = 9.7d and Specific Biogas Production (SBP) = 0.55 m3/kgSV. COD removal achieves values above 70% from HRT>19 d. A comparison with other anaerobic digestion technologies demonstrates that low cost tubular digesters with biofilm carriers, are competitive for OLR<0.5 kg COD/m3d of biogas production, and for 0.25 kgCOD/m3d, when the focus is on COD removal.

Biogas from a full scale digester operated in psychrophilic conditions and fed only with fruit and vegetable waste

María Jesús Samper — Fri, 19 Jun 2020 11:12:57 +0200

The aim of this work was to evaluate the feasibility of treating fruit and vegetable waste from a municipal market in a full-scale anaerobic digester with minimum implementation and operational costs, i.e. without pre-treatment, clean water consumption, active heating or mixing. For this purpose, a 13.9 m3 digester that forces the submersion of solids, gains heat through solar radiation and recirculates effluent was monitored during one year of real operational and weather conditions in Bolivia. The digester was initially loaded with cow rumen (inoculum source), and after eight weeks with only fruit and vegetable waste. The digester operated at an organic loading rate (OLR) of 1 kgVS/(m3d). The normalized specific biogas production averaged 0.26 m3/kgVS and increased to 0.34 m3/kgVS when OLR was halved to 0.57 kgVS/(m3d), while the passive solar design allowed an average digester temperature of 21 °C (4 °C above the average ambient temperature).

Energy use of Jatropha oil extraction wastes: Pellets from biochar and Jatropha shell blends

María Jesús Samper — Fri, 19 Jun 2020 10:51:11 +0200

The aim of this paper is to evaluate the use of biochar (SCB) produced through the pyrolysis of Jatropha seed cake in combination with Jatropha shell waste (JSh) to form pellet combustion fuel. Mixtures using different proportions of SCB and JSh (0, 25, 50, 75 and 100% each), were prepared, with each proportion tested using two particle sizes and two different quantities of water added. Combustion characteristics and mechanical durability were analyzed to identify the best blend, for which a thermogravimetric analysis was made. Pellets composed of 50% JSh and 50% SCB, with 25% of additional water and 4 mm particle size, presented the greatest mechanical durability (96.83%), and higher heating value (22.14 MJ kg−1). Combustion of pellets made with these characteristics in an industrial burner showed average temperatures ranging from 300 to 350 °C, with a maximum temperature of 460 °C. The production cost of the selected pellet is estimated at 0.11 USD kg−1, making the energy cost of the pellet [0.005 USD MJ−1] approximately 28% of the energy cost of LPG, demonstrating the commercial viability of the selected pellet as feasible.

Future oil extraction in Ecuador using a Hubbert approach

María Jesús Samper — Fri, 19 Jun 2020 10:40:07 +0200

Hubbert based models to project future oil extraction in Ecuador were developed. Two values of ultimate recoverable resources (URR) (7860–10,700 million barrels (MMbbl)) are applied to 16 models, considering symmetric and asymmetric Hubbert models and one and two cycles under top-down and bottom-up approaches. Models are discussed based on the best fit to historical data, and year and value of maximum extraction. The peak oil extraction obtained ranges between 196 and 215 MMbbl and would be reached in the years 2014–2025. An analysis of the implications of extraction models in a Business as Usual and Alternative oil demand scenarios up to 2035 was performed. Ecuador could become a net oil importer between 2024 and 2035, depending on the model and demand scenario. Economic oil trade balance could be seriously affected, decreasing from a current positive value of around 2 billion USD to incur deficits of 0.6–16.7 billion USD in 2035. Current and future oil dependence for Ecuador would increase vulnerability and compromise the country in terms of energy security and trade balance. It is critical for Ecuador to consider more ambitious policies focused on energy efficiency, renewables and diversification of the productive structure over the next few years.

Management and design of biogas digesters: A non-calibrated heat transfer model

María Jesús Samper — Fri, 19 Jun 2020 10:20:39 +0200

A thermal balance modeling framework is developed, based on heat transfer-resistance networks. The heat transfer model accounts for effects of digester- design, location and operation, including effects of solar irradiance, external heating and ambient climate. We demonstrate extendibility of the framework by using the model in dynamic simulations of substrate temperature for digesters comprising two very different designs. Digester designs modeled include fixed-dome, buried, uninsulated and unheated household digesters in Hanoi, Vietnam, and an industrial-scale anaerobic digester located at a wastewater treatment plant in Esbjerg, Denmark. The modeled temperature profiles were evaluated against measured substrate temperatures over long periods, from 7 months and up. For the two Hanoi digesters, root-mean-square-error were 1.43 °C and 0.92 °C, with Nash-Sutcliffe model efficiency coefficients (NS-C) of 0.87 and 0.93 respectively. For the industrial digester in Esbjerg root-mean-square-error was 0.48 °C with an NS-C of 0.94. The model was not calibrated prior simulation, suggesting good predictive performance.

Performance Analysis of a Small-Scale Biogas-Based Trigeneration Plant: An Absorption Refrigeration System Integrated to an Externally Fired Microturbine

María Jesús Samper — Fri, 19 Jun 2020 10:00:55 +0200

Trigeneration or combined cooling, heat and power (CCHP) systems fueled by raw biogas can be an interesting alternative for supplying electricity and thermal services in remote rural areas where biogas can be produced without requiring sophisticated equipment. In this sense, this study considers a performance analysis of a novel small-scale CCHP system where a biogas-fired, 5 kWel externally fired microturbine (EFMT), an absorption refrigeration system (ARS) and heat exchangers are integrated for supplying electricity, refrigeration and hot water demanded by Bolivian small dairy farms. The CCHP solution presents two cases, current and nominal states, in which experimental and design data of the EFMT performance were considered, respectively. The primary energy/exergy rate was used as a performance indicator. The proposed cases show better energy performances than those of reference fossil fuel-based energy solutions (where energy services are produced separately) allowing savings in primary energy utilization of up to 31%. Furthermore, improvements in electric efficiency of the EFMT and coefficient of performance (COP) of the ARS, identified as key variables of the system, allow primary energy savings of up to 37%. However, to achieve these values in real conditions, more research and development of the technologies involved is required, especially for the EFMT

Biogas based polygeneration plant options utilizing dairy farms waste: A Bolivian case

María Jesús Samper — Fri, 19 Jun 2020 09:30:00 +0200

This study presents a comparative techno-economic feasibility analysis for two polygeneration plant solutions, applied to low-income dairy farms in Bolivia. The first option considers an internally fired microturbine (IFMT) and, the second, an internal combustion engine (ICE). They are integrated with an absorption refrigeration system and a fertilizer dryer. Biogas, produced with farms waste, fuels these power generators. The levelized costs of biogas for cooking, electricity, cooling and fertilizers were determined. The cost of biogas, for both options, was found to be 0.020 USD/kWh, which is lower than the subsidized price of LPG. The most competitive cost of electricity was determined for the ICE plant option; it was found to be 0.082 USD/kWh and is lower than the subsidized cost of fossil fuel-based electricity. The cost of cooling was found to be around 0.082 USD/kWh, which is slightly higher than the cost of cooling supplied by using grid electricity. In a realistic scenario, the shorter payback period was found to be 4.4 years for the ICE plant option. From this, the ICE-based plant was found as the most feasible option. Additionally, if no subsidies are applied to the fossil fuel-based services, the proposed polygeneration systems are a highly competitive alternative.

Techno-Economic Study of a Biogas-Based Polygeneration Plant for Small Dairy Farms in Central Bolivia

María Jesús Samper — Fri, 19 Jun 2020 09:05:37 +0200

This paper presents a techno-economic feasibility study of a polygeneration plant proposed for an association of dairy farmers in Bolivia. The systems have been designed in an integrated approach and are based on the resource recovery utilizing available cow dung. The production of biogas is proposed using “low cost” tubular digesters. The biogas is used for the production of electricity and heat in a 40 kWel internal combustion engine. Heat from the exhaust gas is recovered to drive an absorption cooling system for milk refrigeration and for a bio-slurry drying system. The final services are biogas, electricity, cooling and fertilizers. The techno-economic analysis focuses on determining the levelized cost of the services and compare them with the subsidized and non-subsidized prices of other competitive services in the market. Sensitivity analyses for the services prices are carried out to see the influence of the feedstock handling cost and the subsidies on investment capital. Once the selling prices of the services are defined, the payback period of investment capital is determined. The results show that the biogas and electricity costs were found lower than the subsidized prices of similar services in the market while the cost of cooling was found slightly higher. From the sensitivity analysis we have that the feedstock handling cost can be increased from 10 to 18 USD/ton while maintaining the biogas price below the subsidized LPG price, and applying subsidies to the investment capital allows reducing the rest of the services costs. The payback period is around 3.5 years when a subsidy of 15% is applied in the investment capital and the services are sold at the subsidized prices of the conventional services. Our study has shown that under the conditions of the Bolivian market it is feasible the installation of a polygeneration plant. On the other hand, a market without or with reduced subsidies for the conventional services makes the proposed system more feasible and competitive.

Hosting solutions and other services for congresses

Scipedia content — Tue, 16 Jun 2020 10:30:35 +0200

Hosting solutions and other services for congresses

Combinación de métodos lagrangianos de elementos finitos y partículas (PFEM) con métodos eulerianos para el análisis del comportamiento de presas de escollera durante un sobrevertido

María Jesús Samper — Thu, 11 Jun 2020 12:19:41 +0200

El objetivo de este trabajo es desarrollar una herramienta de aplicabilidad general que ayude en el estudio de la estabilidad y seguridad de una presa de escollera en una avenida extraordinaria que origina un sobrevertido. se combinan métodos eulerianos para el cálculo de flujo de agua fuera y dentro de la presa con métodos lagrangianos de elementos finitos y partículas para analizar el comportamiento tenso-deformacional de la presa y su posible rotura y eventual desmoronamiento bajo la acción del agua. Finalmente se presenta un método para la proyección de datos entre mallas no coincidentes que permita el acomplamiento fluido-estructura.

Un método de cálculo para el estudio de la seguridad de presas de escollera durante un sobrevertido

María Jesús Samper — Thu, 11 Jun 2020 12:00:37 +0200

El objetivo de este trabajo es desarrollar y validar un nuevo método de cálculo de aplicabilidad general para el estudio de la estabilidad y seguridad de una presa de escollera durante un sobrevertido. Esta nueva herramienta permite analizar con detalle el estado tenso-deformacional de la presa y su posible rotura y eventual desmoronamiento bajo la acción del flujo de agua y combina métodos de elementos finitos avanzados con métodos de partículas. Esta técnica, denominada PFEM (particle finite element method), permite tener en cuenta de las complejas no-linealidades geométricas y mecánicas en problemas de interacción fluido-estructura, incluyendo fenómenos de superficie libre y de erosión. En el trabajo se describen las características esenciales del PFEM y se detalla la aplicación de este método para modelar el sobrevertido en presas de escollera y su eventual desplome. Este tabajo numérico se combina con una parte experimental que se desarrolla en el CEDEX y en la ETS de Ingenieros de Caminos de Madrid en el marco del proyecto XPRES.

Air-Forced Flow in Proton Exchange Membrane Fuel Cells: Calculation of Fan-Induced Friction in Open-Cathode Conduits with Virtual Roughness

Dejan Brkić — Thu, 11 Jun 2020 11:49:02 +0200

Measurements of pressure drop during experiments with fan-induced air flow in the open-cathode proton exchange membrane fuel cells (PEMFCs) show that flow friction in its opencathode side follows logarithmic law similar to Colebrook’s model for flow through pipes. The stable symbolic regression model for both laminar and turbulent flow presented in this article correlates air flow and pressure drop as a function of the variable flow friction factor which further depends on the Reynolds number and the virtual roughness. To follow the measured data, virtual inner roughness related to the mesh of conduits of fuel cell used in the mentioned experiment is 0.03086, whereas for pipes, real physical roughness of their inner pipe surface goes practically from 0 to 0.05. Numerical experiments indicate that the novel approximation of the Wright-ω function reduced the computational time from half of a minute to fragments of a second. The relative error of the estimated friction flow factor is less than 0.5%.

Análisis numérico del proceso de filtración en presas de escollera

María Jesús Samper — Thu, 11 Jun 2020 11:20:55 +0200

Las presas de materiales sueltos funcionan como presas de gravedad, el peso propio de los materiales que la componen aportan la resistencia necesaria para soportar el empuje horizontal y la flexión que producen las aguas en el trasdós. El inconveniente principal de éstas presas está en el sobrevertido. Si se produce una avenida y el agua vierte sobre la presa, ésta se comporta de manera inestable y puede colapsarse y desmoronarse. Se sabe que este fenómeno del sobrevertido es una de las principales razones por las que colapsan este tipo de presas, pero se trata de un fenómeno poco conocido y poco desarrollado. Por ello existe un interés cada vez mayor en el estudio del comportamiento de las presas de materiales sueltos cuando se lleva a cabo el llenado, y el sobrevertido de agua sobre el talud aguas abajo. Este estudio está limitado por la ausencia de un método de cálculo, por lo que es esencial llegar a constituir un método numérico que modelice el comportamiento físico de estos procesos en dichas presas.

A mixed Lagrangian and Eulerian FSI approach for simulation of overtopping on embankment dams

María Jesús Samper — Thu, 11 Jun 2020 10:49:55 +0200

The present work is concerned with some recent advances in FSI simulations with particular emphasis on the simulation of rockfill dams during extreme phenomena. The final objective is the study of the consequences of an overtopping on the stability of such structure and its possible failure mode. A mixed Lagrangian and Eulerian approach is used. The fluid behavior is described using a modified form of the Navier Stokes equations in order to consider the effect of a variable porosity. A non linear Darcy law is included in the momentum equation. A level set function is chosen to follow the movement of the free surface inside and outside the porous medium. The structure is described using a purely lagrangian PFEM formulation. The specific features of PFEM make it appropriate to treat the rockfill material and its large deformations and shape changes. A projection technique allows to perform the data transfer between the fluid and the structure non matching meshes

Particle finite element method and level set method for the simulation of the failure of rockfill dams due to overtopping phenomena

María Jesús Samper — Thu, 11 Jun 2020 10:17:51 +0200

On the Validation of The Particle Finite Element Method (PFEM) for complex engineering fluid flow problems

María Jesús Samper — Thu, 11 Jun 2020 10:11:46 +0200

Several comparisons between experiments and computational models are presented in the following pages. The objective is to verify the ability of Particle Finite Elements Methods (PFEM) to reproduce hydraulic phenomena involving large deformation of the fluid domain.

Metodología de análisis del comportamiento de presas de escollera frente a un sobrevertido

María Jesús Samper — Thu, 11 Jun 2020 09:59:19 +0200

Con el objetivo de llegar a un mejor conocimiento del principio del proceso de rotura de las presas de material suelto y obtener una herramienta que permita un análisis más riguroso de este fenómeno, se está desarrollando el proyecto XPRES financiado por el Ministerio de Educación y Ciencia [XPRES 2007-2010]. El proyecto consta de una parte experimental, llevada a cabo por CEDEX y UPM conjuntamente, y de una parte numérica, desarrollada en el Centro Internacional de Métodos Numéricos en Ingeniería (CIMNE). En el presente trabajo se quiere presentar lo que CIMNE está realizando para crear una herramienta de cálculo para el análisis del comportamiento de una presa de escollera durante un sobrevertido, que, por su incidencia en la seguridad de la presa y su entorno, merece un tratamiento más preciso. La nueva metodología permite analizar con detalle el estado de tensión y deformación de la presa y su posible rotura parcial, o catastrófica, bajo la acción del flujo de agua y combina métodos de elementos finitos avanzados con métodos de partículas.

Physical and numerical modelization of the behaviour of rockfill dams during overtopping scenarios

María Jesús Samper — Thu, 11 Jun 2020 09:52:14 +0200

In recent years the technology on embankments dams has developed sensibly due to the advances in soil mechanic, and in all related sciences. Nevertheless their vulnerability to overtopping is still an open theme of discussion. Such an extreme phenomenon can lead to catastrophic consequences with loss of stability and failure of the entire structure. In spite of that, the consequences of an overtopping are still difficult to analyze and to prevent optimally. The possibility to define a numerical instrument to provide support for analyzing the failure of a dam is an important step ahead for organizing the intervention measures and for optimizing the economic plan. This is the principal reason why the Spanish ministry of Science and Innovation is funding the XPRES project, between the Polytechnic University of Madrid (UPM), the Centre for Hydrographical Studies of CEDEX and the International Centre for Numerical Methods in Engineering (CIMNE). The characterization of the failure process is dealt with by combining new numerical models, developed by CIMNE, with experimental tests on physical models of different sizes and scales carried out by UPM and CEDEX. In the present paper the work carried out by CIMNE is presented. The numerical model is based on combination of Lagrangian Particle Finite Element Method (PFEM) and Eulerian level set techniques in order to optimize the coupling between the dynamic effect of water inside and outside the dam, with the structural deformation and collapse induced on the down stream shoulder. The breaching formation and the consequent beginning of the failure are analyzed with particular interest. The specific features of the adopted method make it appropriate to treat the rockfill material and its large deformations and shape changes. A variable viscosity law and an energetic erosion criterion are used to take into account mass sliding and erosion acting in combined or alternative way

Estudios sobre el comportamiento de presas de escollera ante vertidos por coronación

María Jesús Samper — Thu, 11 Jun 2020 09:43:00 +0200

Como consecuencia del desarrollo de un Proyecto de Investigación, el CEDEX, la Universidad Politécnica de Madrid y el CIMNE han trabajado conjuntamente en el desarrollo de un método para el estudio del proceso de rotura de presas de escollera por sobrevertido combinando técnicas de elementos finitos y partículas. Para ello se han realizado y analizado 80 ensayos en modelo físico en las instalaciones del CEDEX y UPM, utilizando seis tamaños de material, cuatro taludes distintos (1,5H:1V; 2,2H:1V; 3,0H:1V y 3,5H:1V) y tres opciones de impermeabilización (sin pantalla, núcleo interno y pantalla externa). Estos ensayos están permitiendo conocer mejor el fenómeno y calibrar el modelo matemático. El modelo matemático basa su desarrollo en el acoplamiento de dos fenómenos: la filtración del agua en un medio granular (simulada mediante una malla euleriana) y la erosión del propio material granular (simulada mediante una malla lagrangiana). Durante 2010 se prevé finalizar la construcción, en el CEDEX, de un canal de grandes dimensiones que permitirá ensayar presas de hasta 4 m de altura en condiciones de laboratorio, que servirán como prototipo para el análisis del efecto de escala. Este prototipo aumentará considerablemente la fiabilidad de las conclusiones obtenidas.

PFEM application in fluid structure interaction problems

María Jesús Samper — Thu, 11 Jun 2020 09:22:07 +0200

In the current paper the Particle Finite Element Method (PFEM), an inno-vative numerical method for solving a wide spectrum of problems involving the interaction of ﬂuid and structures, is brieﬂy presented. Many examples of the use of the PFEM with GiD support are shown. GiD framework provides a useful pre and post processor for the speciﬁc features of the method. Its advantages and shortcomings are pointed out in the present work

Artificial neural network as a tool for estimation of hydraulic resistance

Dejan Brkić — Wed, 10 Jun 2020 17:44:02 +0200

Design of gas network problem from practice

Dejan Brkić — Wed, 10 Jun 2020 17:34:02 +0200

Variants of indirect and direct natural gas usage for heating of flats

Dejan Brkić — Wed, 10 Jun 2020 17:27:02 +0200

Sedbuk: Britain methodology for determination of seasonal efficiency of gas boilers

Dejan Brkić — Wed, 10 Jun 2020 17:19:02 +0200

Usage of Lambert-W function in calculation of flow friction in pipes

Dejan Brkić — Wed, 10 Jun 2020 17:12:03 +0200

Example of looped gas distributive network calculation using unified node-loop method

Dejan Brkić — Wed, 10 Jun 2020 16:57:02 +0200

Natural gas prices in Serbia and in the world

Dejan Brkić — Wed, 10 Jun 2020 16:47:02 +0200

Natural gas as ecological most acceptable fossil fuel

Dejan Brkić — Wed, 10 Jun 2020 16:37:02 +0200

Condensation technology boilers

Dejan Brkić — Wed, 10 Jun 2020 16:30:02 +0200

Verification of conditional area model of gas heating in setlements

Dejan Brkić — Wed, 10 Jun 2020 16:23:03 +0200

In this paper is shown short particular review of the book by the same author “Natural Gas as Heating Fuel”. First criterions for decision of appending on distribution gas network or district heating system for particular part of town are determinate on conditional urban area base. These conclusions are verified on specific settlement in Belgrade and in this paper are shown results on Karaburma settlement.

Designing of specific class of gas distribution networks

Dejan Brkić — Wed, 10 Jun 2020 16:15:02 +0200

Optimization of pipe diameters in looped pipeline network after the modified loop method

Dejan Brkić — Wed, 10 Jun 2020 16:07:02 +0200

After the calculation of flows in a looped pipeline distribution network is done, very often velocity of fluid in some of the pipeline branches are undersized while in other are oversized. Hence, it is necessary to perform optimization of the pipes’ diameter to fit in range of suggested values. Suggested velocities for water flow in waterworks systems are 0.75-1.5 m/s, while in the case of natural gas flow in gas-pipelines this velocity limit is increased up to20 m/s. Problem of velocity in a gas pipeline is more complex in comparison to water velocity. This issue is caused by compressibility of gas. In this paper is shown procedure of diameters’ optimization after the modified Hardy Cross method. Difference in comparison to classical problem in which flow distribution is unknown is that on the contrary, in optimization problem flows are now constants while the diameters are treated as variables.

Calculation of flow distribution in looped pipeline network after the node-loop method

Dejan Brkić — Wed, 10 Jun 2020 15:55:02 +0200

Calculation of gas i.e. water distribution network is much more complex compared with branch-like distribution pipelines. In this paper is shown one more suitable method in comparison to modified Hardy Cross. Here shown method as well as modified Hardy Cross method are iterative methods. For balancing of network, approximately equal number of iteration are to be calculated after the here presented node-loop method as for the improved Hardy Cross. Main advantage of the node-loop method in comparison to improved Hardy Cross method is that the final result in each calculation is flow which is used directly as input parameter in the next iteration cycle. Disadvantage of the improved Hardy Cross method is that this flows are calculated indirectly using flow corrections which they need to be added after complicated algebraic scheme to the flows calculated in previous iteration.

Classification of domestic gas boilers based on efficiency

Dejan Brkić — Wed, 10 Jun 2020 15:35:03 +0200

Methodology for calculation of Seasonal Efficiency of Comestic Gas Boilers is set by the UK Government's Standard Assessment Procedure for Energy Rating of Dwellings. This methodology is general applicable, and can be applied in our country. This methodology is compatible with EU Council Directive 92/42/EEC on efficiency requirements for new hot-water boilers fired with liquid or gaseous fuels. Thus calculated Seasonal Efficiency is relevant parameter for comparisons of different models of boilers. Energy Efficiency Bands are as signed, in a first place based on verified values of seasonal efficiency. In correlation with seasonal efficiency stands annual fuel consumption of certain boiler, and consequently annual fuel consumption of certain boiler, and consequently annual fuel costs are comparable.

Analysis of characteristic for gasification and district heating on the model of nominal structural area

Dejan Brkić — Wed, 10 Jun 2020 15:24:02 +0200

Criterias of iterative procedure interupt at calculation of gas distribution network with loops

Dejan Brkić — Wed, 10 Jun 2020 15:17:03 +0200

Diagnostic of problems issued at calculation of low pressure gas distribution networks with loops

Dejan Brkić — Wed, 10 Jun 2020 15:09:02 +0200

Optimization of loop method adjusted for calculation of gas distribution networks

Dejan Brkić — Wed, 10 Jun 2020 14:55:05 +0200

Gas distributive network in settlements are usually with contours, which assure that gas is available for consumers from few different directions. By application of concept of looped network least possible number of consumers can remain without gas in the case of malfunction in some branch. But, calculation of these types of networks is much more complicated compared with branch type of network. Here is shown example of one possible way of improvement of iterative procedure during the calculation of network according to Hardy Cross method.

Seasonal efficiency of domestic gas boilers

Dejan Brkić — Wed, 10 Jun 2020 14:43:02 +0200

Methodology for calculating Seasonal Efficiency of Domestic Gas Boilers is set by The UK Government’s Standard Assessment Procedure for Energy Rating of Dwellings. This methodology is compatible with EU Council Directive 92/42/EEC on efficiency requirements for new hot-water boilers fired with liquid or gaseous fuels. Thus calculated Seasonal Efficiency is relevant parameter for comparisons of different models of boilers.

Comparison of gas boilers in households based on seasonal efficiency

Dejan Brkić — Wed, 10 Jun 2020 14:33:02 +0200

This paper presents a review of the boilers according to performed seasonal efficiency. Energy Efficiency Bands are assigned, primarily based on the verified values of seasonal efficiency. In correlation with seasonal efficiency stands annual fuel consumption of a certain boiler, and consequently annual fuel costs are comparable.

Influence factors on natural gas price forming

Dejan Brkić — Wed, 10 Jun 2020 14:16:02 +0200

Condensation technology efficiency

Dejan Brkić — Wed, 10 Jun 2020 14:05:02 +0200