An enhanced direct forcing immersed boundary method implemented in the open-source hydrodynamic framework REEF3D::CFD is used to simulate six-degrees-of-freedom (6DOF) motion response of a 1:30 scale point-absorber wave energy converter(WEC) under extreme wave conditions. The enhancement of the method is achieved with a new density interpolation method that removes unphysical spurious phenomena. The governing equations are solved on a staggered rectilinear grid. REEF3D::CFD uses the level set function to represent the free surface. A ray casting algorithm is employed to get inside-outside information in the vicinity of the body with the underlying Cartesian grid. The enhanced method is tested and validated based on the experimental data from the experimental wave tank campaign carried out in the Ocean and Coastal Engineering Laboratory, at Aalborg University, in Denmark.
Abstract An enhanced direct forcing immersed boundary method implemented in the open-source hydrodynamic framework REEF3D::CFD is used to simulate six-degrees-of-freedom (6DOF) motion [...]
Q. Rakotomalala, L. Rouleau, C. Leblond, M. Abbas, J. Deü
marine2023.
Abstract
In this paper, a simplified numerical approach is applied to the analysis of the linear vibrations of aluminum or composite marine propeller blades with embedded viscoelastic films, around a predeformed nonlinear state. This approach allows to evaluate in a sequential way the nonlinear deformation, induced by the generation of lift (propulsion phenomenon), and the acoustic power emitted (vibroacoustic phenomenon) of a non-cavitating flexible marine propeller blade. For a given incident flow, the deformation of an aluminum blade is much smaller than that of a composite blade. The composite blade’s deformation has a significant positive impact on lift generation whereas the deformation of the aluminum blade tends to reduce traction. The dynamic characteristics of the aluminum blade at rest and under the fluid induced static load are similar, while those of the composite blade are very different in these two cases. The simplified two-step approach proposed in this study is therefore useful to accurately assess the emitted sound power by the vibrating blades.
Abstract In this paper, a simplified numerical approach is applied to the analysis of the linear vibrations of aluminum or composite marine propeller blades with embedded viscoelastic [...]
In this work, our objective is to quantify emission reductions using speed optimization considering a realistic ship route and a broad range of weather conditions. Two representative bulk carriers have been selected for the analysis. An optimization algorithm has been used to minimize voyage fuel consumption while completing the voyage on or before the expected arrival time. A constraint on engine power has been used for realistic estimates of achievable ship speeds in different weather conditions considering the available engine power. Multiple voyages at different ship speeds and in different seasons have been simulated with and without speed optimization to observe the effect of these factors on emission reduction. The effect of wind and waves on engine power has been considered by calculating wind and wave resistance along with propeller efficiency as a function of advance coefficients. Up to 11% reduction in fuel consumption was obtained by optimizing speed as compared to the constant speed profile. It was observed that a significant amount of fuel could be saved especially in seasons with a higher likelihood of heavy weather. Variation in fuel savings in different seasons has been discussed in the context of metocean conditions experienced in the selected months. Additionally, higher fuel savings were obtained for lower average ship speed which means speed reduction combined with speed optimization has greater potential to reduce emissions. Realistic estimates of fuel savings in a range of operating conditions presented in this paper would help ship owners, operators, and policymakers to assess the benefits of speed optimization among other technologies to decarbonize the shipping industry.
Abstract In this work, our objective is to quantify emission reductions using speed optimization considering a realistic ship route and a broad range of weather conditions. Two representative [...]
J. Kemper, J. Mense, K. Graf, U. Riebesell, J. Kröger
marine2023.
Abstract
Artificial Upwelling (AU) of nutrient-rich Deep Ocean Water (DOW) to the ocean’s sunlit surface layer is currently being investigated as a way of increasing the ecosystem productivity and enhancing the natural CO2 uptake of the ocean. AU is thus considered a marine Carbon Dioxide Removal (CDR) option (GESAMP, 2019) in addition to its potential in the context of open ocean fish and macroalgae farming (Kirke, 2003; Wu et al., 2023). A promising technical concept for AU was described by the oceanographer Stommel et al. (1956). Stommel proposed that the counteracting effects of typical open ocean temperature and salinity depth profiles on density can be utilized to drive a self-sustaining upwelling flow in a vertical ocean pipe. He termed this effect the ”perpetual salt fountain”. Despite several research efforts, none of the previous studies were able to reliably predict or demonstrate the potential of Stommel Upwelling Pipes (SUP)s. The growing interest in AU in light of current CDR research poses the need for reliable performance prediction methods and further development of Stommel’s concept. To fill this gap, two models have been developed in the present work. A Reynolds-Averaged Navier-Stokes (RANS) model and a one-dimensional numerical model. While the RANS model enables detailed modeling of the heat transfer and flow phenomena, the onedimensional numerical model allows for fast evaluation of simplified geometries for optimization and large-scale studies. This twofold approach allows for effective performance predictions while ensuring good reliability of the results. The present work shows the results of a number of studies, performed for different geometries and environmental conditions. The results of both models are compared and analyzed, and the respective potential is demonstrated. The presented results provide insight into some key aspects of the performance of SUPs and their potential for AU.
Abstract Artificial Upwelling (AU) of nutrient-rich Deep Ocean Water (DOW) to the ocean’s sunlit surface layer is currently being investigated as a way of increasing the ecosystem [...]
As ships operate under sea wave conditions most of time, it is desirable to consider the wave effect on propeller performance and cavitation safety in the propeller design process. In this work, unsteady cavitation simulations are carried out on a five-bladed propeller of KRISO container ship in calm water and regular waves of five different headings. Bare-hull simulations are made for estimating nominal hull wake fields by URANS solver. Cavitation simulations are made on the propeller and rudder by DES with a cavitation model and an Eulerian multiphase flow model. Nominal hull wake is numerically modelled in cavitation simulations as a propeller inflow instead of including a hull model. The maximum cavity area on the suction side of the blade is increased by 19 – 32% for beam, stern-quartering and following sea waves compared to calm water mostly due to the stronger axial hull wake. As the sheet cavity is more extended, tip vortex cavitation is intensified especially for stern-quartering and following waves. The maximum cavity area is on a similar level with less than 3% differences for head and bow waves as for calm water. The CFD investigation shows that hull wake differs depending on the wave direction and it can lead to significant changes in cavitation safety.
Abstract As ships operate under sea wave conditions most of time, it is desirable to consider the wave effect on propeller performance and cavitation safety in the propeller design [...]
The aim of the current paper is to evaluate the cavitation erosion on a Delft twisted hydrofoil using a coupled Euler-Lagrange methodology. The transport equation modelling approach is introduced to handle the macroscopic liquid-vapor mixture, which is regarded as a homogeneous continuum. The Keller-Herring equation and bubble motion equation are used to track the bubble's dynamics and trajectory. A two-way coupling method is employed to describe the interaction between the mixture and bubbles. A newly developed Lagrangian erosion model is used to assess the cavitation erosion on the hydrofoil. The numerical results are in good agreement with the experimental test data. The statistical results reveal the evolution characteristics of cavitation erosion. The relationship between macroscopic cavitation structure and potential erosion sensitive zone indicates the cavitation erosion intensity at different stages of cloud cavitation. This study contributes to a deeper understanding of the mechanism of cavitation damage from a multi-scale perspective.
Abstract The aim of the current paper is to evaluate the cavitation erosion on a Delft twisted hydrofoil using a coupled Euler-Lagrange methodology. The transport equation modelling [...]