Search

International audience

International audience

International audience

International audience; The flow around the well-known KRISO Very Large Crude Carrier no. 2 (KVLCC2) is characterised by boundary layer thickening, flow separation, and vortex creation on the aft body of the ship, leading to a very nonhomogeneous velocity field in the propeller plane (figure 1). It is known since the 1990s that the numerical simulation of such flows is highly sensitive to the treatment of the turbulence. For RANS equations, standard turbulence models perform poorly; measures like the anisotropic treatment of turbulence production are necessary in order to compute correctly the most famous aspect of the flow, the hook-shaped low-velocity zone in the main longitudinal vortex. The case is therefore justly famous as a test of turbulence models for marine flow simulation. The focus of most studies that use the KVLCC2, such as Larsson et al. [3], is the comparison of different flow solvers or different turbulence models. To keep such studies compact and accessible, comparisons are mostly done for a few physical features only. The propeller plane with its hook shape in the axial velocity is often studied. The objective of this paper, on the contrary, is to use a single numerical simulation of good quality to visualise the entire flow field around the aft body of the KVLCC2.We then attempt a physical analysis of this flow, searching explanations of less-studied features such as the outer boundary layer shape and the flow behind the propeller hub cap.We hope that better knowledge of the flow will allow, in the future, to understand more precisely the behaviour of different turbulence models, which may ultimately lead to better models. The paper starts with a definition of the test case and the computation (section 2). Then section 3 presents the visualisation and analysis of the numerical solution. To assess the validity of the analysis, selected aspects of the solution are compared with experiments in section 4. The conclusion (section 5) addresses the challenges for turbulence modelling which the KVLCC2 test case still presents.

International audience; The accurate modeling of surface tension is an important prerequisite to correctly simulate the phenomena that take place in smaller scales. However, modeling surface tension for complex hydrodynamic flows, such as marine applications, poses several difficulties. More specifically the method has to deliver accurate curvature results for absurdly changing topological features of the free surface. As a result, the topological features of the free surface have to be dynamically recovered during the calculation. In this work, we present a surface tension (ST) method implemented in the VOF-RANS flow solver ISIS-CFD [1], that has the above features and present preliminary results obtained for a flat inclined plane cutting the free-surface in an asymmetric flow. In order to capture the geometry of free-surface, the method uses a marching cube algorithm (see for example [2]). The algorithm dynamically generates the grid of the volume fraction isosurface representing the free surface. The curvature is calculated by the generated grid, using a high-order differencing method based on least squares. The results obtained for the test case studied, demonstrate that the method can be used in practical cases to directly visualize complex three dimensional flow regimes and investigate the surface tension effects for smaller scales. For example, the figure below shows the unstructured surface grid generated for the ventilating air vortex emanating near the plate's (transparent plane) submerged leading edge corner and a detail of the free surface roll-up.REFERENCES[1] P. Queutey and M. Visonneau, “An interface capturing method for free-surface hydrodynamicflows”, Computers and Fluids, Vol. 36, pp. 1481−1510, (2007).[2] T.S. Newman, H. Yi, “A survey of the marching cubes algorithm”, Computers and Graphics,Vol. 30, pp. 854–879, (2006).

International audience; This paper studies if adaptive grid refinement combined with finite-volume simulation of the incompressible RANS equations can be used to obtain grid-independent solutions of realistic flow problems. It is shown that grid adaptation based on metric tensors can generate series of meshes for grid convergence studies in a straightforward way. For a two-dimensional airfoil and the flow around a tanker ship, the grid convergence of the observed forces is sufficiently smooth for numerical uncertainty estimation. Grid refinement captures the details of the local flow in the wake, which is shown to be grid converged on reasonably-sized meshes. Thus, grid convergence studies using automatic refinement are suitable for high-Reynolds incompressible flows.

International audience

International audience

International audience

International audience

Submitted to Ocean Engineering on 15 July 2014

International audience; This article deals with the validation of the modelling and numerical simulation of a rowing stroke, by means of CFD. Simplified but realistic strokes were performed in a towing tank with a rotating arm and a real flexible oar. Those laboratory conditions are better controlled than those of in situ trials. An FSI procedure is developed to take into account the oar bending, which is essential in the physics of this flow. The results show that this numerical framework is able to reproduce qualitatively the real flow including the breaking of the free surface around the blade and the transport of the air cavity behind it. The profiles of forces are well reproduced, with propulsive forces overestimated by 5-12% for their maxima. The study also focuses on the computation of the uncertainties. It is highlighted that, even for this well-controlled experimental equipment, the uncertainties on the quantities of interest are of about 11%. In other words, the experimental uncertainty covers the numerical errors. So, this numerical modelling is validated and can be used for design and optimisation of blades and oars, or to contribute to the better understanding of the boat-oar-rower system and its dynamics.

International audience; This paper studies if adaptive grid refinement combined with finite-volume simulation of the incompressible RANS equations can be used to obtain grid-independent solutions of realistic flow problems. It is shown that grid adaptation based on metric tensors can generate series of meshes for grid convergence studies in a straightforward way. For a two-dimensional airfoil and the flow around a tanker ship, the grid convergence of the observed forces is sufficiently smooth for numerical uncertainty estimation. Grid refinement captures the details of the local flow in the wake, which is shown to be grid converged on reasonably-sized meshes. Thus, grid convergence studies using automatic refinement are suitable for high-Reynolds incompressible flows.

CFD simulation results for self propelled full scale ship are compared to sea trial measurements in this work. Two–phase RANS based CFD numerical model used in this work is based on the Ghost Fluid Method for numerically robust treatment of discontinuities at the free surface and the algebraic Volume–of–Fluid method for interface capturing. The propeller is modelled as a pressure–jump based actuator disc, allowing CPU time efficient simulations while preserving the accuracy of integral results. The numerical model is implemented in foam–extend, a community driven fork of the OpenFOAM software. The comparison with sea trials includes achieved forward speed, thrust and torque for given shaft speed (in RPM) for a general cargo carrier.

Split Trailing Suction Hopper Dredgers (Split TSHD) have longitudinally-split hull, which symmetrically opens when executing gravity-driven unloading of the cargo, while being ex- posed to various environmental conditions. Even though they have variable hull geometry, their hydrostatic and stability characteristics are usually calculated for initial and unchanged loading conditions only, which is a requirement imposed by classification society stability regulations for TSHD ships [2, 3, 4]. In order to investigate the significance of the discharge process dynamics on actual ship stability, unsteady numerical simulations were performed with the Discrete Element Method (DEM) for symmetrical hopper opening during cargo discharge procedure, without the hull opening failure modes examined. The ship hydrostatic properties, which are pre-calculated analytically using Radial Basis Functions (RBF) for all possible states [11], are used in combin- ation with the solver in order to compute the righting moment and the righting arm, which are affected by the dynamics of the cargo and the loss of displacement. The dynamics of the cargo discharge process was simulated with a DEM solver implemented for Graphics Processing Units (GPUs), Blaze-DEMGPU [8]. Spherical shapes of particulate elements were employed to model the soil cargo, with both cohesion and buoyancy effects included for wetted elements. The simu- lations of the discharging were performed for various loading conditions. Numerical simulations indicate that the dynamics of the cargo during its discharging should not be ignored due to its effect on the transverse stability of the ship. Therefore, an incoming wave and other environ- mental loads in combination with a hull opening failure during the discharge could lead to inapt unstable situation of the ship. Non-symmetrical Split TSHD ship openings will be examined in future work, with an investigation of its influence on ship stability and safety of cargo discharge procedures in failure modes.

In this paper a monolithic algorithm for coupling rigid body motion equations and pressure field within a Finite Volume framework is presented. Monolithic coupling enables fewer number of pressure–velocity loops per time–step, thus reducing the required computational time. The presented method is compared to conventional partitioned coupling approach in terms of computational efficiency and accuracy. The results are compared to experimental data as well.

An important consideration in the catamaran design is the distance between the hulls. Arrangement of the hulls in catamaran configuration can have strong influence on the wave making resistance and thus on the total resistance of a catamaran. The hydrodynamic interaction between hulls becomes significant when spacing between hulls is sufficiently small. In this paper, numerical simulations of viscous flow around monohull and catamaran model are performed utilizing commercial software package STAR-CCM+, in order to investigate the influence of spacing between hulls on the interference resistance. A mathematical model based on Reynolds Avaraged Navier-Stokes (RANS) equations, k-ε turbulence model and Volume of Fluid (VOF) method for describing the motion of two-phase media are briefly described. Numerical simulations are performed for Series 60 monohull and two catamaran configurations with CB=0.6 for different values of Froude number. Results of performed numerical simulations are compared with experimental results available in the literature and satisfactory agreement has been achieved. It has been shown that CFD is a very useful tool in preliminary catamaran design.

To realise the full potential of Computational Fluid Dynamics (CFD) within ma- rine science and engineering, there is a need for continuous maturing as well as verification and validation of the numerical methods used for free surface and interfacial flows. One of the distinguishing features here is the existence of a water surface undergoing large deformations and topological changes during transient simulations e.g. of a breaking wave hitting an off- shore structure. To date, the most successful method for advecting the water surface in marine applications is the Volume-of-Fluid (VOF) method. While VOF methods have become quite advanced and accurate on structured meshes, there is still room for improvement when it comes to unstructured meshes of the type needed to simulate flows in and around complex geometric structures. We have recently developed a new geometric VOF algorithm called isoAdvector for general meshes and implemented it in the OpenFOAM interfacial flow solver called interFoam. We have previously shown the advantages of isoAdvector for simple pure advection test cases on various mesh types. Here we test the effect of replacing the existing interface advection method in interFoam, based on MULES limited interface compression, with the new isoAd- vector method. Our test case is a steady 2D stream function wave propagating in a periodic domain. Based on a series of simulations with different numerical settings, we conclude that the introduction of isoAdvector has a significant effect on wave propagation with interFoam. There are several criteria of success: Preservation of water volume, of interface sharpness and shape, of crest kinematics and celerity, not to mention computational efficiency. We demonstrate how isoAdvector can improve on many of these parameters, but also that the success depends on the solver setup. Thus, we cautiously conclude that isoAdvector is a viable alternative to MULES when set up correctly, especially when interface sharpness, interface smoothness and calcula- tion times are important. There is, however, still potential for improvement in the coupling of isoAdvector with interFoam’s PISO based pressure-velocity solution algorithm.