Chapter 6 - CFD VALIDATION FOR FLOW SEPARATION ONSET AND PROGRESSION USING DELFT CATAMARAN 372 IN STATIC DRIFT CONDITIONS

This chapter presents CFD validation efforts for the high-speed, multihull Delft Catamaran 372 advancing in calm water with steady drift angles. Available experimental data include hydrodynamic loads (from BSHC), sinkage and trim measurements (from BSHC and CNR-INSEAN), and stereo-PIV measurements of the velocity field on several transverse planes (from CNR-INSEAN). Three organizations have conducted RANS or DES simulations by using their own codes: CNR-INSEAN using Xnavis; IIHR, The University of Iowa using CFDShip-Iowa and CNRS/ECN using ISIS. Computations have been made using different grid strategies (structured grid with overlapping, unstructured grid with or without an automatic grid refinement), several turbulence models (the isotropic one equation Spalart-Allmaras model, the anisotropic two equations STT k? and EARSM models, and DES simulations) and different free surface approaches (single phase level set and volume of fluid). Comparisons are made in terms of integral quantities (i.e. hydrodynamics loads and vehicle attitudes), local quantities in separated vortex core (i.e. axial vorticity, axial velocity, position of the vortex and TKE), planar data (velocity field, axial vorticity field and TKE on selected planes) and wave patterns. Discussions and comparisons on the onset and progression of the vortical structures separated from the two hulls will be presented. In general computational results have shown that the main features of the separated flow field are well captured by all the computations. No large differences between submissions can be inferred from the cross planar fields; effects of turbuence model and grid resolution have been investigated by the core vortex analysis. Grid resolution effects are dominant in the onset region; only very refined grids (total of order of 100M of grid points or using an automatic grid refinement technique) are able to provide field quantities closer to EFD data. Turbulence model effects are dominant in the progression of the main vortices, with DES simulations and RANS based on non-isotropic models, proved to be superior (stronger and less dissipated vortices) than one equation isotropic models. In terms of hydrodynamic loads, a general good agreement between the submissions has been observed, with the standard deviation on the resistance prediction of about 3.5%; similar differences between CFD have been seen for the lateral force prediction, whereas larger discrepancy is observed for the yaw moment estimation (about 7%). The comparison with measured data revels a rather large overall error (of the order of 15%); however, it has to be pointed out that EFD values where collected for different conditions and a larger model; reference values have been obtained by an interpolation procedure on speed/drift plane, whereas no Reynolds number correction was considered. Differences due to grid resolution and the model adopted for the description of the free surface have been also observed; very fine grids are required to capture wave breaking and rebounds phenomena. Both single phase Level Set and Volume of Fluids approaches are able to capture wave induced vortices; the VOF method shows the formation of foam close to the free surface region and a consequent dissipation of the vortices, whereas the single phase level set provides a long life vortices. The different behaviour is supposed due to the interaction with the underline turbulence model (i.e. the production of turbulent viscosity close to the free surface), rather than the free surface approach itself.