Post Workshop Computations and Analysis for KVLCC2 and 5415

The Workshop submissions for the local flow predictions for straight ahead KVLCC2 and 5415 were on large disparate grids ranging from 0.6M to 300M, which made it difficult to draw concrete conclusions regarding the most reliable turbulence model, appropriate numerical method and grid resolution requirements. In this chapter, additional analysis including grid verification study is performed on intermediate grids to shed more light on these issues. Second order TVD or bounded central difference schemes are found to be sufficient for URANS, whereas fourth or higher order schemes are required for hybrid RANS/LES (HRLES). Resistance predictions show grid uncertainties £ 2.2 % for URANS on 50M grid and HRLES on 300M grid, which suggests that these grids are approaching asymptotic range. URANS with anisotropic turbulence model perform better than URANS with isotropic turbulence model. Grid with 3M points are found to be sufficient for resistance predictions, however, grids with up to 10s M points are required for local flow predictions. Adaptive grid refinement is helpful in generating optimal grids; however available grid refinement technique based on the Hessian of pressure, fails to refine the grid further downstream along the hull. HRLES simulations are promising in providing the details of the flow topology. However, they show limitations such as grid induced separation for bluff body KVLCC2 and inability to trigger turbulence for slender body 5415. Implementation of improved delayed DES and/or physics based RANS/LES transition is required to address these limitations. Grid resolution of 300M shows resolved turbulence levels of > 95 % for bluff body, thus such grids seem sufficiently fine for HRLES. The free-surface predictions do not show significant dependence on boundary layer predictions, and accurate prediction for 5415 at Fr = 0.28 is obtained using just 2M grid points. The free-surface reduces pressure gradients on the sonar dome, causing weaker vortical structures than single phase. Flow over 5415 shows three primary vortices, and all of them originate from the sonar dome surface. Onset analysis shows that all the three vortices have open-type separation, and separate from the surface due to cross flow. Further investigation of the cause of differences in KVLCC2 CFD submissions and experimental data suggests that it could be due to differences in the sharpness of the stern.

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