CFD analysis of a captive bullet entry in calm water with and without turbulence

Simulations are carried out for a simplified lifeboat drop test case, which consists of a captive axisymmetric generic lifeboat shape (bullet), that penetrates the water surface at a constant velocity and angle of attack. The quantities of interest are the body fixed longitudinal force F_X, vertical force F_Z, and pitch moment M _YY. This case was previously used in a verification and validation exercise [1]. Here, a step forward in complexity is taken, as the previous numerical model is now supplemented with the eddy-viscosity based turbulence model k–ω SST. Both approaches are then used to simulate two different cases: Case 1 with minimal wake effects; and Case 3 with flow separation and significant wake. The results are compared with the experimental data. The numerical uncertainty is estimated for both models. It is seen that for Case 1 the difference between both models is mostly within the comparison uncertainty, except for the longitudinal force F_X, where the turbulent flow predicts a larger force, improving the comparison with the experiments. The loads predicted with turbulent flow stayed mostly within 6 % of the laminar flow. For Case 3 small differences between both models are found during/after the wake collapse stage. However, this difference is often within the comparison uncertainty. A reasonable agreement is found with the experimental data, except for F_Z after the bow wake collapse. The turbulent flow improves slightly on the laminar approach regarding the agreement with the experiments, however it can be argued if this difference justifies the increased computational cost of the turbulence model.