NATO STO

The NATO AVT-301 RTG evaluated the state-of-the-art in computational fluid dynamics (CFD) predictions of underwater vehicle hydrodynamics. The use of a single common geometry (the BB2 generic submarine model) and data formats allowed for the direct comparison of CFD predictions of both the flow fields and hydrodynamic loads for steady maneuvering conditions. Relatively good agreement was achieved between individual CFD submissions, and between CFD and wind tunnel experiments, for translation at a moderate drift angle. However, large discrepancies were observed for steady turning manoeuvres. None of the CFD submissions consistently matched all hydrodynamic forces and moments measured in a rotating arm experiment and there was a large scatter between individual CFD submissions. It was determined that the differences in predictions were caused by a wide variation in predicted flow separation from the aft control surfaces, which are exposed to large flow incidence angles during turning manoeuvres. Only the total vehicle forces and moments were measured in the rotating arm experiments so it was difficult to assess local differences between CFD and experiment. The proposed RTG will extend the work of AVT-301 by analyzing the complex flow around marine control surfaces at high angles of attack in more detail. A focus will be on the prediction of flow separation and detached flow but there is also interest in horseshoe vortices generated at hull-rudder junctions. New experiments with an isolated control surface that include flow field measurements will be conducted to better evaluate CFD prediction methods with varying degrees of fidelity. While Reynolds-Averaged Navier-Stokes (RANS) and unsteady RANS methods were primarily used for AVT-301, the use of more focused geometry and the advancement in computing power are expected to allow more participants to evaluate higher-fidelity methods such as hybrid RANS-LES (large eddy simulation) and wall-modeled LES in this follow-on study.