Submarine Motions in Confined Waters. INSEAN Technical Report No. 8

This technical report concerns the work element WE-4.5 within the WEAO EUCLID RTP 10.17 project "Submarine Motions in Confined Waters". The velocity fields predicted by the CFD solver developed at INSEAN around a submarine advancing with constant speed and straight path, in open-water, close-to-bottom and close-to-free surface conditions are compared with the corresponding INSEAN flow measurements. The comparison represents a validation of the numerical method from a local point of view and is used to revise the solver, that is to identify its potentialities and main challenges both in handling the flow features around the vehicle and to estimate the global loads acting on it. These aspects represent the aim of the examined Work Element within the Project. Generally experimental and numerical results are consistent and show a promising agreement. The largest qualitative discrepancies are observed in local regions where very small angles of the hull (i.e. zero submarine drift angle) and/or of the appendages (i.e. zero forward plane angle) occur and no other flow disturbances are present. Such circumstances are very challenging both experimentally and numerically because the flow features are very sensitive to the errors committed. The comparison supports the validity of the basis assumptions of the developed solver and its capabilities in investigating problems with submarines. Further it identifies the lateral velocity component as an important parameter for the differences observed when comparing the numerical lateral and vertical forces with the corresponding towing-tank measurements. The outcome of the present analysis is the suggestion of a dedicated convergence study to relate the grid size with the numerical prediction of the global loads. This is important for fixing the maximum grid size allowable to get results quantitatively reliable. Finally, the presence of the strut in the experimental case with close-to-free surface conditions and non-zero drift should be investigated numerically to quantify the strut influence on the submarine lateral force.