Effects of length-to-diameter ratio on a near free surface underwater vehicle

This paper presents the Computational Fluid Dynamics (CFD)-based study on the effects of length-to-diameter ratio on the interaction forces acting on an underwater vehicle operating near the free surface. The CFD model was validated with experimental data involving the SUBOFF and the Joubert hull undergoing straight-ahead motion near the free surface. The validated model was extended to investigate the SUBOFF with varying length-to-diameter ratios (L/D = 7.3, 8.6, and 9.5) by adjusting the length of the parallel mid-body. The models were tested at a range of forward speeds and submergence depths from 1.1 to 6.6 diameters from the free surface. Results show that the lengthening of the parallel mid-body causes a phase shift in the hydrodynamic coefficients, most notably a delay in the first surge coefficient peak. This is due to an increased distance between the bow and the stern, and thus their respective wave systems and the locations at which they superimpose.