Investigation of flow characteristics occurring by interaction of different torpedo-like geometries with a free water surface

In this study, the flow structures around various torpedo-like geometries near the free-surface were investigated by using dye flow visualization experiments, Particle Image Velocimetry (PIV) measurement method and Computational Fluid Dynamics (CFD). The study was carried out at Reynolds numbers of Re=2x104 and 4x104 based on the characteristic length of the geometry for immersion ratios between 0.5≤h/D≤3.5 and angles of attack α=0°, 4°, 8° and 12°. Passive control methods were applied by using elliptical and spherical nose structures and adding wings to the rear. Free-surface was simulated using VOF multiphase model. Results showed that decreasing the immersion ratios increases the drag coefficient CD. The overall structure of the wake region under the effect of free-surface was observed to be asymmetrical and interacting with the free-surface for all angles of attacks at h/D<1.5. Due to the restriction of the flow between the free-surface and geometry at lower immersion ratios, a jet-like flow occurred between the upper surface of the geometry and the free-surface. Interaction of the spherical nose with the free-surface caused more distortions on the free-surface for all cases than the elliptical one. The effect of the free-surface was found negligible for h/D≥2.5. The elliptical nose was found to be more effective on the flow control than the spherical nose with lower CD and delayed flow separation on its upper surface. Also, the 4-winged stern was observed to be most effective at suppressing the vortex dissipation in the wake.