The impacts of the free-surface and angle of attack on the flow structures around a torpedo-like geometry

This study presents the hydrodynamic characteristics around a torpedo-like geometry under the free-surface effect at different angles of attack using preliminary dye visualization and Particle Image Velocimetry (PIV). The optimized torpedo-like geometry is placed at submersion ratios between 0.5≤h/D≤3.50 where h is the distance to the free-surface from its centerline and D is the diameter. Throughout the experiments, angles of attack were taken as 0°≤α≤12° for two Reynolds numbers, Re = 2.0 × 10⁴ and 4.0 × 10⁴. The PIV method provided instantaneous vorticity and time-averaged velocity components, vorticity, streamline topology, fluctuating velocity components, Reynolds stress correlation, and the turbulent kinetic energy. This study focused on the stern section and the wake structures at h/D=1.0. It is demonstrated that a jet-like flow region occurred between the model and the free-surface for all angles of attack at small submersion ratios of 0.5≤h/D≤1.0 while it is observed at h/D=1.5 for α=8°, a jet-like flow region occurred between the model and the free-surface. The impact of the jet-like flow was more noticeable for α=4° and 8° with velocity fluctuations in lower magnitudes. The nose section partially pierced the free-surface for h/D = 0.5, 0.75, and 1.0 in the range of 4°≤α≤12° and it prevented the wake region from connecting with the free-surface by directing the separated flow region toward the stern section. Pointwise variations of the turbulence data extracted from vertical lines within the wake region for all cases revealed that the effect of the free-surface on the turbulence statistics was negligible beyond h/D=2.0.