Free surface and passive control effects on flow characteristics around various torpedo-like geometries

The present study investigates the effect of free-surface on the flow structure around a model of an underwater vehicle called torpedo-like geometry by using experimental and numerical methods. The numerical study utilizes the pressure-based solver of ANSYS-Fluent software while the experimental study employs the dye flow visualization and particle image velocimetry (PIV) methods. The Large Eddy Simulation (LES) is performed to reveal the unsteadiness of the flow structure around the torpedo-like geometries. The interface-capturing volume of the fluid fraction (VOF) method is used to model the free-surface. The results show that the flow characteristics such as velocity field, streamline topology, vorticity, and kinetic energy becomes asymmetric as the torpedo-like geometry is located close to free-surface. Therefore, the resulted drag force, FD for h/D = 0.75 was found to increase relative to uniform flow condition. It was also found that the free-surface effect becomes negligible for submergence depth ratios higher than 2.5 owing to the near-symmetrical form of the flow structures. Also, when the Reynolds number is increased from Re = 20x103 to 40x103, time-averaged vorticity formation length and drag coefficient, CD decrease for all examined torpedo-like geometries. As a passive control method, modifications were applied by adding 3 and 4 wings to the stern section of the torpedo-like geometry to examine in detail the resulted flow structure and drag force. It is observed from numerical results that the stern with wings favorably organizes the wake region while increasing the drag coefficient, CD.