Numerical investigation on the propulsive performance of flexible flapping fins using CFD/CSD method

Abstract

A FSI (fluid-structure interaction) numerical simulation was performed to investigate the flow field around a flexible flapping fin using an in-house developed CFD/CSD solver. The three-dimensional fluid-structure interaction of the flapping locomotion was achieved by loosely coupling preconditioned Unsteady Reynolds-Averaged Navier-Stokes (URANS) solutions and non-linear co-rotational structural solutions. The CSD solver was developed specifically for high flexible flapping fins by considering the large geometric nonlinear characteristics. Validation of benchmark tests illustrated the high-fidelity of the developed methodology. Then effect of flexural angles, flexural amplitude and flapping frequency in terms of Strouhal number were evaluated. Results demonstrated that different flexural angles will present different flow fields, and thus significantly varied thrust generation and pressure distribution. The thrust does not increase monotonically with flexural angles. The thrust is also found to increase with increasing Strouhal number while propulsive efficiency peaks within the range of 0.2 < 0.4, which lies in the middle of the range observed in nature. The appropriate combination of flexibility and Strouhal number illustrates higher efficiency and gives instruction for further design of flexible flapping fins.