Wake characteristics at the fin tip streamwise cross-section of a manta robot

Mantas exhibit significant deformation at the fin tip during swimming, which results in remarkable hydrodynamic performance. To investigate the wake characteristics at the streamwise tip cross-section of a manta robot, we have developed an experimental platform that utilizes a Particle Image Velocimetry (PIV) system. The physical and geometric characteristics of the wake vortex at the fin tip are analyzed when varying motion parameters and flow velocity conditions. Results indicate that the vortex flux in the wake decreases over time, with attenuation exceeding 50 % by the fifth vortex, while the vortex core area initially increases, reaching a peak at a characteristic length of 3.0 with a 35 % increase before subsequently decreasing. Moreover, only the first two vortices contribute to propulsion momentum. Similarly, the propulsion performance of the single-bone fins is comparable to that of the double-bone designs. Furthermore, the Strouhal number (St) significantly influences the wake dynamics: when St is within [0.2, 0.4], effective wake jets develop, and efficient propulsion appears with St located in [0.37, 0.44], where the jet angle and momentum angle align, thus optimizing hydrodynamic performance. Sensitivity analysis further confirms that amplitude and frequency are the most influential parameters on vortex momentum, while phase difference plays a key role on propulsion efficiency.