Hydrodynamic benefit of cephalic fins in a self-propelled flexible manta ray

The hydrodynamic benefit of cephalic fins in manta ray was explored using the penalty immersed boundary method. When manta rays feed, they open their cephalic fins and lower them to their mouth. As they swim forward, the cephalic fins are straightened in the forward direction. The smooth body of manta rays was modeled as a flexible plate. A self-propelled flexible plate was realized by enforcing a prescribed harmonic oscillation in the vertical direction but allowing the plate to move freely in the horizontal direction. Simulations without cephalic fins were also performed for comparison. Vortical structures and pressure distributions were visualized to elucidate the hydrodynamic benefits of cephalic fins. The fins generated streamwise vortices that resulted in negative pressure and enhanced the average cruising speed and thrust. The effect of the gap distance (g/L, where g is the gap distance between the two cephalic fins and L is the length of the plate) was examined in detail. The underlying propulsion mechanism was analyzed by examining the phase of the heaving stroke. The effects of the g/L were scrutinized by visualizing the contours of vorticity (omega(x), omega(y), omega(z)) and pressure (p) around the flexible plate. A maximum cruising speed was obtained at g/L = 0.6, where the average cruising speed increased by more than 62.8%.