To fill the research gap concerning the influence of cephalic fin posture on the hydrodynamic performance of manta rays during gliding, this study first developed three-dimensional morphological models of four typical cephalic fin postures based on biological observations, namely embracing, spreading, outwardly turned, and curling. In contrast to previous studies that either ignored the cephalic fins or simplified them as two-dimensional rigid plates, the present work preserves the realistic three-dimensional geometry and systematically investigates the effect of posture variations on gliding performance.
This study adopts the immersed boundary method and the sphere function-based gas kinetic scheme (IB-SGKS) to perform numerical simulations, with nine angles of attack ranging from -20° to 20° covering the typical gliding envelope of manta rays. Prior to the simulations, the numerical method was first validated against the classical flow around a sphere, confirming its reliability for resolving pressure fields and vortex structures. A rigorous grid-independence and time-step independence verification was also conducted to ensure that numerical discretization errors were well controlled.
The results reveal that the change in cephalic fin posture has a negligible influence on the lift coefficient, with a variation of less than 1% among the four postures at all angles of attack. Consequently, the variation in the lift-to-drag ratio is primarily determined by drag differences. Depending on the angle of attack range, cephalic fin postures affect drag through two distinct mechanisms, and its effect is significant when the angle of attack is below 10°. Specifically, the curling posture does not cause the rear low-pressure region to shift downstream due to vortex shedding, nor does it lead to the fusion of high- and low-pressure regions near the head caused by the shape, thus achieving the best drag reduction effect. When the angle of attack is -20°, the curling posture reduces drag by 5.6% compared with the outwardly turned posture. When the angles of attack exceed 10°, none of the four postures cause a rearward shift of the low-pressure region. The differences among the postures are mainly affected by the joint action of the isolation of the head high-pressure zone and the expansion of the low-pressure zone, leading to a maximum drag variation of only 1.8% between the embracing and spreading postures when the angle of attack is 20°.
This study provides novel insights into the hydrodynamic mechanisms by which cephalic fin postures influence manta ray gliding. From a biological perspective, the results quantitatively confirm the hypothesis that curling the cephalic fins reduces drag during routine cruising, and show that the embracing posture facilitates feeding. From an engineering perspective, the findings offer direct theoretical guidance for optimizing the gliding performance of manta ray like vehicles and for designing cephalic-fin-inspired actuators in multi-scenario underwater operations: gliding with curled fins for energy saving and switching to the embracing posture for low-energy traction of small objects.