Numerical Study on the Hydrodynamics of Manta Rays Exiting Water

Observation of manta rays exiting water has been rarely reported, as there are various difficulties in observing and obtaining data on their behavior in a marine environment. Therefore, the movement mechanism of manta rays exiting water is still unclear. This paper proposes the idea of using CFD (based on Ansys Fluent, version 2022) to simulate the water-exit process of the manta ray. The study discusses the changes in the mechanical and kinematic parameters of the manta ray over time and obtains the evolution of vortex structures during the underwater movement phase of the manta ray. Time history variations of the mechanical and kinematics parameters in the vertical water-exit motion are discussed. The evolution of vortex structures during the underwater movement of the manta ray is obtained. The direction in which the manta ray approaches the free surface is the X-direction and the direction of its flapping motion is the Z-direction. VX and VZ are the velocities of the manta ray in the X- and Z-directions, respectively. FX and FZ represent the forces acting on the manta ray in the X- and Z-directions, respectively. The results indicate that the vertical water-exit of the manta ray mainly undergoes three stages: underwater acceleration, crossing the free surface, and aerial movement. During the underwater acceleration phase, the force FX of the manta ray fluctuates, but its average value is positive within one cycle. VX also shows a stepwise increase, while FZ and VZ exhibit periodic changes. During the stage of crossing the free liquid surface, FX first increases and then sharply decreases, VX also shows an increase and then decrease, FZ fluctuates greatly, producing a peak, and the swimming speed VZ of the manta ray is negative. During the aerial motion phase, FX is mainly affected by gravity, VX decreases linearly, FZ approaches 0, and VZ remains constant. During the process of swimming underwater, the tail vortex of the manta ray presents a double row staggered structure to generate thrust. Increasing the flapping frequency and decreasing the wave number can improve the swimming speed of the manta ray, and then increase its water-exit height. The findings may provide an important hydrodynamics basis for biomimetic trans-media vehicle designs.