INVESTIGATION ON THE HYDRODYNAMIC PERFORMANCE OF A VERTICAL DOUBLE MANTA RAY GLIDING WITH VARIABLE ATTACK ANGLES

In order to investigate the effects of vertical spacing and angle of attack on the hydrodynamic performance of double manta rays when gliding in clusters along the vertical distribution, a computational model of manta rays was developed based on the actual shape of manta rays. Four spacing arrangements, including 0.25, 0.5, 0.75 and 1 times the body thickness, and nine angle of attack states, namely −8°−8°, were set up. Then, the numerical simulation of the double manta ray with variable attack angle and vertical distance was carried out by Fluent software. The mean lift/drag of the system and the lift/drag of each individual in the cluster were analyzed by combining the flow field pressure and velocity clouds. Numerical calculations showed that the average drag of the two manta rays was higher than that of a single manta ray when they glided in groups with attack angles ranging from −8° to 8° in the vertical direction. When the two manta rays glide at negative attack angle, the drag of the lower manta ray decreases, and the smaller the vertical spacing, the more obvious the drag reduction effect is; when the two manta rays glide at positive attack angle, the upper manta ray gains drag reduction. When the two manta ray glide at negative attack angle, the system average lift is greater than that of the single glider; when the two manta rays glide at negative attack angle, the system average lift is less than that of the single glider, and the system average lift is almost independent of the vertical spacing. The lift of the lower manta ray is always greater than that of the upper manta ray, but the difference in lift decreases as the vertical spacing increases.