Numerical investigation of the hydrodynamics of stingray swimming under self-propulsion

Abstract

Stingrays are different from many other aquatic animals; they have dorsoventrally flattened bodies, with enlarged pectoral ‘wings’ to generate thrust for swimming. In this study, aiming to discover the key features and dynamic response characteristics of stingray swimming, 3D incompressible viscous computational fluid dynamics (CFD) simulations were conducted for self-propelled stingrays. The body geometry was reconstructed based on 3D laser scanner data of a freshwater stingray. The locomotion of the stingray was considered based on the first Fourier mode, and was analyzed using experimental measurements of the 3D kinematics in live stingrays. The amplitude for the body point was obtained using a binary linear function. The swimming velocity, as calculated through the self-propelled simulations, was within 11.9% of the nominal experimental swimming speed. It was found that the Froude efficiency of stingray swimming at slow and fast speeds is approximately 59%, suggesting that the morphological and kinematic characteristics of the stingray allow it to maintain high efficiency during swimming. The wake structures and pressure fields on the dorsal and ventral sides of the stingray were visualized. Moreover, a vortex on the upper surface of the wing’s trough and one on the lower surface of the wing’s crest were newly discovered. Furthermore, two low-speed and two high-speed boundary frequency cases were numerically simulated. It was found that the average swimming speed during the quasi-steady state has a linear relationship with the frequency of the undulatory motion. The newly discovered vortex on the upper surface of the stingray transforms from scattered small balls into several large balls. This indicates that this vortex plays an important role in lubrication.