Effects of the Reynolds number and attack angle on wake dynamics of fish swimming in oblique flows

This study numerically simulated oblique flows around an undulating fish using a ghost cell method with general processing unit (GPU) acceleration to investigate the hydrodynamic mechanism of swimming fish at an angle to the flow direction. The hydrodynamic performance and wake patterns of swimming fish were assessed under different attack angles (α) and Reynolds numbers (Re) over a wide range of Strouhal numbers (St). The results indicate that a higher St for the nonzero α yields a larger lift-to-drag ratio and thrust, which benefits propulsion and maneuverability. As the α grows, the fish can obtain a higher thrust along the tangential direction with reduced energy consumption. Additionally, the force coefficients shift from the nearly periodic variation to the multiple-period oscillations due to the modulation of several harmonic components. Similarly, increasing Re also enhances the propulsive performance, although to a lesser extent than α and St. Correspondingly, the force pattern becomes increasingly unstable and irregular as the viscous force diminishes. A high St can amplify the effects of α and Re. Diverse wake modes are captured, depending on the α, St, and Re including the von Kármán (VK) vortex modes, the interaction of the leading-edge vortex and trailing-edge vortex, and the multiscale vortex pattern. Thrust is not necessarily induced by the reverse von Kármán (RVK) mode, and it can be generated in both the VK and RVK modes. Furthermore, a higher Re intensifies vortex interactions and wake deflection.