The effects of Mach number on turbulence behaviors are investigated by means of direct numerical simulation of compressible turbulent boundary layers with the free-stream Mach numbers 2,4 and 6.Some typical turbulent quantities,including the velocity and vorticity fluctuations,intermittency,sweep and ejection events and turbulent kinetic energy budget,are analyzed.The turbulence intensities are significantly decreased with the increasing Mach number.The onset of intrinsic intermittency occurs nearer the wall for a higher Mach number.The ejection event is more frequent and the sweep event becomes less frequent.Moreover,the magnitudes of turbulent kinetic energy production,diffusion and transport terms are increased and the solenoidal dissipation is slightly decreased as the Mach number is increased.
Flapping plates of typical fishlike tail shapes are simulated to investigate their locomotion performance using the multi-block Lattice Boltzmann Method (LBM) and Immersed Boundary (IB) method. Numerical results show that fishlike forked configurations have better locomotion performance compared with unforked plates. Based on our results, the caudal fin in carangiform mode has greater thrust, and the lunate tail fin in thunniform mode has higher efficiency. These findings are qualitatively consistent with biological observations of fish swimming. Analysis of wake topology shows that the wake of the forked plate consists of a chain of alternating reverse horseshoe-like vortical structures. These structures induce a backward jet and generate a positive thrust. Moreover, this backward jet has a more favorable direction compared with that behind an unforked plate.
