Effect of Mach number on the compressible flow past a wavy-axis cylinder

Abstract

Large-eddy simulations of compressible turbulent flow around wavy-axis cylinder have been carried out at Mach number M∞ varying from 0.55 to 0.95. A fixed Reynolds number is chosen, i.e., Re=2×105. Two different force behaviors as well as flow characteristics are obtained by wavy-axis cylinder, around the critical Mach number 0.9 relevant to transonic flow past a circular cylinder. For M∞<0.9, the effective drag reduction is obtained by wavy-axis cylinder with suppressing vortex shedding, which can be considered as the result of the reduced interaction between high- and low-momentum streams in the near wake. When M∞>0.9, the drag reduction vanishes, associated with the coexistence of unsteady and quasi-steady flow states and formation of strong oblique shock waves in the wake of wavy-axis cylinder. For all considered M∞, the turbulent fluctuations are very small in the near wake of wavy-axis cylinder, resulting in the lower fluctuating force on the body. The instability processes for wavy-axis cylinder are derived from the competition between the oblique waves and Kelvin-Helmholtz instability waves, associated with the large-scale vortex motion attenuating or vanishing as well as the low-intensity turbulence in the near wake. The analysis of dynamical mechanisms shows that wavy-axis cylinder only provide the effective drag reduction for compressible flow past circular cylinder involving large-scale vortex shedding. For the intermittent shock waves and permanent shock waves with large-scale movement, wavy-axis cylinder is an effective way for shock control. While, when the wake shock waves appear, say about M∞≥0.85, the wavy-axis cylinder is also invalid for shock control.