Coupled flow characteristics of high-speed free flow and cylindrical arrays

The interaction between high-speed freestream flows and porous media involves complex coupled phenomena, including shock interactions, rarefaction effects, and unsteady wake dynamics, which are not fully understood. To elucidate these interactions, this study employs pore-scale numerical simulations using a simplified model system of regularly arranged cylinder arrays. The solid fraction (ϕ) is varied by adjusting the cylinder arrangement. The simulations cover a wide parameter space, with ϕ ranging from 0.043 to 0.315, the freestream Mach number (Ma∞) varying from 2 to 6, and the freestream Knudsen number (Kn∞) spanning from 7.4×10−6 to 9.9×10−4. To account for rarefaction effects in the slip-flow regime, velocity-slip, and temperature-jump boundary conditions are implemented at the cylinder surfaces. Key findings are as follows. (1) Variations in ϕ induce significant transitions in both shock and wake structures, leading to the identification of four distinct flow regimes. Among these, the regime characterized by a body-fitted shock structure and integrated supersonic tail jets yields the largest spatial inhomogeneity in time-averaged forces on individual cylinders and the maximum Strouhal number (St). (2) Increasing Ma∞ generally suppresses vortex shedding. A transition from unsteady to steady flow is observed at a critical Mach number. (3) Rarefaction effects, quantified by increasing Kn∞, significantly suppress vortex shedding, leading to a monotonic decrease in St and an increase in the time-averaged drag coefficient. (4) Slip effects significantly alter both global and local drag predictions, with local drag variations exceeding 20% in the trailing region, highlighting the necessity of incorporating slip-wall boundary conditions for accurate modeling of such flows.