Hiroki Nagai

This study presents a numerical investigation of transonic flow characteristics over a reentry capsule at Mach 1.05, employing a detached eddy simulation. The simulated flowfields demonstrate that the recompression shock wave oscillates back and forth at a Strouhal number of the order of 0.01. Frequency analysis of fluctuations in the drag coefficient and base pressure reveals that these fluctuations can be predominantly attributed to oscillations in the recompression shock wave. While the recompression shock wave propagates upstream, local shock waves occur alternately in the upper and lower free-shear layers, with a Strouhal number of the order 0.1, due to the instability of free-shear layers. The repeated occurrence of these local shock waves causes the recirculation region to shrink, resulting in a decrease in the base pressure. Conversely, while the recompression shock wave propagates downstream, no local shock wave occurs and the recirculation region expands, leading to an increase in the base pressure. These results suggest that the interactions among the recompression shock wave, local shock wave, and free shear layer give rise to fluctuations in the base pressure of the capsule, which in turn can affect its dynamic instability.