) using a low-density wind tunnel*

In this study, drag force measurements were conducted for subsonic compressible flows around a rectangular cylinder with a side length ratio from 0.5 to 1.2 near the critical geometry, a side length ratio of which was reported to be 0.65 for incompressible flow in the previous study. The effect of the compressibility on the critical geometry was evaluated using a low-density wind tunnel in the Mach number range from 0.1 to 0.6 at the Reynolds number of O(10³). The side length ratio with the peak drag coefficient was confirmed to change with the Mach numbers. Remarkably, the peak drag coefficient for the side length ratio of 1.0 was observed at the Mach number of 0.6. In addition, the change in the flow field was investigated using the time-resolved schlieren visualization, and the flow field was discussed with the force measurement results. The change in the flow fields behind cylinders that have different side length ratios is related to the back pressure, and it results in a significant change in the drag coefficient.