In the present study, flow over a circular cylinder under compressible low-Reynolds-number conditions was investigated using the low-density wind tunnel. The Reynolds number (Re) based on cylinder diameter was set in the range of \(100\; \le \;{\text{Re}}\; \le \;1000\) , and the Mach number (M) was set in the range of \(0.1\; \le \;M\; \le \;0.7\) . The Schlieren visualization and force measurement were conducted under pressure below 10 kPa (0.81 kPa for the lowest case) with the circular cylinder with 1.2, 3.0, and 5.0 mm in diameter. Although the signal-to-noise ratio of the Schlieren image is very low because of the low-pressure condition, the fluctuation components originating from the flow phenomena were successfully extracted using the denoising technique based on the modal decomposition. As a result, the Mach number effects on the length of the recirculation region and the Strouhal number of the vortex shedding were revealed. The drag coefficient obtained at \(M=0.1\) and 0.2 for \(100\; \le \;{\text{Re}}\; \le \;1000\) was in good agreement with that under the incompressible conditions, and the drag coefficient increases as the Mach number increases.
