Sensing flow separation on a circular cylinder by MEMS thermal-film sensors

By applying a MEMS thermal film sensors array on a circular cylinder and Wavelet analysis on the signals obtained, the unsteady behaviors of laminar flow separation from the cylinder were investigated in this study. The sensor signals obtained at θ=85° revealed that the unsteady behaviors of flow separation are characterized by two time scales, one of which is associated by the frequency of vortex shedding, and the other is at least one order of magnitude longer due to the excursion of flow separation in a circumferential region about 5°. The Wavelet analysis results enabled one to reduce the percentage of time of which the vortex shedding frequency was detectable at the location measured. It is found that the percentage values are very close to 1 in the region upstream of flow separation, followed by a pronounced transition from θ=85° to 100°, at which the percentage values are decreased to about 0.2. Low-frequency variations were noticed in the signals of the thermal films upstream of flow separation, and a hot-wire probe situated in the free stream. Further, a negative correlation between the vortex shedding frequency and its modulus was realized from the hot-wire signals measured in the free stream, inferring that the lower the vortex shedding frequency, the higher the amplitude of the vortex shedding frequency component.