Sensing flow separation on a circular cylinder by micro-electrical- mechanical-system thermal-film sensors

Unsteady characteristics of flow separation from a circular cylinder, at Reynolds numbers of 1.65-1.71 × 10 ⁴, were studied using Wavelet analysis of the signals obtained by a micro-electrical-mechanical-system film sensors array flushed with the cylinder surface. At θ = 85 deg near where flow separation took place, it was found that the unsteady flow behaviors were featured with two time scales, one of which was associated with vortex shedding, and the other was at least 1 order of magnitude longer due to excursion of flow separation in a circumferential region of the extent about 5 deg. The Wavelet analysis results enabled one to reduce the percentage of time during which the vortex shedding frequency was detectable at the location measured. In the region upstream of flow separation the percentage values obtained were very close to 100%, followed by a pronounced decrease from θ = 85 to 100 deg. At θ = 100 deg, the percentage values reduced were about 30%. Moreover, low-frequency modulations were noticed in the signals measured by the thermal-film sensors situated upstream of flow separation and the hot-wire in the freestream. The Wavelet analysis with these signals revealed a trend that the lower the vortex shedding frequency, the larger the amplitude of the vortex shedding frequency component.