Phenomenology of a flow around a circular cylinder at subcritical and critical Reynolds numbers

Kurzfassung

In this work, the flow around a circular cylinder is investigated at Reynolds numbers ranging from 79 000 up to 238 000 by means of a combined acquisition system based on Temperature Sensitive Paint (TSP) and particle velocimetry. The proposed setup allows simultaneous and timeresolved measurement of absolute temperature and relative skin friction fields onto the cylinder surface and nearwake velocity field. Combination of timeresolved surface measurements and planar nearfield velocity data allows the investigation of the profound modifications undergone by the wall shear stress topology and its connections to the nearfield structure as the flow regime travels from the subcritical to the critical regime. Laminar boundarylayer separation, transition, and reattachment are analyzed in the light of temperature, relative skin friction maps, and Reynolds stress fields bringing about a new perspective on the relationship between boundary layer development and shear layer evolution. The fastresponding TSP employed allows high acquisition frequency and calculation of power spectral density from surface data. Correlation maps of surface and nearwake data provide insight into the relationship between boundarylayer evolution and vortex shedding. We find that as the Reynolds number approaches the critical state, the separation line oscillations feature an increasingly weaker spectrum peak compared to the nearwake velocity spectrum. In the critical regime, separation line oscillations are strongly reduced and the correlation to the local vorticity undergoes an overall decrease giving evidence of modifications in the vortex shedding mechanism.