A study of the effect of the jet-like flow on the near wake behind a circular cylinder close to a plane wall

Abstract

The flow around a circular cylinder placed near a plane wall has been investigated experimentally using PIV, with the purpose of studying the jet-like flow that takes place in the gap between the cylinder and the wall and its effects on the wake flow. The Reynolds number based on the diameter of the cylinder, D, and the incoming flow velocity was about 8700. The gap size, G, between the cylinder and the plane wall was varied between G/D=0.15 and 1.00. The results showed the existence of three distinct flow regimes as the cylinder-wall gap size is varied: (1) The large gap flow regime (G/D>0.8) in which the vortex shedding is nearly symmetric. Small influence of the wall has been detected in this flow regime; (2) The intermediate gap flow regime (0.3<G/D<0.8) in which the vortex shedding becomes intermittent with the appearance of small structures due to the transition of the elongated shear layers; and (3) The small gap flow regime (G/D<0.3) in which the jet like-flow destroys the lower shear layer and prevents the onset of the alternating vortex shedding. The proper orthogonal decomposition (POD) analysis showed that at the intermediate gap flow regime the large scale vortices have been bowed and deformed by the vorticity created by the presence of the wall, their formation location was moved downstream as the gap-to-diameter ratio was decreased. At the small gap flow regime the instantaneous flow fields and the POD results revealed the existence of large scale vortices shed from the upper shear layer (other than Von Kármán vortices). The streamline plots showed that the flow structure at this gap regime is identical to the structure of the flow behind a back-step. Phase averaged flow fields showed that the location of the jet-like flow “axis” depended on the upper shear layer vortices formation cycle. In fact, the jet-like flow took different “incident” angles and different downstream locations at different phase instants during the formation of the upper shear layer vortices.