Flow around a porous cylinder subject to continuous suction or blowing

In the present experimental investigation the surface pressure distribution, vortex shedding frequency, and the wake flow behind a porous circular cylinder are studied when continuous suction or blowing is applied through the cylinder walls. It is found that even moderate levels of suction/blowing (less than or similar to 5% of the oncoming streamwise velocity) have a large impact on the flow around the cylinder. Suction delays separation contributing to a narrower wake width, and a corresponding reduction of drag, whereas blowing shows the opposite behaviour. Both uniform suction and blowing display unexpected flow features which are analysed in detail. Suction shows a decrease of the turbulence intensity throughout the whole wake when compared with the natural case, whilst blowing only shows an effect up to five diameters downstream of the cylinder. The drag on the cylinder is shown to increase linearly with the blowing rate, whereas for suction there is a drastic decrease at a specific suction rate. This is shown to be an effect of the separation point moving towards the rear part of the cylinder, similar to what happens when transition to turbulence occurs in the boundary layer on a solid cylinder. The suction/blowing rate can empirically be represented by an effective Reynolds number for the solid cylinder, and an analytical expression for this Reynolds number representation is proposed and verified. Flow visualizations expose the complexity of the flow field in the near wake of the cylinder, and image averaging enables the retrieval of quantitative information, such as the vortex formation length. (C) 2004 Elsevier Ltd. All rights reserved.