Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.
The paper summarizes experimental investigations concerning drag reduction of circular cylinders by porous coating. In order to quantify the effect, wind tunnel experiments with a force balance and flow field analyses have been carried out. In a first experiment, the cylinders were coated completely with a thin porous layer. The results show that the boundary layer over the porous surface is turbulent right from the beginning and thickens faster because of the enhanced vertical momentum exchange when compared to a smooth cylinder surface. The region of flow detachment is widened resulting in a broader area with almost vanishing low flow velocities. All in all, the measurements show that a full porous coating of the cylinders increase the flow resistance. In a second experiment, the cylinders were coated only on the leeward side, which resulted in a reduction of the body's flow resistance. This effect seems due to the fact that the recirculating velocity and the underpressure in the wake is reduced significantly through a leeward porous coating. Thus, combining a smooth non-permeable windward side with a porously coated leeward side can lead to a drag reduction of the body. These findings can be applied advantageously in many technical areas, such as energy saving of moving bodies (cars/trains/planes) or in reducing fluid loads on submersed bodies.