Vortex Shedding Noise from Finite, Wall-Mounted, Circular Cylinders Modified with Porous Material

The aerodynamic noise generated by a circular cylinder in a crossflow is a classical problem in aeroacoustics, which can be found in various practical applications, such as parts of the landing gear of airplanes, train pantographs, and antennas of vehicles. The noise is especially disturbing because it contains strong tonal contributions due to the regular vortex shedding at the cylinder. If the cylinders are not two-dimensional, but finite and wall-mounted at one end and free at the other, additional flow phenomena occur at both the tip and the wall junction, which can lead to additional noise contributions. One method to reduce the aerodynamic noise from such cylinders is to cover them with a flow-permeable, porous material. The present paper describes an experimental study on the effect of porous covers on the reduction of noise generated by wall-mounted, finite, circular cylinders. Thereby, special focus was put on the identification of both the optimum position and the optimum extent of the porous cover regarding a maximum noise reduction. The experiments included both measurements of the far field noise as well as flow measurements in the wake of the cylinders. They were conducted in an aeroacoustic open jet wind tunnel at subsonic flow speeds. It was found that a placement of porous material at the wall end is more efficient in reducing Karman vortex shedding noise than a placement of porous material at the free end.