Reduction of Aeroacoustic Noise Generated from a Flow past a Cylinder by Porous Materials

It is important to reduce the aerodynamic noise generated from high-speed trains in order to achieve their further speed-up in the near future. In recent years, a new method for reducing aerodynamic noise by using porous materials has been proposed, and confirmed to be effective experimentally. In this study, by modeling the porous material by a large number of small cylinders, the flow around the cylinder covered by the porous materials is simulated by direct numerical simulation based on the modified volume penalization (VP) method. It is confirmed that the aerodynamic noise from the cylinder can be reduced by using porous materials. In particular, the aerodynamic sound reduction effect is significant when installing only one row of the small cylinders in the radical direction. In this case, depending on the number of the cylinders, the reduction rate of acoustic power becomes about of the case of a bare cylinder. In contrast to the case of a bare cylinder, the Karman vortex shedding is delayed downstream by installing the small cylinders. In other words, vortex shedding in the wake is suppressed by using the porous material. The essential mechanism of the reduction of aerodynamic sound is that placing small cylinders makes the wake behind the cylinder nearly steady.