A NUMERICAL STUDY ON MECHANISM OF AERODYNAMIC NOISE REDUCTION BY POROUS MATERIAL

摘要

Vortex shedding behind bluff bodies slackens off and the resulting aerodynamic noise is well reduced when they are covered with porous materials. To clarify this mechanism we simulate a flow around a circular cylinder covered with a porous material. We perform a large eddy simulation at Reynolds number 100,000 to obtain the flow field around the circular cylinder. To solve the fluid flow with a porous interface, we impose a permeable boundary condition. The transpiration is assumed to be proportional to the local pressure fluctuation. The simulated flow fields are compared with experimental ones and good agreements are obtained. The computation reveals blowing flows at the porous interface play a vital role in the vortex shedding suppression. The mechanism is summarized as follows: 1) The blowing flow at the cylinder sides makes the flow separation points upstream along the body. It results in weakened the strength of the separated shear layers and increased a distance between the shear layers at both sides. The former eases the shear layer instability and the latter makes positions of the vortices formation downstream. 2) The blowing flow behind the cylinder decreases a reverse flow and therefore hinders the vortices formation.