Numerical Analysis of Aerodynamic Noise Reduction by Applying Porous Materials

Abstract

Aerodynamic noise from bluff bodies is well reduced when they are covered with porous materials. In this study, presence of porous materials is modeled by momentum loss in the equation of fluid motion, and mathematical formulae for evaluating aerodynamic noise based on the theory of vortex sound are modified in order to include the effect. Flow around the cylinder covered with porous materials is calculated by LES techniques. When the inertial resistance factor C2 is set to 200[m-1], no vortex is shed periodically into the wake any longer and a region of zero-velocities is spread behind the cylinder, which forms stable shear layers. Time variations of the force exerted on the cylinder and the total momentum loss in the whole porous areas are also reduced at C2=200[m-1]. Flow through porous materials induces velocities in circumferential direction at the outer edge of the materials which weaken vortices of shear layers, and also induces velocities in radial direction which widen the space between two shear layers behind the model. Numerical analysis predicts the reduction of aerodynamic noise in the case of C2=200[m-1], which agrees qualitatively with experimental results. Although strong sound sources are formed around separation points in the case of the bare cylinder, stable flow around the cylinder with porous materials seems to suppress production of sound sources.