Experimental investigation on surface impedance effect of sound radiation from low mach number flow around a circular cylinder

This paper describes a wind tunnel experiment on aero-acoustical effect of finite surface impedance on low Mach number (Ma) flow around a circular cylinder. Modification of surface acoustical properties of source area is said to result in reduction of radiated sound pressure level. In this paper the effect of the finite impedance not only to the acoustical effect but also to the hydrodynamic effects were examined. Three boundary conditions (BC) of surface impedance were supposed: 1) rigid surface, 2) uniform and 3) optimized impedance distribution by introducing an array of micro-scale perforation with a back space. A wind tunnel experiment was conducted according to the three types of BC with three different experimental models under the Reynolds number (Re) condition from 6 000 to 12 000. The third condition caused reduction of the far field pressure level by 6 dB at largest case compared to the first condition.