Akishita S.

The effect of finite impedance for alleviating sound propagation by porous surface is well known. The authors (2002, 2004, 2005) reported study on the effect for reducing aerodynamic sound source of the Aeolian tones generated by a circular cylinder, where the optimized impedance distribution on the surface was proposed. This effect for reducing aerodynamic source intensity generated by the wing has been reported by M.S. Howe (1979), S. Lee (1994) and C.S. Ventres and Barakat (1978). They investigated the effect to the amplitude of surface pressure fluctuation and radiated sound pressure, when the geometries of the porosity are given. The effect actually has not been utilized in so many products, though it seems favorable in industrial use. The reason is inferred as follows; the porous surface with large enough diameter to affect the boundary layer flow causes the reduced characteristics of wing aerodynamic load. This paper discusses mainly following two aspects of the finite impedance of the wing surface immersed in the low Mach number flow; 1) Realization of the effective finite impedance by introducing perforations of very small diameter which minimizes the effect of porosity to the boundary layer flow, 2) optimal distribution of perforations to yield the reduced sound intensity at the radiation field. Optimization of the surface impedance is carried out representing the sound radiation by the Green�fs function combined with surface pressure fluctuation. The validity of the optimization is verified at the experimental data of the Aeolian tones generated by the circular cylinder above mentioned. The diameters of perforations at the porous surface are supposed not larger than 0.1mm. The surface impedance of the porous surface is estimated from the measurement data for the normally incidental acoustic impedance modified with grazing effect of the flow.