Visualization of Aerodynamic Sound Source with Compact Green's Function

Abstract

The objective of this investigation is to develop a method for detecting the aerodynamic sound source in a low-Mach number turbulent wake. In order to evaluate vorticity contribution for the dipole sound generation, a compact Green's function is utilized for identification of the aerodynamic sound source. The compact Green's function (1) (2) denotes the acoustic efficiency of the surface dipole caused by the interaction between the vorticity fluctuation and solid surfaces. The surface dipole sources and vorticity fluctuation are measured (3)(4) in the wake of a circular cylinder with Reynolds number of 4.0×10 4 . The compact Green's function model can be estimate instantaneous sound source distribution, however, this method requires the spatial derivative of vorticity fluctuations. Therefore, experimental evaluation is difficult. To evaluate the compact Green's function method, we introduce coherent output power (COP) in terms of surface dipole and vorticity fluctuation (4) . This method can be estimated vorticity contribution to the dipole sound generation with measurement of the coherence output power in terms of the vorticity fluctuation and the surface (dipole) pressure. This method is based on the conventional Lighthill-Curle's theory (5) , which shows the aerodynamic sound is calculated with the surface pressure fluctuation. The COP method makes use of the coherent output power instead of the surface pressure fluctuation, it is found that the intensity of COP corresponds to the distribution of the dipole sound source caused by unsteady vorticity fluctuation. The estimated sound source distribution with the compact Green's function model is good agreement with that of the COP method.