A study of aerodynamic sound generated from an airfoil placed in a flow with turbulence (2<sup>nd</sup> Report: In the case of airfoil subjected to circular-cylinder wake)

Abstract

To clarify the mechanism for aerodynamic sound to be generated from a lifting surface placed in a flow with turbulence, wind-tunnel experiments and numerical simulations have been carried out for a flow around an airfoil subjected to the wake of a circular cylinder. The test airfoil has the NACA0012 profile with a chord length of 150 mm and a spanwise length of 500 mm, and it is set in a flow with the cylinder wake at angles of attack of 0 degree and 9 degrees. The wind velocity is set to 30 m/s, which results in an airfoil Reynolds number of 3.0×105. The circular cylinder is set 100 mm upstream of the leading edge of the airfoil, and its diameter as well as installation position in the transverse direction are varied. The numerical simulations are composed of large eddy simulation (LES) and aeroacoustical simulations that solves the Lighthill equation in the frequency domain with the Lighthill tensor computed by the above-mentioned LES as the acoustical source terms. The sound pressure level is also computed by the Curle’s equation with the fluctuation in the airfoil lift force computed by the LES as the acoustical source and compared with the experiments. The computed static-pressure distributions on the airfoil surfaces and aerodynamic sound spectra are in good agreement with the measured equivalents. The pressure fluctuation near the leading edge of the airfoil is remarkably high when the circular cylinder is placed upstream. When they flow near the leading edge of the airfoil, the vortices in the cylinder wake are stretched due to the acceleration of the main flow, form strong source of sound, and radiate intense sound in the upstream direction.