Numerical study of quadrupole noise prediction for moving bodies

The quadrupole sound field generated by a solid surface in arbitrary motion is simulated using a computational aero-acoustic time domain method. This numerical method is able to provide an accurate computation of quadrupole contribution by using the formulation of Farassat and Brentner. In addition, to avoid the use of a fine grid around the bodies, the numerical method is extended to implementation of the permeable surface form of the FW-H equation. This permits the evaluation of the sound field in terms of a distribution of thickness and loading sources over a permeable control surface. By using the case modified by Bailly for LEE problems (2D), a new reference solution is proposed for quadrupole noise calculations in 3D for codes based on Green function solutions in the time domain. This paper is also concerned with the application of the numerical method to a circular cylinder in cross-flow at a Reynolds number of 3900, in which the near field flow was calculated using large eddy simulation. The quadrupole noise from the sources in the volume outside the cylinder is compared with the summation of monopole and dipole noise generated from a suitable permeable control surface. It is shown that by choosing the control surface far enough from the cylinder, the noise from the quadrupole contribution can be neglected