Reverberation Effects on Directionality and Response of Stationary Monopole and Dipole Sources in a Wind Tunnel

Analytical solutions for the three-dimensional inhomogeneous wave equation with flow in a hardwall rectangular wind tunnel and in the free field are presented for a stationary monopole noise source. Dipole noise sources are calculated by combining two monopoles 180 0 out of phase. Numerical calculations for the modal content, spectral response and directivity for both monopole and dipole sources are presented. In addition, the effect of tunnel alterations, such as the addition of a mounting plate, on the tunnels reverberant response are considered. In the frequency range of practical importance for the turboprop response, important features of the free field directivity can be approximated in a hardwall wind tunnel with flow if the mayor lobe of the noise source is not directed upstream. However, for an omnidirectional source, such as a monopole, the hardwall wind tunnel and free field response will not comparable. INTRODUCTION The relatively hiqh fuel economy available from propeller-driven aircraft has renewed interest in high speed, highly loaded multiple blade turboprop propulsion systems. The undesirable features of community noise and, more importantly, the high intensity cabin noise associated with the propellers supersonic helical tip speeds have stimulated new theoretical and experimental research on the acoustic characteristics of turboprops. The acoustic testing of propellers at realistic inflow Mach ovimbers can be carried out in flight using a suitably scaled model, or performed in a wind NM VN w 'i :f