Towards CAA Based Acoustic Wind Tunnel Corrections for Realistic Shear Layers

Abstract

The free shear layers induced by the nozzle in aeroacoustic open jet wind tunnels perform a significant influence on sound wave propagation, causing direction alteration and amplitude reduction (due to velocity gradient and turbulence), as well as spectral redistribution (due to turbulence). The effects need to be seriously considered in measurement data correction, in order to eliminate the misunderstanding or incorrect conclusions drawn from the experiments conducted in such a wind tunnel. A commonly used correction method derived by Amiet for refraction effect has been utilized for many years, since it provides a simplified but practical approach for shear layer correction. However, the zero-thickness shear layer assumption taken by Amiet may cause deviation in application while the real shear layers are finite in thickness and spread as the flow goes downstream. At the same time, the aerodynamic model located in the test section could also affect the shear layer shape. Hence, it will be helpful if the various situations are taken into account and systematically evaluated for the correction method. In this paper an investigation is carried out using Computational Aeroacoustics (CAA) methods to simulate sound propagation through free shear layers representative for the jets of acoustic wind tunnels of rectangular cross section. The analysis is carried out for 1kHz and 10kHz while considering the shear layer thickness, flow curvature due to test model lift and reflection from the nozzle surface. The results are compared with the theoretical correction based on Amiet’s approach. While the effects of shear layer thickness for plane shear layers even with streamwise spreading seems comparatively small, the curvature effects have a larger impact. The presence of the actual nozzle rim also has a considerable influence on the overall sound field. Moreover, 3D simulations of the sound propagation through the shear layers of DLR’s acoustic wind tunnel AWB with rectangular jet cross section show significant effect around the intersection corner of two shear layers.