Study of the refraction of sound by shear and boundary layers

Abstract

Performing accurate acoustic measurements in aerodynamic wind tunnel is a challenge of great interest for the aeroacoustics community. Hence aeroacoustic measurements could be obtained in the same facility under the same aerodynamic conditions. This paper shows the refraction effects to be taken into account during aeroacoustics measurements in a closed test section, including flows up to high subsonic Mach numbers. In closed wind tunnel, the wall-mounted microphones are either flush-mounted or recessed in a cavity to avoid perturbations from the boundary layer. Besides, due to the wall, interference pattern generated by reflections occur. Thus, the acoustical wave front is strongly modified compared to the open test section configuration. Numerical computations, using the ONERA’s sAbrinA-v0 CAA code solving the Euler’s equations, allow us to accurately characterize the influence of a shear layer (recessed microphones) and of a boundary layer (flush-mounted microphones) and measurement setups, on the radiation from a monopolar source in the presence of a rigid wall. The CAA computations are made in the range 1 kHz to 10 kHz and for different boundary/shear layers thicknesses. The paper details a comparison between the Amiet’s model and the CAA computations. The comparisons are in a good agreement despite the absence of a wall in the Amiet’s model.