Sound propagation corrections in open jet wind tunnels

Abstract

Noise produced by aircraft, wind turbines or drones is an increasing societal challenge. In order to develop more silent technology new silent designs are tested in wind tunnels. The wind tunnel environment, however, is different from the real world. Therefore, corrections are made to properly predict the acoustics of the application in the real world. Acoustic corrections, specifically aimed to be used in acoustic imaging techniques, are the topic of this thesis. The work is split into two parts. The first part concerns efficient mean flow corrections. The second part presents a method to improve the resolution of acoustic images when the acoustic waves propagate through a turbulent medium. The thesis begins with a background on sound propagation, including common correction models used within acoustic imaging techniques. Furthermore, the fundamentals of acoustic imaging by means of the beamforming method are described.Localization of aeroacoustic sound sources in open jet wind tunnel experiments is often performed using acoustic imaging techniques. These techniques require the evaluation of the acoustic propagation time between the sound source and observer, which depends on the flow topology. Most conventional predictions use either a ray-tracer, coupled with a modeled continuous velocity field, or use ray diffraction and a discretization of the velocity field by means of vortex sheets. A novel method is proposed in which the continuous velocity field is discretized into blocks of constant velocity separated by velocity discontinuities, thus removing the requirement for the velocity to be parallel to the surface that separates the blocks. The acoustic ray is solved by minimization of the acoustic propagation time. The computational effort is low compared to ray-tracing methods, while maintaining an improvement in accuracy compared to methodologies using vortex sheets. An empirical continuous velocity field is derived that models a selfsimilar shear layer expanding asymmetrically from a rectangular nozzle. Subsequently, this velocity field is discretized to compute the acoustic rays. Experimental results with a loudspeaker source placed in the open jet of a large industrial wind tunnel showed a decrease in source localization uncertainty compared to techniques based on vortex sheets. This is attributed to the inclusion of the shear layer slanting.A similar method is applied to an open jet wind tunnel experiment in which sound sources on high-lift devices are localized using acoustic imaging techniques. The lift force deflects the wind tunnel jet and therefore affects the flow topology. A method is presented to evaluate the propagation time when the jet is deflected. The deflected flow is modeled using potential flow theory and used as input for a ray-tracer to obtain the propagation time. Although accurate, a ray-tracer significantly increases the computational effort of acoustic imaging. Therefore, approximate ray solutions are again computed using the principle of least propagation time, significantly lowerin...