Using wavenumber analysis from flight testing (to improve acoustic wind tunnel measurements)

Abstract

A large number of aeroacoustic wind tunnel measurements is performed in research facilities around the world aiming at locating noise sources on scaled models. In the following, the focus is however not set on the issue of locating noise sources on the research model, but rather to utilize a bybroduct of such measurements in closed test sections: the hydrodynamic pressure fluctuations from the turbulent boundary layer. In the majority of acoustic measurements, a phased array is being used to locate the sources on the research model. Such arrays consist of hundreds of microphones that are placed at carefully chosen locations either outside the flow in open test section wind tunnels, or underneath the turbulent boundary layer in closed test section wind tunnels. In the latter case, the pressure fluctuations recorded by the microphones are of both, hydrodynamic and of acoustic nature. In acoustic beamforming analysis, oftentimes the hydrodynamic pressure fluctuations are regarded as noise which mostly affects the main diagonal elements of the cross-spectral matrix. Therefore, the diagonal elements are commonly set to zero in order to avoid the effects of this noise on the beamforming map. In the following analysis, it is this "noise" that the focus will be set on. Analysis of the turbulent boundary layer pressure fluctuations can be used to both, learn about its characteristics, and to design better arrays for future measurements. A successful methodology for this "noise" evaluation will open up new possibilities of evaluating existing measurements for the the characterization of turbulent boundary layer pressure fluctuations.