Analytical shear layer corrections for acoustic transmission in Kevlar walled wind tunnels and their experimental validation

Kevlar walled aeroacoustic wind tunnels have become important research facilities to evaluate low noise wind turbine designs. In this paper a semi-analytical Green’s function for the transmission of sound through Kevlar walls and shear layers was developed in order to provide accurate acoustic measurements at high flow speeds. The semi-analytical Green’s function was evaluated experimentally with a monopole sound source. The levels of measured auto spectra could be successfully restored to the reference value obtained from the measurement without flow. It was possible to find the correct source location when the Green’s functions was used to formulate the steering vectors for the beamforming technique. However, the source levels were not predicted correctly when using the beamforming technique. The reduction of the peak level was due to a loss of coherence across the array and cannot be compensated even though the formulation of the Green’s function was correct. A parametric study showed that the analytical transmission loss was very sensitive to a parameter related to the aerodynamic loading of the Kevlar walls. The experimental setup to validate the green’s function could be improved if aerodynamic loading was applied to the walls.