An innovative Fast Fourier Transform array technique for low wave-number measurements of the turbulent boundary layer fluctuating pressure field

A method has been developed to measure the parameters of the longitudinal wavenumber and frequency spectrum corresponding to wall pressure in a turbulent boundary layer. The method is based on a Fast-Fourier-Transform (FFT) approach for both the frequency and wavenumber domains. Instead of electrically summing the output of the pressure transducers to form a single array output, individual outputs in the FFT array are kept distinct and used with a FFT algorithm to produce a wavenumber spectrum. The method is analogous to the transformation from the time domain to the frequency domain by temporal sampling and FFT calculations in digital frequency analyzers. A spatial FFT filter response in the low-wavenumber region of a turbulent boundary layer was estimated in order to demonstrate the effectiveness of the method. The spatial filter response (in dBs) is plotted against the three-dimensional wavenumber spectrum phi sub p(k sub 1, k sub 3; omega) in a graph. Some applications of the method to measurements of large scale flow structure with structural modes, structural radiation, and sonar detection are discussed.