Causality correlation analysis on a cold jet by means of simultaneous particle image velocimetry and microphone measurements

The aeroacoustic sound generation processes of a cold jet were investigated by means of simultaneous Particle-Image-Velocimetry (PIV) in the near-field and microphone measurements in the far field. The measurements were conducted in a synchronized manner so as to enable the calculation of the cross correlation coefficient between the acoustic pressure and flow quantities derived from the measured velocity fluctuations. In this manner the regularities in the near-field fluctuations which are related to the radiated sound field could be identified. The tests were run for different Mach numbers M=0.5, 0.7 and 0.9 in air. Five microphones arranged at angles of 24 ° < θ < 88 ° to the jet centre line were used. The correlation with the axial velocity was highest. The spatial as well as the temporal distribution of the coefficient was observed to be dominated by coherent structures and showed a strong dependency on M and θ . For the M=0.9 jet the correlation coefficients fell below the confidence interval for θ = 67 ° .