Experimental Investigation of pressure fluctuations in the near field of subsonic jets at different Mach and Reynolds numbers

摘要

Advanced signal processing methods based on wavelet transform are applied to pressure data measured in the near field of a free jet at subsonic Mach numbers (up to 0.9) and for Reynolds numbers around 10~5. The wavelet technique allows for the separation of nearly Gaussian background fluctuations, interpreted as acoustic pressure, from intermittent pressure peaks induced by the hydrodynamic components. The effect of the Reynolds number is evaluated by comparing the results of two different nozzles at the same Mach number. Thanks to the adopted wavelet technique, the acoustic and hydrodynamic pressure statistics are analyzed separately in the two cases. An interpretation of the observed statistics is given in terms of the potential core fluctuations and of the correlation between acceleration and pressure fluctuations. A conditional analysis is also performed to extract the signature of the most energetic acoustic pressure events. Preliminary results seem to indicate that the near field acoustic pressure is dominated by the trace of instability waves.