Analysis of simultaneous measurement of acoustic pressure in the far-field and density gradient in the near field in a cold jet

Abstract

The purpose of the investigation presented here is to study the degree of correlation between the density gradient in the near eld and the aeroacoustic sound generation in the far field of a cold jet. The variations of the density gradient in the flow are visualized and recorded by the Background Oriented Schlieren (BOS) technique. In this method high power LEDs are used as illumination source. Simultaneously the pressure fluctuations in the acoustic far-field are recorded by a set of microphones. Measurements on a round nozzle and a chevron nozzle are conducted and the obtained results are compared. Large structures emitting noise are localized, being in the case of the chevron nozzle further upstream due to the higher level of entrainment of the jet with the surrounded ambient fluid. In addition, it is demonstrated that the noise produced by these structures is radiated mainly downstream, having the highest correlations with the pressure fluctuations recorded with the microphone located at an angle Theta = 26°respect to the jet-axis.