Experimental investigation of the Aero-Acoustics of a rectangular jet impinging a slotted plate for different flow regimes

Impinging jets are found in many industrial applications and specifically in ventilation systems. Mostly, these jets are turbulent and under certain conditions can be a source of discomfort in closed areas due to the high level of noise it can generates. Therefore, it is very important to understand the flow dynamics that is responsible of generating the acoustic field in order to control and reduce such phenomenon. In this paper, an experimental study of a rectangular impinging air jet on a slotted plate is considered for three different Reynolds numbers producing self-sustaining tones (Re = 4100, 5100, and 5900). The sound pressure and spatial velocity field are obtained simultaneously using four microphones located at different locations and SPIV technique. Results show that Re = 5100 correspond to a critical regime where there is a significant drop in the sound pressure level (SPL) that reached 5 dB when compared to a lower Reynolds number Re = 4100. The flow dynamic analysis suggests that this drop in SPL could be contributed to the path followed by the large coherent structure at Re = 5100, where they are deviated in the transverse direction along the wall leading to low energy transfer from the dynamic field to the acoustic one. However, at Re = 4100 and 5900 the peak in SPL could be contributed to the two paths followed by the large coherent structures and particularly to the path where vortices hit the slotted plate before escaping through it. This path is responsible for the optimization of energy transfer from the dynamic field to the acoustic field. Moreover, at Re = 5900 the spectrogram of the instantaneous frequency and instantaneous flow dynamics results show that the acoustic frequencies (160 Hz and 320 Hz) are generated by symmetric and anti-symmetric modes respectively. This unusual aspect is related to the sudden changes in vortex mode.