Alexandria Engineering Journal (Sep 2023)
Control of a rectangular impinging jet: Experimental investigation of the flow dynamics and the acoustic field
Abstract
Passive control techniques of impinging jets are of high interest for many industrial applications and particularly for noise generation issues encountered in such configurations. Thus, an experimental study was carried out to simultaneously show the effect of a mechanism of control on the acoustic and the dynamic fields involved in a rectangular jet of air impinging on a slotted plate. A Reynolds number of Re = 5900 presenting an intense acoustic level was considered. The mechanism of control consists on a thin rod which was introduced in different positions of the flow. A total number of 1085 spatial positions of the rod were tested in order to identify the optimal position for noise reduction. Combined Stereoscopic Particle Image Velocimetry measurements were performed to obtain the kinematic field in the whole area of interest from the both sides of the introduced rod. A new representation of the acoustic levels (cartography of acoustic level as function of the location of the rod) is provided to identify the optimal positions of control. It was found that when the self-sustaining tone loop disappears, the sound pressure levels can drop by almost 23% depending on the location of the rod. A Dynamic Mode Decomposition (DMD) was established and cross-correlations were calculated between temporal modes and acoustic signals for both controlled and not controlled cases. The cross-correlations between the acoustic signal and the temporal modes were found to be insignificant in case of controlled flow. Moreover, in case of controlled flow, spatial modes were found to be significant far from the slot which plays a principal role in the self-sustaining tones by interacting with the passage of vortices through it. These results are of interest since the visualization of the flow dynamics and the corresponding vortex activity explains the disappearance of the self-sustaining loop and the sound pressure level changes. Such results are of high interest for developing new strategies of noise control.
