Mitigation of impinging tones using central protrusion

This article demonstrates that a simple modification such as installing a protrusion on an impingement plate can lead to reduction of jet impingement tones. This experimental work compares the acoustic variations in jets impinging on a flat plate, and plates with a central protrusion. The key strategy here is the modification of the stagnation flow region brought about by the introduction of the central protrusion. It is observed that the toneless acoustic power is almost the same for jet impinging on the plate with and without the central protrusion. It is also found that the protrusion is more effective in reducing tones in supersonic impinging jets than the tones in subsonic impinging jets. However, there is a slight increase in acoustic power when the jet impinges on a larger protrusion (greater than jet diameter) at high nozzle pressure ratios. Further, high speed flow visualization is carried out to relate the shock oscillations with the tonal frequencies. The noise source location is estimated by performing FFT on the flow visualization images. The acoustic and optical measurements are compared in terms of tonal frequency and found to be in agreement with each other.