Effects of boundary-layer tripping on the acoustic tones of impinging jets

The influence of boundary-layer tripping on the acoustic tones generated by a jet at a Mach number of 0.9 impinging on a flat plate is investigated for two nozzle-to-plate distances. Tripping the jet boundary-layer is found not to appreciably alter the frequencies but to significantly affect the levels of the tones. These effects strongly depend on the tone number and on the nozzle-to plate distance. For a given tone, in particular, the levels can decrease or increase depending on the distance. Therefore, tripping the boundary layers of impinging jets can result to less or more noise, and cannot be considered as a reliable method to reduce noise for these jets.