This study investigated the noise characteristics of supersonic impinging jets by measuring the three-dimensional directivity of the acoustic field and acoustic efficiency, which is defined as the ratio of radiated acoustic power to total jet power. A Mach 1.8 ideally expanded jet was impinged on an inclined flat plate, and microphone array measurements were conducted for four nozzle–plate distances ranging from 5D to 20D, where D was the nozzle-exit diameter. The results revealed that the acoustic efficiency reached a maximum at a nozzle–plate distance of 10D. As the nozzle–plate distance increases from 5D to 10D, the Mach waves from the jet between the nozzle and plate become stronger, leading to increased acoustic efficiency, because these waves are primarily radiated from the region approximately 6D to 9D downstream of the nozzle. At 15D and 20D, impingement-related acoustic waves weaken due to reduced flow velocity at the impingement region, resulting in lower acoustic efficiency. The peak directivity was observed in a side direction rather than the symmetry plane. At 15D and 20D, this is primarily caused by reflected Mach waves, while at 5D and 10D, this is not the case, suggesting that the impingement-related noise has a strong impact.
Ohyama et al. (Wed,) studied this question.