Supersonic impinging jets have been of interest both from an applications and a fundamental fluid mechanics point of view for several decades. The vertical hot jet used by aircraft capable of vertical landing or take-off during hover is perhaps the most significant application. When the distance between the ground and the aircraft is small, the impinging jet has been shown to produce lift loss, hot gas ingestion and a highly unsteady flow field producing ground erosion and damaging vibrational loads on aircraft, structures and personnel in the vicinity. Previous tests using an ideally-expanded, axisymmetric Mach 1.5 primary jet heated up electrically to a total temperature of up to 500K have been carried out to establish both baseline acoustic features of this flow field and noise suppression using microjets. Pressure spectra acquired in this setup showed discrete, high-amplitude acoustic impingement tones at frequencies varying with jet temperature. These tones were found to grow more pronounced at elevated versus ambient conditions. In these tests, microjets produced substantial suppression of both tones and broadband noise. Of recent interest is the case of a jet impinging on an angled surface, as seen on the decks of aircraft carriers using blast deflectors. With such geometry, a new opportunity to effect noise control downstream of the nozzle arises. The present paper describes experiments carried out in a high-temperature, supersonic jet facility in which a heated jet produced by the combustion of ethylene was allowed to impinge on a flat plate outfitted with oblique aqueous microjets. Steady microjet suppression was tested in this configuration. Water flow rates of up to 10% of the main jet flow were tested for ground plane locations of 8 and 10 nozzle throat diameters downstream of the nozzle exit and at jet exhaust temperatures from 288K to 1033K. Measurements were carried out to quantify the noise suppression in the acoustic far-field for various stagnation temperatures and ground plane locations. It was found that the angled ground plane microjets suppressed the impingement tones as well as produced additional broadband noise reduction at low temperatures. Reductions of up to nearly 4 dB were observed in the far-field.
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