Control of High-Temperature Supersonic Impinging Jets Using Microjets

The flowfield associated with supersonic impinging jets has been of interest to both engineers and researchers for some time due to its wide range of practical applications and its complex nature from a fundamental fluid dynamic point of view. An example of supersonic impinging jets occurs in short takeoff and vertical landing aircraft, for which the highly oscillatory flowfield and the associated acoustic loads are also accompanied by a dramatic loss in lift during hover, severe ground erosion of the landing surface, and hot gas ingestion into the engine inlets. Another characteristic feature of this flowfield is an intensive heat transfer between the jet and the impingement surface. In the past we have examined impinging jets and their control using microjets at cold conditions; the present study is a step toward examining this flowfield and the effectiveness of microjet control at increasingly realistic thermal conditions. An ideally expanded, Mach 1.5 primary jet issuing from an axisymmetric nozzle was heated up to a stagnation temperature of similar to 500 K. Mean and unsteady temperature and pressure measurements were obtained on a lift plate representative of the undersurface of an aircraft and on the ground plane over a range of nozzle-to-plate distances (representing aircraft hover conditions). In addition, near-field noise was also measured using a microphone. The velocity field of the impinging jet for both cold and hot conditions was mapped using particle image velocimetry. Our results show that the temperature recovery factor at the stagnation point on the ground plane is strongly dependent on the temperature ratio and nozzle-to-plate distance, similar to observations in subsonic impinging jets. The hover lift loss for hot jets is much higher than for cold jets, nearly 75% of the primary jet thrust at small nozzle-to-plate distances. The pressure fluctuations generated by hot impinging jets are also substantially higher than their cold counterparts. As in cold jets, pressure and noise spectra for hot jets show discrete, high-amplitude acoustic tones (generally known as impinging tones) at frequencies varying with jet temperature. The activation of microjet control shows a substantial reduction in pressure fluctuations both in terms of overall sound pressure levels (up to 20 dB on the ground plane and 15 dB on the lift plate) and the attenuation of discrete, high-amplitude impinging tones (up to 32 dB). High-temperature peaks were observed in the temperature spectra at frequencies corresponding to impingement tones in the pressure and noise spectra; these were also substantially attenuated with microjet control. As much as 50% of the lift loss was recovered by using control for hot,jets at smaller nozzle-to-plate distances. In general, the results provide evidence of the feasibility of using this active control approach under increasingly realistic conditions to achieve desired reductions in noise, unsteady pressures, and thermal loads.