Unsteady pressures under impinging jets in crossflows
HE in-ground effect of the flowfield surrounding jet-lift V/STOL aircraft provides many problems for the aircraft designer and operator.1 High pressure ratio jets striking the ground are a well-known source of noise and jet instability2; with two jets an unsteady fountain (or upwash) is formed between the jets by the collision of the two opposing wall jets. In the presence of a crossflow the ground jet sheet formed from the impingement region separates and rolls up to form a ground vortex.1 The ground vortex is one source of hot gas ingestion (HGI) as well as influencing airframe airloads; the fountain is another important source of HGI and can lead to airframe fatigue problems. Both of these sources of HGI are known to be highly unsteady.1 It has been argued by Williams3 that the statistical extrema of the flowfield conditions are as important as the mean, from the point of view of safe engine operations. What is not clear is the nature and origin of this unsteady behavior. Single impinging jets have long received attention from the research community for a variety of reasons, including the generation of impingement tones (e.g., Refs. 2 and 4). These tones appear to be due to a feedback mechanism between the impingement plane and the nozzle lip. There has been some experimental disagreement about the precise feedback route—either outside the jet or through the jet shear layer, although the latter seems to be more likely.4 Little experimental work has been published on twin impinging jets, particularly from the point of view of aeroacoustics. The unsteadiness of the ground vortex surrounding a single, low-pressure-ratio, normally impinging jet in crossflow has been investigated experimentally by Cimbala et al.5 They used hot-wire anemometer measurements in and around the jet, in the crossflow, and in the ground vortex region to try to characterize the vortex unsteadiness and identify its origin. Are such low pressure ratio tests representative of underexpanded-jet flowfield fluctuations? This paper reports measurements of ground plane pressure fluctuations, both near the impinging jet(s) and under the ground vortex and fountain for a range of nozzle pressure ratios (prn = 1.054.0), nozzle heights above the ground (h/dn = 2-8) and crossflow velocities (V*, = 0-20 ms"1).