Multiple Jet/Wall/Crossflow Interactions

Abstract

This paper summarizes thirty years of research on ground vortex and upwash flows generated by single and multiple impinging jets in a crossflow with relevance to vertical/Short Take-Off and Landing Aircrafts (V/STOL). Laser Doppler measurements and flow visualization are presented for turbulent circular jets emerging into a low-velocity cross-stream and, then, impinging on a flat surface perpendicular to the geometrical jet-nozzle axis. The experiments were performed for single, twin side-by-side., twin in tandem, and three jets through a crossflow for velocity ratios between the jet exit and the crossflow VR =Vj /Uo of 7.5 to 90. The multiple jets configurations include interject spacings of S=5D and L=6D. The impinging heights were varied from 5 to 20 jet diameters and include mean and turbulent velocity characteristics along the two normal directions parallel to the nozzle axis. The largest velocity ratios are characterized by a large penetration of the impinging jets, giving rise to a ground vortex due to the collision of the radial wall jet and the crossflow that wraps around the impinging point like a scarf. The results help to understand the flow around a V/STOL aircraft operating in ground vicinity with front wind or small forward movement that may result in enhanced negative pressures in the underside of the aircraft causing a pitching moment and suction down force towards the ground.