Multiple Jet/Wall/Cross-Wind Interaction Relevant to VSTOL Ground Effects

Abstract

The flowfield of ground vortex and upwash flows generated by single and multiple impinging jets in a crossflow is studied in detail. Laser Doppler measurements and flow visualization are presented for turbulent circular jets emerging into a low-velocity crossstream and, then, impinging on a flat surface perpendicular to the geometrical jet-nozzle axis. The experiments were performed for Reynolds numbers based on the jet-exit conditions of 43,000 to 105,000, a jet-to-crossflow velocity ratio of 30 and for an impinging height of 5 to 20 jet diameters and include mean and turbulent velocity characteristics along the two normal directions parallel to the nozzle axis. The mean velocity, velocity fluctuation and visualization in the impingement region were obtained for velocity ratios between the jet exit and the crossflow VR =Vj /Uo of 7.5 to 90 with interject spacings of S=5D and L=6D. 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 VSTOL 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.