Jet ground vortex formation by annular jets

Abstract

An experimental study has been conducted of the impingement of a single circular jet on a ground plane in a crossflow. This geometry is a simplified model of the interaction of propulsive jet exhaust from a V/STOL aircraft with the ground in forward flight. Jets have been oriented normal to the crossflow and ground plane. Jet size, crossflow-to-jet velocity ratio, ground plane-to-jet board spacing, jet exit turbulence level, and mean velocity profile shape have all been varied to determine their effects on the size of the ground vortex interaction region which forms on the ground plane, using smoke injection into the jet. Variation of observed ground vortex size with crossflow-to-jet velocity ratio was consistent with previous studies. Observed effects of jet size and ground plane-to-jet board spacing were relatively small. Jet exit turbulence level effects were also small. However, an annular jet with a low velocity central core was found to have a significantly smaller ground vortex than an equivalent uniform jet at the same values of crossflow-to-jet velocity ratio and jet exit-to-ground plane spacing. This may suggest a means of altering ground vortex behavior somewhat, and points out the importance of proper simulation of jet exit velocity conditions. Laser velocimeter data indicated unsteady turbulence levels in the ground vortex as high as 80%.

Inverted-profile coaxial jet flows relevant to Astovl applications

“…Thus it might be expected that the flowfield from an annular jet would resemble that from a circular jet at a large nozzle height. This is supported by Kuhlman and Cavage< 16 > who noted a decrease in the ground vortex penetration into a crossflow using an annular nozzle. It is difficult, however, to deduce the effects for a coaxial jet when the nozzle height is low, and the jet is not fully developed.…”