Laser Doppler Velocimeter Measurements in the Ground Interaction Region of an Impinging Scale Model Jet

Two-component Laser Doppler Velocimetry measurements were made in the jet plume, impingement region, and outwash region of a scale model jet impinging on a ground plane. The model used for this study is representative of a generic military-style STOVL aircraft in hover. A 13.5mm contoured de Laval nozzle was operated at its design nozzle pressure ratio of 2.93. Velocity measurements were made for ground plate to model separations of 22.9, 12, and 6Dr. Mean velocity measurements in the jet plume and wall jet were compared to previous pitot rake measurements. In general there was good agreement with past data although there was a slight mismatch in measured peak velocities between the two measurement techniques. For plate separations of 12 and 6Dr, axially-directed velocities as high as 250m/s were measured within 0.5Dr of the ground plane. Turbulence intensities as high as 15% of the jet exit velocity were found to be in the shear layer of all jet plumes as well as out to 4Dr in the wall jet for a plate separation of 12Dr. Outwash velocities as high as 25% of the jet exit velocity were found to extend radially out to 7Dr. The decay of peak velocity in the wall jet was found to be approximately inversely proportional to radial position. Spectra obtained for a plate separation of 6Dr showed a distinct tone at several locations in the flow field that agreed well the impingement tone measured previously in the acoustic field. These are large scale motions that have been observed with flow visualizations but have been previously difficult to quantify for supersonic impinging jets.