Abstract
This chapter examines large-scale instabilities in a short take-off and vertical landing (STOVL) upwash fountain. The wall jets created by the impingement on the ground of the individual jet flows from a jet-lift STOVL aircraft (with two or more nozzles) meet at a stagnation line and interact to form an upward-flowing “fountain” that interacts with the airframe. In some cases this provides a beneficial lift-generating ground cushion. In this chapter, the fountain flow created by two under-expanded axisymmetric, turbulent jets impinging on a ground plane is focused through the use of laser-based experimental techniques. Velocity and turbulence data are acquired in the jet and fountain flow regions using laser Doppler velocimetry and particle image velocimetry. Profiles of mean and rms velocities along the jet centerline are presented for nozzle pressure ratios of two, three, and four. The unsteady nature of the fountain flow is examined and the presence of large-scale coherent structures is identified. A spectral analysis of the fountain flow data is performed using the Welch method. The results obtained have relevance to ongoing studies of the fountain flow using large eddy simulation techniques.
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