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TRANSITION EXPERIMENTS WITH STREAMWISE VORTICES IN SUBCRITICAL BOUNDARY LAYERS

K. V. Manu
Department of Aerospace Engineering, Indian Institute of Science Bangalore 560012 INDIA

J. Dey
Department of Aerospace Engineering, Indian Institute of Science Bangalore 560012 INDIA

Joseph Mathew
Department of Aerospace Engineering, Indian Institute of Science, Bangalore, 560012, INDIA

Résumé

Experiments were conducted by affixing smooth hills, with a short gap, on a flat plate mounted in a low turbulence tunnel. Configurations with three gap-widths of the order of the hill height (also of the order of the incoming boundary layer) and a range of freestream speeds were examined. PIV and hotwire data were obtained. At the hill location the Reynolds number was always subcritical. Flow past the hill develops streamwise vorticity that develops into a pair of streamwise vortices with the common flow between them directed away from the plate. At low freestream speeds of about 2 m/s, the lift-up action of these vortices creates inflectional profiles but there is no transition. Above a threshold speed, which increases with gap width, transition was observed. Between the streamwise vortices a low speed streak forms, flow separates and then reattaches. A reverse flow index shows unsteady reattachment over an extended region and the concomitant rise in velocity fluctuations. A high shear layer appears, which then rolls up. The unsteady reattachment is in this roll-up region. There are similarities with recent studies of transition induced by freestream turbulence. Present results indicate the common feature of late stages of transition due to freestream turbulence or wall protrusions to be streak breakdown.