ON CONTROLLING THE FLOW IN A MIXING LAYER OR WAKE CREATED DOWNSTREAM OF A 'Λ' NOTCH SIMULATING THE FLOW DOWNSTREAM OF A CHEVRON NOZZLE OR A λ WING

摘要

Mean flow, wave propagation and information about large vortex structures was obtained for a mixing layer generated by two parallel streams merging downstream of a splitter plate whose trailing edge simulates a chevron nozzle. The "Λ" shaped trailing edge was equipped with small and light oscillating fliperons that oscillated uniformly along the span thus enhancing the natural instabilities in the flow. In the absence of periodic excitation the mixing layer evolved in the direction of streaming as if it were generated by a two-dimensional splitter plate provided the origin of the flow coincided with the local location of the trailing edge. Mean velocity profiles measured parallel to the trailing edge were all identical suggesting that the boundary layer independence principle could apply to this highly turbulent flow. This led to extensive examinations of similar flows like a wake of a yawed airfoil or a yawed flat plate boundary layer since the applicability of this principle to turbulent flows was dismissed for many years. It was also discovered that the mixing layer downstream of the Λ notch generates two counter rotating streamwise vortices that force the turbulent flow to penetrate into the high speed stream. This penetration enhances the mixing occurring between the two streams and reduces the noise generated by a typical jet emanating from a chevron nozzle. The effects of periodic excitation on this flow were measured using PIV, Pitot-probe wake rakes and hot wire anemometers. Extensive two point correlation measurements determined the interaction between wave crests emanating from the two trailing edges either together or separately. The sensitivity of the flow to forcing amplitudes and frequencies was explored. Periodic excitation affected the rate of growth of the mixing layer particularly when the latter was carried out on both sides of the A notch at identical phase angle. Some of the data is discussed in the report and journal publications associated with this research are enclosed as appendices.