Oscillatory excitation of unsteady compressible flows over airfoils at flight Reynolds numbers

Abstract

An experimental investigation, aimed at delaying flow separation due to the occurrence of a shock-waveboundary-layer interaction, is reported. The experiment was performed using a NACA 0012 airfoil and a NACA 0015 airfoil at high Reynolds number incompressible and compressible flow conditions. The effects of Mach and Reynolds numbers were identified, using the capabilities of the cryogenic-pressurized facility to maintain one parameter fixed and change the other. Significant Reynolds number effects were identified in the baseline compressible flow conditions even at Reynolds number of 10 and 20 million. The main objectives of the experiment were to study the effects of periodic excitation on airfoil drag-divergence and to alleviate the severe unsteadiness associated with shock-induced separation (known as "buffeting"). Zero-mass-flux oscillatory blowing was introduced through a downstream directed slot located at 10% chord on the upper surface of the NACA 0015 airfoil. The effective frequencies generated 2-4 vortices over the separated region, regardless of the Mach number. Even though the excitation was introduced upstream of the shock-wave, due to experimental limitations, it had pronounced effects downstream of it. Wake deficit (associated with drag) and unsteadiness (associated with buffeting) were significantly reduced. The spectral content of the wake pressure fluctuations indicates of steadier flow throughout the frequency range when excitation was applied. This is especially important at low frequencies which are more likely to interact with the airframe. have shown that periodic vortical excitation introduced into a separating boundary layer, slightly upstream of the average separation location, can effectively delay boundary layer separation. The improved ability of the boundary layer to overcome an adverse pressure gradient is attributed to enhanced mixing between the low momentum fluid near the wall and the external high momentum flow. The successful application of the method increases the lift while maintaining low drag. At low Mach numbers, where high-lift for take-off, landing or loiter is required, the delay of boundary layer separation allows increased loading of a multi-element high-lift airfoil system.It was recently demonstrated 4 that periodic excitation of the boundary layer upstream of separation can delay the occurrence of the adverse effects associated with boundary layer separation and significantly enhance the performance of airfoils at flight Reynolds numbers and incompressible speeds. Low Reynolds number experiments, where control was applied from the LE region of the airfoils, were repeated at a chord Reynolds number of 37.6x10 6 . Using a flapped NACA 0015 airfoil, where control was applied at the flap shoulder, it was shown that the method is essentially independent of Reynolds number 4 , as long as the appropriate dimensionless control parameters are applied.A recently published numerical simulation 5 shows that oscillatory excitation of a separated boundary layer, at ...

Synthetic Jet Actuation - Modeling, Actuator Development and Application to Separation Control

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