No 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 ...

The effects of periodic excitation on the evolution of a turbulent jet were studied experimentally. A short, wide-angle diffuser was attached to the jet exit and excitation was introduced at the junction between the jet exit and the diffuser inlet. The introduction of high amplitude periodic excitation at the jet exit enhances the mixing and promotes attachment of the jet shear-layer to the diffuser wall. Vectoring is achieved by applying the excitation over a fraction of the circumference of the circular jet, enhancing its spreading rate on the excited side and its tendency to reattach to that side. Static deflection studies demonstrate that the presence of the wide-angle diffuser increases the effectiveness of the added periodic momentum due to a favorable interaction between the excitation, the jet shear-layer and the diffuser wall. This point was further demonstrated by the evolution of a wave packet that was excited in the jet shear-layer. Strong amplification of the wave packet was measured with a diffuser attached to the jet exit. The turbulent jet responds quickly (10-20 msec) to step changes in the level of the excitation input. The response scales with the jet exit velocity and is independent of the Reynolds number. Jet deflection angles were found to be highly sensitive to the relative direction between the excitation and the jet flow and less sensitive to the excitation frequency. The higher jet deflection angles were obtained for a diffuser length of about two diameters and for diffusers with half-angles greater than 15 degrees.

No abstract

No abstract