Nonlinear Growth and Breakdown of the Hypersonic Crossflow Instability
Abstract
A sharp, circular 7° half-angle cone was tested in the Boeing/AFOSR Mach-6 Quiet Tunnel at 6° angle of attack, extending several previous experiments on the growth and breakdown of stationary crossflow instabilities in the boundary layer. Measurements were made using infrared imaging and surface pressure sensors. Detailed measurements of the stationary and traveling crossflow vortices, as well as various secondary instability modes, were collected over a large region of the cone. The Rod Insertion Method (RIM) roughnescfvuse on a flared cone, was adapted for application to crossflow work. It was demonstrated that the roughness elements were the primary factor responsible for the appearance of the specific pattern of stationary streaks downstream, which are the footprints of the stationary crossflow vortices. In addition, a roughness insert was created with a high RMS level of normally-distributed roughness to excite the naturally most-amplified stationary mode. The nonlinear breakdown mechanism induced by each type of roughness appears to be different. When using the discrete RIM roughness, the dominant mechanism seems to be the modulated second mode, which is significantly destabilized by the large stationary vortices. This is consistent with recent computations. There is no evidence of the presence of traveling crossflow when using the RIM roughness, though surface measurements cannot provide a complete picture. The modulated second mode shows strong nonlinearity and harmonic development just prior to breakdown. In addition, pairs of hot streaks merge together within a constant azimuthal band, leading to a peak in the heating simultaneously with the peak amplitude of the measured secondary instability. The heating then decays before rising again to turbulent levels. This nonmonotonic heating pattern is reminiscent of experiments on a flared cone and earlier computations of crossflow on an elliptic cone. The nonlinear breakdown mechanism induced by each type of roughness appears to be different. When using the discrete RIM roughness, the dominant mechanism seems to be the modulated second mode, which is significantly destabilized by the large stationary vortices. This is consistent with recent computations. There is no evidence of the presence of traveling crossflow when using the RIM roughness, though surface measurements cannot provide a complete picture. The modulated second mode shows strong nonlinearity and harmonic development just prior to breakdown. In addition, pairs of hot streaks merge together within a constant azimuthal band, leading to a peak in the heating simultaneously with the peak amplitude of the measured secondary instability. The heating then decays before rising again to turbulent levels. This nonmonotonic heating pattern is reminiscent of experiments on a flared cone and earlier computations of crossflow on an elliptic cone. When using the distributed roughness there are several differences in the nonlinear breakdown behavior. The hot streaks appear to be much more uniform and form at a higher wavenumber, which is expected given computational results. Furthermore, the traveling crossflow waves become very prominent in the surface pressure fluctuations and weakly nonlinear. In addition there appears in the spectra a higher-frequency peak which is hypothesized to be a type-I secondary instability under the upwelling of the stationary vortices. The traveling crossflow and the secondary instability interact nonlinearly prior to breakdown.
Degree
Ph.D.
Advisors
Schneider, Purdue University.
Subject Area
Fluid mechanics|Mechanics|Thermodynamics
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