Supersonic boundary-layer response to optically generated freestream disturbances
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Controlled, localized disturbances were introduced into the supersonic freestream upstream of a 4:1 elliptic cross-section cone. The response of the initially laminar boundary layer to the laser-generated freestream perturbation was measured above the cone minor axis. The experiment was conducted in the Mach-4 Purdue Quiet-flow Ludwieg tube at a freestream unit Reynolds number of 4.5 million/m. The focused beam from a frequency-doubled Nd:YAG laser was used to generate the disturbance. The perturbation existed in the flowfield as a region of locally heated air, referred to here as the thermal spot. Constant-temperature anemometry was used to characterize the boundary-layer response to the introduction of the thermal spot. The response was largest and most complex near the boundary-layer edge. The duration of the measured boundary-layer response was an order of magnitude greater than the measured duration of the disturbance in the freestream. Within the boundary layer, the mass-flux deviation introduced by the thermal spot was of the same magnitude as the local mean mass flux. The optically generated disturbance is potentially useful as a perturbation source in future boundary-layer receptivity experiments.
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