A focused laser differential interferometer (FLDI) is applied to Caltech's Hypervelocity Expansion Tube (HET). FLDI is shown to be sensitive enough to make high-bandwidth measurements of flow features at low-density conditions. The freestream disturbance level is quantified as a function of the sound speed ratio across the primary contact surface. This is done to experimentally investigate whether the freestream disturbances obey the acoustic wave theory proposed by l'aull and Stalker. It is found that disturbance magnitudes generally decrease with reductions in this sound speed ratio, as predicted by the theory, but some anomalous results are observed that suggest additional controlling parameters beyond the acoustic model.
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