Differential Interferometric Measurement of Instability in a Hypervelocity Boundary Layer

A quantitative nonintrusive optical scheme, focused laser differential interferometry (FLDI), used to investigate disturbances in a hypervelocity boundary layer on a 5 deg half-angle cone, is described. All experiments were performed in T5, the reflected shock tunnel at the California Institute of Technology. The test article was a 1 m long 5 deg half-angle aluminum cone, and the test gas was air. A relation between the fluctuations in density and output voltage from the photodetector is used for postprocessing. The spectral content of the turbulent spot shows broadband response; the wave packet has a strong peak in response at 1.11 MHz. The largest sources of systematic error are considered to be the uncertainty introduced by the assumed integration length, assumed to be 20%. Random error from electrical noise and mechanical vibrations can be estimated by inspecting the spectral content of the signal immediately preceding the test time.