Differential interferometric measurement of instability at two points in a hypervelocity boundary layer

The focused laser differential interferometer (FLDI) was used to investigate disturbances in a hypervelocity boundary layer on a sharp five degree half-angle cone. The T5 hypervelocity free-piston driven reflected-shock tunnel was used as the test facility; in such a facility the study of thermo-chemical/fluid-dynamic energy exchange is emphasized. Two sensitive FLDI probe volumes were aligned along a generator of the cone that recorded time-traces of density fluctuation at sufficient time resolution, spatial resolution, and signal to noise ratio, so that the boundary layer instability could be resolved. This arrangement of the FLDI allows for the interpretation of disturbances at two points and the correlation between them. The acoustic instability is detected with narrow-band peaks in the spectral response at a number of frequencies (500 kHz to 1.29 MHz). The data indicate that the instability driving the boundary layer to turbulence is acoustic in nature. Preliminary analysis indicates that there is not a significant difference between N₂ and air acoustic boundary layer disturbance amplification factors for the representative cases presented. Computation of acoustic damping by thermo-chemical relaxation processes is presented for the same representative cases, and indicates that there is a negligible amount of absorption for both air and N₂ at the observed disturbance frequencies.