Differential Interferometric Measurement of Instability in a Hypervelocity Boundary Layer

T HE prediction of laminar–turbulent transition location in highspeed boundary layers is critical to hypersonic vehicle design because of the weight implications of increased skin friction and surface heating rate after transition. Current work in T5 (the California Institute of Technology’s free piston reflected shock tunnel) includes the study of problems relevant to hypervelocity boundary layer transition on cold-wall slender bodies. With the ability to ground-test hypervelocity flows, the study of energy exchange between the boundary layer instability and the internal energy of the fluid is emphasized. The most unstable mode on a cold-wall slender body at zero angle of incidence is not the viscous instability (as in low-speed boundary layers) but the acoustic instability [1,2]. Quantitative characterization of this disturbance is paramount to the development of transition location-prediction tools. Traditionally, fast-response piezoelectric pressure transducers, heat-flux gauges, or hot-wire anemometry techniques are used in this type of study [3–5]; however, the high frequency and small wavelength of the disturbances render these techniques inadequate above 1 MHz for conditions in T5. Recently, time-resolved visualization of the acoustic instability atmoderate reservoir enthalpy (3–4 MJ∕kg) has been reported [6]. That study used a dual-field-lens schlieren system with an extended light source, which was used to reduce the depth of focus of the system to reduce the contribution of disturbances outside of the boundary layer; however, even at the high frame rate (500 kHz) available, the exposure time (500 ns) is too long to adequately capture the acoustic instability at the boundary layer edge velocities of the current work in T5. Resonantly enhanced focused schlieren work in T5 has yielded some promising results [7]. Peaks in the spectral content at frequencies consistent with the acoustic instability were found along with detection of turbulent bursts; however, the method of resonantly enhanced focused schlieren makes quantitative interpretation of the results difficult. This note describes a quantitative nonintrusive optical scheme that is used to investigate disturbances in a hypervelocity boundary layer on a 5 deg half-angle cone. The technique, focused laser differential interferometry (FLDI), has been successfully implemented to make quantitative measurements of density perturbations with high temporal (20 MHz) and spatial (700 μm) resolution. The acoustic instability is detected, with a peak in the spectral response at over 1 MHz. The experimental setup and results are presented, and future plans are discussed.