Investigation of hypersonic boundary-layer transition using high speed visualization techniques

Abstract

Transition from laminar to turbulent flow in hypersonic boundary layers leads to highly elevated levels of surface heat transfer and frictional drag. Understanding and predicting this transition process is thus critical to the design of high-speed flight vehicles. For slender two-dimensional or axisymmetric geometries above Mach 4, the dominant instability leading to transition is typically the acoustic (second-mode or Mack) instability, with most-amplified frequencies that can vary from ∼100kHz to over 1MHz. At the latter limit, typical of the hypervelocity regime, traditional sensors such as fast-response pressure transducers, heat-flux gages, or hot-wires have inadequate time response. An attractive class of alternatives are optical or visualization-based techniques, which are typically capable of higher spatial resolutions and frequency bandwidths. Additional benefits include the ability to measure away from the model surface and the non-intrusive nature of the techniques, allowing simultaneous measurements at multiple downstream locations in the the flow field. In this study, we outline the development of several such techniques for use in a shock tunnel.