Wind Tunnel Experiments Relating to Supersonic and Hypersonic Boundary-Layer Transition

TLDR

The study investigates how physical mechanisms influence transition by comparing fluctuation growth with Mack’s sound‑forcing and stability theories. Hot‑wire anemometry was employed to measure natural fluctuation growth in zero‑pressure‑gradient boundary layers over Mach 1.6–8.5, enabling comparison with Mack theory. Significant laminar‑layer fluctuations were detected ahead of expected instability amplification, cross‑correlated with sound fields at higher supersonic speeds, with growth rates matching Mack theory at Mach 4.5 and qualitatively at Mach 2.2 and 8.5, and the second instability mode dominating at Mach 8.5.

Abstract

Hot-wire anemometry is used to study the origin and growth of natural fluctuations in zero pressure-gradient boundary layers for several Mach numbers between 1.6 and 8.5. The importance to transition of certain physical mechanisms is examined through comparison of the fluctuation growth with the sound-forcing and stability theories of Mack. Flow fluctuations of substantial amplitude were observed within the laminar layer ahead of stations where instability amplification is expected to be important. These fluctuations were found to be cross-correlated with the sound field for the higher supersonic speeds, but not for the lower ones. The fluctuation growth rates in the unstable Reynolds number range ahead of the nonlinearity region were in reasonably close agreement with the theory for Mach 4.5; the agreement for Mach 2.2 and 8.5 was qualitative. The second mode of instability was predominant at Mach 8.5.