Observations of hypervelocity boundary-layer instability

Abstract

A novel optical method is used to measure the high-frequency (up to 3 MHz) density fluctuations that precede transition to turbulence within a laminar boundary layer in a hypervelocity flow. This optical method, focused laser differential interferometry, enables measurements of short-wavelength, high-frequency disturbances that are impossible with conventional instrumentation such as pressure transducers or hot wires. In this work, the T5 reflected-shock tunnel is used to generate flows in air, nitrogen and carbon dioxide with speeds between 3.5 and 5 km s ), density fluctuations are observed to grow in amplitude and transition from a single narrow band of frequencies consistent with the Mack or second mode of boundary-layer instability to bursts of large-amplitude and spectrally broad disturbances that appear to be precursors of turbulent spots. Disturbances that are sufficiently small in initial amplitude have a wavepacket-like signature and are observed to grow in amplitude between the upstream and downstream measurement locations. A cross-correlation analysis indicates propagation of wavepackets at speeds close to the edge velocity. The free stream flow created by the shock tunnel and the resulting boundary layer on the cone are computed, accounting for chemical and vibrational non-equilibrium processes. Using this base flow, local linear and parabolized stability (PSE) analyses are carried out and compared with the experimental results. Reasonable agreement is found between measured and predicted most unstable frequencies, with the greatest differences being approximately 15 %. The scaling of the observed frequency with the inverse of boundary-layer thickness and directly with the flow velocity are consistent with the characteristics of Mack's second mode, as well as results of previous researchers on hypersonic boundary layers.

Effects of Shock-Tube Cleanliness on Hypersonic Boundary Layer Transition at High Enthalpy

“…During each T5 experiment, a pistoncompressed helium/argon driver ruptures a scored stainless-steel primary diaphragm. Following the primary diaphragm rupture, a shock wave propagates in the shock tube; is reflected off the endwall, breaking the secondary diaphragm; and reprocesses the test gas, which is then expanded through a converging-diverging contoured nozzle to ∼Mach 5.5 in the test section [24,26,29].…”

“…The focused laser differential interferometry is an optical technique, developed by Smeets [39] and recently used in T5 [21,27], which permits the high-speed and nonintrusive interrogation of small-amplitude density perturbations within a small sensitive region of the beam path while rejecting perturbations outside of the sensitive region, including those resulting from the nozzle shear layer. The effect of cleaning on transition location and repeatability is examined in a series of tests using the FLDI to measure disturbances within the boundary on a 5 deg half-angle smooth cone at a zero angle of attack.…”

“…To reduce the effects of particulates of BLT on slender cones [20][21][22][23][24][25][26][27], we devised a new cleaning and fill procedure for the shock tunnel, which is the subject of this Note. Possible sources of particles include piston buffer material, piston brakes, test gas impurities, and the Mylar secondary diaphragm.…”

Hypersonic Boundary Layer Transition on a 7 Degree Half-Angle Cone at Mach 10

“…This has been achieved thanks to the improved bandwidth of the measurements techniques (several hundreds of kilohertz are typically required), using hot-wires, [9][10][11] pressure sensors, [12][13][14][15][16] heat flux sensors 17,18 or optical diagnostics. 19,20 These investigations at hypersonic Mach numbers between 6 and 8, have shown the dominance of the so-called second mode of instabilities on conical geometries. This confirms the earlier Linear Stability Theory results of Mack 21,22 who predicted the amplification of an inviscid mode distinct from the one responsible for transition at lower speeds.…”

Growth and breakdown of wave packets in a high-speed boundary layer

“…Rapid recent advances in sensor technology and optical methods have enabled the dramatic development of measurement techniques over the past few years, which now allow the direct study of instabilities (e.g. Bountin et al 2015;Parziale, Shepherd & Hornung 2015).…”