The prediction of laminar–turbulent transition location in high-speed
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.
Quantitative characterization of this disturbance is paramount to the
development of transition location-prediction tools.

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Differential Interferometric Measurement of Instability in a Hypervelocity Boundary Layer

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^(−1) (Mach numbers between 4 and 6) over a 5° half-angle cone at zero angle of attack. Simultaneous measurements are made at two locations approximately midway along a generator of the 1-m-long cone. With increasing Reynolds number (unit values were between 2 and 5×10^6 m^(−1)), 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.

/pdf/observations-of-hypervelocity-boundary-layer-instability-5fw5hnkxoj.pdf

Observations of hypervelocity boundary-layer instability

Focused laser differential interferometry is used to quantify the free-stream density perturbations in the T5 reflected-shock tunnel. The investigation of reflected-shock tunnel disturbances is motivated by the study of hypervelocity boundary-layer instability and transition. Past work on hypersonic wind-tunnel noise is briefly reviewed. New results are reported for hypervelocity air flows at reservoir enthalpies between 5 and 18 MJ/kg at Mach ≈ 5.5. Statistical analysis finds no correlation of RMS density perturbations with tunnel run parameters (reservoir pressure, reservoir mass-specific enthalpy, free-stream unit Reynolds number, free-stream Mach number, and shot number). Spectrograms show that the free-stream disturbance level is constant throughout the test time. Power spectral density estimates of each of the experiments are found to collapse upon each other when the streamwise disturbance convection velocity is used to eliminate the time scale. Furthermore, the disturbance level depends strongly on wavelength. If the disturbance wavelength range of interest is between 700 μm and 10 mm, the tunnel noise is measured to be less than 0.5 % with the focused laser differential interferometer.

/pdf/free-stream-density-perturbations-in-a-reflected-shock-23twsfiw8b.pdf

Free-stream density perturbations in a reflected-shock tunnel

A computational method for predicting the output of a focused laser differential interferometer (FLDI) given an arbitrary density field is presented. The method is verified against analytical predictions and experimental data. The FLDI simulation software is applied to the problem of measuring Mack-mode wave packets in a hypervelocity boundary layer on a 5° half-angle cone. The software is shown to complement experiments by providing the necessary information to allow quantitative density fluctuation magnitudes to be extracted from experimental measurements.

/pdf/analysis-of-focused-laser-differential-interferometry-23r3hmtedb.pdf

Analysis of focused laser differential interferometry

A survey is made of interferometric techniques receiving the optical path ? = ?nds in real time. Using differential interferometry or a new phase coupling technique, ?(t) can be obtained in a wide range of conditions. A variety of applications to gasdynamic problems is described.

Flow Diagnostics by Laser Interferometry

A He-Ne laser beam is sent through a common path differential interferometer and the interference signal is detected by a p-i-n photodiode. Weak phase objects can be examined because optical path changes can be resolved to 0.1 A with a time resolution of about 20 ns. Examples for different gas-dynamic investigations show good results and demonstrate the wide field of application for this type of laser interferometer.

G. Smeets

Papers

Laser Interferometer for High Sensitivity Measurements on Transient Phase Objects