The popularity of plasma actuators as flow control devices has sparked a flurry of diagnostic efforts towards their characterisation. This review article presents an overview of experimental investigations employing diagnostic techniques specifically aimed at AC dielectric barrier discharge, DC corona and nanosecond pulse plasma actuators. Mechanical, thermal and electrical characterisation techniques are treated. Various techniques for the measurement of induced velocity, body force, heating effects, voltage, current, power and discharge morphology are presented and common issues and challenges are described. The final part of this report addresses the effect of ambient conditions on the performance of plasma actuators.

https://iopscience.iop.org/article/10.1088/0957-0233/26/9/092001/pdf

Diagnostics for characterisation of plasma actuators

The stability of a hypersonic boundary layer on a flared cone was analysed for the same flow conditions as in earlier experiments (Zhang et al., Acta Mech. Sinica, vol. 29, 2013, pp. 48–53; Zhu et al., AIAA J., vol. 54, 2016, pp. 3039–3049). Three instabilities in the flared region, i.e. the first mode, the second mode and the Gortler mode, were identified using linear stability theory (LST). The nonlinear-parabolized stability equations (NPSE) were used in an extensive parametric study of the interactions between the second mode and the single low-frequency mode (the Gortler mode or the first mode). The analysis shows that waves with frequencies below 30 kHz are heavily amplified. These low-frequency disturbances evolve linearly at first and then abruptly transition to parametric resonance. The parametric resonance, which is well described by Floquet theory, can be either a combination resonance (for non-zero frequencies) or a fundamental resonance (for steady waves) of the secondary instability. Moreover, the resonance depends only on the saturated state of the second mode and is insensitive to the initial low-frequency mode profiles and the streamwise curvature, so this resonance is probably observable in boundary layers over straight cones. Analysis of the kinetic energy transfer further shows that the rapid growth of the low-frequency mode is due to the action of the Reynolds stresses. The same mechanism also describes the interactions between a second-mode wave and a pair of low-frequency waves. The only difference is that the fundamental and combination resonances can coexist. Qualitative agreement with the experimental results is achieved.

Interactions between second mode and low-frequency waves in a hypersonic boundary layer

Developing mechanism-based methods for estimating hypersonic boundary-layer transition in flight: The role of quiet tunnels

Direct numerical simulations (DNS) were carried out to investigate the laminar–turbulent transition for a flared cone at Mach 6 at zero angle of attack. The cone geometry of the flared cone experiments in the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University was used for the simulations. In the linear regime, the largest integrated spatial growth rates (. Low grid-resolution simulations were carried out in order to identify the azimuthal wavenumber that led to the strongest growth rates with respect to the secondary instability for a fundamental and subharmonic resonance scenario. It was found that for the BAM6QT conditions the fundamental resonance is much stronger compared to the subharmonic resonance. Subsequently, for the case which led to the strongest fundamental resonance onset, detailed investigations were carried out using high-resolution DNS. The simulation results exhibit streamwise streaks of very high skin friction and of high heat transfer at the cone surface. Streamwise ‘hot’ streaks on the flared cone surface were also observed in the experiments carried out at the BAM6QT facility using temperature sensitive paint. The presented findings provide strong evidence that the fundamental breakdown is a dominant and viable path to transition for the BAM6QT conditions.

Direct numerical simulations of hypersonic boundary-layer transition for a flared cone: fundamental breakdown

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.

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Observations of hypervelocity boundary-layer instability

Focused schlieren is used in combination with a small-diameter fiber optic and avalanche photodetector to measure second-mode instabilities on a Mach 6 flared-cone model, within the unit Reynolds number range 7.8 to 11.0×10 m. The second-mode instability is readily observed at f0 ~ O(250 kHz), as is harmonic content at 2f0 and 3f0. A bispectral analysis shows that after sufficient amplification of the second mode, several nonlinear mechanisms become significant. A self-excited nonlinear interaction of the second mode takes place, resulting in an energy transfer to generate higher harmonics. A variety of nonlinear interactions occur, including ones involving 3f0, which have not hitherto been reported in the literature. With increasing Reynolds number, phase-coupled interactions are found to be increasingly intermittent, and there is a significant amplitude modulation of the secondmode disturbance at frequencies significantly lower than f0. At a freestream unit Reynolds number coinciding with the loss of the tunnel's quiet-flow environment, the redistribution of the available energy in the interacting modes eventually involves frequency components throughout much of the spectrum, and there is a filling in of the valleys in between the spectral peaks as the cone boundary layer becomes intermittently turbulent. A parametric study is considered in order to determine the sensitivity of f0 to small angles-of-attack of the test article about 0o and provides a convenient measure of angle misalignment for numerical simulations.

High-Bandwidth Optical Measurements of the Second-Mode Instability in a Mach 6 Quiet Tunnel

The Mach 6 Quiet Tunnel (M6QT), the hypersonic low-disturbance facility formerly at NASA Langley, has been permanently installed as part of the Texas A&M National Aerothermochemistry Laboratory (TAMU-NAL) in College Station, Texas. The tunnel now operates interchangeably with the Adaptively Controlled Expansion Mach 5-8 tunnel (ACE) in a blowdown configuration with a maximum runtime of approximately 50 seconds. An initial series of tests to determine the quiet-flow capabilities of the nozzle indicate performance largely consistent with its former operation at NASA Langley, with evidence of a full-length quiet test core at stagnation pressures up to 145 psia. Intermittency present in the pressure signals at total pressures above 80 psia is quantified and discussed. In addition, the capability of the tunnel to support future flared-cone experimental models is demonstrated.

The Mach 6 Quiet Tunnel at Texas A&M: Quiet Flow Performance

Measurements of boundary-layer transition location and boundary layer profiles on a sharp-tipped 5o-half-angle flared cone were made in a low-disturbance Mach 6 wind tunnel. Uncalibrated boundary-layer profiles of mean and fluctuating voltage representative of mass flux are obtained using constant temperature hot-wire anemometry at several axial locations, and are notionally compared with preliminary simulations. Spectral energy content is observed between 250 and 310 kHz – the first measurements of frequencies typical of the second mode instability at Texas A&M. Growth of this high-frequency content is compared with N-factor results from linear parabolised stability equation (LPSE) computations. Possible sources of disagreement between the experimental and computed frequencies for second-mode growth are discussed, as are future improvements to the hotwire anemometry technique. Nevertheless, the successful measurement of high-frequency content highlighted here constitutes an important step toward acquisit...

Boundary-layer instability & transition on a flared cone in a Mach 6 quiet wind tunnel

Measurements of boundary-layer transition location and boundary layer profiles on a sharp-tipped 5o-half-angle flared cone were made in a low-disturbance Mach 6 wind tunnel. Uncalibrated boundary-layer profiles of mean and fluctuating voltage representative of mass flux are obtained using constant temperature hot-wire anemometry at several axial locations, and are notionally compared with preliminary simulations. Spectral energy content is observed between 250 and 310 kHz – the first measurements of frequencies typical of the second mode instability at Texas A&M. Growth of this high-frequency content is compared with N-factor results from linear parabolised stability equation (LPSE) computations. Possible sources of disagreement between the experimental and computed frequencies for second-mode growth are discussed, as are future improvements to the hotwire anemometry technique. Nevertheless, the successful measurement of high-frequency content highlighted here constitutes an important step toward acquisition of calibrated measurements of hypersonic boundary-layer instabilities to be used as code validation.

Boundary-layer instability and transition on a flared cone in a Mach 6 quiet wind tunnel

Measurements of boundary layer transition location and boundary-layer profiles on a sharp-tipped 5o-half-angle flared cone were made in a low-disturbance Mach 6 wind tunnel. Following a recent nozzle repolishing effort, an updated flow quality characterization for the facility is presented for freestream unit Reynolds numbers of 8 and 10 × 10/m. At the high Reynolds number condition, transition onset on the flared cone model was observed to occur only very near its base, consistent with the previous observations on the same test article in the quiet tunnel’s former installation at NASA Langley. Uncalibrated boundary-layer profiles of mean and rms mass flux are obtained using constant temperature hotwire anemometry at several axial locations, and are compared with available historical data. Locations of maximum fluctuating content within the boundary layer are consistent with previous experiments and expectation, although boundary-layer thicknesses are observed to be 16% higher than previously, most likely due to a slightly heated-wall condition on the cone. Several imminent improvements to the current hotwire technique are discussed. This early work represents a key milestone toward the acquisition of highly-resolved, calibrated measurements of hypersonic transitional boundary layers to be used as code validation.

Jerrod W. Hofferth

Papers

High-Bandwidth Optical Measurements of the Second-Mode Instability in a Mach 6 Quiet Tunnel

The Mach 6 Quiet Tunnel at Texas A&M: Quiet Flow Performance

Boundary-layer instability & transition on a flared cone in a Mach 6 quiet wind tunnel

Boundary-layer instability and transition on a flared cone in a Mach 6 quiet wind tunnel

Boundary-Layer Transition on a Flared Cone in the Texas A&M Mach 6 Quiet Tunnel