Mack-mode waves are measured in a hypersonic boundary layer using high-frequency focusing schlieren deflectometry. Experiments are performed using a $5^{\circ }$ flared cone at $0^{\circ }$ angle of attack in the low-disturbance Mach 6 Quiet Tunnel at Texas A&M University across a free-stream unit Reynolds number range of $7.8\times 10^{6}~\text{m}^{-1}\leqslant \mathit{Re}^{\prime }\leqslant 11.0\times 10^{6}~\text{m}^{-1}$. The high-frequency response of the measurement system allows harmonics and other nonlinear behaviour to be measured. Mack-mode waves and several harmonics are clearly observed at a frequency of $f_{0}\approx 250~\text{kHz}$. Bispectral analysis is used to show that these waves undergo several quadratic phase-coupled sum and difference interactions with themselves to produce harmonics, as well interact with a relatively low-frequency wave that results in amplitude modulation. Bispectral analysis is used to highlight these interactions.

No abstract

A 2:1 elliptic cone model was tested in a variable-Mach-number conventional hypersonic wind tunnel. Freestream Reynolds number, Mach number, model streamwise location, and model wall temperature were all varied to ascertain the effect of each on measured disturbances. A low-frequency disturbance was observed at Mach 5.8. It experienced some growth in excess of the increasing freestream pressure and noise for a narrow Reynolds number range. Disturbance properties were similar to what was measured in another conventional hypersonic wind tunnel. For Mach 6.5 and 7, there was no evidence of traveling crossflow waves. However, higher-frequency disturbances were observed. These disturbances were nearly two-dimensional and had phase speeds near the expected edge velocity. It is possible that these disturbances were second mode waves. None of the measured disturbances corresponded to any feature of the freestream Pitot spectra.

The spatiotemporal structure of a millimetric annular dielectric barrier discharge plasma actuator is investigated using a photomultiplier tube, a high-sensitivity camera, particle image velocimetry, and electrohydrodynamics simulations. Plasma actuators have typically demonstrated their utility in flow separation control, but on a millimetric scale they have also shown to be promising in the control of crossflow instabilities in crossflow-dominated laminar-turbulent boundary-layer transition. In view of the subtleties associated with creating an initial disturbance to excite subcritical wavelengths, it is desirable to characterize the local plasma discharge structure, body force organization, and induced velocity field in detail. The results show that, similar to their linear centimetric counterpart, the plasma discharge has a highly dynamic and somewhat organized spatiotemporal structure. Under quiescent flow conditions, the actuator induces a velocity field that consists of two counter-rotating vortices, accompanied by a wall-normal synthetic jet region, which in three-dimensions describes a toroidal vortex around the aperture's periphery. The surprising result, however, is that these vortices rotate in the opposite direction to vortices generated by similar centimetric annular designs. Three-dimensional electrohydrodynamics simulations correctly reproduce this behavior. Because the body force organization may be qualitatively perceived as being the axisymmetric counterpart of the more classical linear actuator, this flow reversal is thought to be due to the actuator scale. When an array of millimetric actuators is considered in close proximity, an interaction takes place between the vortices created from each actuator and those of neighboring actuators, resulting in a significant reduction in vortex size compared with the single aperture case, accompanied by an increase in the maximum induced flow velocity magnitude.