A. A. Maslov

Stabilization of a hypersonic boundary layer using an ultrasonically absorptive coating

Experimental and theoretical studies of the effect of an ultrasonically absorptive coating (UAC) on hypersonic boundary-layer stability are described. A thin coating of fibrous absorbent material (felt metal) was selected as a prototype of a practical UAC. Experiments were performed in the Mach 6 wind tunnel on a

Leading-edge receptivity of a hypersonic boundary layer on a flat plate

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Nonlinear Aspects of Hypersonic Boundary-Layer Stability on a Porous Surface

half-angle sharp cone whose longitudinal half-surface was solid and other half-surface was covered by a porous coating. Hot-wire measurements of ‘natural’ disturbances and artificially excited wave packets were conducted on both solid and porous surfaces. Stability analysis of the UAC effect on two- and three-dimensional disturbances showed that the porous coating strongly stabilizes the second mode and marginally destabilizes the first mode. These results are in qualitative agreement with the experimental data for natural disturbances. The theoretical predictions are in good quantitative agreement with the stability measurements for artificially excited wave packets associated with the second mode. Stability calculations for the cooled wall case showed the feasibility of achieving a dramatic increase of the laminar run using a thin porous coating of random microstructure.

Evolution of nonlinear processes in a hypersonic boundary layer on a sharp cone

Experimental investigations of the boundary layer receptivity, on the sharp leading edge of a at plate, to acoustic waves induced by two-dimensional and three- dimensional perturbers, have been performed for a free-stream Mach number M ∞ = 5.92. The fields of controlled free-stream disturbances were studied. It was shown that two-dimensional and three-dimensional perturbers radiate acoustic waves and that these perturbers present a set of harmonic motionless sources and moving sources with constant amplitude. The disturbances excited in the boundary layer were measured. It was found that acoustic waves impinging on the leading edge generate Tollmien–Schlichting waves in the boundary layer. The receptivity coefficients were obtained for several radiation conditions and intensities. It was shown that there is a dependence of receptivity coefficients on the wave inclination angles.

Influence of a counterflow plasma jet on supersonic blunt-body pressures

The nonlinear aspects of the stabilization of the second-mode disturbance using a passive, ultrasonically absorptive coating (UAC) of regular microstructure are studied using bispectral analysis. The experimental data consist of hot-wire measurements made in artificially excited wave packets that are introduced into the hypersonic boundary layer on both solid and porous surfaces. The bispectral measurements show that the subharmonic and harmonic resonances of the second mode are significantly modified. The harmonic resonance, which is quite pronounced in the latter stages of the hypersonic boundary layer on solid surfaces, is completely absent on the porous surface. The degree of nonlinear phase locking that is associated with the subharmonic resonance and identified on the solid surface is substantially weakened on the porous surface. This nonlinear interaction persists farther downstream on theporous surface than on the solid surface; however, unlike on the solid surface, there are no strongly preferred interaction modes. The spectral measurements, made in previous work, show that the first mode is moderately destabilized on the porous surface. The bispectral measurements presented here identify a nonlinear interaction that is associated with the destabilized first mode; however, this is observed to be a very weak nonlinear interaction that has no deleterious effect on the performance of the UAC.

Stabilization of a Hypersonic Boundary Layer Using a Wavy Surface

Nonlinear processes in a hypersonic boundary layer on a sharp cone are considered using the bicoherence method. The experiments are performed for a Mach number M ∞ = 5.95 with introduction of artificial wave packets at the frequency of the second mode. It is shown that the basic mechanism of nonlinear interaction at the location of the maximum r.m.s. voltage fluctuation is the subharmonic resonance; all nonlinear interactions in the maximum r.m.s. voltage fluctuation layer are related to the second mode of disturbances; nonlinear processes above and below that layer are much more intense than those in it. The effect of artificial disturbances on nonlinear interactions in the boundary layer is shown to be insignificant.

Mach 6 Boundary-Layer Stability Experiments on Sharp and Blunted Cones

Aerodynamic augmentation in the presence of a thin high-temperature onboard plasma jet directed upstream of a slightly blunted cone was studied experimentally and numerically. The flow around a truncated cone cylinder at zero incidence was considered for Mach numbers M∞ = 2.0, 2.5, and 4.0. For the first time, computationally validated experimental pressure distributions over the model surface in the presence of the plasma jet were obtained. As in the conventional (nonplasma) counterflow jet, two stable operational regimes of the plasma jet were found. These were a short penetration mode and a long penetration mode (LPM) aerospike into the opposing supersonic freestream. The greatest drag reduction occurred in the moderate LPM regime. LPM strong overblowing reduces the benefits. The experimental pressure results were approximately validated against an Euler computational fluid dynamics simulation, modeling a perfect gas hot jet, counterflowing against a perfect gas supersonic freestream. Plasma effects such as electron pressure, radiation, electric field interactions, Joule heating, and induced vorticity, streamers, and plasmoids have been identified that, if accounted for, may improve the comparison. Procedures for the use of these experimental results have been outlined as a baseline that will be useful in separating fluid dynamic/thermal effects from plasma processes in understanding the physics of onboard plasma jets for aerodynamic augmentation.

Hypersonic Laminar Flow Control Using a Porous Coating of Random Microstructure

Stability of a supersonic near-wall flow over a shallow grooved plate in the freestream of Mach 6 is investigated by means of numerical simulations and wind-tunnel experiments. Numerical solutions of two-dimensional Navier–Stokes equations are used to model propagation of artificial disturbances of several fixed frequencies generated by an actuator placed on the wall. It is shown that the high-frequency forcing excites unstable waves in the flat-plate boundary layer. These waves are relevant to the second-mode instability. The wavy wall damps the disturbances in a high-frequency band while it enhances them at lower frequencies. Stability experiments are conducted in the Institute of Theoretical and Applied Mechanics Tranzit-M shock tunnel under natural freestream conditions. The measured disturbance spectra are similar to those predicted numerically. They contain a peak associated with the second-mode instability. This peak is damped by the wavy wall, while a marginal increase of the disturbance amplitude...

An experimental study of density waves in a hypersonic shock layer on a flat plate

Recent studies of hypersonic boundary-layer stability and transition on cones with sharp and blunted nosetips are presented. The experiments were carried out on a 7-deg half-angle cone at freestream Mach number 5.95. Laminar‐turbulent transition locations are measured for various flow parameters and model nose bluntness. Mean and fluctuation characteristics of the flow are obtained using constant-temperature hot-wire anemometry. The spectral content and amplification rates of natural disturbances are obtained. The method of artificial wave packets is applied to obtain detailed information on the disturbances. Data on development of both natural and artificial finite-amplitude disturbances are compared. It is experimentally shown that the wave vector of the most unstable waves of the first mode have an inclination angle of 40‐49 deg. In the frequency range of the second mode, plane waves appear to be dominant and have the highest amplification. The bluntness of the cone nosetip results in an increase of the disturbance amplification rate downstream of the entropy layer swallowing point. At the same time, nose bluntness dramatically increases the transition Reynolds number because of the strong damping of initial disturbances.

Nonlinear Mechanisms of the Initial Stage of the Laminar–Turbulent Transition at Hypersonic Velocities

It is shown that a passive porous coating of random structure (felt metal) significantly delays transition on a sharp cone at zero angle of attack in the Mach=12 wind tunnel. A semi-empirical method is developed to predict acoustic properties of randomly structured coatings including effects of gas rarefaction. This method simplifies calculations of the boundary conditions on the porous coating and solving the boundary-layer stability problem. The transition onset points on coated and uncoated cone surfaces are calculated using the -method. With this approach theoretical predictions agree satisfactorily with the experimental data. For the first time it is demonstrated that porous coatings of random microstructure, which are synergistic with fiber-ceramic thermal protection systems (TPS), can be used for hypersonic laminar-flow control. This provides symbiotic reduction of aeroheating and reduced skin friction drag. It also leads to a new family of lightweight TPS. N e