卷 53, 编号 1 (2018)

The results of an experimental investigation of the gasdynamic structure of supersonic underexpanded air jets flowing out of a sonic nozzle into a low-pressure medium are presented. This setting of the experimentmakes it possible to achieve high values of the nozzle-to-ambient pressure ratio at moderate outflow Reynolds numbers characteristic of underexpanded jets issuing from micronozzles. The data on the supersonic core length, the laminar-turbulent transition location, and the jet flow characteristics are obtained. The results are compared with those obtained in microjets flowing out of sonic nozzles. Emphasis is placed on the earlier discovered effect of inverse transition of a turbulent jet into the laminar flow regime with increase in the Reynolds number.

Free inertia-gravity internal waves in a two-dimensional stratified flow of an ideal fluid with a vertical velocity shear are considered in the Boussinesq approximation. The boundary-value problem for the amplitude of the vertical velocity of internal waves has complex coefficients; therefore, the wave frequency has an imaginary correction and the eigenfunction is complex. It is shown that the wave is weakly damped, the vertical wave momentum fluxes being nonzero and can be greater than the turbulent fluxes. The Stokes drift velocity component transverse to the direction of wave propagation is nonzero and less than the longitudinal component by an order of magnitude. The dispersion curves of the first two modes are cut off in the low-frequency domain due to the influence of critical layers in which the wave frequency taken with the Doppler shift is equal to the inertial frequency.

The three-dimensional problem of steady-state forced vibrations of fluid and semiinfinite ice sheet under the action of a local external load traveling along the rectilinear sheet edge at a constant velocity is considered. Two cases are analyzed. In the first case the fluid surface outside the ice sheet is free and in the second the fluid is confined by a rigid vertical wall and the ice sheet edge adjacent to the wall can be both clamped and free. The ice sheet is simulated by a thin elastic isotropic plate floating on the surface of fluid of finite depth. The load traveling velocity is assumed to be not higher than the minimum phase velocity of the flexural-gravity waves (subcritical regime). The solution to the linear problem is obtained by means of the integral Fourier transform and matching the expansions of the velocity potential in the vertical eigenfunctions. Examples of the numerical investigation of the ice sheet and fluid displacements are given.

The solution of the Navier–Stokes equations which reproduces some qualitative features of localized turbulent structures developed in circular pipes at transitional Reynolds numbers is numerically investigated. In the phase space this solution corresponds to the limiting state of the solution which evolves along the separatrix dividing the regions of attraction of the solutions corresponding to the laminar and turbulent flow regimes. Relative simplicity of the spatial and temporal behavior of the limiting solution on the separatrix makes it possible to investigate it in detail. In particular, the nonlinear mechanism of the onset of streamwise vortices responsible for sustainment of near-wall streaks whose instability ensures the presence of fluctuations is revealed.

Laminar boundary layer flow over an infinite-span, finite-length flat plate is investigated in the regime of strong interaction with a hypersonic gas flow. Under the assumption that an additional condition dependent on the transverse coordinate can be imposed on the trailing edge of the plate the flow functions are expanded in power series in the vicinity of the leading edge. It is shown that these expansions include an indefinite function dependent on the transverse coordinate. The corresponding boundary value problems are formulated and solved and the eigenvalues are determined. It is established that in this case the two-dimensional boundary layer can rearrange itself into a three-dimensional boundary layer.

The purpose of the study is to numerically, experimentally, and analytically investigate the characteristics of plasma pulsed thermal actuators (PT actuators) and to assess their possibilities in controlling flow around airfoils, wings, and configurations at large subsonic freestream velocities. For the PT actuators of the types considered the mathematical models adequately describing their effect on flow past bodies are developed. The characteristics of a prototype PT actuator are experimentally investigated on a specially developed rig. A new type of the PT actuator equippedwith a channel (PTC actuator) is proposed; it is designed to operate at a high pulse repetition frequency and at large flow velocities. Numerical investigations show that the PTC actuators are free of the essential and fundamental shortcoming of the PT actuators which consists in the working zone superheating at high pulse repetition frequencies.

The pattern of the two-dimensional vortex flow of a nonhomogeneous fluid in a confined domain is studied using two-dimensional numerical calculations. It is found that in the case of a nonhomogeneous initial density distribution the kinetic energy decay rates are proportional to the square root of viscosity at the active stage of flow restructuring. The correlation functions of the velocity and the density are derived for different moments of time in the inertial range. All these results indicate the choice of the two-dimensional turbulence development scenario in a nonhomogeneous fluid.

The two-dimensional time-dependent problem of rarefied gas flow in a plane channel, formed by parallel plates of finite length and closed at one end, is solved on the basis of the kinetic S-model. The flow develops as a result of rupture of a diaphragm which separates the gas at rest in the channel and the gas at rest in a reservoir of infinite volume. The effect of gas deceleration at the channel walls under the conditions of diffuse molecular reflection from the channel walls and end face is studied. Decay of a shock wave and disappearance of a homogeneous flow zone behind the shock wave is traced for three variants of conditions at the channel inlet: (1) gas enters the channel from a reservoir of infinite length and width (as the basic variant), the simultaneous motion in the reservoir and channel being studied; (2) the high-pressure reservoir represents a usual channel section; and (3) the motion of the gas in the reservoir is not considered at all, instead of this, the boundary conditions of the evaporation-condensation type under the conditions of gas at rest in the reservoir are imposed in the inlet cross-section.

An experimental study of supersonic flow over two-dimensional surface-mounted prisms is carried out in a Mach 3 low-noise wind tunnel. The noise level of this supersonic wind tunnel, defined as the root mean-square Pitot pressure fluctuation normalized by the mean Pitot pressure, can be reduced to about 0.37%. The nanotracer planar laser scattering (NPLS) technique is used to analyze the influence of the prism geometry and the oncoming flow conditions on the typical flow structures including separation and reattachment shocks. With increase in the prism height the induced shocks move upstream. At a constant streamwise length L of a prism the timeaveraged NPLS images show that the length of the downstream recirculation region increases from 0.8L to 1.2L, when the prism height H changes from 3 to 5 mm. As compared with the flow structures occurring downstream of the prisms, the upstream flow structures are more susceptible to the oncoming boundary layer and are considerably different in laminar and turbulent flows. The separation shock wave is clearly visible in turbulent flow even for the 1-mm prism, whereas in the case of laminar flow there is no a distinct shock wave upstream of this prism. At the same time, the location of the flow reattachment and the angle of the reattachment shock wave in the downstream flow remain almost the same in both two flow regimes.