Home – Home – Jornals – Thermophysics and Aeromechanics 2002 number 2

A review of most popular direct and indirect methods of skin friction measurement and their brief characteristic are given. A comparative analysis of measurement results is presented; the results were obtained using some of the methods in an incompressible turbulent boundary layer on a flat plate under conditions of the formation of a negative (favorable) streamwise pressure gradient on the plate surface and in a gradient-free flow. The degree of effectiveness of the methods under the conditions described is evaluated, and the limits of their applicability and limitations are shown.

In the first part of this work [1], results of an experimental study of swept-wing boundary layer receptivity to localized wing-surface vibrations were reported, which, in particular, made it possible to obtain complex values of vibrational receptivity coefficients for the swept-wing flow as functions of spanwise wavenumber for six disturbance frequencies. In the present part of the work we present a detailed analysis of the frequency dependence of all disturbance characteristics obtained previously. By extrapolating the experimental data to the zero frequency, we obtained quantitative estimates of characteristics of swept-wing boundary layer receptivity to stationary surface roughness. The validity of this approach is substantiated.

Wind tunnel data on a vortex shedding at laminar boundary-layer separation behind a backward-facing step on a flat plate are reported. The periodic vortices induced by controlled flow excitation in the region of reattachment are dealt with. Amplitude and phase characteristics of the velocity oscillations at the vortex-shedding frequency are determined and compared with those of the short-wave convective perturbations growing behind the point of separation. The results support the conclusion inferred earlier from experimental data on natural vortex motion that shedding of the vortices and amplification of the wavy disturbances in the separated shear layer may be essentially different instability phenomena.

A qualitative difference between dynamics of the inner regions of vortex pairs with one-way and opposite rotation of elements is shown experimentally. Typical spatial scales of vortex clusters with two elements and velocity distribution inside them were determined. Experiments were carried out in the air jets using hot-wire probe equipment with a block for registration of the evolution phase of nonstationary vortex structures. It was shown that the well-known formulas for dipole and quadrupole vortices may be used for description of velocity distribution.

A flow on the windward side of a rectangular wing at high angle of attack was visualized using liquid-crystalline coatings. It was discovered that at an increase in a degree of flow turbulence, stationary longitudinal structures, whose number depends on the flow regime, appear above the wing surface.

The results of numerical computations are presented for three-dimensional supersonic flow around the lifting elements of flying vehicles on the basis of the Euler equations. The flow around different configurations of the lifting body of the ELAC 1 type (M_∞ = 2.02; 4.04, -3

The waveriders are considered, which are `designed on the basis of the flows behind the planar shocks whose compression surface is described with the aid of power functions and polynomials. The possibilities are analysed for such a specification of the surfaces, and the effect of the contour parameters is studied, which determine the cross section shape, on the lift-to-drag ratio and the integral heat fluxes for a given body temperature. The results of a comparison of the optimal (with respect to the lift-to-drag ratio) configurations of waveriders for different laws of the description of surfaces are presented.

The method of direct simulation of Monte Carlo (DSMC) was applied for a study of axisymmetric outflow of monoatomic gas into vacuum from a short-pulse source for the range of Knudsen numbers which provides a flow pattern with transitional and free-molecular modes. This problem statement is the approximate model for laser ablation of a solid target into vacuum after impact of a short laser pulse of moderate intensity. The stages of outflow and dispersion were considered for corresponding states of the source turn-on and turn-off. The main emphasis was put on the study of flow structure, spatial and temporal evolution of gas dynamic parameters and laws of translational relaxation.

The paper continues the study of gasdynamic track in supersonic heterogeneous setups using a mathematical model, which was initiated in [1 - 4]. The effect of combustion gas pressure in the prechamber p₀₀  , on the kinetic energy of particles W_pw  bombarding the bottom of erosion-produced cavity was analysed. The top levels of pressure p^*₀₀  were calculated: the further pressure growth does not bring a gain in W^*_pw  for the case of a target placed at a fixed distance from the nozzle exit. The criterial relationship was formulated for limiting kinetic energy flux as a function of the nozzle-target distance and the dimensionless depth of cavity.

A dispersion equation for the growth rate of small disturbances was obtained, and 1D stability was analysed for different regimes of stretched flame. In contrast to the classical diffusion-thermal instability occurring only at high Lewis numbers, this new type of instability may take place for Lewis numbers either higher or lower than one. Flame pulsations take place at moderate values of velocity gradient; there are no pulsations at low or high values of velocity gradient. The diagram of pulsation instability was obtained. Analytical results are in agreement with results of numerical simulation study [11] and help to explain experimental data [19] obtained under microgravity conditions.

A new family of the exact solutions of the Navier — Stokes equations for incompressible fluid is presented in the work. Within its framework, the problems of the fluid flow over one plane or between two infinite solid planes moving and rotating arbitrarily while remaining orthogonal to a fixed direction can be considered. The found family of the solutions includes as special cases two well-known families of the exact solutions of the Navier — Stokes equations: Karman

Transient free convection flow of a viscous fluid between two infinite vertical parallel plates is investigated. The temperature at one of the plates is assumed to be Td, at another plate the heat flux is assumed to be constant. The velocity and temperature distributions are found using the Laplace transform technique and are shown for small values of time t, because the initial stage of the unsteady process corresponding to the one-dimensional heat-transfer mechanism is under investigation.

Formation of temperature fields and radiation fluxes at melting and solidification of a semitransparent medium was simulated with consideration of optical properties of material dependency on radiation frequency. Two different models of rectangular bands for the absorption factor were considered.

The probes for pressure measurement are usually mounted at the butt end of a narrow duct adhering to the tested volume. A question of the agreement between the pressure measured in this way and the true pressure in the volume is considered. The problem is solved for short ducts within the framework of the Euler equations. The disturbance is created in the tested volume by an instant local release of energy. The MacCormack method is used for the solution together with a grid procedure, which enables us to introduce in some subregions a grid with the cell sizes less by an order. This technique does not produce oscillations and nonphysical waves. A considerable excess of the mean pressure near the narrow duct bottom over the mean pressure in the volume has been revealed, which is related to the effect of anomalous aerodynamic heating. The dispersion of gas density and temperature in the tested volume is very significant during a long time. The research results cast doubt on the possibility of using only one pressure measured with the aid of a probe in a narrow duct to determine the flow parameters in hot-shot wind tunnels and in the ducts of aggregates with rapid processes.

On the basis of the equations of magnetogasdynamics the characteristics of an open high-current arc in argon at atmospheric pressure were computed versus the interelectrode distance, current strength, and geometry of electrodes. The numerical results agree satisfactorily with experiment.