Vol 54, No 6 (2016)

The stability of relatively small perturbations of a plane stationary interface between a two-phase thundercloud and a humid turbulent atmosphere has been analyzed with regard to medium viscosity. It has been indicated that two mechanisms could result in medium interface instability. The criteria characterizing the instability origination conditions have been obtained. It has been qualitatively indicated how the development of the instability could result in the formation of “a trunk” that forms a tornado funnel, if viscosity is taken into account. The obtained relationships make it possible to identify critical atmospheric conditions under which a tornado funnel can be formed. The results can be used as an initial state in the numerical calculations of the flow characteristics within a tornado funnel and during laser and microwave sounding used to analyze an electromagnetic signal reflected from the mother cloud surface.

Implementation of plasma technologies widely introduced into modern industrial processes requires plasma generators with a high efficiency, great economy, and long lifetime. The last requirement is caused by the necessity of an increase in the lifetime of the most easily worn plasmatron elements, i.e., the electrodes and, primarily, their cathode units. Thermoemission cathodes made of tungsten, the most refractory metal with the melting point T_m = 3695 K, are widely exploited in high-current (I = 300–1000 A) plasmatrons operating in oxygen-free media (inert gases like nitrogen and hydrogen). By the middle of the 1980s, due to the discovery and application of the phenomenon of the recirculation of electrode material in the cathode zone, erosion of the cooled thermocathode was successfully reduced to G = 10^–10 g/C. The present study performed by means of a plasmatron with a self-adjusting arc length concerns important problems, such as measurement of the emission current, determination of cathode material erosion, and analysis of means of enhancement of the thermoemission cathode lifetime. Through the investigations performed and the resource tests of the above-mentioned plasmatron type with currents of 300–500 A, with nitrogen as the working gas, the requirements for the cathode and the plasmatron operating regimes providing for its low erosion losses, G ≤ 10^–10 g/C, at a cathode surface temperature close to its melting point are determined. The experimentally obtained densities of the electron emission currents exceed, by an order of magnitude, those calculated according to the Richardson–Dushman theory with the Schottky correction and with account for the photoemission on the cathode under the action of the resonance emission generated by the positive column of the arc. In that connection, attention is paid to the mechanism of the anomaly electron emission proposed by S.V. Lebedev and caused by occurrence of the Frenkel defect accompanied by crystal lattice deformation, Fermi energy increase, and the respective decrease in the electron-from-metal work function. The obtained estimates of the Frenkel defect concentration in tungsten at the premelting temperatures are a cogent argument in favor of the anomaly electron emission concept.

A resistively heated emissive probe has been developed to work in low-pressure air plasma produced by 100 Hz pulsed DC source. The evolution of the discharge and consequent rapid changes in plasma potential and electron temperature are characterized for different fill pressures at constant input voltage of 300 V. The floating point method in the strong emission regime is applied to determine the plasma potential. Emissive probe responds to rapid changes in the discharge current during different stages of the pulse cycle. The electron temperature is determined from the potential difference of hot probe in the strong emission regime and the cold one incorporating the space charge effects of the hot probe. Temporal measurements of V_p and T_e describe the development and characteristics of the emissive probe technique for fast measurements in pulsating discharges.

Using the earlier proposed relations for the viscosity coefficient and internal energy of a thermodynamic system, a simple unified few-parameter equation for describing the viscosity coefficient of argon, nitrogen, and carbon dioxide in a wide range of the state parameters has been derived.

Experimental values are presented for the 1-chloroalkanes viscosity within the temperature range of 253.15–423.15 K, obtained by means of standard quartz viscometer. According to investigation results, grounded suppositions and generalizations are made concerning temperature dependence characteristics of the dynamic viscosity of 1-chloroalkanes.

An analytical solution to the problem of heating anisotropic half-space by the environment with a spatially and temporally variable temperature (boundary conditions of the third kind at an anisotropic body) has been obtained for the first time based on the construction of the boundary influence function (the Green’s function), which is determined using the Fourier and Laplace integral transforms. Nonstationary temperature fields in anisotropic blunt bodies have been found under the conditions of aero-gas-dynamic heating of hypersonic aircrafts with different heat-transfer coefficients and incoming-flow temperatures. The solution obtained is recommended for determining the state of thermal protection fabricated from composites which are generally anisotropic.

On the basis of the Navier–Stokes equations, a parametric study of the influence of various factors on the development of initial perturbations caused by starting an air-blown annular nozzle is performed for the laminar model of flow. Flow regimes are found in which the start perturbations accompanying the initiation of the nozzle do not attenuate but pass to a quasi-periodic mode. The frequency Fourier spectrum of pressure fluctuations at the center of the thrust wall and the value of the thrust of the nozzle are determined. Typical pulsed pressure signals obtained in the calculation model and detected in the experiments are presented.

On the basis of theoretical and known experimental results, an improved mathematical model of thermochemical destruction of a multilayer thermal-protective coating is presented. Taking into account the heat transfer across the body made it possible to predict with a higher accuracy the state of a protected structure in fire conditions. The numerical results are compared with known experimental data.

The behavior of spherical particles with various geometric and physical parameters is experimentally investigated at the stimulated longitudinal gas oscillations in closed and opened tubes as well as in the external wave field near subharmonic resonances. The temporal dependences of the oscillating particle coordinate are obtained for different tube lengths and excitement frequencies. It is shown that, inside the tube, a particle moves, performing the longitudinal oscillations, from the opened end to the piston. Outside the tube, a particle moves from the opened end to the external wave field, without oscillations and with nonlinear coordinate increase in time. Also investigated is the influence of the particle weight and diameter and of the gas excitement frequency on the oscillation amplitude of the particle and its average velocity. The nonmonotone character revealed the dependence of the average velocity of the spherical particle on the gaseous column oscillation frequency at passing through the subharmonic resonance frequencies.

The experimental data on studying the tangential swirling injection effect on the effectiveness of a conical diffuser with an aperture angle of 18° and the expansion ratio equal to four are presented. It has been indicated that the effect becomes maximal for a given diffuser at an insignificant initial thickness of the boundary layer, when the injected jet swirling angle is 15° and the relative injection rate is 7.7%, as a result of which the pressure increases from 0.48 to 0.79 if the injected jet energy is taken into account.

Methods for determining the combustion efficiency, thermodynamics parameters of propellants, and characteristics of ramjets are analyzed. Energetic and multizone models taking into account various physical processes of incomplete combustion are considered. An approach based on comparison of the combustion efficiency evaluated by different models is developed, and recommendations on their application are given. The results and mathematical models presented can be used in numerical computational and experimental studies of the working processes in ramjets.

Complex permittivity of metals being in the two-temperature state of electron and ion sub-systems excited by the femtosecond laser pulses is measured. Diagnostics of the optical properties was performed in the sub-picosecond range, in the visible spectral range at the wavelengths of 800, 620, and 400 nm for gold, silver, nickel, and vanadium under electron sub-system heating up to 10–30 kK.

Pressure decrease is investigated at reflection, from the end of the shock tube channel, of the shock wave passing through its sand layer. The influence of the layer location on the pressure pulse extension and on the decrease of the maximal pressure amplitude behind the shock wave is studied.