Parametric instability in a weakly ionized magnetoactive plasma

The stability of a weakly ionized homogeneous plasma, situated in a weak superhigh frequency (shf) electric field and a constant magnetic field, is studied. Expressions are obtained for longitudinal wave deviation increments in the plasma, and for the threshold values of the external shf field, at which the system begins to develop instability. It is shown that the presence of the external magnetic field produces either a stabilizing or a destabilizing effect on the system, dependent on the orientation of the shf field. In particular, when the direction of the magnetic field B is perpendicular to the electric fieldE and the Langmuir electron frequency _Le= (4n_ee_e ²/m_e)^1/2 is less than the cyclotron electron frequency _e = e_eB/m_ec, the threshold value of shf electric field intensity in (Le/)³ is lower than the corresponding value for an isotropic plasma.Translated from Zhumal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 12–16, September–October, 1971.In conclusion, the author thanks A. A. Rukhadze for his interest in the study.     

Structure of a steady-state ionization front in the plasma accelerator channel

A theoretical approach to studying ionizable gas flows with the formation of an ionization front in the plasma accelerator channel is proposed. The study is based on the MHD equations supplemented with the ionization and recombination kinetics equation. As a result, the structure of an ionization front is investigated.     

Flow instability in baffled channel flow

Flow instability in baffled channel flow, where thin baffles are mounted on both channel walls periodically in the direction of the main flow, has been numerically investigated. The geometry considered here can be regarded as a simple model for finned heat exchangers. The aim of this investigation is to understand how baffle interval (L) and Reynolds number (Re) influence the flow instability. With a fixed baffle length of one quarter of channel height (H), ratios of baffle interval to channel height (R_B = L/H) between 1 and 4 are considered. The critical Reynolds number of the primary instability, a Hopf bifurcation from steady flow to time-periodic flow, turned out to be minimum when R_B = 3.08. The friction factor (f) is strongly correlated with the critical Reynolds number for R_B ⩽ 2.5. For the particular cases of R_B = 1.456 and R_B = 1.0, we performed Floquet stability analysis in order to study the secondary instability through which time-periodic two-dimensional flow bifurcates into three-dimensional flow. The results obtained in this investigation are in good agreement with those computed from full simulations, and shed light on understanding and controlling flow characteristics in a finned heat exchanger, quite beneficial to its design.     

Experiments on the linear instability of flow in a wavy channel

The effects of wall corrugation on the stability of wall-bounded shear flows have been examined experimentally in plane channel flows. One of the channel walls has been modified by introduction of the wavy wall model with the amplitude of 4% of the channel half height and the wave number of 1.02. The experiment is focused on the two-dimensional travelling wave instability and the results are compared with the theory [J.M. Floryan, Two-dimensional instability of flow in a rough channel, Phys. Fluids 17 (2005) 044101 (also: Rept. ESFD-1/2003, Dept. of Mechanical and Materials Engineering, The University of Western Ontario, London, Ontario, Canada, 2003)]. It is shown that the flow is destabilized by the wall corrugation at subcritical Reynolds numbers below 5772, as predicted by the theory. For the present corrugation geometry, the critical Reynolds number is decreased down to about 4000. The spatial growth rates, the disturbance wave numbers and the distribution of disturbance amplitude measured over such wavy wall also agree well with the theoretical results.     

Effect of dissipative processes on compression flows in a plasma accelerator channel

Plasma flows in coaxial channels with a truncated central electrode are accompanied by compression and heating of the plasma on the channel axis [1–4]. Such flows were calculated in [1, 4] within the framework of a simple MHD model and by simple numerical methods and, accordingly, the results reflect only the basic qualitative characteristics of compression flows. Below, these flows are investigated in greater detail on the basis of a more accurate physical model with allowance for the finite conductivity, heat conduction and radiation of the plasma and impurities. The cases of anisotropic and classical isotropic heat conduction are considered. The numerical method employed is based on two finite-difference schemes: SHASTA-FCT [5–7] and TVD [8, 6]. The main advantage of these methods is the high resolution of the shock waves and contact discontinuities, which is highly desirable in describing compression flows. The calculations relate to the case of a fully ionized hydrogen plasma.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 102–109, May–June, 1991.In conclusion, the author wishes to express his gratitude to K. V. Brushlinskii and A. I. Morozov for frequent discussions and to K. P. Gorshenin for the use of his calculation results.     

Interfacial instability in turbulent flow over a liquid film in a channel

We revisit the stability of a deformable interface that separates a fully-developed turbulent gas flow from a thin layer of laminar liquid. Although this problem has received considerable attention previously, a model that requires no fitting parameters and that uses a base-state profile that has been validated against experiments is, as yet, unavailable. Furthermore, the significance of wave-induced perturbations in turbulent stresses remains unclear. To address these outstanding issues, we investigate this problem and introduce a turbulent base-state velocity that requires specification of a flow rate or a pressure drop only; no adjustable parameters are necessary. This base state is validated extensively against available experimental data as well as the results of direct numerical simulations. In addition, the effect of perturbations in the turbulent stress distributions is investigated, and demonstrated to be small for cases wherein the liquid layer is thin. The detailed modelling of the liquid layer also elicits two unstable modes, ‘interfacial’ and ‘internal’, with the former being the more dominant of the two. We show that it is possible for interfacial roughness to reduce the growth rate of the interfacial mode in relation to that of the internal one, promoting the latter, to the status of most dangerous mode. Additionally, we introduce an approximate measure to distinguish between ‘slow’ and ‘fast’ waves, the latter being the case for ‘critical-layer’-induced instabilities; we demonstrate that for the parameter ranges studied, the large majority of the waves are ‘slow’. Finally, comparisons of our linear stability predictions are made with experimental data in terms of critical parameters for onset of wave-formation, wave speeds and wavelengths; these yield agreement within the bounds of experimental error.     

Mixing of hot gases in an axisymmetric channel with a pre-swirled flow

The problem of the mixing of hot turbulent gases in an axisymmetric channel with a lateral surface of arbitrary shape and a pre-swirled flow is considered. The flowfields and the temperature and concentration distributions are calculated for various inlet conditions.     

Numerical investigation of static flow instability in a low-pressure subcooled boiling channel

A three-dimensional two-fluid model to predict subcooled boiling flow at low pressure is presented. The model is adopted to investigate the two-phase flow and heat transfer characteristics in a heated channel. The presence of bubbles as a consequence of heating flow through a vertical rectangular channel has a significant effect on the overall pressure drop along the channel. Numerical results were compared against a series experimental data performed at various conditions – mass flux, heat flux, inlet temperature and exit pressure. Good agreement on the overall pressure drop was achieved. The onset of flow instability velocity was also accurately determined when compared against measurements. Predicted results of void fraction provided useful information towards a more fundamental understanding of the occurrence of onset of nucleate boiling, onset of significant voiding and onset of flow instability. The phenomenon of boiling onset oscillations was also predicted through the use of the two-fluid model.     

Longwave instability of a binary mixture flow in a vertical channel in the presence of vibration

The longwave instability of a hybrid (thermogravitational and thermovibrational) flow of a binary incompressible liquid mixture occurring in a plane vertical channel, whose boundaries are maintained at constant but different temperatures, is studied. The investigation is carried out with account for the Soret thermal-diffusion effect on the ranges of normal and anomalous values of the mixture separation coefficient. It is shown that, owing to the properties of the system, the subharmonic response to an external action is absent. The ranges on which secondary flows arise are analytically determined using the asymptotic expansion method, both under weightlessness conditions and in the presence of the gravity effect. The parameter ranges on which longwave disturbances present the greatest danger for the main flow stability are determined.     

Buoyancy driven flow of light gases in atmosphere compared to that of hot gases

The transient formation and subsequent dispersion of the plumes of a fixed mass of lighter than air gases emerging out of open cylindrical enclosures with negligible pressure difference was investigated using 3-D and 2-D CFD models. Subsequently, the dispersion into atmosphere of a similar amount of equally buoyant hot air was also considered. The structure and dynamics of the resulting thermally driven hot air plumes were compared to the corresponding characteristics of the mass-transfer driven isothermal plumes. Some cases were investigated in which the dispersing gases were both lighter than air and at a different temperature from that of the atmosphere. The similarities and differences of these double heat-mass-transfer driven problems with the other cases were discussed. It was shown that a criterion developed previously for judging the validity of the 2-D model relative to the more complex 3-D approach for mass-transfer driven problems could be equally applied for the thermally driven or double heat and mass-transfer driven plume flow characteristics.     

On the breakdown potential of highly ionized gases

Summary For practical applications of gas-discharge tubes their re-ignition characteristics are often required. The increase of the re-ignition potentialV _B after a previous discharge depends upon the rates of deionization and cooling of the discharge path during the time of repose . The author obtainedV _B - curves of TR-tubes by impulse voltage on a Braun-tube screen. He also made some numerical calculations and explained the characteristic feature of theV _B - curves as well as the breakdown process manifested therein.The writer wishes to express his cordial thanks to Mr. H. Tamagawa for his help during the experiments and to various colleagues for helpful discussions. Part of this work was supported by the Grant in Aid for Fundamental Scientific Research of the Ministry of Education.     

Two-dimensional subcritical and supercritical open channel flow calculation using a time-marching method

A time-marching method is presented for the calculation of two-dimensional, high-speed channel flow, including the usually neglected terms of slope and bottom friction. Time-marching methods are potentially the most flexible means of calculating flow through geometrically complex channel passages, since they can readily deal with subcritical and supercritical flows. The adopted numerical scheme comes straight from gas flow computations in turbomachines. The flow is assumed to be fully mixed in the vertical direction, so that vertical variations may be neglected. Comparisons with other numerical solutions for various open channel configurations show that the proposed approach is a comparatively accurate, reliable and fast technique. It can be utilized for open channel designs.