Instability and Receptivity of Laminar Wall Jets

Results of eigenvalue analysis based on global and local eigenvalue considerations are presented. A collocation method with the Chebyshev polynomial approximation has been used for the global eigenvalue analysis. The results explain the appearance of a second unstable mode. In the case of real frequencies with Reynolds number R < 381 there is only one unstable mode. This mode coalesces at R≈ 381 with a stable mode. At R > 381 they become separated by interchannging their branches, then the second unstable mode occurs. The receptivity problem has been considered with respect to perturbations emanating from a wall. The results illustrate that high-frequency modes have a stronger response than low-frequency modes. It is shown that the method of expansion in a biorthogonal eigenfunction system and the method used by Ashpis and Reshotko are equivalent with regard to the receptivity problem solution. Received: 29 March 1996 and accepted 26 November 1996     

Laminar convection with an absorbing and emitting gas

The heat transfer and fluid flow are determined for fully developed laminar flow in a vertical channel. The effects of natural and forced convection and thermal radiation are included in the analysis. The spectral variation of the radiation properties are included through the use of the total band absorptance. The analysis is general and specific numerical results have been obtained for the 15 micron band of carbon dioxide. A simplified approach for large path is also presented which is in excellent agreement with the more exact results.     

Effect of Condensation on the Length of Strongly Underexpanded Jets Exhausting Into a Rarefied Submerged Space

Exhaustion of supersonic argon and nitrogen jets through sonic and supersonic nozzles into a rarefied submerged space at high stagnation pressures is studied experimentally. The shapes and lengths of the jets are visualized by means of detecting radiation excited in the considered flow by an electron beam. Dependences of the geometric parameters of the jets on exhaustion and clusterization conditions at low Reynolds numbers based on the reference length of the jet are obtained. It is found that the coefficient of proportionality between the length of the first “barrel” of the supersonic jet and the degree of jet expansion increases with an increase in the stagnation pressure. Empirical dependences of the proportionality coefficient on the size of clusters formed in supersonic flows are derived for the first time.     

附面层中马蹄形旋涡的实验研究

本文用烟流显示和热线方法研究附面层中马蹄形旋涡的形态、生成频率、配对和涡系之间的相互作用。     

A Numerical Investigation on the Laminar Boundary Flow Layer with Transpiration Cooling

Heat transfer characteristics in the laminar boundary layer with transpiration cooling function are numerically analyzed by an integral method. The effects of coolant injection ratio, and the Re and Pr numbers of the exterior hot flow on the temperature at porous plate surface are discussed. The numerical results and discussions indicate that the surface temperature falls with an increase of coolant injection ratio, the temperature distribution on the surface is not uniform, and the effects of the Re number under lower Pr number condition are distinctly different to that under the higher Pr number condition.     

Investigation of Different Condensation Regimes in Isobaric Turbulent Air-Steam Jets

Condensation in axisymmetric turbulent air-steam jets is studied theoretically and experimentally under bench experiment conditions in which a hot mist jet is injected from a nozzle into air. On the basis of the physico-mathematical model developed, four problems are considered: homogeneous condensation in the jet at a fairly low ambient air temperature, heterogeneous condensation on particles introduced into the jet at the nozzle outlet, heterogeneous condensation on particles ejected into the jet from the surrounding space, and condensation on ions entering the jet from a corona point on the flow axis. The local characteristics of the dispersed phase (mean particle size, standard deviation of the particle size, particle number and volume concentrations) and its integral characteristics (coefficient of vapor conversion into condensed phase and the optical thickness of the jet in different sections) are determined. The calculation results are compared with experimental data. As an application of the model developed, the characteristics of heterogeneous condensation in the jets of certain modern aircraft engines (IL-96-300, Tu-204, MiG-29, Boeing-707) are found on the assumption that the condensation occurs on particles entering the jet at the nozzle outlet and the particle growth rate in all stages (including the initial stage of particle irrigation) coincides with the growth rate of liquid drops.     

Laminar flow through porous channels with an applied magnetic field

Summary The two-dimensional steady flow of an incompressible and electrically conducting viscous fluid through a porous channel with a transverse magnetic field is discussed. It is assumed that there is constant suction at one wall and constant blowing at the other wall. A perturbation solution is obtained where the perturbation parameter is equal to the difference between the two wall velocities. The behaviour of the solution for various suction Reynolds numbers and magnetic Reynolds number is considered. Finally, the skin friction at one wall is given and is found to increase with the increase of the magnetic field.     

群沙射流结构的实验研究

本文以90－150μm的标准细沙为研究对象，利用超声波速度剖面测量仪和数码摄录机进行了垂直入水群沙射流的实验研究，给出了的群沙入水后的运动规律。我们发现，群沙湍射入水的一霎那，底部瞬时产生反射波，由它带动的流速与群沙入水带动的流速同量级。     

Interaction Between a Time-Dependent Laminar Boundary Layer and an Inviscid Subsonic Flow Near a Region of Local Energy Supply

The results of numerical modeling of the time-dependent flows of a viscous heat-conducting gas occurring in the region of interaction between an external inviscid flow and a laminar boundary layer near a zone of local energy supply at high subcritical Reynolds numbers are presented. The solution of the Navier-Stokes equations is constructed on the basis of the method of matched asymptotic expansions. Numerical solutions of the nonlinear boundary-value problem describing the flow in the wall region of the boundary layer are given in similarity variables. It is shown that time- and space-localized energy supply results in the formation of a self-consistent flow disturbance, whose downstream propagation is accompanied by a disturbance amplitude growth during a short time interval, even after the energy supply has stopped. Calculations of the flows induced by two heat sources placed in tandem make it possible to conclude that the time lag for the second energy supply zone and the distance between the sources can be so chosen that superposition of the disturbances induced by the first and second sources leads, due to nonlinear effects, to a considerable increase in the amplitude of the total flow disturbance.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, 2005, pp. 64–75.Original Russian Text Copyright © 2005 by Kazakov.     

Characteristics of Sonic Jets with Tabs

The results of an experimental investigation on the effect of a vortex generator in the form of a mechanical tab placed at the nozzle exit on the evolution of jet and its decay are reported in this paper. Jets from a sonic nozzle with and without tabs operated at nozzle pressure ratios from 2 to 7 were studied. Tabs with two combinations of length-to-width ratio were investigated by keeping the blockage area constant. The tabs offered a blockage of 10.18% of the nozzle exit area. The centerline pitot pressure decay shows that for the tabbed jet a maximum core reduction of about 75% can be achieved at a nozzle pressure ratio (NPR) 7 compared to an uncontrolled jet. The shadowgraph pictures show that the tabs drastically weaken the shock structure in the jet core and disperse the supersonic zone of the flow making them occupy a greater zone of the flow field compared to the plain nozzle. This causes the waves to become weaker and the jet to spread faster. The tabs are found to shed counter-rotating vortices all along the edges, resulting in enhanced mixing. Isobaric contours reveal that the streamwise vortices cause an inward indentation of the entrained mass into the jet core and an outward ejection of core flow. To understand the distortion introduced by tabs on the jet cross-section and its growth leading to bifurcation of the jet, a surface coating visualization method was developed and employed.     

Phenomenological Modeling of Electrorheological Fluids with an Extended Casson-Model

In this paper we propose a phenomenological theory for electrorheological fluids. In general these are suspensions which undergo dramatic changes in their material properties if they are exposed to an electric field. In the context of continuum mechanics these fluids can be modeled as non-Newtonian fluids. Recalling the governing equations of rational thermodynamics and electrodynamics of moving media (Maxwell-Minkowski-equations), we derive suitable governing equations of electrorheology using essentially two assumptions concerning magnetic quantities. Furthermore we introduce a 3-dimensional nonlinear constitutive equation for the Cauchy stress tensor which is an extension of the model proposed by Ružička (see [14]). Assuming a viscometric flow, we compare the shear stress of our model with other well known models and fit the parameters by using measurements that were obtained in a rotational viscometer. Excellent agreement between model and measurements is achieved. On the basis of these results we propose a 3-dimensional model, the so-called extended Casson -model. This model is investigated further for a channel flow configuration with a homogeneous electric field. We determine analytical solutions for the electric field, the velocity and the volumetric flow rate and illustrate the velocity profiles and the predicted pressure drop. The velocity profiles are flattened compared to parabolic profiles and become more flat if the electric field increases. Received March 21, 2000     

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

Weakly nonlinear development of waves in an axisymmetric hypersonic boundary layer is studied by the method of bispectral analysis. The type of nonlinear interaction that was not observed previously in such flows is found. The possibility of subharmonic resonance of the second mode at the nonlinear stage of transition is demonstrated. The previously discovered nonlinear generation of the harmonic of the fundamental wave of the second mode of disturbances is observed.     

含初始损伤岩石的冻融损伤试验研究

在实际工程中,岩石大多是含初始损伤的,分析含初始损伤岩石的力学性能有重要的工程意义.预先统计岩石试块的初始抗压强度,然后根据其抗压强度给岩石试块施加压力.分别施加了极限抗压强度的10％、30％、50％、70％、80％、90％,以形成不同的初始损伤.然后通过低温冻融实验和单轴压缩实验,将岩石在传统循环荷载作用后的损伤与低温冻融损伤进行比较,得出这两种损伤的异同点,建立了岩石材料在不同环境条件下的损伤表达式,研究了初始损伤对岩石冻融的影响,对今后估测含初始损伤岩石的冻融寿命有一定参考价值.     

Laminar flow through channels with porous walls and with an applied transverse magnetic field

Summary The problem of two-dimensional steady laminar flow of a viscous incompressible and electrically conducting fluid through a channel with two equally porous walls in the presence of a transverse magnetic field has been extended to include all values of Hartmann number and small suction velocity at the walls. Expressions for the velocity components, the pressure and the wall friction in terms of the Hartmann number and the suction Reynolds number are given. It is found that the pressure drop in the major flow direction and the wall friction decrease with the increase in suction and increase with the increase in the strength of the magnetic field.     

Laminar flow in a uniformly porous channel with an applied transverse magnetic field

Summary The flow of a viscous incompressible and electrically conducting fluid in a two-dimensional uniformly porous channel, having fluid sucked or injected with a constant velocity through its walls, is considered in the presence of a transverse magnetic field. A solution for small Reynolds number has been given by the authors in a previous paper. A solution valid for large suction Reynolds number and all values of Hartmann number is presented here and the resulting boundary layer is discussed. Also Yuan's solution for large negativeR is extened to include small values ofM ₂/R.Nomenclature x, y distances parallel and perpendicular to the channel walls - u, v velocity components inx, y directions - p pressure - density - U(0) entrance velocity atx=0 - V suction velocity at the wall - V velocity field - J current density - E electric field - H magnetic field - H ₀ applied magnetic field - electrical conductivity - _m magnetic permeability - 2h distance between the porous walls - kinematic viscosity - y/h - B _m H - B _{0 m}H₀ - R Vh/, Reynolds number - M _mH₀ h(/)^1/2, Hartmann number - M/R - a - b - z 1–     

Laminar boundary layer in a flow past an aerofoil with a circular cavity

The paper describes a numerical study of a method of preventing the separation of a laminar boundary layer from the forward section of a symmetric aerofoil, the flow past which does not separate at zero angle of incidence. In order to increase the maximum angle of incidence at which the flow has still not separated, a circular cavity (vortex cell) located almost completely inside the aerofoil is introduced on the segment vulnerable to separation. The asymptotics of the corresponding flow at high Reynolds number are described using the Prandtl-Batchelor model. Krasnodar. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 52–57, March–April, 1998. The work was financially supported by the International Science Foundation (grants M4K000 and M4K300) and by the Russian Foundation for Fundamental Research (project No. 96-01-01290).     

Investigation of Mixing Models and Conditional Moment Closure Applied to Autoignition of Hydrogen Jets

The present paper is focused on performing a thorough investigation of first order Conditional Moment Closure (CMC) including an inhomogeneous turbulent mixing model for the conditional scalar dissipation rate to predict autoignition. Autoignition of a hydrogen and nitrogen fuel mixture in a heated coflow of air is examined. A sensitivity analysis is proposed for the autoignition length with respect to the mixing field, as well as a comparison of the effects of the inhomogeneous turbulent and Amplitude Mapping Closure (AMC) mixing models. The choice of turbulence constants only change predicted ignition length by approximately 5 % when applied to the AMC mixing model. Predictions of ignition length performed by the inhomogeneous model are lower than that of the AMC model by up to 15 %. The current ignition predictions are in reasonable agreement with the experimental data and previous simulation results. Two of the four regimes observed experimentally are reproduced qualitatively. Further improvement may be gained by using large eddy simulation and a gradient model for the conditional velocity in the inhomogeneous turbulent mixing model.