Thermal slip of a nonuniformly heated gas along a solid planar surface

Two methods are used to study the solution of a linearized model of the Boltzmann equation in the problem of thermal slip of a nonuniformly heated gas along a solid flat wall.The first method involves analytic solution of the integral equation for the average gas velocity. In the case of purely diffuse or purely specular reflection of the molecules from the wall surface the first method makes it possible to obtain analytically two important results; namely, the average gas velocity at the surface and at a large distance from the wall. The average gas velocity profile cannot be constructed analytically with this method. The second approximate method involves expanding the distribution function into a series in Sonine polynomials in velocity space and formulation of half-space moment equations from which the correction to the distribution function is determined. This method is used to obtain a simple analytic expression for the distribution function, from which we can find the average velocity profile for the gas for any arbitrary tangential momentum accommodation coefficient. In particular cases in which analytic solution of the problem by the first method is possible, good agreement is obtained between the two computational methods.It is known that a gas in a temperature gradient field tangent to the wall must begin to move in the direction of the temperature gradient (thermal slip). The first attempt to solve the thermal slip problem was made by Maxwell [1]. In his analysis Maxwell assumed that the distribution function of the molecules incident on the wall near the surface does not differ from the bulk distribution at a large distance from the wall. As a result Maxwell obtained the following expression for the thermal slip velocity for any tangential momentum accommodation coefficient u ^*=3/4 grad lnT.Here is the kinematic viscosity.However, in the case of molecular reflection from the wall which is not purely specular, the distribution of the incident molecules in the Knudsen layer differs from the bulk distribution because of collisions with the molecules reflected from the wall. Thus, Maxwell's assumption is not valid in the general case.For the exact solution of the problem it is necessary to find the distribution function in the Knudsen layer by solving the Boltzmann equation. Several investigators have used the Grad method [2] to find the distribution function in the Knudsen layer. However, the use of Grad's method in the thermal-slip problem leads to Maxwell's result [3].The solution of the thermal-slip problem obtained by Sone [4] is more exact than the analyses noted above. A comparison of the results obtained by Sone with those of this investigation is given at the end of our paper.     

Calculation of the transport coefficients and slip velocities for molecules in the form of solid spheres

A solution of the Boltzmann equation is carried out by the Monte Carlo method for problems of rarefied gasdynamics in a linear formulation. The problems are solved by calculating the transport coefficients and slip velocities on a solid wall for molecules in the form of solid spheres. The accuracy of the method due to various parameters of the computational scheme in the solution of the problem is investigated by calculating the transport coefficients for pseudo-Maxwellian molecules.The Boltzmann kinetic equation is a complex integro-differential equation which is very difficult to solve and analyze. Hence, the solution of even one-dimensional problems and for the linearized Boltzmann equation turns out to be quite difficult, and such problems are solved by approximate methods (the expansion in Knudsen numbers, the method of moments, the expansion in series, etc. [1]). A method of solving the linearized Boltzmann equation by the Monte Carlo method is proposed in [2]. An exact solution of a number of problems of rarefied gas dynamics has been obtained by this method [3, 4]. However, the method was applied for pseudo-Maxwellian molecules, for which the collision cross section is inversely proportional to the relative velocity of the colliding particles =₀/g.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 155–158, March–April, 1971.In conclusion, the author is grateful to M. N. Kogan for formulating the problem and for great assistance provided during the research, and also to V. I. Vlasov, S. L. Gorelov and V. A. Perepukhov for assistance in compiling the program.     

Couette problem for the generalized Krook equation stress-peak effect

The Couette problem is the simplest problem of steady shear flow of rarefied gas in a region bounded by solid surfaces. This problem has been examined in the linear formulation by many authors, using either the linearized Krook equation or the moment methods (see [1]). It has recently been solved by the Monte Carlo method [2].The nonlinear problem of Couette flow with heat transfer for the Krook equation has been solved by reducing the problem to a system of integral equations [3] over a wide range of flat-plate velocities and temperature ratios and by the discrete-velocity method [4] for moderate plate velocities. In this article we solve the same problem for the generalized Krook equation [5] which approximates the Boltzmann equation for a pseudo-Maxwellian gas in accordance with the method suggested by the author [6, 7]. The generalized Krook equation was solved numerically by a modified discrete-velocity method which has been used by the author previously to solve the problem of shock wave structure [8].The primary case examined is that of pseudo-Maxwellian molecules, in which the viscosity is proportional to the temperature. The computations were made for Prandtl numbers of 1 and 2/3 over a wide range of Mach and Knudsen numbers as well as flat-plate temperature ratios. As we would expect, the Prandtl number effect is greatest for small Knudsen numbers. The flow velocity profiles are not very sensitive to variation of the Prandtl number (at least for pseudo-Maxwellian molecules).However, the most interesting result of the study is independent of the Prandtl number. Specifically, it was found that for any sufficiently high flat-plate velocities the friction stress, referred to the corresponding free molecular value, does not change monotonically with variation of the Knudsen number; instead, there is a peak. As far as the author is aware, this nonlinear effect has not been discussed previously in the literature (including articles [3, 4]).     

Macroscopic description of steady and unsteady rarefaction effects in boundary value problems of gas dynamics

Four basic flow configurations are employed to investigate steady and unsteady rarefaction effects in monatomic ideal gas flows. Internal and external flows in planar geometry, namely, viscous slip (Kramer’s problem), thermal creep, oscillatory Couette, and pulsating Poiseuille flows are considered. A characteristic feature of the selected problems is the formation of the Knudsen boundary layers, where non-Newtonian stress and non-Fourier heat conduction exist. The linearized Navier–Stokes–Fourier and regularized 13-moment equations are utilized to analytically represent the rarefaction effects in these boundary-value problems. It is shown that the regularized 13-moment system correctly estimates the structure of Knudsen layers, compared to the linearized Boltzmann equation data.     

Strong recondensation in a one-component and a two-component rarefied gas for an arbitrary value of Knudsen number

As is known, surface phenomena such as evaporation, absorption, and reflection of molecules from the surface of a body depend strongly on its temperature [1–5]. This leads to the establishment of a flow of a substance between two surfaces maintained at different temperatures (recondensation). The phenomenon of recondensation was studied in kinetic theory comparatively long ago. However, up to the present, only the case of small mass flows in a onecomponent gas has been investigated completely [3,4]. Meanwhile it is clear that by the creation of appropriate conditions we can obtain considerable flows of the recondensing substance, so that the mass-transfer rate will be of the order of the molecular thermal velocity. Such a numerical solution of the problem with strong mass flows along the normal to the surface for small Knudsen numbers for a model Boltzmann kinetic equation was obtained in [7]. In this study we numerically solve the problem of strong recondensation between two infinite parallel plates over a wide range of Knudsen numbers for a one-component and a two-component gas, on the basis of the model Boltzmann kinetic equation [6] for a one-component gas and the model Boltzmann kinetic equation for a binary mixture in the form assumed by Hamel [8], for a ratio of the plate temperatures equal to ten. We also investigate the effect of the relative plate motion on the recondensation flow.Moscow. Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 130–138, September–October, 1972.     

Solution of the linearized couette-flow problem in a rarefied gas by the integral-diffusion method

In ordinary diffusion theory the transfer of properties is determined by the local gradients of the corresponding fields. As the mean free path increases, the flux density becomes an integral quantity and is determined by a neighborhood of the point under consideration of the order of a few mean free paths. In a previous article [1], the author proposed a model for a one-dimensional transfer process in linear rarefield-gas problems based on the analogy with radiative transfer. The same approach, though without directional averaging, is used in the present paper to analyze the linearized Couette flow problem. The solution obtained here has the properties of the solution obtained by more exact methods based on the solution of the Boltzmann equation [3-4].Nomenclature p_xy shear stress - c mean thermal velocity of molecules - 2/3 A mean free path - d half-width of channel - ±w₀ plate velocity - c nonequilibriumvalue of momentum flux density - y transverse coordinate - ratio of specific heats - W dimensionless velocity - P_xy shear stress scaled with respect to the shear stress in free-molecule flow - Y dimensionless coordinate - W₁(y) velocity distribution according to Millikan's solution - coefficient of viscosity - R Reynolds number - K Knudsen number     

Asymptotic theory of the Boltzmann system,for a steady flow of a slightly rarefied gas with a finite Mach number: General theory

A steady rarefied gas flow with Mach number of the order of unity around a body or bodies is considered. The general behaviour of the gas for small Knudsen numbers is studied by asymptotic analysis of the boundary-value problem of the Boltzmann equation for a general domain. The effect of gas rarefaction (or Knudsen number) is expressed as a power series of the square root of the Knudsen number of the system. A series of fluid-dynamic type equations and their associated boundary conditions that determine the component functions of the expansion of the density, flow velocity, and temperature of the gas is obtained by the analysis. The equations up to the order of the square root of the Knudsen number do not contain non-Navier–Stokes stress and heat flow, which differs from the claim by Darrozes (in Rarefied Gas Dynamics, Academic Press, New York, 1969). The contributions up to this order, except in the Knudsen layer, are included in the system of the Navier–Stokes equations and the slip boundary conditions consisting of tangential velocity slip due to the shear of flow and temperature jump due to the temperature gradient normal to the boundary.     

Numerical simulation of shock wave propagation in microchannels using continuum and kinetic approaches

Numerical simulations of shock wave propagation in microchannels and microtubes (viscous shock tube problem) have been performed using three different approaches: the Navier–Stokes equations with the velocity slip and temperature jump boundary conditions, the statistical Direct Simulation Monte Carlo method for the Boltzmann equation, and the model kinetic Bhatnagar–Gross–Krook equation with the Shakhov equilibrium distribution function. Effects of flow rarefaction and dissipation are investigated and the results obtained with different approaches are compared. A parametric study of the problem for different Knudsen numbers and initial shock strengths is carried out using the Navier–Stokes computations.      

超薄膜磁头滑块气动力特性

采用有限差分法对广义润滑方程进行数值求解,计算出计算机磁头滑块压强场的分布情况。分析、研究了其稳态和动态气动力特性,并将计算结果分别与求解一阶、二阶修正雷诺方程所得到的结果进行了比较,得到如下三个结论:(1)当飞行高度很小,飞行速度较低时,必须采用广义润滑方程进行磁头滑块的气动力计算,(2)与广义润滑方程结果比较,求解一阶修正雷诺方程所得到的计算结果总是偏高,而求解二阶修正雷诺方程所得到的计算结果总是偏低。此外,还解决了大压缩数下数值失稳问题,使得压缩数可以计算到120万,足以适应任何实际工程的需要。     

High-altitude aerothermodynamics

Hypersonic high-altitute flight can be conventionally divided into three regimes: the continuum regime, when the Knudsen number Kne1, the free-molecule regime (Kn1), and the transitional regime (K1). In general, each of these regimes differs with respect to both the structure of the flow and the method of determining the aerodynamic and thermal characteristics. For Knudsen numbers Kne1 the Navier-Stokes equations or models with slip and temperature jump boundary conditions are widely used. When Kn1 the methods employed are mainly directed towards determining the distribution function of the molecules reflected from the surface of the body. On the transition interval between these two limiting regimes numerical methods of solving the Boltzmann equation and its model equations are being used with success. Together with the experimental techniques, these various methods, which complement each other, make it possible to investigate gas flows fairly effectively from the continuum to the free-molecule regime (see, for example, [1]).Based on a paper presented to the Fluid Mechanics Section of the Seventh Congress on Theoretical and Applied Mechanics, Moscow, August 1991.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.2, pp. 142–152, March–April, 1993.     

Forced convection gaseous slip flow in circular porous micro-channels

Laminar forced convection of gaseous slip flow in a circular micro-channel filled with porous media under local thermal equilibrium condition is studied numerically using the finite difference technique. Hydrodynamically fully developed flow is considered and the Darcy–Brinkman–Forchheimer model is used to model the flow inside the porous domain. The present study reports the effect of several operating parameters (Knudsen number (Kn), Darcy number (Da), Forchhiemer number (Γ), and modified Reynolds number ) on the velocity slip and temperature jump at the wall. Results are given in terms of the velocity distribution, temperature distribution, skin friction , and the Nusselt number (Nu). It is found that the skin friction is increased by (1) decreasing Knudsen number, (2) increasing Darcy number, and (3) decreasing Forchheimer number. Heat transfer is found to (1) decrease as the Knudsen number, or Forchheimer number increase, (2) increase as the Peclet number or Darcy number increase.     

Database for flows of binary gas mixtures through a plane microchannel

A binary mixture of rarefied gases between two parallel plates is considered. The Poiseuille flow, thermal transpiration (flow caused by a temperature gradient of the plates) and concentration-driven flow (flow caused by a gradient of concentration of the component species) are analyzed on the basis of the linearized model Boltzmann equation with the diffuse reflection boundary condition. The analyses are first performed for mixtures of virtual gases composed of the hard-sphere or Maxwell molecules and the results are compared with those of the original Boltzmann equation. Then, the analyses for noble gases (He–Ne, He–Ar and Ne–Ar) are performed assuming more realistic molecular models (the inverse power-law potential and Lennard-Jones 12,6 models). By use of the results, flux databases covering the entire ranges of the Knudsen number and of the concentration and a wide range of the temperature are constructed. The databases are prepared for the use in the fluid-dynamic model for mixtures in a stationary nonisothermal microchannel derived in [S. Takata, H. Sugimoto, S. Kosuge, Eur. J. Mech. B/Fluids 26 (2007) 155], but can also be incorporated in the generalized Reynolds equation [S. Fukui, R. Kaneko, J. Tribol. 110 (1988) 253] in the gas film lubrication theory. The databases constructed can be downloaded freely from Electronic Annex 2 in the online version of this article.     

Couette flow of a binary mixture of rigid-sphere gases described by the linearized Boltzmann equation

A concise and accurate solution to the problem of plane Couette flow for a binary mixture of rigid-sphere gases described by the linearized Boltzmann equation and general (specular-diffuse) Maxwell boundary conditions for each of the two species of gas particles is developed. An analytical version of the discrete-ordinates method is used to establish the velocity, heat-flow, and shear-stress profiles for both types of particles, as well as the particle-flow and heat-flow rates associated with each of the two species. Accurate numerical results are given for the case of a mixture of helium and argon confined between molybdenum and tantalum plates.     

Multiscale Expansion Method in the Hilbert Problem

The problem of constructing an asymptotic approximation to the solution of the kinetic Boltzmann equation is considered for the hydrodynamic region of low Knudsen numbers. The problem is linearized for one-dimensional perturbations in a gas at rest. The distribution function is sought in the form of a multiscale expansion of the Hilbert asymptotic series type. The construction of a solution uniformly suitable as t is demonstrated with reference to a particular example of sonic wave propagation. It is shown that the multiscale technique makes it possible to extend the domain of applicability of the Hilbert expansion to the entire interval of dissipative relaxation.     

Burnett simulation of flow and heat transfer in micro Couette flow using second-order slip conditions

The conventional Burnett equations with second-order velocity slip and temperature jump conditions were applied to the steady-state micro Couette flow of a Maxwellian monatomic gas. An analytical approach as well as a relaxation method was used to determine the velocity slip and temperature jump at the wall. Convergent solutions to the Burnett equations were obtained on arbitrary fine numerical grids for all Knudsen numbers (Kn) up to the limit of the equations’ validity. The Burnett equations with second-order slip conditions indicate a much better agreement with DSMC data over the first-order slip conditions at high Kn. The convergent Burnett solutions were obtained in orders of magnitude quicker than that with the corresponding DSMC simulation. The augmented Burnett equations were also introduced to model the flow but no obvious improvement in the results was found.     

Kramers’ problem and the Knudsen minimum: a theoretical analysis using a linearized 26-moment approach

A set of linearized 26 moment equations, along with their wall boundary conditions, are derived and used to study low-speed gas flows dominated by Knudsen layers. Analytical solutions are obtained for Kramers’ defect velocity and the velocity-slip coefficient. These results are compared to the numerical solution of the BGK kinetic equation. From the analysis, a new effective viscosity model for the Navier–Stokes equations is proposed. In addition, an analytical expression for the velocity field in planar pressure-driven Poiseuille flow is derived. The mass flow rate obtained from integrating the velocity profile shows good agreement with the results from the numerical solution of the linearized Boltzmann equation. These results are good for Knudsen numbers up to 3 and for a wide range of accommodation coefficients. The Knudsen minimum phenomenon is also well captured by the present linearized 26-moment equations.     

Intense evaporation of a gas from a two-dimensional periodic surface

Evaporation (or condensation) of a gas is said to be intense when the normal component of the velocity of the gas in the Knudsen layer has a value of the order of the thermal velocity of a molecule, c_T=(2kT/m)^1/2. In this case the distribution function of the molecules with respect to their velocities in the Knudsen layer differs from the equilibrium (Maxwellian) value by its own magnitude. As a result of this, over the thickness of the Knudsen layer the macroparameters also vary by their own magnitudes. So in order to obtain the correct boundary conditions for the Euler gas dynamic equations, it is necessary to solve the nonlinear Boltzmann equation in the Knudsen layer. The problem of obtaining such boundary conditions for the case of a plane surface was considered in [1–11]. In the present study this problem is solved for a two-dimensional periodic surface in the case when the dimensions of the inhomogeneities are of the order of the mean free path of the molecules and the inhomogeneities have a rectangular shape. The flow in the Knudsen layer becomes two-dimensional, and this leads to a considerable complication of the solution of the problem.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 132–139, March–April, 1985.In conclusion the author would like to express his gratitude to V. A. Zharov for his valuable advice, and also V. S. Galkin, M. N. Kogan, and N. K. Makashev for discussion of the results obtained.     

Degeneracy of Chapman-Enskog series for the transport properties in the case of slow steady flows of weakly rarefied gases

The structure of the Chapman-Enskog solution of the Boltzmann equation linearized with respect to the absolute Maxwell equilibrium is studied. Under the assumption of uniqueness and existence of a solution it is shown that in the steady case the series describing the transport phenomena consist of a finite number of terms, and the heat fluxes and diffusion rates are given by the Burnett approximation and the stresses by the super-Burnett approximation, the following terms of the series vanishing. At the same time, the gas-dynamic variables in all approximations in the small Knudsen number K satisfy the conservation equations in the Stokes approximation; the forces and moments acting on bodies placed in a mixture of gases can be calculated from the Navier-Stokes stresses without allowance for their reprocessing in Knudsen layers. A problem is formulated for a simple gas, and the transport properties are analyzed by using the invariance properties of the linearized Boltzmann equation and by means of the algorithm of the Chapman-Enskog method, and then the results are generalized to a mixture of gases, and the question of the forces and moments is considered.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 157–163, July–August, 1988.I thank M. N. Gaidukov and O. G. Fridlender for fruitful discussions.