Relaxation processes in hypersonic rarefied binary gas mixture flow around a cylinder

Using the direct simulation Monte Carlo method, the hypersonic flow of a binary gas mixture around a cylinder is investigated over a wide rarefaction range: from an almost continuum regime (at the Knudsen number Kn = 0.01) to free-molecular flow. The effect of a small admixture of heavy diatomic particles in a light gas flow on the relaxation processes near the cylinder and the heat flux is studied.     

Direct Monte Carlo Simulation of Transverse Supersonic Rarefied Gas Flow around a Cylinder

Supersonic flow around a cylinder is investigated using the direct simulation Monte Carlo method over a wide rarefaction range: from the Knudsen number Kn = 0.1 to free-molecular flow. The effect of the cylinder temperature on the region of sharp nonequilibrium near the cylinder and the heat flux is studied.     

Effect of incomplete thermal accommodation on intensive subsonic gas condensation

The direct Monte Carlo simulation method is used for investigating the effect of the thermal accommodation coefficient α _E on the relation for the Knudsen layer in the presence of intensive subsonic condensation. It is shown that the deviation of α _E from unity may significantly affect the flow parameters, in particular, M_lim, the Mach number value limiting for subsonic condensation. It is shown that a decrease in α _E leads to an increase in M_lim (M_lim < 1) if the relative flow temperature (ratio of the outer Knudsen layer boundary to the surface temperature) T < 1 and to a decrease in M_lim if T > 1. It is shown that for mirror reflection of molecules from the surface this effect may intensify.     

Transformation of energy and composition of gas mixtures in a collision of rarefied supersonic flows

Transformation of energy and composition of gas mixtures in a convergent flow from a strip source on a cylindrical surface toward the axis is systematically studied with the use of the direct simulation Monte Carlo method. Information on the influence of gas-flow rarefaction and system geometry on the temperature and concentration of the heavy gas in a dense cloud formed on the axis is obtained. The use of convergent supersonic flows is demonstrated to offer new possibilities for research in the field of physical gas dynamics. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 2, pp. 198–204, March–April, 2009     

Direct Statistical Simulation of a Supersonic Flow of a Binary Mixture of Rarefied Gases around a Transversely Positioned Cylinder

A supersonic flow of a binary mixture of gases in a wide range of rarefaction (from a flow with a Knudsen number K _n = 0.1 to a free-molecular flow) around a cylinder is studied by means of direct statistical Monte Carlo simulations (DSMC method). The influence of a small fraction of heavy particles in a light gas flow on the region of significant nonequilibrium near the cylinder and on the heat flux is considered.__________Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 5, pp. 53–59, September–October, 2005.     

基于重叠网格与结构网格的圆柱绕流数值模拟

为研究重叠网格与结构网格在圆柱绕流数值模拟中的区别,以二维圆柱为例,利用有限元分析软件ANSYS 19.2中的DM与Mesh建立模型并划分重叠网格,利用ANSYS 19.2中的ICEM建立模型并划分结构网格。采用FLUENT 19.2中laminar模型模拟分析系统中的平均升力系数、平均阻力系数、斯特劳哈尔数St等流体动力特性。通过改变流体流速得到两种不同网格下各6组雷诺数Re,这6组雷诺数在60~160之间。结果表明:结构网格与重叠网格的St都随着Re的增加而增加,但相同雷诺数下重叠网格对应的St数值更大,St的增长速度更快;重叠网格与结构网格的平均升力系数与平均阻力系数随着Re的增加趋于稳定的速度都加快,但结构网格的平均升力系数与平均阻力系数趋于稳定的速度更快,且两种网格的平均升力系数与平均阻力系数趋于稳定速度的差距逐渐缩小,当Re=160时,两种网格的平均升力系数与平均阻力系数趋于稳定的速度几乎相同;当雷诺数在60~160之间时,采用重叠网格计算出来的斯特劳哈尔数比结构网格更加接近理论值;从升力功率谱密度分布曲线中可以看出,随着雷诺数的增加,两种网格下的频率逐渐变大,并且相同雷诺数下重叠网格的频率比结构网格大。     

Charge transfer by high-energy electrons onto the leeward surfaces of a solid in a supersonic rarefied plasma flow

Numerical and experimental dependence of equilibrium potentials of the leeward surfaces on the ratio of concentrations of high-energy electrons and positive ions in a supersonic rarefied plasma flow around a solid are obtained. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 1, pp. 13–23, January–February, 2008.     

Effect of a Dispersed Admixture on the Flow Pattern and Heat Transfer in a Supersonic Dusty-Gas Flow Around a Cylinder

A mathematical model of two-phase (gas-solid particle) flow which takes into account particle-particle collisions and the feedback effect of the admixture on the gas parameters is proposed. The dispersed phase is described by a kinetic equation of the Boltzmann type and the carrier gas by modified Navier-Stokes equations. Using this model, a supersonic uniform dusty-gas flow past a cylinder is calculated. The fields of the macroparameters of the admixture and the carrier medium are obtained. The dependence of the heat transfer at the stagnation point on the relative particle size and the free-stream admixture concentration is studied in detail. The ranges of these parameters on which particle collisions and the feedback effect of the admixture on the carrier-gas flow are important are found.     

Investigation of the rarefied gas flow around a circular cylinder in stationary and oscillatory regimes

Numerically, on the basis of the Krook kinetic equation, the rarefied gas flow around a circular cylinder is investigated in stationary and oscillatory regimes. The flows around a rotating cylinder and a cylinder with a nonuniformly heated surface are considered. The Knudsen numbers at which the lift force acting on the rotating cylinder changes sign are calculated. It is shown that at low Knudsen numbers a lift force acts on the nonuniformly heated cylinder.     

Series solutions of annular axisymmetric stagnation flow and heat transfer on moving cylinder

The steady, laminar, incompressible flow and heat transfer of a viscous fluid between two circular cylinders for two different types of thermal boundary conditions are investigated. The governing Navier-Stokes and thermal equations of the flow are reduced to a nonlinear system of ordinary differential equations. The equations are solved analyt- ically using the homotopy analysis method （HAM）. Convergence of the HAM solutions is discussed in detail. These solutions are then compared with recently obtained numericM and perturbative solutions. Plots of the velocity and temperature profiles are provided for various values of the relevant parameters.     

NUMERICAL STUDY ON THE FLOW AROUND A CIRCULAR CYLINDER WITH SURFACE SUCTION OR BLOWING USING VORTICITY-VELOCITY METHOD

A vorticity-velocity method was used to study the incompressible viscous fluid flow around a circular cylinder with surface suction or blowing. The resulted high order implicit difference equations were effeciently solved by the modified incomplete LU decomposition conjugate gradient scheme (MILU-CG). The effects of surface suction or blowing ' s position and strength on the vortex structures in the cylinder wake, as well as on the drag and lift forces at Reynoldes number Re = 100 were investigated numerically. The results show that the suction on the shoulder of the cylinder or the blowing on the rear of the cylinder can effeciently suppress the asymmetry of the vortex wake in the transverse direction and greatly reduce the lift force; the suction on the shoulder of the cylinder, when its strength is properly chosen , can reduce the drag force significantly , too.     

Analytical treatment on magnetohydrodynamic (MHD) flow and heat transfer due to a stretching hollow cylinder

This paper studied on magnetohydrodynamics flow and heat transfer outside a stretching cylinder. Momentum and energy equations are reduced using similarity transformation and converted into a system of ordinary differential equations which are solved analytically by the homotopy analysis method. The effects of the parameters involved, namely the magnetic parameter (M), Prandtl number (Pr) and Reynolds number (Re) on the velocity and temperature fields are investigated. The obtained results are valid for the whole solutions' domain with high accuracy. These methods can be easily extended to other linear and nonlinear equations and so can be found widely applicable in engineering and sciences. Copyright © 2009 John Wiley & Sons, Ltd.     

Vortex structure generation on the frontal surface of a cylinder set transversely in a hypersonic flow

The problem of formation of spatially periodic structures on the frontal surface of a cylindrically blunted body set transversely in a hypersonic flow is studied. Within the framework of the model adopted, a possible mechanism of vortex structure generation on the frontal surface of the blunt body is proposed and confirmed by calculations; in this mechanism, the curved bow shock produces a vortex flow, while in its turn the vortex, which persists under weak dissipation, acts on the shock thus maintaining its curved shape. It is shown that the spatially periodic mode of hypersonic flow past a cylinder can exist in the case of a uniform incident flow and under homogeneous boundary conditions on the body surface.     

Numerical simulation of the flow around a porous covering square cylinder in a channel via lattice Boltzmann method

In this paper, a detailed investigation on the flow past a porous covering cylinder is presented through the lattice Boltzmann method. The Brinkman‐Forchheimer‐extended Darcy model is adopted for the entire flow field with the solid, fluid, and porous medium. The effects of several parameters, such as porous layer thickness, Darcy number, porosity, and Reynolds number on flow field are discussed. Compared with the case of a solid cylinder, the present work shows that the porous layer may play an important role on the flow, the lift and drag force exerted on the cylinder. The numerical results indicate that the maximal drag coefficient C_d and maximal amplitude of lift coefficient C_l exist at certain Darcy number which is in the range of 10^?6–10^?2. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of magnetic Reynolds number on the two‐dimensional hydromagnetic flow around a cylinder

Numerical experiments have been conducted to study the effect of magnetic Reynolds number on the steady, two‐dimensional, viscous, incompressible and electrically conducting flow around a circular cylinder. Besides usual Reynolds number Re, the flow is governed by the magnetic Reynolds number R_m and Alfvén number β. The flow and magnetic field are uniform and parallel at large distances from the cylinder. The pressure Poisson equation is solved to find the pressure fields in the entire flow region. The effects of the magnetic field and electrical conductivity on the recirculation bubble, drag coefficient, standing vortex and pressure are presented and discussed. For low interaction parameter (N<1), the suppression of the flow‐separation is nearly independent of the conductivity of the fluid, whereas for large interaction parameters, the conductivity of the fluid strongly influences the control of flow‐separation. Copyright © 2008 John Wiley & Sons, Ltd.     

Numerical study on the dynamics of droplet passing through a cylinder obstruction in confined microchannel flow

The droplet dynamics passing through a cylinder obstruction was investigated with direct numerical simulations with FE-FTM (Finite Element-Front Tracking Method). The effect of droplet size and capillary number (Ca) was studied for both Newtonian and viscoelastic fluids. In the case of Newtonian droplet immersed in Newtonian medium, the droplet breakup induced by the geometric hindrance depends on the droplet size. As Ca increases, the short droplets (1.3 times longer than the channel width) break up while passing through the obstruction. However, the breakup does not occur for longer droplets (1.8 times longer than the channel width). When the viscoelastic fluid characterized by the Oldroyd-B model is considered, the Newtonian droplet immersed in viscoelastic medium breaks up into two smaller droplets while passing through the cylinder obstruction with increasing De_m (Deborah number of the medium). We also show that the normal stress difference plays a key role on the droplet breakup and the droplet extension. The normal stress difference is enhanced in the negative wake region due to the droplet flow, which also promotes droplet extension in that region. This numerical study provides information not only on underlying physics of the droplet flows passing through a cylinder obstruction but also on the useful guidelines for microfluidic applications.     

Effects of dimensional wall temperature on Reynolds stress budgets in a supersonic turbulent channel flow with thermally perfect gas

ABSTRACT

Direct numerical simulations of temporally evolving supersonic turbulent channel flow of thermally perfect gas are conducted at Mach number 3.0 and Reynolds number 4800, combined with constant dimensional wall temperatures from 149.075 to 1788.90?K to study the influence of dimensional wall temperature on the characteristics of Reynolds stress budgets. It is found that, as the dimensional wall temperature increases, the production, diffusion, pressure–velocity gradient correlation and dissipation terms increase, whereas the compressibility-related term decreases. This is mainly due to variations in mean flow properties. The mechanism of inter-component transfer (ICT) is insensitive to the dimensional wall temperature. The ICT relating to the pressure–velocity gradient correlation term can be divided into inner and outer regions, and the critical position separating these regions is at the semi-local scaling of approximately 16 irrespective of the different dimensional wall temperature.     

Optical investigation of the heat transfer from a rotating cylinder in a cross flow

The heat transfer from a rotating cylinder in an air-cross flow was investigated by purely optical measuring techniques. Flow velocities were measured by a two-dimensional LDV both in the vicinity of the cylinder and in the boundary layer. A new optical device based on light-deflection in a temperature field was developed to examine local temperature gradients in the boundary layer of the rotating cylinder. Finally, a Michelson-interferometer was installed to produce real-time pictures of isothermal lines around the heated cylinder. The impact of rotation on flow patterns, boundary layer behaviour and heat transfer could be clearly identified. It appears that the velocity-ratio acts like an independent parameter, in that flow patterns correspond to this dimensionless number. Furthermore, it seems that rotation dominates over cross flow, both fluid-dynamically and thermally above = 2.This work was carried out at the University of the Federal Armed Forces in Munich/Germany.     

Experimental study on the heat transfer coefficient of water flow boiling in mini/microchannels

The heat transfer coefficients of the evaporative water flow in mini/microchannels are studied experimentally to explore the novel heat dissipation for high power electronics. Two sets of parallel channels which are 61 channels with hydraulic diameter of 0.293 mm and 20 channels with hydraulic diameter of 1.2 mm are investigated respectively. The inlet and outlet temperatures of fluids, and the temperatures beneath the channels are measured to calculate the heat dissipation of the evaporative water in channels. The experiments are carried out with the mass flow rates range from 11.09 kg/(m² s) to 44.36 kg/(m² s) for minichannels and 49.59 kg/(m² s) to 198.37 kg/(m² s) for microchannels. The effective heat flux range from 5 W/cm² to 50 W/cm², and the resulted outlet vapor qualities range from 0 to 0.8. The relations of the heat transfer coefficient with heat flux and vapor quality are analyzed according to the results. The experimental heat transfer coefficients are compared with the prediction of latest developed correlations. A new correlation takes the effect of Bond number is proposed, and be verified that it is effective to predict the heat transfer coefficient of both minichannels and microchannels in a large range of vapor qualities.     

Numerical investigation of fully developed turbulent fluid flow and heat transfer in a square duct

Three-dimensional fully developed turbulent fluid flow and heat transfer in a square duct are numerically investigated with the author's anisotropic low-Reynolds-number k-ε turbulence model. Special attenton has been given to the regions close to the wall and the corner, which are known to influence the characteristics of secondary flow a great deal. Hence, instead of the common wall function approach, the no-slip boundary condition at the wall is directly used. Velocity and temperature profiles are predicted for fully developed turbulent flows with constant wall temperature. The predicted variations of both local wall shear stress and local wall heat flux are shown to be in close agreement with available experimental data. The present paper also presents the budget of turbulent kinetic energy equation and the systematic evaluation for existing wall function forms. The commonly adopted wall function forms that are valid for two-dimensional flows are found to be inadequate for three-dimensional turbulent flows in a square duct.