Simulations for gas flows in microgeometries using the direct simulation Monte Carlo method

Micro gas flows are often encountered in MEMS devices and classical CFD could not accurately predict the flow and thermal behavior due to the high Knudsen number. Therefore, the gas flow in microgeometries was investigated using the direct simulation Monte Carlo (DSMC) method. New treatments for boundary conditions are verified by simulations of micro-Poiseuille flow, compared with the previous boundary treatments, and slip analytical solutions of the continuum theory. The orifice flow and the corner flow in microchannels are simulated using the modified DSMC codes. The predictions were compared with existing experimental phenomena as well as predictions using continuum theory. The results showed that the channel geometry significantly affects the microgas flow. In the orifice flow, the flow separation occurred at very small Reynolds numbers. In the corner flow, no flow separation occurred even with a high driving pressure. The DSMC results agreed well with existing experimental information.     

Molecular models for simulation of rarefied gas flows using direct simulation Monte Carlo method

The direct simulation Monte Carlo (DSMC) method is a technique for the numerical simulation of the rarefied gas flows by employing simulated molecules in simulated physical spaces. In the procedures involved in DSMC, the accuracy of the simulation of intermolecular collisions depends on the collision model adopted in the collision routine. The simplest molecular model is the hard-sphere model. In order to improve the accuracy of the simulations, more and more refined collision models were introduced for the use in DSMC. Thus, the variable hard-sphere, the variable soft-sphere, the generalised hard-sphere, the generalised soft-sphere and the variable sphere models were put forward by various researchers. And, all these models have met with varying degrees of success. Meanwhile, the Borgnakke-Larsen model, statistical inelastic cross-section models for both continuous and discrete internal energy and the dynamic molecular collision model were proposed for the treatment of polyatomic molecules in which transfer of energy among translational, rotational and vibrational degrees of freedom is possible. This paper gives a brief introduction to the intermolecular potentials based on which the molecular models have been constructed. Then the various models are introduced in the chronological sequence; finally concluding with a brief summary of the progress that has been made so far in this area.     

气固两相流中颗粒弥散的拉格朗日模拟

本文提出了一种对于均匀,稳定及各向同性气固两相紊流场中圆形固体颗粒弥散的拉格朗日模拟计算方法,应用该方法对带有网栅的垂直与水平管道中均匀,稳定的气固两相流模拟计算结果与Snyder及Wells等人所做的相同情况下的试验结果进行了比较,以证明该模拟计算方法的有效性,。     

The direct simulation Monte Carlo method using unstructured adaptive mesh and its application

The implementation of an adaptive mesh‐embedding (h‐refinement) scheme using unstructured grid in two‐dimensional direct simulation Monte Carlo (DSMC) method is reported. In this technique, local isotropic refinement is used to introduce new mesh where the local cell Knudsen number is less than some preset value. This simple scheme, however, has several severe consequences affecting the performance of the DSMC method. Thus, we have applied a technique to remove the hanging node, by introducing the an‐isotropic refinement in the interfacial cells between refined and non‐refined cells. Not only does this remedy increase a negligible amount of work, but it also removes all the difficulties presented in the originals scheme. We have tested the proposed scheme for argon gas in a high‐speed driven cavity flow. The results show an improved flow resolution as compared with that of un‐adaptive mesh. Finally, we have used triangular adaptive mesh to compute a near‐continuum gas flow, a hypersonic flow over a cylinder. The results show fairly good agreement with previous studies. In summary, the proposed simple mesh adaptation is very useful in computing rarefied gas flows, which involve both complicated geometry and highly non‐uniform density variations throughout the flow field. Copyright © 2002 John Wiley & Sons, Ltd.     

Comparison of molecular tagging velocimetry data and direct simulation Monte Carlo simulations in supersonic micro jet flows

We present results of a combined experimental computational study of free jet flow produced by a 1 mm (height)ǹ mm (span) nominally Mach 2 supersonic jet. Two-dimensional maps of u_x, the component of velocity parallel to the principal flow axis, are obtained experimentally, by acetone molecular tagging velocimetry (MTV), and computationally, by the direct simulation Monte Carlo (DSMC) method, at a stagnation pressure and temperature of 10 torr and 300 K, respectively. In all cases, direct comparison of the experimental data and the predictions from DSMC showed excellent agreement, with only minor deviations which, in most cases, can be ascribed to either the inherent uncertainty in the MTV or small uncertainties in the measured wall pressures.     

考虑颗粒间碰撞的气固两相流拉格朗日模拟

在均匀,稳定的各向同性气固两相紊流场颗粒弥散的拉格朗日模拟计算方法基础上,进一步考虑了流场中颗粒之间的碰撞对于模拟计算结果的影响。与Lavieville用大涡模拟所做的计算结果进行了对比,以对本方法进行验证,并考察了颗粒间的碰撞分别对流体相和颗粒相的影响。     

结构随机分析的Monte Carlo加权残值法

本文提出一种结构随机分析的Monte Carlo加权残值法。文中建立了这种方法的基本列式,并通过静力挠度、固有频率和屈曲荷载等算例,表明本文方法理论简捷,计算工程量少,精度较高,是随机结构数值分析的有效方法。     

Coupled Particle-In-Cell and Direct Simulation Monte Carlo method for simulating reactive plasma flows

Plasma flows with high Knudsen numbers cannot be treated with classic continuum methods, as represented for example by the Navier–Stokes or the magnetohydrodynamic equations. Instead, the more fundamental Boltzmann equation has to be solved, which is done here approximately by particle based methods that also allow for thermal and chemical non-equilibrium. The Particle-In-Cell method is used to treat the collisionless Vlasov–Maxwell system, while neutral reactive flows are treated by the Direct Simulation Monte Carlo method. In this article, a combined approach is presented that allows the simulation of reactive, partially or fully ionized plasma flows. Both particle methods are briefly outlined and the coupling and parallelization strategies are described. As an example, the results of a streamer discharge simulation are presented and discussed in order to demonstrate the capabilities of the coupled method.     

气固两相流中颗粒碰撞的Monte-Carlo数值模拟

利用颗粒碰撞动力学模型和颗粒几何碰撞率模型,采用Monte—Carlo算法来模拟颗粒之间碰撞,把该算法与求解雷诺应力-概率密度函数模型的有限差分-Monte Carlo算法耦合起来,对轴对称突扩通道内的两相旋流场进行了数值模拟,模拟结果表明,由于颗粒碰撞使颗粒的动能和湍动能在三个坐标方向上进行了再分配,从而导致颗粒的动能和湍动能在三个坐标方向上趋于各向同性;另外,由于颗粒碰撞破坏了颗粒-颗粒、颗粒-流体微团之间的速度关联,从而造成颗粒湍动能及两相速度脉动关联的降低。     

A multi-scale architecture for multi-scale simulation and its application to gas-solid flows

A multi-scale hardware and software architecture implementing the EMMS （energy-minimization multi-scale） paradigm is proven to be effective in the simulation of a two-dimensional gas-solid suspension. General purpose CPUs are employed for macro-scale control and optimization, and many integrated cores （MlCs） operating in multiple-instruction multiple-data mode are used for a molecular dynamics simulation of the solid particles at the meso-scale. Many cores operating in single-instruction multiple- data mode, such as general purpose graphics processing units （GPGPUs）, are employed for direct numerical simulation of the fluid flow at the micro-scale using the lattice Boltzmann method. This architecture is also expected to be efficient for the multi-scale simulation of other comolex systems.     

Effect of collision‐partner selection schemes on the accuracy and efficiency of the direct simulation Monte Carlo method

The effect of the collision‐partner selection scheme on the accuracy and the efficiency of the Direct Simulation Monte Carlo method is investigated. Several schemes that reduce the mean collision separation, including the fixed sub‐cell scheme, the transient adaptive sub‐cell scheme, and the virtual sub‐cell scheme, are evaluated. Additionally, a new scheme is proposed that limits the population from which collision partners are selected based on the distance traveled by a simulator and performs near‐neighbor collisions using this population. These collision‐partner selection schemes are assessed for Fourier flow (heat conduction between parallel plates) and a standard hypersonic benchmark problem (Mach 15.6 nitrogen flow over a 25–55° biconic). The new limited‐selection near‐neighbor scheme has superior performance compared to the other schemes for both flows and reduces both the spatial and temporal discretization errors relative to random‐selection and nearest‐neighbor collision‐partner selection schemes. Copyright © 2010 John Wiley & Sons, Ltd.     

Dynamic Monte Carlo simulation of surface recombination

A dynamic model for the Monte Carlo method is developed to analyze the atom recombination on a catalytic surface. A numerical method for the study of this model is considered. The concentrations of physically and chemically adsorbed atoms obtained using this approach are in good agreement with experimental data and with the numerical results obtained on the basis of the phenomenological model and by other authors with the aid of the Monte Carlo method.     

Enhancement of the accuracy of the finite volume particle method for the simulation of incompressible flows

A finite volume particle (FVP) method for simulation of incompressible flows that provides enhanced accuracy is proposed. In this enhanced FVP method, a dummy neighbor particle is introduced for each particle in the calculation and used for the discretization of the gradient model and Laplacian model. The error‐compensating term produced by introducing the dummy neighbor particle enables higher order terms to be calculated. The proposed gradient model and Laplacian model are applied in both pressure and pressure gradient calculations. This enhanced FVP scheme provides more accurate simulations of incompressible flows. Several 2‐dimensional numerical simulations are given to confirm its enhanced performance.     

计算复杂边界渗流的蒙特卡罗方法

本文运用不规则游动网格的蒙特卡罗方法求解复杂边界渗流问题。与以往各种数值方法相比,蒙特卡罗方法在计算渗流问题时要更灵活、更方便,可以根据需要独立地计算出渗流区域内任意一点的渗透压力、渗流流速。     

大尺度网格下DSMC仿真碰撞概率的修正方法

从DSMC碰撞概率的实现途径出发,通过引入最小二乘法及亚松弛的思想给出一种大尺度网格下分子仿真碰撞概率的修正方法,将该方法应用于非结构四面体网格下的DSMC模拟中,计算分析了文献中钝锥模型的绕流结果,数值结果初步验证了本文方法的合理性和有效性。     

Incompressible smoothed particle hydrodynamics simulation of multifluid flows

This paper presents an incompressible SPH (ISPH) technique to simulate multifluid flows. The SPH method is a mesh‐free particle modeling approach that can treat free surfaces and multi‐interfaces in a simple and efficient manner. The ISPH method employs an incompressible hydrodynamic formulation to solve the fluid pressure that ensures a stable pressure field. Two multifluid ISPH models are proposed following different interface treatments: the coupled ISPH model does not distinguish the different fluid phases and applies the standard ISPH technique across the interface, whereas the decoupled ISPH model first treats each fluid phase separately and then couples the different phases by applying pressure and shear stress continuities across the interface. The two proposed models were used to investigate a gravity underflow with a low density ratio in a Generalized Reservoir Hydrodynamics (GRH) flume and a horizontal lock exchange flow with a high density ratio. Comparisons with data and relevant numerical error analysis indicated that the decoupled model performed well in cases of both low and high density ratios because of the accurate treatment of interface boundaries. Copyright © 2011 John Wiley & Sons, Ltd.