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

Prasanth, P S; Kakkassery, Jose K

doi:10.1016/j.fluiddyn.2007.10.001

Cited by 17 publications

(8 citation statements)

References 36 publications

(39 reference statements)

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“…This statistical method is a proven tool for computational molecular dynamics of transitional regimes [15], although the standard DSMC does not always conserve AM [16,17]. For this reason, we designed an advanced variant of the DSMC simulation in which angular momentum is strictly conserved.…”

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confidence: 99%

Laser-Induced Gas Vortices

2012

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Properly polarized and correctly timed femtosecond laser pulses have been demonstrated to create rotational states with a preferred sense of rotation. We show that due to conservation of angular momentum, collisional relaxation of these unidirectionally rotating molecules leads to the generation of macroscopic vortex flows in the gas. The emerging velocity field converges to a self-similar Taylor vortex, and repeated laser excitations cause a continuous stirring of the gas.PACS numbers: 37.10.Vz, 47.32.Ef Nowadays, femtosecond lasers are routinely used to manipulate and control molecular rotation and orientation in space [1,2]. Particularly, diverse techniques have been developed to bring gas molecules to a rapidly spinning state with a preferred sense of rotation. These schemes include the optical centrifuge [3][4][5] and the molecular propeller [6,7] methods, excitation by a chiral pulse train [8] and other techniques that are currently discussed [9,10]. In rarefied gas, coherent effects such as alignment revivals have been widely investigated. Most recently, the optical centrifuge method was applied to excite molecules to rotational levels with angular momentum of hundreds of in dense gas samples [5], where collisions play an important role. In this paper we analyze the behavior of a dense gas of unidirectionally spinning molecules after many collisions have occurred. The statistical mechanics and equilibration process of such a system are not trivial, as known for other ensembles in which angular momentum (AM) is a conserved quantity in addition to energy [11,12]. We show that due to the AM conservation, the collisions in such a gas transform the laser induced molecular rotation into macroscopic vortex flows. The lifetime of the generated vortex motion exceeds the typical collision time by orders of magnitude, and the emerging velocity field eventually converges to a self-similar Taylor vortex [13]. The viscous decay of this whirl can be overcome by repeated laser excitations that produce a continuous stirring effect.Consider a gas sample excited by laser pulses causing the molecules to rotate, on average, unidirectionally. Our analysis of the subsequent gas dynamics was performed in two steps. First we used a Monte Carlo approach to simulate directly the kinetics of molecular collisions just after the excitation. Then, after the motion had taken a collective character, we used continuum hydrodynamic equations to investigate the gas dynamics and vortex flow evolution.We analyzed the initial stages of the transformation of the individual molecular rotation into collective gas motion with the help of the Direct Simulation Monte Carlo technique (DSMC [14]). This statistical method is a proven tool for computational molecular dynamics of transitional regimes [15], although the standard DSMC does not always conserve AM [16,17]. For this reason, we designed an advanced variant of the DSMC simulation in which angular momentum is strictly conserved. The simulation was run in axisymmetric geometry, and handled co...

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confidence: 99%

Laser-Induced Gas Vortices

2012

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“…It can be seen that the differences of the molecular collisions among the HS, VHS and VSS models are significant. Note that VSS model can not only reflect the molecular diffusion effect well, but also has almost the same computational simplicity as the VHS model [ 32 ]. Thus, applying the VSS model can illustrate the gas movement rules better for gas mixtures, so as to improve the accuracy of the simulation results.…”

Section: Theoretical Models and Numerical Methodsmentioning

confidence: 99%

“…In previous research on the Knudsen pump, the hard sphere (HS) model [ 19 ] and variable hard sphere (VHS) model [ 20 , 21 , 22 , 23 , 24 , 31 ] are two common molecular models for particle simulations. In fact, the HS model has disadvantages of constant collision cross-section and unrealistic scattering law [ 32 ]. Thus, the HS model is not physically realistic or recommended.…”

Section: Introductionmentioning

confidence: 99%

“…However, in practice, the VHS model is more often used for gas mixtures compared to single gases. Note that in the VHS model, the ratio of diffusion collision cross-section

to viscosity cross-section

is constantly 1.5, which results in a large deviation from practice when multi-component gas mixtures with diffusion playing an essential role are considered [ 32 , 34 ]. In contrast, the problem can be effectively avoided by applying variable soft sphere (VSS) model [ 35 , 36 ].…”

Section: Introductionmentioning

confidence: 99%

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Numerical Investigation into the Flow Characteristics of Gas Mixtures in Knudsen Pump with Variable Soft Sphere Model

Wang

Han

et al. 2020

Micromachines

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In Knudsen pumps with geometric configuration of rectangle, gas flows are induced by temperature gradients along channel walls. In this paper, the direct simulation Monte Carlo (DSMC) method is used to investigate numerically the flow characteristics of H2–N2 mixtures in the Knudsen pump. The variable soft sphere (VSS) model is applied to depict molecular diffusion in the gas mixtures, and the results obtained are compared with those calculated from a variable hard sphere (VHS) model. It is demonstrated that pressure is crucial to affecting the variation of gas flow pattern, but the gas concentration in H2–N2 mixtures and the collision model do not change the flow pattern significantly. On the other hand, the velocity of H2 is larger than that of N2. The velocities of H2 and N2 increase if the concentration of H2 rises in the gas mixtures. The results of velocity and mass flow rate obtained from VSS and VHS models are different. Finally, a linear relation between the decrease of mass flow rate and the increase of H2 concentration is proposed to predict the mass flow rate in H2–N2 mixtures.

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“…Currently, a variety of molecular models for nonreaction collisions have been developed [18]. The hard-sphere (HS) model is the simplest molecular model, in which a gas molecule is regarded as a rigid ball whose radius is constant and a collision occurs only when two molecules touch each other.…”

Section: Molecule Collision Modelmentioning

confidence: 99%

DSMC Simulation of Non-Premixed Combustion of H₂/O₂in a Y-shaped Microchannel

Zhang

Xie

2015

Nanoscale and Microscale Thermophysical Engineering

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Combustion at the microscale shows an immeasurable potential in the fields of energy utilization and microelectromechanical systems (MEMS) due to its novel combustion characteristics. Studying the mechanism of this kind of combustion is of great significance for deepening the understanding of microcombustion phenomena and designing related devices. In this article, the non-premixed combustion of H 2 /O 2 in a two-dimensional Y-shaped microchannel with a height of 10 µm was numerically studied using the direct simulation Monte Carlo (DSMC) method. The total collision energy (TCE) model and a kinetic mechanism including six species and seven reversible reactions were employed. Predicted distributions of velocity, temperature, heat flux, and components inside the microchannel are presented and analyzed. Influences of the Knudsen number and wall surface conditions on the combustion characteristics are discussed. The results show that the exothermic reaction mainly takes place in the junction area of the branch channels and in the first half of the main channel, and the wall heat flux at the microscale is much higher than that at the conventional scale. This is helpful to effectively heat and ignite the gaseous H 2 /O 2 mixture. Moreover, the conversions and distributions of individual components in the flow field are mainly controlled by the chemical kinetics; the Kn number and different wall conditions have a sophisticated influence on the combustion process.

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

Cited by 17 publications

References 36 publications

Laser-Induced Gas Vortices

Laser-Induced Gas Vortices

Numerical Investigation into the Flow Characteristics of Gas Mixtures in Knudsen Pump with Variable Soft Sphere Model

DSMC Simulation of Non-Premixed Combustion of H₂/O₂in a Y-shaped Microchannel

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

Cited by 17 publications

References 36 publications

Laser-Induced Gas Vortices

Laser-Induced Gas Vortices

Numerical Investigation into the Flow Characteristics of Gas Mixtures in Knudsen Pump with Variable Soft Sphere Model

DSMC Simulation of Non-Premixed Combustion of H2/O2in a Y-shaped Microchannel

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DSMC Simulation of Non-Premixed Combustion of H₂/O₂in a Y-shaped Microchannel