Molecular Dynamics Simulation Of Rarefied Gas Flow Mechanism

Rarefied gas flow is widely existed in aerospace flights and micro/nano electromechanical systems.Rarefied environment has significant effects on the aerodynamic force and heat of aircrafts,and is of great importance of the control and thermal protection of the aircrafts.Due to the high specific surface area and surface force dominated of micro/nano scale flows,the calculation of heat dissipation and resistance of micro/nano electronic components are closely related to these effects.So it needs to pay great attention and efforts to research.According to Knudsen number,the rarefied gas can be divided into slip flow region,transition flow region and free molecular region.Because the interactions between gas molecules and gas molecules with wall molecules are varied with different flow region,it is necessary to use different methods for modeling and solving flow processes,which leads to the current problems and difficulties of rarefied gas flow in the slip boundary conditions are not unified form,Flow law of Knudsen layer is still not clear,and the molecular surface interaction model is not accurate,the research about these questions is of great help for the accurate modeling of rarefied gas flows.The molecular dynamics(MD)method is a deterministic particle simulation method,which is different from the DSMC.It is starting from the most basic laws of physics,using Newton’s motion theorem to calculate each particle’s positions,velocites as well as trajectories,and then using statistical methods to abtain the macro physical quantity of the system.Therefore,the MD method is more suitable for the simulation research and analysis of the rarefied gas flow mechanism.Based on the above background,in this paper,we used molecular dynamics method to model and simulate rarefied gas flows.First,we built a computing platform with open source LAMMPS program.Then the Couette flow in micro/nano channels was simulated with it,under different Knudsen numbers,shear velocities,temperatures as well as wall parameters.Wall slip of each case was calculated and changes of slip velocity with these parameters was analysed.We found that the slip velocity was proportional to shear velocity but inversely proportional to Knudsen number and temperature.As to the effect of wall parameters,it’s relatively weaker,and more complex,but still can not be ignored.