Molecular block model direct simulation Monte Carlo method for low velocity microgas flows

When the characteristic velocities of microgas flows become very small (<3 m s-1), the use of conventional direct simulation Monte Carlo (DSMC) methods often incur large statistical errors that can overwhelm the realistic physical information. To overcome this difficulty, a molecular block (or `big molecule') model is developed here to replace the real molecular model. The mass of the `big molecule' is the sum of the real mass of molecules contained within the molecular block. The reference diameter and number density of this `big molecule' are determined by ensuring that the mean free path and dynamic viscosity of the `big molecule' are equal to that of the real constituent molecules. These molecular blocks are then used to replace the particles in the DSMC method, and a molecular block direct simulation Monte Carlo (MB-DSMC) method is established, the computational process of which is similar to that of the DSMC method. As the mass of the molecular block is larger, the statistical error incurred by the MB-DSMC method is smaller. The numerical verification has shown that when the molecular number in a molecular block is taken to be 100, the velocity and pressure distributions obtained are very close to analytical solutions. The simulation of the temperature field shows that the present method has the same accuracy as the original DSMC method.