Comparison analysis between two continuum breakdown parameters applied in coupled NS-DSMC method

Navier Stokes-direct simulation Monte Carlo (NS-DSMC) coupled method is a principal method for the computation of continuum-rarefied transitional gas flows. How to locate the interface between continuum and rarefied domains is one of the key aspects in using this method, which is generally achieved by applying a continuum breakdown parameter. To choose the continuum breakdown parameter more reasonably, theoretical analysis and comparisons were carried out for two major continuum breakdown parameters: Kn_Q and B, which show that they have similar mathematical forms to a certain extent, although they are established from different basis. Numerical test was conducted by using an over-cylinder flow problem which also indicates the relationship between them. Kn_Q and B parameters have few differences when applied in NS-DSMC coupled method, except that Kn_Q parameter has a widely used threshold value of 0.05, but the threshold value of B parameter will vary for different flows.