Investigation on the Interface Smoothing of Coupled N–S/DSMC Method Using Image Processing Filters

Gas flow problems involving both continuum and rarefied regimes are common in many scientific and engineering applications. Coupled N–S/DSMC method is a major solution to simulate the continuum–rarefied transitional flow. Interface determination is one of the key aspects in developing coupled N–S/DSMC solver, which is often implemented using continuum breakdown parameters to indicate the rarefication level. Due to the statistics characteristic of DSMC method, distribution of the continuum breakdown parameters usually fluctuates, resulting in difficulty in locating the interface. Considering the similarity between continuum breakdown parameter smoothing and noise reduction in image processing, a general smoothing method is proposed in this paper, which employs the image filters to smoothen the distribution of continuum breakdown parameters. Two commonly used image filters, the mean filter and the median filter, are investigated. Several comparison studies are implemented by simulating a typical vacuum plume impingement flow problem. The median filter with 5 × 5 mask provides the best performance. The flow problem of flow over a 2D cylinder is used to validate the coupled N–S/DSMC solver using median filter to smoothen continuum breakdown parameter, which proves the accuracy of the coupled solver and validity of the smoothing method.