On the consequences of successively repeated collisions in no-time-counter collision scheme in DSMC

• Published in: Computers & fluids Vol. 161; pp. 23 - 32
• Main Authors: Akhlaghi, Hassan, Roohi, Ehsan, Stefanov, Stefan
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Ltd 15.01.2018; Elsevier BV
• Subjects: Collision avoidance; Collision dynamics; Collision-pair selection scheme; Collisional parameters; Collisions; Computer simulation; Direct simulation Monte Carlo method; Distribution functions; Fourier analysis; Fourier heat transfer problem; Heat flux; Heat transfer; Monte Carlo simulation; Parameters; Predictions; Probability distribution; Probability distribution functions; Separation; Solvers; Studies; Successive duplicated collisions; Successive duplicated collisions; Collisional parameters; Fourier heat transfer problem; Collision-pair selection scheme
• ISSN: 0045-7930; 1879-0747
• DOI: 10.1016/j.compfluid.2017.11.005

•A detailed study on repeated collisions in NTC collision scheme in DSMC is presented.•Effects of duplicated collisions on heat flux error in Fourier problem is reported.•We showed that direct avoiding of repeated collisions leads to systematic errors. The current paper investigates the appropriateness and consequences of avoiding successively repeated collisions usually recommended in the Direct Simulation Monte Carlo (DSMC) solvers, which use the No Time Counter (NTC) collision technique. A sensitive test case, i.e., Fourier heat transfer problem at the early slip regime, is considered in the presence and absence of the successively repeated collisions. The effects of repeated collisions and avoiding them on different parameters such as heat flux, collision frequency, collision separation distance, accepted-to-selected collisions ratio, and probability distribution functions for the number of collisions and relative velocities of collision pairs are evaluated and discussed. The investigations are performed considering contributions of different computational parameters, i.e., cell sizes, time step, and number of particles. In addition to collision frequency and collision separation distance, which were reported previously as crucial parameters affecting the accuracy of the DSMC solution, we found that the level of repeated collisions also plays a pivotal role in the accuracy of the heat flux prediction in the Fourier problem. We show that direct avoiding of repeated collisions can lead to a distortion of collision probability distribution and consequently, to introduction of a systematic error in collision frequency and predicted heat flux. This error is negligible if one uses a large number of particles per cell, but it is of considerable importance if there are a few particles per cell.