This work concerned about modifications that help the Direct Simulation of Monte Carlo (DSMC) method to employ smaller amount of simulators/particles in unsteady flow conditions. As it is clear in flow fields with high Knudsen numbers, the statistical noise due to the rarefication and non-equilibrium effects force the DSMC method to provide excessive particles. To overcome this numerical obstacle and reduce the number of particles as small as possible two policies should be followed. First the viewpoint toward the concept of collision scheme should be altered to prevent from repeated collisions. Parallel to this aim, here we suggested using the Simplified Bernoulli Trials (SBT) collision scheme. Furthermore, part of the noise and perturbation of the DSMC results are engendered due to the smallness of sample-sizes. In steady state problems the sample size growth is readily possible if the numerical simulation continues, however, in unsteady time-accurate solutions there would not be such a possibility. Instead of waiting for a cell to grow its sample-size, we proposed the idea of mixing sample sizes, in which a cell or a surface properties can reach to its neighbor values that are weighted by their sample-sizes. The obtained results demonstrated that this policy in combination with a collision scheme that prevents from repeat collisions will notably help the DSMC method to improve the results in unsteady solutions, and employ smaller number of particles compared with other methods, e.g. the NTC-TAS or NN.

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