  The Direct Simulation Monte Carlo (DSMC) method was widely used to simulate
low density gas flows with large Knudsen numbers. However, DSMC encounters
limitations in the regime of lower Knudsen numbers (Kn<0.1). In such cases,
approaches from classical computational fluid dynamics (CFD) relying on the
continuum assumption are preferred, offering accurate solutions at acceptable
computational costs. In experiments aimed at imaging aerosolized nanoparticles
in vacuo a wide range of Knudsen numbers occur, which motivated the present
study on the analysis of the advantages and drawbacks of DSMC and CFD
simulations of rarefied flows in terms of accuracy and computational effort.
Furthermore, the potential of hybrid methods is evaluated. For this purpose,
DSMC and CFD simulations of the flow inside a convergent-divergent nozzle
(internal expanding flow) and the flow around a conical body (external shock
generating flow) were carried out. CFD simulations utilize the software
OpenFOAM and the DSMC solution is obtained using the software SPARTA. The
results of these simulation techniques are evaluated by comparing them with
experimental data (1), evaluating the time-to-solution (2) and the energy
consumption (3), and assessing the feasibility of hybrid CFD-DSMC approaches
(4).
  Keywords: DSMC; SPARTA; Continuum assumption; Transition regime; Rarefied
flow; high-performance computing
