Two-dimensional adaptive mesh refinement simulations of colliding flows

Context. Colliding flows are a commonly used scenario for the formation of molecular clouds in numerical simulations. Turbulence is produced by cooling, because of the thermal instability of the warm neutral medium. Aims. We carried out a two-dimensional numerical study of colliding flows to test whether statistical properties inferred from adaptive mesh refinement (AMR) simulations are robust with respect to the applied refinement criteria. Methods. We compare probability density functions of various quantities, as well as the clump statistics and fractal dimension of the density fields in AMR simulations to a static-grid simulation. The static grid with 20482 cells matches the resolution of the most refined subgrids in the AMR simulations. Results. The density statistics are reproduced fairly well by AMR. Refinement criteria based on the cooling time or the turbulence intensity appear to be superior to the standard technique of refinement by overdensity. Nevertheless, substantial differences in the flow structure become apparent. Conclusions. In general, it is difficult to separate numerical effects from genuine physical processes in AMR simulations.