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Context. Supersonic disordered flows accompany the formation and evolution ofMCs. It has been argued that turbulence can support against gravitationalcollapse and form hierarchical sub-structures. Aims. We study the timeevolution of simulated MCs to investigate: What physical process dominates thedriving of turbulent flows? How can these flows be characterised? Do thesimulated flows agree with observations? Methods. We analyse three MCs thathave formed self-consistently within kpc-scale numerical simulations of theISM. The simulated ISM evolves under the influence of physical processesincluding self-gravity, stratification, magnetic fields, supernova-driventurbulence, and radiative heating and cooling. We characterise the flows usingVSFs with/out density weighting or a density cutoff, and computed in one orthree dimensions. Results. In regions with sufficient resolution, thedensity-weighted VSFs initially appear to follow the expectations for uniformturbulence consistent with Larson's size-velocity relationship. SN blast waveimpacts produce short-lived coherent motions at large scales, increasing thescaling exponents for a crossing time. Gravitational contraction drivessmall-scale motions, producing scaling coefficients that drop or even turnnegative as small scales become dominant. Conclusions. We conclude that twodifferent effects coincidentally reproduce Larson's size velocity relationship.Initially, uniform turbulence dominates, so the energy cascade produces VSFsconsistent with Larson's relationship. Later, contraction dominates, thedensity-weighted VSFs become much shallower or even inverted, but therelationship of the global average velocity dispersion of the MCs to theirradius follows Larson's relationship, reflecting virial equilibrium orfree-fall collapse. The injection of energy by shocks is visible in the VSFs,but decays within a crossing time.

How do velocity structure functions trace gas dynamics in simulated molecular clouds?