Molecular clouds are to a great extent influenced by turbulent motions in the gas .
Numerical and observational studies indicate that the star formation rate and efficiency crucially depend on the mixture of solenoidal and compressive modes in the turbulent acceleration field , which can be quantified by the turbulent driving parameter b .
For purely solenoidal ( divergence–free ) driving previous studies showed that b = 1 / 3 and for entirely compressive ( curl–free ) driving b = 1 .
In this study , we determine the evolution of the turbulent driving parameter b in magnetohydrodynamical simulations of molecular cloud formation and evolution .
The clouds form due to the convergence of two flows of warm neutral gas .
We explore different scenarios by varying the magnitude of the initial turbulent perturbations in the flows .
We show that the driving mode of the turbulence within the cloud strongly fluctuates with time and exhibits no clear correlation with typical cloud properties , such as the cloud mass and the ( Alfvén ) Mach number .
We specifically find that b strongly varies from b \sim 0.3 to b \sim 0.8 on timescales t \lesssim 5 Myr , where the timescale and range of variation can change from cloud to cloud .
This rapid change of b from solenoidal to compressive driving is primarily associated with global contraction of the cloud and subsequent onset of star formation .
We conclude that the effective turbulence driving parameter should be treated as a free parameter that can vary from solenoidal to compressive in both time and space .