What is the Strouhal number of turbulence driven by supernovae?

The Strouhal number, St = t_cor/t_out, measures the temporal coherence of turbulent driving relative to the outer-scale eddy turnover time. In turbulence-box models one commonly sets St = 1, although recent work has shown that turbulence statistics, especially the mass-density distribution in compressively driven turbulence, are sensitive to this choice. In this Letter, we compute St directly from the measured two-time correlation tensor and outer-scale eddy time in stratified multiphase ISM simulations of Milky Way-like and starburst disks. We find isotropic median values St = 0.26 for the Milky Way-like model and St = 0.25 for the starburst model. These values are consistent with the picture that supernova remnants drive turbulence locally near R_cool, where the unstable contact discontinuity in the expanding remnant sets comparable forcing and eddy times. The reconstructed scale-dependent curves reach St = 1 at a nearly universal outer-scale fraction, ell_*/ell_out = 0.12–0.13, so the standard St = 1 prescription is not an outer-scale model of SN-driven ISM turbulence, but a local-scale approximation tied to injection near the cooling radius of the remnant.