Upper: Number density profiles for different snapshots of the simulation. Time in free-fall time units is colour-coded. The surface density of the cloud is ${\mathrm{\Sigma }}_g={10}^4{M}_{\odot }{\mathrm{pc}}^{-2}$, with metallicity log(Z/Z_⊙) =  − 2 and SFE ϵ_* = 30%. Lower: Same for the velocity profiles. The black dashed line shows the time evolution of the shell velocity, corresponding to the velocity of the cell with highest density.