Structure of shearless turbulent merging

The merging of two turbulent fronts without mean shear is investigated by direct numerical simulations. The turbulent streams are created by prescribing instantaneous velocity fields from precursor simulations of homogeneous isotropic turbulence (HIT) as inlet conditions for spatially evolving turbulent merging. The fronts are initially separated by a distance H and convected with a uniform free stream velocity U∞. The inlet turbulence intensity varies in the range of 0.24 ≤ u'/U∞ ≤ 0.47, while the inlet Taylor-scale Reynolds number is in the range of 151 ≤ Reλ ≤ 317. As the flow develops in the streamwise direction, two distinct regions are identified: (i) an initial linear decay region, where the two turbulent fronts gradually approach each other without noticeable interaction; and (ii) a rapid decay region, where the opposing turbulent fronts influence one another and eventually merge. The flow statistics collapse once the streamwise coordinate is rescaled as x+ = (x/H) (u'/U∞), suggesting that the merging location is imposed by large scales. An analysis conditioned to the developing turbulent/non-turbulent interfaces (TNTIs) reveals that, within the merging region, conditional mean enstrophy profiles deviate from those observed in ‘classical’ TNTIs, indicating a locally more homogenous flow. Within this region of interaction, the surface area of the TNTI increases while the volume of irrotational fluid steadily decreases, resulting in the generation of fine-scale structures. These findings support that turbulent merging is a multiscale process, where both the largest and smallest scales of motion intervene.