Self-similarity of turbulent jet flows with internal and external intermittency

The combined effect of internal and external intermittency on the statistical properties of small-scale turbulence is investigated in temporally evolving, planar turbulent jet flows at different Reynolds numbers using highly resolved direct numerical simulations. In turbulent jet flows, the phenomenon of external intermittency originates from a sharp layer, known as the turbulent/non-turbulent interface, that separates the turbulent core from the surrounding irrotational fluid. First, it is shown that low-order and higher-order structure functions in both the core and the shear layer of the jet satisfy complete self-preservation, which means that structure functions are invariant with time and collapse over the entire range of scales, regardless of the set of length and velocity scales used for normalization. Next, the impact of external intermittency on small-scale turbulence is studied along the cross-wise direction by the self-similarity of structure functions. It is shown that structure functions exhibit from the centre toward the edge of the flow a growing departure from self-similarity and the prediction of classical scaling theories. By analysing statistics conditioned on the turbulent portion of the jet, it is demonstrated that this departure is primarily due to external intermittency and the associated similarity-breaking effect.