On the impact of the turbulent/non-turbulent interface on differential diffusion in a turbulent jet flow
2016
The effect of differential diffusion of two passive scalars having Schmidt numbers
of unity and 0.25, respectively, is investigated using direct numerical simulation of a
temporally evolving jet. The objective of the research is twofold: (i) to compare the
turbulent/non-turbulent (T/NT) interface position using the scalar criterion between
the unity- and low-Schmidt-number scalar; and (ii) to determine the impact of
the T/NT interface on differential diffusion. For the latter, the T/NT interface is
detected using the vorticity criterion. To quantify the effect of differential diffusion, a
normalised differential diffusion parameter is analysed, clearly showing the dominance
of differential diffusion at the T/NT interface. A transport equation for the scalar
differences is then evaluated, which shows that differential diffusion originates at the
interface. Further, the separation between the passive scalars, arising due to differential
diffusion, is studied using conventional and conditional statistics with respect to the
interface distance. Since differential diffusion is known to be present at large and
small scales, the connection between them is analysed using the scalar dissipation rate.
Moreover, the physical mechanism responsible for the departure of the two scalars is
analysed using the scalar gradient alignment, the ratio of the diffusive fluxes and by
a transport equation for the scalar gradients.
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