Passive scalar mixing at finite correlation times and the Batchelor spectrum [CL]

An elegant model for passive scalar mixing was given by Kraichnan (1968) assuming the velocity to be delta-correlated in time. For realistic random flows this assumption becomes invalid. We generalize the Kraichnan model to include the effects of a finite correlation time, $\tau$, using renewing flows. The generalized evolution equation for the 3-D passive scalar spectrum $\hat{M}(k,t)$ or its correlation function $M(r,t)$, gives the Kraichnan equation when $\tau \to 0$, and extends it to the next order in $\tau$. It involves third and fourth order derivatives of $M$ or $\hat{M}$ (in the high $k$ limit). For small-$\tau$ (or small Strouhl number), it can be recast using the Landau-Lifshitz approach, to one with at most second derivatives of $\hat{M}$. We present both a scaling solution to this equation neglecting diffusion and a more exact solution including diffusive effects. Interestingly, to leading order in $\tau$, we show that the steady state 1-D passive scalar spectrum, preserves the Batchelor (1959) form, $E_\theta(k) \propto k^{-1}$, in the viscous-convective limit, independent of $\tau$. When passive scalar fluctuations decay, we show that the decay rate is reduced for finite $\tau$, but the spectrum $E_\theta(k) \propto k^{1/2}$ independent of $\tau$ . We also present results from high resolution ($1024^3$) direct numerical simulations of passive scalar mixing. We find reasonable agreement with predictions of the Batchelor spectrum, during steady state. The scalar spectrum during decay is however shallower than what theory predicts, a feature which remains intriguing.

Read this paper on arXiv…

A. Aiyer, K. Subramanian and P. Bhat
Mon, 30 Nov 15

Comments: 20 pages, 2 figures, Submitted to JFM