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arXiv · 1606.04812

Combined role of molecular diffusion, mean streaming and helicity in the eddy diffusivity of short-correlated random flows

Marco Martins Afonso·Andrea Mazzino·Sílvio Gama
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Abstract

We analytically investigate the effective-diffusivity tensor of a tracer particle in a fluid flow endowed with a short correlation time. By means of functional calculus and a multiscale expansion, we write down the main contributions to the eddy diffusivity due to each single physical effect and to their interplays. Namely, besides molecular diffusivity and a constant uniform mean streaming, we take into account the possibility for the (incompressible, Gaussian, stationary, homogeneous, isotropic) turbulent fluctuations to break parity invariance. With respect to the classical turbulence-driven diffusivity amplification for delta-correlated flows, we find that the presence of a short temporal correlation induces a diminution even when coupled with such effects, with two principal exceptions. Notably, the diffusivity is --- perturbatively --- enlarged not only by the helical contribution itself, but also by the interference between molecular diffusion and mean flow.

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Marco Martins Afonso, Andrea Mazzino, Sílvio Gama. 2016-06-15. Combined role of molecular diffusion, mean streaming and helicity in the eddy diffusivity of short-correlated random flows. https://doi.org/10.1088/1742-5468/2016/10/103205

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