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Plumes and turbulent transport

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ADIW01 - Layering — A structure formation mechanism in oceans, atmospheres, active fluids and plasmas

Plumes, thermals and jets are ubiquitously observed in convection in nature, and play an important role in effectively mixing the mass, momentum, and heat there. Plumes are similar to streamers in fusion plasmas in that they are basically generated in the direction parallel to the gradient of large-scale structure in turbulence. In the framework of multiple-scale renormalised perturbation expansion theory, where the response-function formalism is adopted as well as the multiple-scale analysis, non-equilibrium properties of turbulence are naturally incorporated into the expression of turbulence correlations. Utilising theses analytical results, deviations of the timescale and length scale from the expressions based on the local equilibrium turbulence can be argued. Examples of such non-equilibrium effects on turbulent transport are presented in the talk. To apply this non-equilibrium effect to the transport associated with multiple plumes, the double filtering approach in the time and space domains is introduced. In this formulation, any fluctuation in the space domain is divided into the coherent/dispersion and the incoherent/random ones. Energy transfer mechanism between the coherent and incoherent fluctuation energies is also discussed on the basis of transport equations of them. Regarding plume motions as coherent/dispersion fluctuations, a turbulence model for the convection with multiple plumes is proposed. Application to stellar convections with the aid of numerical simulations shows that the turbulent heat flux in the convection driven by surface cooling, which could not be reproduced by the standard gradient diffusion with the mixing-length model at all, can be properly reproduced by the present non-equilibrium transport model. Possible suggestions towards studies on plume formation mechanism will be made.

This talk is part of the Isaac Newton Institute Seminar Series series.

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