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Turbulent Rayleigh-Bénard convection with polymers: Understanding how heat flux is modified

Benzi, Roberto, Ching, Emily S. C. and De Angelis, Elisabetta 2016. Turbulent Rayleigh-Bénard convection with polymers: Understanding how heat flux is modified. Physical Review E 94 (6) , 063110. 10.1103/PhysRevE.94.063110

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Abstract

We study how polymers affect the heat flux in turbulent Rayleigh-Bénard convection at moderate Rayleigh numbers using direct numerical simulations with polymers of different relaxation times. We find that heat flux is enhanced by polymers and the amount of heat enhancement first increases and then decreases with the Weissenberg number, which is the ratio of the polymer relaxation time to the typical time scale of the flow. We show that this nonmonotonic behavior of the heat flux enhancement is the combined effect of the decrease in the viscous energy dissipation rate due to the viscosity of the Newtonian fluid and the increase in the energy dissipation rate due to polymers when Weissenberg number is increased. We explain why the viscous energy dissipation rate decreases with the Weissenberg number. Then by carrying out a generalized boundary layer analysis supplemented by a space-dependent effective viscosity from the numerical simulations, we provide a theoretical understanding of the change of the heat flux when the viscous energy dissipation rate is held constant. Our analysis thus provides a physical way to understand the numerical results.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Engineering
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Publisher: American Physical Society
ISSN: 2470-0045
Date of First Compliant Deposit: 1 December 2017
Last Modified: 11 Jul 2019 09:43
URI: http://orca-mwe.cf.ac.uk/id/eprint/98490

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