Effects of polymer additives on Rayleigh-Taylor turbulence

TL;DR

This study uses direct numerical simulations to explore how polymer additives influence Rayleigh-Taylor turbulence, revealing enhanced large-scale mixing, reduced small-scale turbulence, and increased heat transport due to viscoelastic effects.

Contribution

It demonstrates the impact of polymers on turbulent mixing and heat transfer in Rayleigh-Taylor flows, highlighting effects previously unobserved without boundary layers.

Findings

Polymer additives enhance large-scale mixing.

Thermal plumes become more coherent with polymers.

Heat transport is significantly increased.

Abstract

The role of polymers additives on the turbulent convective flow of a Rayleigh--Taylor system is investigated by means of direct numerical simulations (DNS) of Oldroyd-B viscoelastic model. The dynamics of polymers elongation follow adiabatically the self-similar evolution of the turbulent mixing layer, and shows the appearance of a strong feedback on the flow which originate a cut off for polymer elongations. The viscoelastic effects on the mixing properties of the flow are twofold. Mixing is appreciably enhanced at large scales (the mixing layer growth-rate is larger than that of the purely Newtonian case) and depleted at small scales (thermal plumes are more coherent with respect to the Newtonian case). The observed speed up of the thermal plumes, together with an increase of the correlations between temperature field and vertical velocity, contributes to a significant {\it enhancement of heat transport}. Our findings are consistent with a scenario of {\it drag reduction} between falling and rising plumes induced by polymers, and provide further evidence of the occurrence of drag reduction in absence of boundary layers. A weakly non-linear model proposed by Fermi for the growth of the mixing layer is reported in the Appendix.