Bounds on Heat Transport in Rapidly Rotating Rayleigh-B\'{e}nard Convection

TL;DR

This paper establishes rigorous upper bounds on heat transport in rapidly rotating Rayleigh-Bénard convection, revealing the limitations of current mathematical methods and the influence of thermal plumes on heat transfer limits.

Contribution

It derives a new upper bound on heat transport using the background method and analyzes the constraints on improving this bound in rapidly rotating convection.

Findings

Bound on Nusselt number: Nu ≤ 20.56 Ra^3 E^4

Bound cannot be improved with piece-wise monotonic background profiles

Small-scale thermal plumes limit the upper bound on heat transport

Abstract

The heat transport in rotating Rayleigh-B\'enard convection is considered in the limit of rapid rotation (small Ekman number $E$) and strong thermal forcing (large Rayleigh number $Ra$). The analysis proceeds from a set of asymptotically reduced equations appropriate for rotationally constrained dynamics; the conjectured range of validity for these equations is $Ra \lesssim E^{-8/5}$. A rigorous bound on heat transport of $Nu \le 20.56Ra^3E^4$ is derived in the limit of infinite Prandtl number using the background method. We demonstrate that the exponent in this bound cannot be improved on using a piece-wise monotonic background temperature profile like the one used here. This is true for finite Prandtl numbers as well, i.e. $Nu \lesssim Ra^3$ is the best upper bound for this particular setup of the background method. The feature that obstructs the availability of a better bound in this case is the appearance of small-scale thermal plumes emanating from (or entering) the thermal boundary layer.