Evidences for the existence of the ultimate regime in supergravitational turbulent thermal convection

TL;DR

This study provides experimental evidence supporting the existence of Kraichnan's ultimate regime in supergravitational turbulent thermal convection, revealing a new high-Rayleigh-number scaling behavior and boundary layer characteristics.

Contribution

It demonstrates the existence of the ultimate turbulence regime in a supergravitational system, extending the understanding of thermal turbulence at extremely high Rayleigh numbers.

Findings

Kraichnan-like ultimate scaling of Nu vs. Ra observed

Turbulent velocity boundary layer profile identified

Enhanced shear Reynolds number and new temperature fluctuation statistics

Abstract

What is the final state of turbulence when the driving parameter approaches to infinity? For thermal turbulence, in 1962, Kraichnan proposed a so-called ultimate scaling dependence of the heat transport (quantified by the Nusselt number $\text{Nu}$) on the Rayleigh number ($\text{Ra}$), which can be extrapolated to arbitrarily high $\text{Ra}$. The existence of Kraichnan ultimate scaling has been intensively debated in the past decades. In this work, using a supergravitational thermal convection system, with an effective gravity up to 100 times the Earth's gravity, both Rayleigh number and shear Reynolds number can be boosted due to the increase of the buoyancy driving and the additional Coriolis forces. Over a decade of $\text{Ra}$ range, we demonstrate the existence of Kraichnan-like ultimate regime with four direct evidences: the ultimate scaling dependence of $\text{Nu}$ versus $\text{Ra}$; the appearance of turbulent velocity boundary layer profile; the enhanced strength of the shear Reynolds number; the new statistical properties of local temperature fluctuations. The present findings will greatly improve the understanding of the flow dynamics in geophysical and astrophysical flows.