In turbulent thermal convection in cylindrical samples with an aspect ratio $\Gamma \equiv D/L$ ($D$ is the diameter and $L$ the height), the Nusselt number Nu is enhanced when the sample is rotated about its vertical axis because of the formation of Ekman vortices that extract additional fluid out of thermal boundary layers at the top and bottom. We show from experiments and direct numerical simulations that the enhancement occurs only above a bifurcation point at a critical inverse Rossby number 1/Ro$_c$, with 1/Ro$_c \propto 1/\Gamma$. We present a Ginzburg-Landau–like model that explains the existence of a bifurcation at finite 1/Ro$_c$ as a finite-size effect. The model yields the proportionality between 1/Ro$_c$ and $1/\Gamma$ and is consistent with several other measured or computed system properties.
- EC Grant Agreement nr.: FP6/031513
- EC Grant Agreement nr.: FP7/222919