Thermal boundary layer measurements in turbulent rotating Rayleigh-B\'enard convection

We report measurements of the temperature boundary layer (BL) profile T($z$) in rotating Rayleigh-B\'enard convection along the centerline of a cylindrical sample. The measurements are taken for two Prandtl numbers Pr=6 and 40, with Rayleigh numbers in the range $5.8{\times}10^8{\le}$Ra${\le}6.3{\times}10^9$ and inversed Rossby numbers $0{\le}$1/Ro${\le}8$. In this parameter range pronounced enhancement of the heat transport (up to $22{\%}$ in Nu) is observed. Measurements of T($z$) reveal two regimes in which the effects of the applied rotations on the BL profiles differ markedly. In a low-Ra and high-Pr regime, the thermal BL thickness $\lambda$ decreases with increasing 1/Ro in accord with the global Nu-enhancement. In the other regime with relatively high Ra$>2.2{\times}10^9$ and low Pr, however, $\lambda$ is found to be independent of 1/Ro. The root-mean-square temperature prfiles $\sigma(z)$ exhibit power-low dependence $\sigma(z){\propto}z^{\beta(Ro)}$ when $z>\lambda$. The exponent $\beta$(Ro) is $-0.6{\pm}0.05$ when 1/Ro=0, but increases with increasing 1/Ro and is asymptotic to $\beta(\infty)$=$-0.2{\pm}0.02$ at large 1/Ro, indicating temperature fluctuation remains dominant in a broader domain outside the thermal BL that is ascribed to the Ekman pumping effect.