Inclined turbulent thermal convection in liquid sodium*

Inclined turbulent thermal convection at large Rayleigh numbers (Ra) in extremely small Prandtl-number (Pr) fluids is studied by measurements and high-resolution numerical simulations. The working fluid is liquid sodium (Pr about 0.0094) and the considered Ra is around $1.5\times 10^7$. The convection cell is a cylinder with equal height and diameter, where one circular surface is heated and another one cooled. For the limiting inclination angle $\beta$, which correspond to Rayleigh-Benard convection ($\beta=0$) and vertical convection ($\beta=\pi/2$), the scaling relations of the mean heat flux (Nusselt number Nu) with Ra are studied. Any inclination of the RBC cell leads to an increase of Nu; the maximal Nu is obtained, however, for a certain intermediate value of $\beta$. For small $\beta$, the large-scale circulation (LSC) exhibits a complex dynamics, with torsion and sloshing modes, which are suppressed for large $\beta$. When the LSC is twisted, the volume-average vertical heat flux is minimal, and it is maximal, when the LSC sloshing brings together the hot and cold streams of the LSC.