Mean Wind in Convective Turbulence of Mercury

The large-scale circulation, often called “wind,” in the confined thermal turbulence of mercury is studied experimentally. The instantaneous velocity profile at 128 points is directly measured using ultrasonic velocimetry. The periodic velocity oscillation is observed in the case of the aspect-ratio Γ=1,2 but not in Γ=0.5. Its peak frequency is scaled by f_c∝Ra^γ_c, where Ra is the Rayleigh number and γ_c=0.43,0.45 for Γ=1,2. f_c is close to the wind circulation frequency f_p, and has the same order of transit time from the bottom to the top of the convection cell. A single roll circulation is expected in Γ=1; however, axisymmetric toroidal rings may exist near the upper and lower plate for Γ=0.5, which are stable up to Ra=7×10¹⁰.