  Using ultrasonic velocimetry we measured the vertical profile of the velocity
fluctuation in high Rayleigh number thermal convection in a cell with aspect
ratio of 0.5, filled with a low Prandtl number fluid, mercury. Intriguing
fluctuating dynamics of the mean flow and universal nature of the kinetic
energy cascade are elucidated utilizing spectral decomposition and
reconstruction. Scaling properties of the structure functions and the energy
spectrum are directly calculated without the use of Taylor's frozen-flow
hypothesis for the first time. Despite the complex nature of the mean flow, it
is found that the energy cascade process exhibits universal laws in thermal
turbulence.
