Instantaneous Measurement of Velocity Fields in Developed Thermal Turbulence in Mercury

TL;DR

This study employs ultrasonic velocimetry to directly measure velocity fluctuations in high Rayleigh number thermal convection in mercury, revealing universal energy cascade behaviors despite complex mean flow dynamics.

Contribution

First direct measurement of velocity fields in thermal turbulence without Taylor's hypothesis, demonstrating universal energy cascade laws in low Prandtl number fluids.

Findings

Universal energy cascade laws observed in thermal turbulence.

Spectral decomposition reveals complex mean flow dynamics.

Scaling properties of structure functions and energy spectrum calculated directly.

Abstract

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.