Combined measurement of velocity and temperature in liquid metal convection


Combined measurement of velocity and temperature in liquid metal convection

Zürner, T.; Schindler, F.; Vogt, T.; Eckert, S.; Schumacher, J.

Combined measurements of velocity components and temperature in a turbulent Rayleigh-Benard convection flow at a low Prandtl number and for large Rayleigh numbers are conducted in a series of experiments with durations of more than a thousand free fall time units. Multiple crossing ultrasound beam lines and an array of thermocouples at mid-height allow for a detailed analysis and characterization of the complex three-dimensional dynamics of the single large-scale circulation (LSC) roll in the cylindrical convection cell of unit aspect ratio which is filled with the liquid metal alloy GaInSn. We extract the superposition of short-term oscillations of the LSC with different orientation angles close to the top/bottom plates and the related sloshing motion in the mid-plane with the slow azimuthal drift of the mean roll orientation as a whole that proceeds on a hundred times slower time scale, and measure the internal temporal correlations of this complex large-scale flow. The coherent LSC drives a vigorous turbulence in the whole cell that is quantified by direct Reynolds number measurements at different locations in the cell. The velocity increment statistics in the bulk of the cell displays characteristic properties of intermittent small-scale fluid turbulence. We also show that the impact of the symmetry-breaking large-scale flow persists to small-scale velocity fluctuations thus preventing the establishment of isotropic turbulence in the cell center. Reynolds number amplitudes depend sensitively on beam line position in the cell such that different definitions have to be compared. The global momentum and heat transfer scalings with Rayleigh number are found to agree with those of direct numerical simulations and other laboratory experiments.

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Publ.-Id: 29059