Combined measurement of velocity and temperature in liquid metal convection

Abstract

Combined measurements of velocity components and temperature in a turbulent Rayleigh-Bénard convection flow at a low Prandtl number of Pr = 0.029 and Rayleigh numbers between 10 6 ≤ Ra ≤ 6×10 7 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 measure the internal temporal correlations of the complex large-scale flow and distinguish between short-term oscillations associated with a sloshing motion in the mid-plane with varying orientation angles of the velocity close to the top/bottom plates and the slow azimuthal drift of the mean orientation of the roll as a whole that proceeds on a hundred times slower time scale. 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 centre. 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.arXiv:1907.03472v1 [physics.flu-dyn]

Turbulent convection for different thermal boundary conditions at the plates

“…The agreement is still good for their 19.1 ± 1.3 with the present value of 21.35 ± 1.83 at Ra = 10 8 even though differences are larger, most probably caused by the different aspect ratios. The magnitude of the turbulent heat transfer agrees also well with measurements by Cioni et al (1997) and Glazier et al (1999) in mercury (Pr = 0.025) and by Zürner et al (2019) in GaInSn for Pr = 0.029, all with Γ = 1.…”

“…As already said in the introduction, low-Pr experiments typically apply copper plates at the top and bottom (e.g. Cioni et al 1997;Horanyi et al 1999;Huang et al 2015;Vasil'ev et al 2015;Zürner et al 2019), which have thermal conductivities that are only an order of magnitude larger than those of the liquid metal (see table 1). The rapid re-establishment of a uniform plate temperature after local plume detachments should ideally proceed by heat conduction inside the plate, and not via the working fluid.…”

Connections between nonrotating, slowly rotating, and rapidly rotating turbulent convection transport scalings

“…Thus, its scaling behaviors are of broad interest and should be considered in future studies. An array of new convection and rotating convection devices have been recently built at research centers worldwide [15,63,107]. These next-generation laboratory devices and associated state-of-the-art numerical simulations, will allow investigations into the efficacy and applicability ranges of the turbulent scaling predictions presented here (Tables I and II).…”

Thermal boundary layer structure in low-Prandtl-number turbulent convection

“…2016; Zürner et al. 2019); and (iii) high Rayleigh numbers can be achieved even with moderate computational resources; the highest explored in this work has not been achieved in 3-D DNS at this (Scheel & Schumacher 2017). Note that 2-D RBC has been utilized to better understand some of the important phenomena in convection, e.g.…”

“…1999; Zürner et al. 2019). On the other hand, numerical investigations of low- convection require massive computational resources, as very small length and time scales need to be resolved accurately to prudently study them (Pandey & Verma 2016; Scheel & Schumacher 2016; Schumacher et al.…”