Abstract
We present laboratory measurements of the interaction between thermoelectric currents and turbulent magnetoconvection. In a cylindrical volume of liquid gallium heated from below and cooled from above and subject to a vertical magnetic field, it is found that the large-scale circulation (LSC) can undergo a slow axial precession. Our experiments demonstrate that this LSC precession occurs only when electrically conducting boundary conditions are employed, and that the precession direction reverses when the axial magnetic field direction is flipped. A thermoelectric magnetoconvection (TEMC) model is developed that successfully predicts the zeroth-order magnetoprecession dynamics. Our TEMC magnetoprecession model hinges on thermoelectric current loops at the top and bottom boundaries, which create Lorentz forces that generate horizontal torques on the overturning large-scale circulatory flow. The thermoelectric torques in our model act to drive a precessional motion of the LSC. This model yields precession frequency predictions that are in good agreement with the experimental observations. We postulate that thermoelectric effects in convective flows, long argued to be relevant in liquid metal heat transfer and mixing processes, may also have applications in planetary interior magnetohydrodynamics.
Highlights
The classical set-up for magnetoconvection (MC) is that of Rayleigh–Bénard convection (RBC) in an electrically conductive fluid layer occurring in the presence of an externally imposed magnetic field (e.g. Nakagawa 1955; Chandrasekhar 1961)
Three behavioural regimes are identified primarily using sidewall temperature measurements: (i) a turbulent large-scale circulation ‘jump rope vortex (JRV)’ regime in the weak magnetic field regime (Vogt et al 2018a); (ii) a magnetoprecessional (MP) regime in which the large-scale circulation (LSC) precesses around its vertical axis is found for moderate magnetic field strengths and electrically conducting boundary conditions; (iii) a multi-cellular magnetoconvection (MCMC) regime is found in the highest magnetic field strength cases
2.5 % in the Insulating RBC case. (The broad lower frequency peaks correspond to the slow meanderings of the LSC plane.) The distinct sharp peaks in both the Insulating MC+ and the Insulating MC− fast Fourier transforms (FFTs) are ≈ 25 % lower than fJRV
Summary
The classical set-up for magnetoconvection (MC) is that of Rayleigh–Bénard convection (RBC) in an electrically conductive fluid layer occurring in the presence of an externally imposed magnetic field (e.g. Nakagawa 1955; Chandrasekhar 1961). Three behavioural regimes are identified primarily using sidewall temperature measurements: (i) a turbulent large-scale circulation ‘jump rope vortex (JRV)’ regime in the weak magnetic field regime (Vogt et al 2018a); (ii) a magnetoprecessional (MP) regime in which the large-scale circulation (LSC) precesses around its vertical axis is found for moderate magnetic field strengths and electrically conducting boundary conditions; (iii) a multi-cellular magnetoconvection (MCMC) regime is found in the highest magnetic field strength cases This is the first systematic study of the MP mode, this is not the first time that it has been experimentally observed.
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