Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session B14: Magnetohydrodynamics: Turbulence |
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Chair: P.K. Yeung, Georgia Institute of Technology Room: 307/308 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B14.00001: Rayleigh-B\'{e}nard Convection in Liquid Metal under Influence of Vertical Magnetic Fields Felix Schindler, Till Zuerner, Tobias Vogt, Sven Eckertz, Joerg Schumacher \noindent In the presented Rayleigh-B\'{e}nard convection experiments the turbulent 3d-flow of the liquid gallium-indium-tin alloy is investigated by use of ultrasound Doppler velocimetry, temperature and contactless inductive flow tomography measurements. We reconstruct for the first time near-wall as well as bulk flow, momentum and heat transport as well as long-term behaviour of the large-scale liquid metal flow at a low Prandtl number of $0.029$ and high Rayleigh numbers up to $5\cdot 10^{9}$. Also the influence of a strong magnetic field on the turbulent liquid metal is investigated. The results of the experiments are compared to direct numerical simulations and other experiments. Simulations are also considered for the interpretation of the measured turbulence statistics.\\ Our experiments aim to provide a deeper understanding of the turbulent convection and its interaction with magnetic fields in turbulent low Prandtl number flows as those in molten steel, aluminum or in geo- and astrophysical flows. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B14.00002: Mixing in magnetohydrodynamic turbulence X.M. 'Shine' Zhai, P. K. Yeung, Kiran Ravikumar Turbulence is characterized by its ability to efficiently promote scalar mixing, which occurs at the molecular level as large scale non-uniformities are broken into ever smaller pieces accompanied by a classical energy cascade. Yet for electrically conducting fluid subject to a strong magnetic field, anisotropy develops in all scales of turbulence and energy cascade is inhibited (Zhai \& Yeung, PRF 2018). As a result, scalar mixing in magnetohydrodynamic (MHD) turbulence is expected to show distinctly different behaviors. We have performed direct numerical simulations of scalar mixing in MHD turbulence under a mean scalar gradient. When a magnetic field is present, mixing of scalars in liquid metals, which has a Schmidt number of $\mathcal{O}$(0.01), is weakened as scalar variances grow more slowly. Variances of scalar gradients grow faster for less diffusive scalars, but gradient fluctuations in the direction of the magnetic field are suppressed. The largest simulations for passive scalars were performed at a resolution of $8192 \times 2048^2$ on a domain size of $32\pi \times (4\pi)^2$. The case of active scalars is also briefly addressed. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B14.00003: Decay of turbulence in a duct with transverse magnetic field Oleg Zikanov, Dmitry Krasnov, Thomas Boeck, Semion Sukoriansky Decay of honeycomb-generated turbulence in a duct with a static transverse magnetic field is studied via high-resolution direct numerical simulations. The simulations follow the experimental study of Sukoriansky et al, 1986, in particular the paradoxical observation of high-amplitude velocity fluctuations, which exist in the downstream portion of the flow when the strong transverse magnetic field is imposed in the entire duct including the honeycomb exit, but not in other configurations. It is shown that the fluctuations are caused by the large-scale quasi-two-dimensional structures forming in the flow at the initial stages of the decay and surviving the magnetic suppression. Statistical turbulence properties, such as the energy decay curves, two-point correlations and typical length scales are computed. The study demonstrates that turbulence decay in the presence of a magnetic field is a complex phenomenon critically depending on the state of the flow at the moment the field is introduced. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B14.00004: Laboratory Measurement of Non-Rotating Magnetoconvection in Liquid Gallium: Wall-mode Onset and Supercritical Precessional Mode Yufan Xu, Susanne Horn, Jonathan Aurnou Turbulent flows in the Earth's molten outer core, driven by convection, generate a planetary-scale, nearly axial, dipole-dominated magnetic field. The behaviors of strongly turbulent convection in the presence of strong Lorentz forces are mostly unknown. Thus, we present results of laboratory experiments on non-rotating Rayleigh-B\'enard convection of liquid gallium in the presence of a vertical magnetic field. Our heat transfer survey in a diameter-to-height aspect ratio $\Gamma=1$ tank, with $10^6 < Ra<10^8$ and $0< Ch< 3\times 10^5$, shows that the convection onsets well below the predicted $Ra$ for an infinite fluid layer. Magnetoconvection in our finite cylindrical tanks likely onsets via stationary wall-attached modes (Houchens et al., 2002; Busse, 2008). In a $\Gamma=2$ tank and with $Ra\approx 2\times10^6$, we vary the applied magnetic field corresponding to interaction parameter numbers $N$ from $0$ to $10$. Our thermal measurements show the existence of a novel precessional mode at $N\approx 0.5$ with electrically conducting boundaries (Cu), but not with electrically insulated boundaries (Teflon coated Al). This finding suggests the possibility of slowly traveling magneto-precessional modes attached to the electrically conductive boundaries of Earth's outer core. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B14.00005: Experimental Investigation of MHD Turbulent Heat Transfer for Molten Salts at Low Magnetic Field Caroline Sorensen, Zachary Hartwig In fusion power plants, a component known as the blanket is required to extract the thermal energy for energy production. MIT has proposed an innovative blanket concept based on the use of molten salts. One of the principal functions of the blanket is to cool components in close proximity to the confined fusion plasma, requiring the forced flow of molten salts through very high magnetic fields (15$+$ T). While the electrical conductivity of the molten salt is orders of magnitude lower than that of liquid metals (an alternative blanket fluid), it is still expected to experience significant magnetohydrodynamic effects in the high magnetic field regions. The degree to which turbulent heat transfer in the high Prandtl number salt will be degraded by the MHD effect is not known at high fields (Hartmann number in the 100s to 1000s), and it is not fully characterized for low fields. A new flow loop at MIT is used to validate and extend existing low field data (up to 2T) and Hartmann numbers in the low 10s using a molten salt simulant fluid. This paper will present temperature profiles and Nusselt number reductions for several angles with respect to the magnetic field. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B14.00006: Numerical simulation and experimental validation of MHD Turbulent Heat Transfer for Molten Salts at Low Magnetic Field Edoardo Prato, Caroline Sorensen, Zachary Hartwig The molten salt FLiBe is a candidate for the liquid breeder blanket in the design of the ARC fusion reactor [Sorbom et al 2015]. FLiBe's electrical conductivity is multiple orders of magnitude lower than those of liquid metals used in fusion designs. It considerably reduces the magnetohydrodynamic (MHD) effects that arise due to the presence of high magnetic field. Turbulence suppression and heat transfer degradation are observed in MHD flows. Considering these constraints is necessary for the ARC design to guarantee the heat extraction capability of the molten salt coolant. MHD simulations for laminar MHD flows have been run by using several CFD commercial tools and have been verified following Smolentsev 2015 guidelines. Turbulence is treated with hydrodynamic RANS models for which coefficients have been modified tanking into account MHD effects. At MIT an experimental campaign has been launched to measure MHD heat transfer reduction using a FLiBe simulant fluid flowing through a 2T magnet. The aim of the investigation is to observe ARC relevant conditions which are still unexplored (ie: low Rm, high Ha,Re,Pr). The proposed MHD RANS models such as Yamamoto 2017 based on DNS simulation can be validated and/or new ones can be built with the experimental data. [Preview Abstract] |
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