Search

The upcoming DRESDYN (DREsden Sodium facility for DYNnamo and thermohydraulic studies) precession experiment will test the possibility to achieve magnetohydrodynamic dynamo action solely driven by precession. Here, after the description of the experimental facility, we present the results from direct numerical simulations with the aim to understand the flow behavior and its dynamo capability. The main conclusion is that in the nonlinear regime the nutation angle is an essential governing parameter which determines the flow structures and the possibility of dynamo action. We obtain clear indications about the optimum configuration for the future experimental runs. [ABSTRACT FROM AUTHOR]

Magnetic fields are key ingredients for heating the solar corona to temperatures of several million Kelvin. A particularly important region with respect to this is the so-called magnetic canopy below the corona, where sound and Alfvén waves have roughly the same speed and can, therefore, easily transform into each other. We present the results of an Alfvén-wave experiment with liquid rubidium carried out in a pulsed field of up to 63T. At the critical point of 54T, where the speeds of Alfvén waves and sound coincide, a new 4 kHz signal appears in addition to the externally excited 8 kHz torsional wave. This emergence of an Alfvén wave with a doubled period is in agreement with the theoretical predictions of a parametric resonance between the two wave types. We also present preliminary results from numerical simulations of Alfvén and magneto-sonic waves using a compressible MHD code. [ABSTRACT FROM AUTHOR]

A possible explanation for the apparent phase stability of the 11.07-year Schwabe cycle of the solar dynamo was the subject of a series of recent papers [1-3]. The synchronization of the helicity of an instability with the azimuthal wavenumber m = 1 by a tidal m = 2 perturbation played a key role here. To analyze this type of interaction in a paradigmatic setup, we study a thermally driven Rayleigh-Bénard convection of a liquid metal under the influence of a tide-like electromagnetic forcing. As shown previously, the time-modulation of this forcing emerges as a peak frequency in the m = 2 mode of the radial flow velocity component. In this paper, we present new numerical results on the interplay between the large-scale circulation of a Rayleigh-Bénard convection flow and the time modulated electromagnetic forcing. [ABSTRACT FROM AUTHOR]

Lack of appropriate flow measurement techniques for liquid steel during continuous casting limits the application of control strategies that could improve the quality of the end product. Contactless Inductive Flow Tomography (CIFT) is a promising measurement technique that can provide information about the flow structure in the mould to a real time controller. On this basis, electromagnetic actuators can be used to react on undesired flow conditions in the mould. However, because of their nature, these actuators pose a significant challenge for inductive measurement methods. In this work we describe the influence of an electromagnetic brake on CIFT in a laboratory environment. We also show how this influence can be fully compensated, which facilitates the viability of CIFT as a key ingredient of real time control of continuous casting. [ABSTRACT FROM AUTHOR]

The synchronization of the helicity of an instability with azimuthal wavenumber m=1 by a weak tidal m=2 perturbation might play a key role in explaining the phase-stable Schwabe cycle of the solar dynamo [1]. To elucidate this type of interaction, we study a thermally driven Rayleigh-Bénard convection under a tide-like influence. We focus first on the generation of the m=2 mode flow by electromagnetic forcing and then on its low-frequency modulation. In the last section, we present preliminary results on the interaction of this perturbation with the sloshing/torsional motion of the Large Scale Circulation (LSC) of a Rayleigh-Bénard convection flow. While the main focus of the paper is on the numerical side, some comparisons with experimental results are also made. [ABSTRACT FROM AUTHOR]

The time dependent behaviour of the large-scale circulation in a Rayleigh-Bénard convection cell exhibits a rich set of different three-dimensional flow features like rotations or torsional modes. In this paper, the applicability of contactless inductive flow tomography to visualise these flow features in a cylindrical cell filled with GaInSn is investigated numerically. The simulated flow in the cylinder with a diameter of 320mm and a height of 640mm serves as a basis to investigate the quality of the reconstructed velocity field by contactless inductive flow tomography. [ABSTRACT FROM AUTHOR]

Contactless inductive flow tomography (CIFT) can reconstruct the complex 3-dimensional flow structure of the large-scale circulation in liquid metal filled Rayleigh-Bénard (RB) convection cells. The method relies on the precise measurement of weak magnetic fields induced by currents in the conducting liquid arising from the fluid motion in combination with primary excitation fields. The velocity distribution is reconstructed from the magnetic field measurements by solving a linear inverse problem using the Tikhonov regularization and L-curve method. A number of technical challenges have to be overcome to reach the desired accuracy of the measurement signals. In this paper we will describe our design of a new CIFT set-up for a large RB vessel with a diameter of 320mm and a height of 640 mm. We outline the major factors perturbing the measurement signal of several tens of nanoteslas and describe solutions to decrease mechanical drifts by thermal expansion to a sub-critical level to enable CIFT measurements for high-Rayleigh number flows. [ABSTRACT FROM AUTHOR]

This study is a continuation of the combined experimental and numerical investigation [1] of the flow of the eutectic GaInSn alloy inside a cylindrical vessel exposed to a constant electrical current. The emerging electrovortex flow driven by the interaction of the current, which is applied through a tapered electrode, with its own magnetic field might have both detrimental and advantageous effects in liquid metal batteries. While the former work [1] was mainly concerned with time-averaged results, this paper focuses on the transient behaviour of the electrovortex flow which becomes most relevant under the influence of an external axial field. The additional Lorentz force generated by the interaction of the imposed current with the vertical component of the geomagnetic field bz drives the ordinary electrovortex jet flow into a swirling motion. Velocity distributions and motion characteristics, such as spiral streamlines and shortened and irregularly swinging jet regions, are investigated. The mechanism is analysed in detail for bz =--25.5 µT. The maximum angular velocity of the rotating jet is basically linearly dependent on bz, at least for the values studied here. A good agreement between the transient simulation and the experimental result is shown. [ABSTRACT FROM AUTHOR]

Aiming at a consistent planetary synchronization model of both short-term and long-term solar cycles, we start with an analysis of Schove's historical data of cycle maxima. Their deviations (residuals) from the average cycle duration of 11.07 years show a high degree of regularity, comprising a dominant 200-year period (Suess-de Vries cycle), and a few periods around 100 years (Gleissberg cycle). Encouraged by their robustness, we support previous forecasts of an upcoming grand minimum in the 21st century. To explain the long-term cycles, we enhance our tidally synchronized solar dynamo model by a modulation of the field storage capacity of the tachocline with the orbital angular momentum of the Sun, which is dominated by the 19.86-year periodicity of the Jupiter-Saturn synodes. This modulation of the 22.14-year Hale cycle leads to a 193-year beat period of dynamo activity which is indeed close to the Suess-de Vries cycle. For stronger dynamo modulation, the model produces additional peaks at typical Gleissberg frequencies, which seem to be explainable by the non-linearities of the basic beat process, leading to a bi-modality of the Schwabe cycle. However, a complementary role of beat periods between the Schwabe cycle and the Jupiter-Uranus/Neptune synodic cycles cannot be completely excluded. [ABSTRACT FROM AUTHOR]

Fluid flows in liquid metal batteries can be generated by a number of effects. We start with a short overview of different driving mechanisms and then address issues specific to the metal pad role instabilities in three-layer systems. We focus on the role of the conductivity distribution in the cell, noting at the same time that interfacial tension should be considered for smaller cells as well. Following this discussion, numerical results on the excitation of interfacial waves in two-layer liquid metal systems with miscibility gaps bearing an interface normal electric current are presented. Confirming recent results from the literature, we find that magnetic damping plays a decisive role for strong vertical magnetic fields. In addition, boundary conditions for the electric field strongly influence critical currents and growth rates. [ABSTRACT FROM AUTHOR]

We report the results of a liquid metal Taylor-Couette experiment in the centrifugally stable (Rayleigh stable) regime under the influence of an azimuthal magnetic field. In the present experimental setup, the azimuthal magnetorotational instability (AMRI) occurs as a travelling wave in a rotational shear flow. The observed symmetry breaking and a systematic deviation of the characteristic properties of the AMRI wave from theoretical prediction are caused by thermal convection. The central current-carrying rod was identified to be responsible for driving the radial heat flux. Preliminary numerical investigations point towards an interaction between AMRI and thermal convection, which supports our experimental findings. [ABSTRACT FROM AUTHOR]

Contactless Inductive Flow Tomography is able to reconstruct the three-dimensional flow field in conducting liquids by measuring the flow induced deformation of an applied magnetic field outside the liquid and by solving subsequently a linear inverse problem. In this paper, we will present a design of the arrangement of excitation coils and magnetic field sensors for a new Rayleigh-B'enard cylindrical convection cell with a diameter of 320mm and a height of 640mm filled with the eutectic alloy GaInSn. Based on this new setup first reconstructions will be shown. [ABSTRACT FROM AUTHOR]

In a cylindrical container filled with an eutectic GaInSn alloy, an electro-vortex flow (EVF) is generated by the interaction of a non-uniform current with its own magnetic field. In this paper, we investigate the EVF phenomenon numerically and experimentally. Ultrasound Doppler Velocimetry (UDV) is applied to measure the velocity field in a cylindrical vessel. Second, we enhance an old numerical solver by considering the effect of Joule heating and employ it for the numerical simulation of the EVF experiment. Special attention is paid to the role of the magnetic field, which is the combination of the current induced magnetic field and the external geomagnetic field. For a higher computational efficiency, the so-called parent-child mesh technique is applied in OpenFOAM when computing the electric potential, the current density and the temperature in the coupled solid-liquid conductor system. The results of the experiment are in good agreement with those of the simulation. This study may help to identify the factors that are essential for the EVF phenomenon and for quantifying its role in liquid metal batteries. [ABSTRACT FROM AUTHOR]

In this paper, we consider a liquid metal flow generated in a cylindrical volume by AC currents in various coil configurations. The final aim of this study is to design and optimize a Rayleigh-Bénard experiment with a large scale circulation, the helicity oscillation of which is synchronized by a periodically modulated tide-like m=2 perturbation. [ABSTRACT FROM AUTHOR]

First results of a new spherical Couette experiment are presented. The liquid metal flow in a spherical shell is exposed to a homogeneous axial magnetic field. For a Reynolds number Re =1000, we study the effect of increasing Hartmann number Ha. The resulting flow structures are inspected by ultrasound Doppler velocimetry. With a weak applied magnetic field, we observe an equatorially antisymmetric jet instability with the azimuthal wave number m =3. As the magnetic field strength increases, this instability vanishes. When the field is increased further, an equatorially symmetric return flow instability arises. Our observations are shown to be in good agreement with linear stability analysis and non-linear flow simulations. [ABSTRACT FROM AUTHOR]

The current-driven, kink-type Tayler instability (TI) is a key ingredient of the Tayler-Spruit dynamo model for the generation of stellar magnetic fields, but it is also discussed as a mechanism that might hamper the up-scaling of liquid metal batteries. Under some circumstances, the TI involves a helical flow pattern which goes along with some α-effect. Here we focus on the chiral symmetry breaking and the related impact on the α-effect that would be needed to close the dynamo loop in the Tayler-Spruit model. For low magnetic Prandtl numbers, we observe intrinsic oscillations of the α-effect. These oscillations serve then as the basis for a synchronized Tayler-Spruit dynamo model, which could possibly link the periodic tidal forces of planets with the oscillation periods of stellar dynamos. [ABSTRACT FROM AUTHOR]

Helical and azimuthal magnetorotational instabilities operate in rotating magnetized flows with relatively steep negative or extremely steep positive shear. The corresponding lower and upper Liu limits of the shear, which determine the threshold of modal growth of these instabilities, are continuously connected when some axial electrical current is allowed to pass through the rotating fluid. We investigate the nonmodal dynamics of these instabilities arising from the non-normality of shear flow in the local approximation, generalizing the results of the modal approach. It is demonstrated that moderate transient/nonmodal amplification of both types of magnetorotational instability occurs within the Liu limits, where the system is stable according to modal analysis. We show that for the helical magnetorotational instability this magnetohydrodynamic behavior is closely connected with the nonmodal growth of the underlying purely hydrodynamic problem. [ABSTRACT FROM AUTHOR]

The increasing deployment of renewable energies requires three fundamental changes to the electric grid: more transmission lines, a flexibilization of the demand and grid scale energy storage. Liquid metal batteries (LMBs) are considered these days as a promising means of stationary energy storage. Built as a stable density stratification of two liquid metals separated by a liquid salt, LMBs have three main advantages: a low price, a long life-time and extremely high current densities. In order to be cheap, LMBs have to be built large. However, battery currents of the order of kilo-amperes may lead to magnetohydrodynamic (MHD) instabilities, which - in the worst case - may short-circuit the thin electrolyte layer. The metal pad roll instability, as known from aluminium reduction cells, is considered as one of the most dangerous phenomena for LMBs. We develop a numerical model, combining fluid- and electrodynamics with the volume-of-fluid method, to simulate this instability in cylindrical LMBs. We explain the instability mechanism similar to that in aluminium reduction cells and give some first results, including growth rates and oscillation periods of the instability. [ABSTRACT FROM AUTHOR]

In the continuous casting industry, electromagnetic brakes (EMBr) are used to influence the mould flow, although their effect on the flow cannot be directly examined due to a lack of market-ready measurement techniques for liquid metal flows. Contactless inductive flow tomography (CIFT) is a technique that is able to reconstruct the mean flow structure of an electrically conducting melt by measuring the flow-induced perturbations of an applied magnetic field outside the melt and solving the linear inverse problem. Since CIFT relies on the measurement of magnetic fields, the question arises: Does CIFT work in the presence of a strong static magnetic field like that of an EMBr that superimposes and distorts the applied excitation magnetic field? In this paper, we will examine the effects of EMBr on CIFT with simulations and accompanying measurements. [ABSTRACT FROM AUTHOR]

The most ambitious project within the DREsden Sodium facility for DYNamo and thermohydraulic studies (DRESDYN) at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is the set-up of a precession-driven dynamo experiment. After discussing the scientific background and some results of water pre-experiments and numerical predictions, we focus on the numerous structural and design problems of the machine. We also outline the progress of the building's construction and the status of some other experiments that are planned in the framework of DRESDYN. [ABSTRACT FROM AUTHOR]

The next generation dynamo experiment currently under development at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) will consist of a precessing cylindrical container filled with liquid sodium. We perform numerical simulations of kinematic dynamo action applying a velocity field that is obtained from hydrodynamic models of a precession driven flow. So far, the resulting magnetic field growth rates remain below the dynamo threshold for magnetic Reynolds numbers up to Rm = 2000. [ABSTRACT FROM AUTHOR]

Magnetohydrodynamic instabilities can constitute a serious hazard to the functionality of liquid metal batteries. Here we consider the Tayler instability, which appears when the electric current, passing through a conducting fluid, reaches a critical value. The experiment discussed in this article involves a column of a eutectic GalnSn alloy, along whose axis an electric current passes. Ultrasound transducers encased in a copper electrode bounding the top of the column were used to obtain the vertical component of fluid flow, once a noise suppression system had been devised. The data thus retrieved will be discussed here. [ABSTRACT FROM AUTHOR]

The DREsden Sodium facility for DYNamo and thermohydraulic studies (DRESDYN) is intended as a platform both for large-scale experiments related to geo- and astrophysics as well as for experiments related to thermohydraulic and safety aspects of liquid metal batteries and liquid metal fast reactors. The most ambitious projects in the framework of DRESDYN are a homogeneous hydromagnetic dynamo driven solely by precession and a large Taylor-Couette type experiment for the combined investigation of the magnetorotational instability and the Tayler instability. In this paper, we give a short summary about the ongoing preparations and delineate the next steps for the realization of DRESDYN. [ABSTRACT FROM AUTHOR]