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Role of soft-iron impellers on the mode selection in the VKS dynamo experiment
Andre Giesecke,Frank Stefani,Gunter Gerbeth
Physics , 2009, DOI: 10.1103/PhysRevLett.104.044503
Abstract: A crucial point for the understanding of the von-K\'arm\'an-Sodium (VKS) dynamo experiment is the influence of soft-iron impellers. We present numerical simulations of a VKS-like dynamo with a localized permeability distribution that resembles the shape of the flow driving impellers. It is shown that the presence of soft-iron material essentially determines the dynamo process in the VKS experiment. % An axisymmetric magnetic field mode can be explained by the combined action of the soft-iron disk and a rather small $\alpha$-effect parametrizing the induction effects of unresolved small scale flow fluctuations.
Kinematic simulations of dynamo action with a hybrid boundary-element/finite-volume method
Andre Giesecke,Frank Stefani,Gunter Gerbeth
Physics , 2008,
Abstract: The experimental realization of dynamo excitation as well as theoretical and numerical examinations of the induction equation have shown the relevance of boundary conditions for a self-sustaining dynamo. Within the interior of a field producing domain geometric constraints or varying material properties (e.g. electrical conductivity of the container walls or localized high-permeability material) might also play a role. Combining a grid based finite volume approach with the boundary element method in a hybrid FV-BEM scheme offers the flexibility of a local discretization with a stringent treatment of insulating magnetic boundary conditions in almost arbitrary geometries at comparatively low costs. Kinematic simulations of dynamo action generated by a well known prescribed mean flow demonstrate the reliability of the approach. Future examinations are intended to understand the behavior of the VKS-dynamo experiment where the field producing flow is driven by ferrous propellers and the induction effects of conductivity/permeability inhomogeneities might provide the required conditions for the measured dynamo characteristics.
Magnetic material in mean-field dynamos driven by small scale helical flows
Andre Giesecke,Frank Stefani,Gunter Gerbeth
Physics , 2014, DOI: 10.1088/1367-2630/16/7/073034
Abstract: We perform kinematic simulations of dynamo action driven by a helical small scale flow of a conducting fluid in order to deduce mean-field properties of the combined induction action of small scale eddies. We examine two different flow patterns in the style of the G.O. Roberts flow but with a mean vertical component and with internal fixtures that are modelled by regions with vanishing flow. These fixtures represent either rods that lie in the center of individual eddies, or internal dividing walls that provide a separation of the eddies from each other. The fixtures can be made of magnetic material with a relative permeability larger than one which can alter the dynamo behavior. The investigations are motivated by the widely unknown induction effects of the forced helical flow that is used in the core of liquid sodium cooled fast reactors, and from the key role of soft iron impellers in the Von-K\'arm\'an-Sodium (VKS) dynamo. For both examined flow configurations the consideration of magnetic material within the fluid flow causes a reduction of the critical magnetic Reynolds number of up to 25%. The development of the growth-rate in the limit of the largest achievable permeabilities suggests no further significant reduction for even larger values of the permeability. In order to study the dynamo behavior of systems that consist of tens of thousands of helical cells we resort to the mean-field dynamo theory (Krause & R\"adler 1980) in which the action of the small scale flow is parameterized in terms of an $\alpha$- and $\beta$-effect. We compute the relevant elements of the $\alpha$- and the $\beta$-tensor using the so called testfield method. We find a reasonable agreement between the fully resolved models and the corresponding mean-field models for wall or rod materials in the considered range $1\leq \mu_r \leq 20$.
Impact of time-dependent non-axisymmetric velocity perturbations on dynamo action of von-Kármán-like flows
Andre Giesecke,Frank Stefani,Javier Burguete
Physics , 2012, DOI: 10.1103/PhysRevE.86.066303
Abstract: We have performed numerical simulations of the kinematic induction equation in order to examine the dynamo efficiency of an axisymmetric von-K\'arm\'an-like flow subject to time-dependent non-axisymmetric velocity perturbations. The numerical model is based on the setup of the French Von-K\'arm\'an-Sodium dynamo (VKS) and on the flow measurements from a model water experiment conducted at the University of Navarra in Pamplona, Spain. Our simulations show that the interactions of azimuthally drifting flow perturbations with the fundamental drift of the magnetic eigenmode (caused by the inevitable equatorial symmetry breaking of the basic flow) essentially determine the temporal behavior of the dynamo state. We find two distinct regimes of dynamo action that depend on the (prescribed) drift frequency of an ($m=2$) vortex-like flow perturbation. For comparatively slowly drifting vortices we observe a narrow window with enhanced growth-rates and a drift of the magnetic eigenmode that is synchronized with the perturbation drift. The resonance-like enhancement of the growth-rates takes place when the vortex drift frequency roughly equals the drift frequency of the magnetic eigenmode in the unperturbed system. Outside of this small window, the field generation is hampered compared to the unperturbed case, and the field amplitude of the magnetic eigenmode is modulated with approximately twice the vortex drift frequency. The abrupt transition between the resonant regime and the modulated regime is identified as an spectral exceptional point where eigenvalues (growth-rates and frequencies) and eigenfunctions of two previously independent modes collapse.
Inferring basic parameters of the geodynamo from sequences of polarity reversals
Michael Fischer,Gunter Gerbeth,Andre Giesecke,Frank Stefani
Physics , 2008, DOI: 10.1088/0266-5611/25/6/065011
Abstract: The asymmetric time dependence and various statistical properties of polarity reversals of the Earth's magnetic field are utilized to infer some of the most essential parameters of the geodynamo, among them the effective (turbulent) magnetic diffusivity, the degree of supercriticality, and the relative strength of the periodic forcing which is believed to result from the Milankovic cycle of the Earth's orbit eccentricity. A time-stepped spherically symmetric alpha^2-dynamo model is used as the kernel of an inverse problem solver in form of a downhill simplex method which converges to solutions that yield a stunning correspondence with paleomagnetic data.
Forward and inverse problems in fundamental and applied magnetohydrodynamics
Andre Giesecke,Frank Stefani,Thomas Wondrak,Mingtian Xu
Physics , 2012, DOI: 10.1140/epjst/e2013-01793-3
Abstract: This Minireview summarizes the recent efforts to solve forward and inverse problems as they occur in different branches of fundamental and applied magnetohydrodynamics. As for the forward problem, the main focus is on the numerical treatment of induction processes, including self-excitation of magnetic fields in non-spherical domains and/or under the influence of non-homogeneous material parameters. As an important application of the developed numerical schemes, the functioning of the von-K\'{a}rm\'{a}n-sodium (VKS) dynamo experiment is shown to depend crucially on the presence of soft-iron impellers. As for the inverse problem, the main focus is on the mathematical background and some first practical applications of the Contactless Inductive Flow Tomography (CIFT), in which flow induced magnetic field perturbations are utilized for the reconstruction of the velocity field. The promises of CIFT for flow field monitoring in the continuous casting of steel are substantiated by results obtained at a test rig with a low melting liquid metal. While CIFT is presently restricted to flows with low magnetic Reynolds numbers, some selected problems of non-linear inverse dynamo theory, with possible application to geo- and astrophysics, are also discussed.
Subcritical transition to turbulence of a precessing flow in a cylindrical vessel
Johann Herault,Thomas Gundrum,Andre Giesecke,Frank Stefani
Physics , 2015,
Abstract: The transition to turbulence in a precessing cylindrical vessel is experimentally investigated. Our measurements are performed for a { nearly-resonant} configuration with an initially laminar flow dominated by an inertial mode with azimuthal wave number $m=1$ superimposed on a solid body rotation. By increasing the precession ratio, we observe a transition from the laminar to a non-linear regime, which then breakdowns to turbulence for larger precession ratio. Our measurements show that the transition to turbulence is subcritical, with a discontinuity of the wall-pressure and the power consumption at the threshold $\epsilon_{LT}$. The turbulence is self-sustained below this threshold, describing a bifurcation diagram with a hysteresis. In this range of the control parameters, the turbulent flows can suddenly collapse after a finite duration, leading to a definitive relaminarization of the flow. The average lifetime $\langle \tau \rangle$ of the turbulence increases rapidly when $\epsilon$ tends to $\epsilon_{LT}$.
Experimental realization of dynamo action: present status and prospects
Andre Giesecke,Frank Stefani,Thomas Gundrum,Gunter Gerbeth,Caroline Nore,Jacques Leorat
Physics , 2012, DOI: 10.1017/S1743921313002846
Abstract: In the last decades, the experimental study of dynamo action has made great progress. However, after the dynamo experiments in Karlsruhe and Riga, the von-Karman-Sodium (VKS) dynamo is only the third facility that has been able to demonstrate fluid flow driven self-generation of magnetic fields in a laboratory experiment. Further progress in the experimental examination of dynamo action is expected from the planned precession driven dynamo experiment that will be designed in the framework of the liquid sodium facility DRESDYN (DREsden Sodium facility for DYNamo and thermohydraulic studies). In this paper, we briefly present numerical models of the VKS dynamo that demonstrate the close relation between the axisymmetric field observed in that experiment and the soft iron material used for the flow driving impellers. We further show recent results of preparatory water experiments and design studies related to the precession dynamo and delineate the scientific prospects for the final set-up.
Dise?o industrial y estética del movimiento: Influencia de los valores estéticos en la historia de los medios gráficos en Europa y Japón
Palabra Clave , 2011,
Abstract: the essay shows how the aesthetic predominant values in a culture have a bearing on the acceptance or rejection of certain products, independently of their practical functionality. the specific case is that of the printing of gutenberg, which does not reach acceptance in the japan of the xviith-century, because the ideals of perfection and production that the western printing represents, collide with the aesthetic conceptions of the japanese chiropraxis.
Dise o industrial y estética del movimiento Influencia de los valores estéticos en la historia de los medios gráficos en Europa y Japón
Michael Giesecke
Palabra Clave , 2011,
Abstract: El ensayo muestra cómo los valores estéticos predominantes en una cultura inciden en la aceptación o el rechazo de ciertos productos, independientemente de su funcionalidad práctica. El caso específico es el de la imprenta de Gutenberg, que no alcanza aceptación en el Japón del siglo XVI, porque los ideales de perfección y producción que la imprenta occidental representa chocan con las concepciones estéticas de la quiropraxis japonesa.
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