Turbulent Rayleigh-Bénard convection in strong vertical magnetic field. - In: Conference proceedings, PAMIR 2019, Reims, (2019), S. 27-31
Rayleigh-Bénard convection in a vertical magnetic field at low Prandtl number. - In: Conference proceedings, PAMIR 2019, Reims, (2019), S. 2-7
Thermal convection experiments in liquid metal flows with and without magnetic field. - Ilmenau : Universitätsbibliothek, 2019. - 1 Online-Ressource (x, 113 Seiten)
Technische Universität Ilmenau, Dissertation 2019
Die Wechselwirkung zwischen elektrisch leitfähigen Fluiden und Magnetfeldern tritt in zahlreichen natürlichen Phänomenen und technischen Anwendungen auf. Weil die dabei relevanten Medien - meist Flüssigmetalle oder Plasmen - im Allgemeinen sehr heiß sind, werden die Strömungen meist von thermischer Konvektion begleitet oder werden sogar von dieser getrieben. Das Phänomen der sogenannten Magnetokonvektion ist damit von Interesse für eine große Anzahl physikalischer Systeme. Die vorliegende Arbeit untersucht hierbei zwei Aspekte. Zum einen wird der Fall betrachtet, wenn ein aufgeprägtes Magnetfeld das Strömungsfeld nicht verändert. Zum anderen werden die Modifizierungen von Strömungsstruktur und globalen Transporteigenschaften durch starke Magnetfelder untersucht. Der erste Fall ist wichtig für induktive Messtechniken, welche die Bewegung eines Mediums untersuchen müssen, ohne dieses dabei zu stören. Die Größe des Fluidvolumens, welches von einem örtlich begrenzten Magnetfeld beeinflusst wird, ist hier ein äußerst wichtiger Faktor. Dieses Thema wird untersucht, indem die Eindringtiefe des Magnetfeldes in das Medium theoretisch hergeleitet wird. Das erlaubt die Vorhersage einer Magnetfeldstärke, oberhalb derer eine Strömung maßgeblich gestört wird. Die theoretischen Ergebnisse werden mittels experimenteller Messungen überprüft. Dazu wird die Messmethode der lokalen Lorentzkraft-Anemometrie auf eine vertikale Konvektionsströmung angewandt. Für den zweiten Fall wird das System der Rayleigh-Bénard Konvektion unter einem homogenen, vertikalen Magnetfeld experimentell untersucht. Der Aufbau besteht aus einer zylindrischen Zelle mit einem Aspektverhältnis von eins. Die großskalige Struktur der Strömung wird mittels Temperaturmessungen und Ultraschall Doppler Anemometrie überwacht. Die Entwicklung der Strömung mit ansteigender Magnetfeldstärke kann in verschiedene Regime kategorisiert und mit theoretischen Vorhersagen sowie numerischen Simulationen verglichen werden. Globale Transporteigenschaften des Systems bezüglich Impuls und übertragener Wärme werden analysiert und ihr Verhalten anhand der zuvor gefundenen Strömungsregime interpretiert. Zusätzlich wird ein theoretisches Modell entwickelt um den turbulenten Wärme- und Impulstransport vorherzusagen. Dazu wird die Großmann-Lohse Theorie für klassische Rayleigh-Bénard Konvektion durch den Effekt eines vertikalen Magnetfeldes erweitert. Die experimentellen Daten aus der vorliegenden Arbeit und aus der Literatur werden genutzt, um dieses Modell zu verifizieren und zu optimieren. Dabei werden physikalische Prozesse identifiziert, welche maßgeblich zu den beobachteten Ergebnissen beitragen.
https://www.db-thueringen.de/receive/dbt_mods_00040236
Decay of turbulence in a duct with transverse magnetic field. - In: ASME digital collection, (2019), AJKFluids2019-4688, V001T01A063, 5 pages
https://doi.org/10.1115/AJKFluids2019-4688
Lagrangian coherent sets in turbulent Rayleigh-Bénard convection. - In: Physical review, ISSN 2470-0053, Bd. 99 (2019), 5, 053103, insges. 15 S.
Coherent circulation rolls and their relevance for the turbulent heat transfer in a two-dimensional Rayleigh-Bénard convection model are analyzed. The flow is in a closed cell of aspect ratio four at a Rayleigh number Ra = 10^6 and at a Prandtl number Pr = 10. Three different Lagrangian analysis techniques based on graph Laplacians (distance spectral trajectory clustering, time-averaged diffusion maps, and finite-element based dynamic Laplacian discretization) are used to monitor the turbulent fields along trajectories of massless Lagrangian particles in the evolving turbulent convection flow. The three methods are compared to each other and the obtained coherent sets are related to results from an analysis in the Eulerian frame of reference. We show that the results of these methods agree with each other and that Lagrangian and Eulerian coherent sets form basically a disjoint union of the flow domain. Additionally, a windowed time averaging of variable interval length is performed to study the degree of coherence as a function of this additional coarse graining which removes small-scale fluctuations that cause trajectories to disperse quickly. Finally, the coherent set framework is extended to study heat transport.
https://doi.org/10.1103/PhysRevE.100.053103
Comparison of subgrid-scale models for large-eddy simulation of hydrodynamic and magnetohydrodynamic channel flows. - In: International journal of numerical methods for heat & fluid flow, ISSN 1758-6585, Bd. 29 (2019), 7, S. 2224-2236
Purpose - This paper aims to address the performance of different subgrid-scale models (SGS) for hydro- (HD) and magnetohydrodynamic (MHD) channel flows within a collocated finite-volume scheme. Design/methodology/approach - First, the SGS energy transfer is analyzed by a priori tests using fully resolved DNS data. Here, the focus lies on the influence of the magnetic field on the SGS energy transport. Second, the authors performed a series of 18 a posteriori model tests, using different grid resolutions and SGS models for HD and MHD channel flows. Findings - From the a priori analysis, the authors observe a quantitative reduction of the SGS energy transport because of the action of the magnetic field depending on its orientation. The a posteriori model tests show a clear improvement because of the use of mixed-models within the numerical scheme. Originality/value - This study demonstrates the necessity of improved SGS modeling strategies for magnetohydrodynamic channel flows within a collocated finite-volume scheme.
https://doi.org/10.1108/HFF-09-2018-0500
Visual exploration of circulation rolls in convective heat flows. - In: 2019 IEEE Pacific Visualization Symposium, (2019), S. 202-211
We present techniques to improve the understanding of pattern forming processes in Rayleigh-Bénard-type convective heat transport, through visually guided exploration of convection features in timeaveraged turbulent flows. To enable the exploration of roll-like heat transfer pathways and pattern-forming anomalies, we combine feature extraction with interactive visualization of particle trajectories. To robustly determine boundaries between circulation rolls, we propose ridge extraction in a z-averaged temperature field, and in the extracted ridge network we automatically classify topological point defects hinting at pattern forming instabilities. An importance measure based on the circular movement of particles is employed to automatically control the density of 3D trajectories and, thus, enable insights into the heat flow in the interior of rolls. A quantitative analysis of the heat transport within and across cell boundaries, as well as investigations of pattern instabilities in the vicinity of defects, is supported by interactive particle visualization including instant computations of particle density maps. We demonstrate the use of the proposed techniques to explore direct numerical simulations of the 3D Boussinesq equations of convection, giving novel insights into Rayleigh-Bénard-type convective heat transport. Keywords: flow visualization, particle-based visualzation, convective heat transport
https://doi.org/10.1109/PacificVis.2019.00031
Combined measurement of velocity and temperature in liquid metal convection. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 876 (2019), S. 1108-1128
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 of 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 roll in a 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 as well as 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 time scale up to a hundred times slower. The coherent large-scale circulation 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 fully 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.
https://doi.org/10.1017/jfm.2019.556
Simultaneous measurements of velocity and temperature fields in large aspect ratio Rayleigh-Bénard convection. - In: 13th International Symposium on Particle Image Velocimetry, (2019), S. 457-467
https://athene-forschung.unibw.de/128915
On the challenges for reliable measurements of convection in large aspect ratio Rayleigh-Bénard cells in air and sulfur-hexafluoride. - In: Experimental thermal and fluid science, Volume 109 (2019), article 109841
https://doi.org/10.1016/j.expthermflusci.2019.109841