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Projekt Druckansicht

Übergang zwischen 2-dimensionaler und 3-dimensionaler MHD Turbulenz unter Einwirkung eines starken Gleich-Magnetfeldes: Experiment mit Flüssigmetal in einer rechteckigen Box

Fachliche Zuordnung Strömungsmechanik
Förderung Förderung von 2007 bis 2010
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 36534768
 
Erstellungsjahr 2012

Zusammenfassung der Projektergebnisse

During the course of the project, the following tasks were successfully achieved: 1. Characterization of three-dimensionality and transition to turbulence of an electrically driven vortex pair. 2. Construction of two successive versions of a modular experimental facility with extensive diagnosis techniques to study liquid metal flows in high magnetic fields. 3. Experimental characterization of the mechanisms of the transition to three-dimensionality in a wall bounded array of electrically driven vortices. 4. Discovery of three-dimensional steady MHD vortices. 5. Derivation of an analytical model to explain the appearance of weak three-dimensionality in MHD, which applies to other flows such as rotating flows. Points 2-4 follow the initial layout of the project rather closely while points 1, 5-6 arose partly in the light of important new results obtained in this project and in projects led in parallel to the present one, that were therefore not known when the initial plan was established. Firstly the preliminary experimental setup which was used in 1 was initially planned as a test for the technologies used in the construction of the main setup. However, this early implementation proved to be of sufficiently high quality to enable us to characterize two-dimensional regimes of an electrically driven vortex pair as well as the effects of three-dimensionality in this configuration. We therefore seized this early opportunity to partially achieve one of the main goals of the project and reveal the previously unknown mechanism of transition to turbulence of a wall bounded vortex pair, which wasn’t known in this configuration. We published this work in Phys. Rev. E. Secondly, the first phase of the project, where the flow was electrically forced gave precise answers on the aspects of the transition between two and three-dimensional MHD turbulence which this project aimed at addressing, and which we were able to publish in the prestigious Phys. Rev. Lett. In the process, further questions were raised, in particular on the influence of 3D recirculations, which the mechanical forcing initially planned in the second phase would not have helped to address. It was therefore deemed more important to build the second version of the experiment with the same electrical forcing but to focus on ultrasound velocimetry to be able to diagnose the flow in the bulk. Finally we were able to derive a theoretical model which proves that some of the mechanisms found in the first phase were common to other types of flows such as flows in rotation. This model was published in EPL and broadens the impact of the results found in MHD to a much wider scientific community. In summary, it is fair to say that this project has delivered well above initial expectations. Not only did it allow us to unveil some of the previously unknown mechanisms acting at the transition between two and three-dimensional turbulence in the presence of walls but it made a very clear point that walls are a key component in the 2D-3D transition not only in MHD but also in non-MHD and rotating flows. Two types of three-dimensionalities were distinguished: a strong and a weak one. The strong one involves disruption of 2D structures through instability mechanism and was known from numerical simulations without walls. The first one on the other hand involves mild velocity variations in the third direction and we made clear that it was triggered by the presence of walls through a complex mechanism involving 3D recirculation that is active well beyond the field of MHD. This project has not only brought significant progress in the question of the dimensionality of MHD flows but shown that the problem was more universal and that it could not be realistically be considered without taking boundaries into account. On the top of this scientific achievement, the experimental method which we developed, based on a high number of simultaneous measurements of electric potential offers a way of extensively exploring liquid metal flows which opens vast possibilities for research in the fields of turbulence and vortex dynamics in general.

Projektbezogene Publikationen (Auswahl)

 
 

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