15:00
Magnetohydrodynamics 3
Chair: Alexandros Alexakis
15:00
15 mins
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MEAN MAGNETIC FIELD GENERATION IN IDEAL FLUID
Krzysztof Mizerski
Abstract: The so-called alpha-effect for large scale fields occurring in turbulent MHD flows is a typically invoked mechanism for mean field generation in all astrophysical objects such as stars, planets, accretions disks, galaxies etc. Although the resistivity of the fluid tends to damp the magnetic fields growth it is helpful in generation of the large scale electromotive force. However, high resolution simulations of Tobias & Cattaneo Nature, 497, 463-465, 2013 show that the resistivity is not a necessary factor for large scale field generation. Here it is demonstrated that there exists a mechanism for mean electromotive force generation based on interactions of turbulent small scale fields, which does not require the resistivity to operate and which is potentially very effective.
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15:15
15 mins
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The Decay of Wall Bounded MHD Turbulence at Low $Rm$
Kacper Kornet, Alban Potherat
Abstract: We have developed a new spectral method to simulate flows with very fine boundary layers present. We apply it to calculate the evolution of freely decaying MHD turbulence between isolating walls. By comparison them with results obtained in fully periodic domain we quantify the influence of the channel walls on the character of freely decaying MHD turbulence.
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15:30
15 mins
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Turbulent MHD channel flows under streamwise magnetic field
Thomas Boeck, Dmitry Krasnov
Abstract: A streamwise magnetic field leads to turbulent drag reduction in channel flow of a conducting liquid due to the selective Joule damping of certain flow structures. Near the walls, the turbulent mean velocity profile retains the logarithmic layer but the von Karman constant decreases with increasing magnetic field strength. In the outer region, the flow is characterized by persistent streaky structures of large streamwise extent, which lead to a rather flat mean velocity profile. In addition, the streamwise velocity fluctuations develop a pronounced second peak upon increasing the magnetic induction as well as a second logarithmic layer that increases in steepness.
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15:45
15 mins
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Magnetohydrodynamic turbulence in a Hartmann duct flow at finite magnetic Reynolds number
Vinodh Kumar Bandaru, Thomas Boeck, Dmitry Krasnov, Jörg Schumacher
Abstract: The dynamics of turbulent flow at finite magnetic Reynolds numbers can be very complex due to the coupled nature of the evolution equations for the flow and magnetic fields. In this regime, the Hartmann flow in a straight rectangular duct with streamwise periodicity is studied with the help of direct numerical simulations (DNS) and the effect of magnetic Reynolds number on turbulent statistics is quantified by comparing the results with the numerical results obtained using the quasistatic approximation.
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16:00
15 mins
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Helical mode interactions and spectral energy transfer in magnetohydrodynamic turbulence
Moritz F. Linkmann, Arjun Berera, Mairi E. McKay, Julia Jäger
Abstract: Spectral transfer processes in magnetohydrodynamic (MHD) turbulence are investigated by decomposition of the velocity and magnetic fields in Fourier space into helical modes. In 1992, Waleffe (Phys. Fluids A, 4:350 (1992)) used this decomposition to calculate triad interactions for isotropic hydrodynamic turbulence and determined whether a given triad contributed to forward or reverse energy transfer depending on the helicities of the interacting modes. The problem becomes more difficult in MHD due to the need to treat a coupled system of partial differential equations and the energy transfers between the magnetic and velocity fields. This requires the development of techniques that extend Waleffe's work, which are subsequently used to calculate the direction of energy transfer processes originating from triad interactions derived from the MHD equations. In order to illustrate the possible transfer processes that arise from helical mode interactions, we focus on simplified cases and putting special emphasis on interactions resulting in reverse spectral energy transfer. This approach also proves to be helpful in determining the nature of certain energy transfer processes, where transfer of energy between different fields and between the same field can be distinguished.
Reverse transfer of magnetic energy was found if the helicities of two modes corresponding to the smaller wavenumbers are the same, while for reverse transfer of kinetic energy Waleffe's result is recovered. Reverse transfer of kinetic to magnetic energy is facilitated if the interacting magnetic field modes are of opposite helicity, and no reverse transfer of magnetic to kinetic energy was found.
More generally, the direction of energy transfer not only depends on helicity but also on the ratio of magnetic to kinetic energy. For the magnetically dominated case reverse transfer occurs of all helicities are the same, the kinetically dominated case two modes need to have the same helicity while the third mode is of opposite helicity to allow reverse transfer.
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