10:30
Magnetohydrodynamics 1
Chair: Axel Brandenburg
10:30
15 mins
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Turbulent reconnection in astrophysical plasmas and quantum fluids
Yasuhito Narita
Abstract: Understanding the origin and the mechanism of reconnection process in collisionless media such as astrophysical plasmas and superfluids remains one of the major challenges in physics. By comparing the induction equation for astrophysical plasmas with the smoothed vorticity equation for superfluids, the possible role of turbulence in triggering collisionless reconnection is highlighted.
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10:45
15 mins
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Magnetic condensate in 2D MHD
Stefano Musacchio
Abstract: By means of numerical simulations, we investigate the phenomenon of self-organization of the magnetic field in a large-scale coherent structure, which occurs in two-dimensional Magneto-Hydro-Dynamic (2D MHD) in presence of a magnetic forcing. We show that the magnetic condensate is not able to induce a large-scale motor effect on the velocity field.
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11:00
15 mins
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Hall effects on scale-hierarchy in MHD turbulence
Hideaki Miura, Tomoharu Hatori, Keisuke Araki
Abstract: Direct numerical simulations of MHD turbulence with and without uniform magnetic field are carried out to study the Hall effects on scale-hierarchy in MHD turbulence. It is observed that vortex and current sheets are filamented either by the Kelvin-Helmholtz instability or magnetic reconnection in case of Hall MHD turbulence, while the filamentation is not observed without the Hall term. We show that the filamentation occurs not only for scales smaller than the ion skin depth, which is indicated by the Hall parameter, but also for scales larger than the ion skin depth, affecting turbulence statistics. It is also shown that the Hall effects can be modelled by a Smagorinsky-type model effectively for high wave number regions.
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11:15
15 mins
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Robust energy transfer mechanism via precession resonance in nonlinear turbulent wave systems
Miguel D. Bustamante, Brenda Quinn, Dan Lucas
Abstract: A robust energy transfer mechanism is found in nonlinear wave systems, which favours transfers towards modes interacting via triads with nonzero frequency mismatch, applicable in meteorology, nonlinear optics and plasma wave turbulence. We emphasise the concepts of truly dynamical degrees of freedom and triad precession. Transfer efficiency is maximal when the triads' precession frequencies resonate with the system's nonlinear frequencies, leading to a collective state of synchronised triads with strong turbulent cascades at intermediate nonlinearity. Numerical simulations confirm analytical predictions.
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11:30
15 mins
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On the edge of an inverse cascade
Alexandros Alexakis
Abstract: We demonstrate that systems with a parameter-controlled inverse cascade can exhibit critical behavior for which at the critical value of the control parameter the inverse cascade stops. In many dynamical systems in nature energy is transferred to small or to large length scales by a forward or inverse cascade, respectively. In three-dimensional hydrodynamic (HD) turbulence energy cascades forward from large to small scales while in two-dimensional HD turbulence energy cascades inversely from small scales to large scales. There are some examples, however, that have a mixed behavior such as fast rotating fluids, conducting fluids in the presence of strong magnetic fields, flows in constrained geometry, and others. In these examples the injected energy cascades both forward and inversely in fractions that depend on the value of a control parameter (rotation rate/magnetic field/aspect ratio ect). In the presented work we demonstrate using the 2D-MHD model that the transition from a forward to an inverse cascade can occur by a critical transition, . In the absence of any external magnetic forcing the system reduces to hydrodynamic fluid turbulence with an inverse energy cascade. In the presence of strong magnetic forcing the system behaves as 2D-MHD turbulence with forward energy cascade. As the amplitude of the magnetic forcing is varied a critical value is met for which the energy flux towards the large scales becomes zero. Close to this point the energy flux scales as a power law with the departure from the critical point and the normalized amplitude of the fluctuations diverges. The generality of this behavior to other systems with variable inverse cascades will be discussed.
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11:45
15 mins
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Energy transfers in small-scale and large-scale dynamos
Rohit Kumar, Mahendra K. Verma
Abstract: We study energy transfers during magnetic energy growth in small-scale and large-scale dynamos. We perform direct numerical simulations for magnetic Prandtl number Pm =20 and 0.2 in a periodic box on 1024^3 grid. Energy fluxes and shell-to-shell energy transfers indicate that in small-scale dynamo for Pm =20, the magnetic energy growth takes place due to a non-local energy transfer from large-scale velocity field to small-scale magnetic field. On the other hand, in large-scale dynamo for Pm =0.2, local energy transfers from large-scale velocity field to large-scale magnetic field takes place.
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12:00
15 mins
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Intermittency in Weak Magnetohydrodynamic Turbulence
Romain Meyrand, Khurom Kiyani, Sébastien Galtier
Abstract: Three-dimensional incompressible magnetohydrodynamic (MHD) turbulence
with a strong uniform magnetic field b0 may be governed by the regime of
weak turbulence. At leading order, it is known that the asymptotic regime
of weak MHD turbulence is achieved via three-wave resonant interactions
with the scattering of two of these waves on a third/2D mode for which
k//=0. For zero cross-helicity, the expected exact solution is an energy
spectrum in $k_perp^{-2}$. Higher-order statistics has, however, never been
reported in the literature. Therefore, we have recently investigated this
question with high resolution direct numerical simulations (Meyrand et al.,
2014). We found the presence of strong intermittency when the vector
separation of structure functions is taken transverse to b0. This result may
be explained by the influence of the 2D modes whose regime belongs to
strong turbulence. In addition to shed light on the origin of this intermittency,
we derived a log-Poisson law, $\zeta_p = p/8 +1 -(1/4)^{p/2}$, which fits
perfectly the data and highlights the important role of parallel current sheets.
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12:15
15 mins
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TURBULENT 2.5D DYNAMOS
Kannabiran Seshasayanan
Abstract: We study the kinematic dynamo problem of a two dimensional turbulent flow with the third velocity component being advected as a passive scalar (2.5D flow). Both helical and nonhelical forcing is considered. The low-dimensionality of the system allows us to study it for a wide range of parameters of the system, here specifically the Reynolds number and the magnetic Reynolds number. We show that the small scale dynamo action depends on the Reynolds number. The critical magnetic Reynolds number after which small magnetic perturbations starts to grow for the nonhelical forcing case is found to be independent of the Reynolds number.
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