24 research outputs found
Nonlinear closures for scale separation in supersonic magnetohydrodynamic turbulence
Turbulence in compressible plasma plays a key role in many areas of
astrophysics and engineering. The extreme plasma parameters in these
environments, e.g. high Reynolds numbers, supersonic and super-Alfvenic flows,
however, make direct numerical simulations computationally intractable even for
the simplest treatment -- magnetohydrodynamics (MHD). To overcome this problem
one can use subgrid-scale (SGS) closures -- models for the influence of
unresolved, subgrid-scales on the resolved ones. In this work we propose and
validate a set of constant coefficient closures for the resolved, compressible,
ideal MHD equations. The subgrid-scale energies are modeled by Smagorinsky-like
equilibrium closures. The turbulent stresses and the electromotive force (EMF)
are described by expressions that are nonlinear in terms of large scale
velocity and magnetic field gradients. To verify the closures we conduct a
priori tests over 137 simulation snapshots from two different codes with
varying ratios of thermal to magnetic pressure () and sonic Mach numbers (). Furthermore, we make a
comparison to traditional, phenomenological eddy-viscosity and
closures. We find only mediocre performance of the
kinetic eddy-viscosity and closures, and that the
magnetic eddy-viscosity closure is poorly correlated with the simulation data.
Moreover, three of five coefficients of the traditional closures exhibit a
significant spread in values. In contrast, our new closures demonstrate
consistently high correlation and constant coefficient values over time and and
over the wide range of parameters tested. Important aspects in compressible MHD
turbulence such as the bi-directional energy cascade, turbulent magnetic
pressure and proper alignment of the EMF are well described by our new
closures.Comment: 15 pages, 6 figures; to be published in New Journal of Physic
A morphological analysis of the substructures in radio relics
Recent observations of radio relics - diffuse radio emission in galaxy
clusters - have revealed that these sources are not smooth but consist of
structures in the form of threads and filaments. We investigate the origin of
these filamentary structures and the role of projection effects. To this end,
we have developed a tool that extracts the filamentary structures from
background emission. Moreover, it is capable of studying both two-dimensional
and three-dimensional objects. We apply our structure extractor to, both,
observations and cosmological simulations of radio relics. Using Minkowski
functionals, we determine the shape of the identified structures. In our 2D
analysis, we find that the brightest structures in the observed and simulated
maps are filaments. Our analysis of the 3D simulation data shows that radio
relics do not consist of sheets but only of filaments and ribbons. Furthermore,
we did not find any measurable projection effects that could hide any
sheet-like structures in projection. We find that, both, the magnetic field and
the shock front consist of filaments and ribbons that cause filamentary radio
emission.Comment: 20 pages, 22 figures, accepted for publication in MNRA