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Register a new userA comparison between grid and particle methods on the small-scale dynamo in magnetised supersonic turbulence
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We perform a comparison between the smoothed particle magnetohydrodynamics (SPMHD) code, Phantom, and the Eulerian grid-based code, Flash, on the small-scale turbulent dynamo in driven, Mach 10 turbulence. We show, for the first time, that the exponential growth and saturation of an initially weak magnetic field via the small-scale dynamo can be successfully reproduced with SPMHD. The two codes agree on the behaviour of the magnetic energy spectra, the saturation level of magnetic energy, and the distribution of magnetic field strengths during the growth and saturation phases. The main difference is that the dynamo growth rate, and its dependence on resolution, differs between the codes, caused by differences in the numerical dissipation and shock capturing schemes leading to differences in the effective Prandtl number in Phantom and Flash.
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Supersonic turbulence is believed to be at the heart of star formation. We have performed smoothed particle magnetohydrodynamics (SPMHD) simulations of the small-scale dynamo amplification of magnetic fields in supersonic turbulence. The calculations use isothermal gas driven at rms velocity of Mach 10 so that conditions are representative of star-forming molecular clouds in the Milky Way. The growth of magnetic energy is followed for 10 orders in magnitude until it reaches saturation, a few percent of the kinetic energy. The results of our dynamo calculations are compared with results from grid-based methods, finding excellent agreement on their statistics and their qualitative behaviour. The simulations utilise the latest algorithmic developments we have developed, in particular, a new divergence cleaning approach to maintain the solenoidal constraint on the magnetic field and a method to reduce the numerical dissipation of the magnetic shock capturing scheme. We demonstrate that our divergence cleaning method may be used to achieve $ abla cdot {bf B}=0$ to machine precision, albeit at significant computational expense.
We present non-radiative, cosmological zoom-simulations of galaxy cluster formation with magnetic fields and (anisotropic) thermal conduction of one very massive galaxy cluster with a mass at redshift zero that corresponds to $M_mathrm{vir} sim 2 times 10^{15} M_{odot}$. We run the cluster on three resolution levels (1X, 10X, 25X), starting with an effective mass resolution of $2 times 10^8M_{odot}$, subsequently increasing the particle number to reach $4 times 10^6M_{odot}$. The maximum spatial resolution obtained in the simulations is limited by the gravitational softening reaching $epsilon=1.0$ kpc at the highest resolution level, allowing to resolve the hierarchical assembly of the structures in very fine detail. All simulations presented, have been carried out with the SPMHD-code Gadget-3 with a heavily updated SPMHD prescription. The primary focus is to investigate magnetic field amplification in the Intracluster Medium (ICM). We show that the main amplification mechanism is the small scale-turbulent-dynamo in the limit of reconnection diffusion. In our two highest resolution models we start to resolve the magnetic field amplification driven by this process and we explicitly quantify this with the magnetic power-spectra and the magnetic tension that limits the bending of the magnetic field lines consistent with dynamo theory. Furthermore, we investigate the $ abla cdot mathbf{B}=0$ constraint within our simulations and show that we achieve comparable results to state-of-the-art AMR or moving-mesh techniques, used in codes such as Enzo and Arepo. Our results show for the first time in a fully cosmological simulation of a galaxy cluster that dynamo action can be resolved in the framework of a modern Lagrangian magnetohydrodynamic (MHD) method, a study that is currently missing in the literature.
We investigate the clustering and dynamics of nano-sized particles (nano-dust) in high-resolution ($1024^3$) simulations of compressible isothermal hydrodynamic turbulence. It is well-established that large grains will decouple from a turbulent gas flow, while small grains will tend to trace the motion of the gas. We demonstrate that nano-sized grains may cluster in a turbulent flow (fractal small-scale clustering), which increases the local grain density by at least a factor of a few. In combination with the fact that nano-dust grains may be abundant in general, and the increased interaction rate due to turbulent motions, aggregation involving nano dust may have a rather high probability. Small-scale clustering will also affect extinction properties. As an example we present an extinction model based on silicates, graphite and metallic iron, assuming strong clustering of grain sizes in the nanometre range, could explain the extreme and rapidly varying ultraviolet extinction in the host of GRB 140506A.
This is a brief review of the main results of our recent studies of the nonlinear evolution of the small-scale MHD dynamo in the high-Prandtl-number regime and of the structure of the resulting saturated state of the isotropic homogeneous MHD turbulence. It is emphasized that the MHD regime without a uniform mean field (as is the case in our studies) is fundamentally different from the one in which such a field is externally imposed. The ability of the turbulence to bend and fold the magnetic-field lines leads to the emergence of a distinctive small-scale structure. The fields are organized in folds of characteristic length comparable to the size of the largest turbulent eddies with spatial-direction reversals at the resistive scale. These folds are very hard to destroy. In the nonlinear regime, the folding structure coexists with Alfven waves propagating along the folds. The turbulent energy injected by the forcing is dissipated in part resistively via the small-scale magnetic fields, and in part viscously via the Alfven waves.
We consider the problem of incompressible, forced, nonhelical, homogeneous, isotropic MHD turbulence with no mean magnetic field. This problem is essentially different from the case with externally imposed uniform mean field. There is no scale-by-scale equipartition between magnetic and kinetic energies as would be the case for the Alfven-wave turbulence. The isotropic MHD turbulence is the end state of the turbulent dynamo which generates folded fields with small-scale direction reversals. We propose that the statistics seen in numerical simulations of isotropic MHD turbulence could be explained as a superposition of these folded fields and Alfven-like waves that propagate along the folds.
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