Research in astrophysics
My most recent project: resolving and modelling the turbulent flow in the cluster outskirtsThe outskirts of galaxy clusters are characterised by the interplay of gas accretion and dynamical evolution involving turbulence, shocks, magnetic fields and diffuse radio emission. The density and velocity structure of the gas in the outskirts provide an effective pressure support and affect all processes listed above. Therefore it is important to resolve and properly model the turbulent flow in these mildly overdense and relatively large cluster regions; this is a challenging task for hydrodynamical codes.
Many astrophysical fluids, presumably including the Intracluster Medium (ICM), have a very large Reynolds number. As a result, the length scale separation between the integral length scale for turbulence injection and the Kolmogorov scale for its viscous dissipation is so large that it cannot be resolved in state-of-the-art direct numerical simulations, even using adaptive mesh refinement. In many fields of computational fluid dynamics, the influence of unresolved turbulence on the resolved scales is therefore modelled by means of heuristic subgrid-scale models, coupled to the large scales of the system, for which the fluid equations are solved.
In my recent research activities I use grid-based simulations of galaxy clusters and focus mainly on two numerical tools:
- Adaptive mesh refinement (AMR) suitable for refining the injection of turbulence in the ICM. This is based on the regional variability of vorticity and is more effective in resolving the turbulent flow in the cluster outskirts than any previously used criterion based on overdensity.
- Subgrid scale model (SGS) for unresolved turbulence. The turbulence SGS model provides a quantity, the SGS turbulence specific energy, which is a useful indicator of small-scale turbulence and its time evolution during a merger event, in combination with the kinetic energy used as a large-scale diagnostic.
The volume-filling factor of the flow with large vorticity is about 60% at low redshift in the cluster outskirts, and thus smaller than in the cluster core. In the framework of modelling radio relics, this point suggests that upstream flow inhomogeneities might affect pre-existing cosmic-ray population and magnetic fields, and the resulting radio emission.
In the Figure below one can get a visual impression of the effectiveness of the AMR criterion based on vorticity (upper row) in refining the underdense filamentary network located around the virial radius (the blue circle in the slices), when compared with the traditional AMR based on gas and DM overdensity (other rows). For details, please refer to Iapichino, Federrath & Klessen 2017.