Viscosity, pressure, and support of the gas in simulations of merging
  cool-core clusters
  Major mergers are considered to be a significant source of turbulence in
clusters. We performed a numerical simulation of a major merger event using
nested-grid initial conditions, adaptive mesh refinement, radiative cooling of
primordial gas, and a homogeneous ultraviolet background. By calculating the
microscopic viscosity on the basis of various theoretical assumptions and
estimating the Kolmogorov length from the turbulent dissipation rate computed
with a subgrid-scale model, we are able to demonstrate that most of the
warm-hot intergalactic medium can sustain a fully turbulent state only if the
magnetic suppression of the viscosity is considerable. Accepting this as
premise, it turns out that ratios of turbulent and thermal quantities change
only little in the course of the merger. This confirms the tight correlations
between the mean thermal and non-thermal energy content for large samples of
clusters in earlier studies, which can be interpreted as second self-similarity
on top of the self-similarity for different halo masses. Another long-standing
question is how and to which extent turbulence contributes to the support of
the gas against gravity. From a global perspective, the ratio of turbulent and
thermal pressures is significant for the clusters in our simulation. On the
other hand, a local measure is provided by the compression rate, i.e. the
growth rate of the divergence of the flow. Particularly for the intracluster
medium, we find that the dominant contribution against gravity comes from
thermal pressure, while compressible turbulence effectively counteracts the
support. For this reason it appears to be too simplistic to consider turbulence
merely as an effective enhancement of thermal energy.
