Turbulence in the intergalactic medium: solenoidal and dilatational motions and the impact of numerical viscosity

Weishan Zhu Nanjing University Long-long Feng Purple Mountain Observatory, Nanjing, 210008, China Yinhua Xia University of Science and Technology of China Chi-Wang Shu Brown University Qiusheng Gu Nanjing University Li-Zhi Fang University of Arizona

Numerical Analysis and Scientific Computing mathscidoc:1610.25049

The Astrophysical Journal, 777, 48, 2013
We use a suite of cosmological hydrodynamic simulations, run by two fixed grid codes, in the context of LCDM model to investigate the properties of solenoidal and dilatational, i.e, curl and compressive, motions of the intergalactic medium (IGM), and the impact of numerical viscosity on turbulence. The codes differ only in the spatial difference discretization, one is a second-order TVD scheme and the other is a fifth-order positivity-preserving WENO scheme. We find that (1) The vortical motion grows rapidly since $z=2$, and reaches $\sim 10 km/s -75 km/s$, depending on the gas density, at low redshifts. The small-scale compressive ratio $r_{CS}$ drops from $0.8$ to $0.5$ during the same period. (2) Power spectra of the solenoidal velocity possess two regimes, $\propto k^{-0.89}$ and $\propto k^{-2.02}$, while the total velocity and dilatational velocity follow the scaling $k^{-1.88}$ and $k^{-2.20}$ respectively all through the turbulent scale $\sim 2-0.2 h^{-1}Mpc$. The IGM turbulence may contain two different phases, the supersonic phase and the post-supersonic phase, simultaneously along the hierarchical structure formation history. (3) The non-thermal pressure support, measured by the vortical kinetic energy, is comparable with the thermal pressure for $\rho_b \sim 10$, or $T \sim 10^5 K$ at $z=0.25$. The deviation of the baryon fraction from the cosmic mean shows a positive correlation with the turbulence pressure support. (4) A relatively higher numerical viscosity would artificially dissipate both the compressive and vortical motions in the IGM more seriously, resulting in less developed density fluctuation and vorticity, and leading to remarkably shortened turbulence scale. Shocks in regions out of the clusters are outstandingly suppressed by the numerical viscosity since $z=2$, which could directly cause the different levels of turbulence in the IGM between the codes. We also discuss the impact of ingredients not included in our simulation.
intergalactic medium, large-scale structure of the universe, numerical
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@inproceedings{weishan2013turbulence,
  title={Turbulence in the intergalactic medium: solenoidal and dilatational motions and the impact of numerical viscosity},
  author={Weishan Zhu, Long-long Feng, Yinhua Xia, Chi-Wang Shu, Qiusheng Gu, and Li-Zhi Fang},
  url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20161012051831395839167},
  booktitle={The Astrophysical Journal},
  volume={777},
  pages={48},
  year={2013},
}
Weishan Zhu, Long-long Feng, Yinhua Xia, Chi-Wang Shu, Qiusheng Gu, and Li-Zhi Fang. Turbulence in the intergalactic medium: solenoidal and dilatational motions and the impact of numerical viscosity. 2013. Vol. 777. In The Astrophysical Journal. pp.48. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20161012051831395839167.
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