Title: Anisotropic hydrodynamic turbulence in accretion disks,
Abstract: Recently, the vertical shear instability (VSI) has become an attractive
purely hydrodynamic candidate for the anomalous angular momentum transport
required for weakly ionized accretion disks. In direct three-dimensional
numerical simulations of VSI turbulence in disks, a meridional circulation
pattern was observed that is opposite to the usual viscous flow behavior. Here,
we investigate whether this feature can possibly be explained by an anisotropy
of the VSI turbulence. Using three-dimensional hydrodynamical simulations, we
calculate the turbulent Reynolds stresses relevant for angular momentum
transport for a representative section of a disk.
We find that the vertical stress is significantly stronger than the radial
stress. Using our results in viscous disk simulations with different viscosity
coefficients for the radial and vertical direction, we find good agreement with
the VSI turbulence for the stresses and meridional flow; this provides
additional evidence for the anisotropy. The results are important with respect
to the transport of small embedded particles in disks.