  Accretion disks in which angular momentum transport is dominated by the
magnetorotational instability (MRI) can also possess additional, purely
hydrodynamic, drivers of turbulence. Even when the hydrodynamic processes, on
their own, generate negligible levels of transport, they may still affect the
evolution of the disk via their influence on the MRI. Here, we study the
interaction between the MRI and hydrodynamic turbulence using local MRI
simulations that include hydrodynamic forcing. As expected, we find that
hydrodynamic forcing is generally negligible if it yields a saturated kinetic
energy density that is small compared to the value generated by the MRI. For
stronger hydrodynamic forcing levels, we find that hydrodynamic turbulence
modifies transport, with the effect varying depending upon the spatial scale of
hydrodynamic driving. Large scale forcing boosts transport by an amount that is
approximately linear in the forcing strength, and leaves the character of the
MRI (for example the ratio between Maxwell and Reynolds stresses) unchanged, up
to the point at which the forced turbulence is an order of magnitude stronger
than that generated by the MRI. Low amplitude small scale forcing may modestly
suppress the MRI. We conclude that the impact of hydrodynamic turbulence on the
MRI is generically ignorable in cases, such as convection, where the additional
turbulence arises due to the accretion energy liberated by the MRI itself.
Hydrodynamic turbulence may affect (and either enhance or suppress) the MRI if
it is both strong, and driven by independent mechanisms such as self-gravity,
supernovae, or solid-gas interactions in multiphase protoplanetary disks.
