We present results from the first global 3D MHD simulations of accretion disks in Cataclysmic Variable ( CV ) systems in order to investigate the relative importance of angular momentum transport via turbulence driven by the magnetorotational instability ( MRI ) compared to that driven by spiral shock waves .
Remarkably , we find that even with vigorous MRI turbulence , spiral shocks are an important component to the overall angular momentum budget , at least when temperatures in the disk are high ( so that Mach numbers are low ) .
In order to understand the excitation , propagation , and damping of spiral density waves in our simulations more carefully , we perform a series of 2D global hydrodynamical simulations with various equation of states and both with and without mass inflow via the Lagrangian point ( L1 ) .
Compared with previous similar studies , we find the following new results .
1 ) Linear wave dispersion relation fits the pitch angles of spiral density waves very well .
2 ) We demonstrate explicitly that mass accretion is driven by the deposition of negative angular momentum carried by the waves when they dissipate in shocks .
3 ) Using Reynolds stress scaled by gas pressure to represent the effective angular momentum transport rate \alpha _ { eff } is not accurate when mass accretion is driven by non-axisymmetric shocks .
4 ) Using the mass accretion rate measured in our simulations to directly measure \alpha defined in standard thin-disk theory , we find 0.02 \lesssim \alpha _ { eff } \lesssim 0.05 for CV disks , consistent with observed values in quiescent states of dwarf novae ( DNe ) .
In this regime the disk may be too cool and neutral for the MRI to operate and spiral shocks are a possible accretion mechanism .
However , we caution that our simulations use unrealistically low Mach numbers in this regime , and therefore future models with more realistic thermodynamics and non-ideal MHD are warranted .