  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 < alpha_{eff} < 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.
