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We present results from the first global 3D MHD simulations of accretiondisks in Cataclysmic Variable (CV) systems in order to investigate the relativeimportance of angular momentum transport via turbulence driven by themagnetorotational instability (MRI) compared to that driven by spiral shockwaves. Remarkably, we find that even with vigorous MRI turbulence, spiralshocks are an important component to the overall angular momentum budget, atleast when temperatures in the disk are high (so that Mach numbers are low). Inorder to understand the excitation, propagation, and damping of spiral densitywaves in our simulations more carefully, we perform a series of 2D globalhydrodynamical simulations with various equation of states and both with andwithout mass inflow via the Lagrangian point (L1). Compared with previoussimilar studies, we find the following new results. 1) Linear wave dispersionrelation fits the pitch angles of spiral density waves very well. 2) Wedemonstrate explicitly that mass accretion is driven by the deposition ofnegative angular momentum carried by the waves when they dissipate in shocks.3) Using Reynolds stress scaled by gas pressure to represent the effectiveangular momentum transport rate alpha_{eff} is not accurate when mass accretionis driven by non-axisymmetric shocks. 4) Using the mass accretion rate measuredin our simulations to directly measure alpha defined in standard thin-disktheory, we find 0.02 < alpha_{eff} < 0.05 for CV disks, consistent withobserved values in quiescent states of dwarf novae (DNe). In this regime thedisk may be too cool and neutral for the MRI to operate and spiral shocks are apossible accretion mechanism. However, we caution that our simulations useunrealistically low Mach numbers in this regime, and therefore future modelswith more realistic thermodynamics and non-ideal MHD are warranted.

Global MHD Simulations of Accretion Disks in Cataclysmic Variables (CVs): I. The Importance of Spiral Shocks