Using two-dimensional computational fluid dynamics (CFD) simulations, this study examines the physical mechanisms of performance improvement in a double rotor vertical axis wind turbine (VAWT) as compared to a single rotor VAWT. It was found that the primary mechanism for the approximately 10% improvement in power coefficient (C-p) for the double rotor VAWT was a reduction in the bypass flow that passes around the outside of the single rotor VAWT. The single rotor VAWT is shown to cause greater than 54% of the incoming flow to bypass around the outside of the VAWT. By placing a second rotor in the system, the cross-stream flow that would normally exist to one side is suppressed due to the opposing cross-stream flow from the second rotor. The collision of cross-stream velocities results in a high pressure field upstream of the rotors. This high pressure field re-directs more flow through the rotors, reducing the total bypass down to below 51%. This reduction in total bypass is solely due to the reduction in bypass on the interior of the double rotor system as the exterior side actually experiences an increase in bypass of about 83%. (C) 2019 Elsevier Ltd. All rights reserved.