Flanged diffuser shrouding small wind turbine, is among the most tested devices for increasing wind energy. The height of the flange is between the geometric futures of the diffuser that contributes efficiently in improving diffuser performances. Results obtained from numerical simulations and Ply visualizations show that when a flange is mounted at the outlet area of the diffuser, two contra-rotating vortices were created at this location. These two vortices move away from each other in the flow direction as the flange height increases and they seem to lengthen in the streamwise direction and to extend in the two directions when the flange height becomes taller. A critical ratio (Flange height/Inlet section diffuser diameter = 0.1) has been found. Beyond this value, due to the remoteness of vortices from the flange, the flange height seems to be without significant effect on increasing wind velocity. (C) 2012 Elsevier Ltd. All rights reserved.