Computational analysis of diffuser-augmented turbines is performed using high resolution computations of the Reynolds Averaged Navier-Stokes equations supplemented with a transition model. Shroud geometries, generated by the extrusion of airfoil profiles into annular wings, are assessed based on their ability to capture mass-flow through the interior of the shroud. To this end, axisymmetric calculations of high-lift airfoil sections are performed. The amplification of mass flow through a shroud is found to increase nearly linearly with radial lift force, and nonlinear effects are examined in terms of the location of the leading edge stagnation point. Of the shapes considered, the Selig S1223 high-lift low-He airfoil is found to best promote mass flow rate. Following this, full three-dimensional simulations of shrouded wind turbines are performed for selected shroud geometries. The results are compared to bare turbine solutions. Augmentation ratios (defined as the ratio of the power generated by a shrouded turbine to the Betz limit) of up to 1.9 are achieved by the shrouded turbines. Peak augmentation occurs at the highest wind speed for which the flow over the bare turbine blade stays attached. Flow fields are examined in detail and the following aspects are investigated: regions with flow separation, the development of averaged velocity profiles, and the interaction between the helical turbine wake and shroud boundary layer. Finally, power augmentation is demonstrated to continue increasing at high wind velocities, at which the turbine blade would otherwise stall, if a constant tip speed ratio is maintained.
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