Blade element momentum modeling of inflow with shear in comparison with advanced model results

There seems to be a significant uncertainty in aerodynamic and aeroelastic simulations on megawatt turbines operating in inflow with considerable shear, in particular with the engineering blade element momentum (BEM) model, commonly implemented in the aeroelastic design codes used by industry. Computations with advanced vortex and computational fluid dynamics models are used to provide improved insight into the complex flow phenomena and rotor aerodynamics caused by the sheared inflow. One consistent result from the advanced models is the variation of induced velocity as a function of azimuth when shear is present in the inflow. This gives guidance to how the BEM modeling of shear should be implemented. Another result from the advanced vortex model computations is a clear indication of influence of the ground, and the general tendency is a speed up effect of the flow through the rotor giving a higher power than in uniform flow. On the basis of the consistent azimuthal induction variations seen in the advanced model results, three different BEM implementation methods are discussed and tested in the same aeroelastic code. A full local BEM implementation on an elemental stream tube in both azimuth and radial direction seems to be closest to the advanced model results. Copyright (C) 2011 John Wiley & Sons, Ltd.