## Blade Element Momentum

The BEM method consists of axial and angular momentum equations of an annular stream tube coupled to the sectional aerodynamic forces in the rotor plane. These forces are determined from a look-up table containing the sectional aerodynamic coefficients (lift, drag and moment) as a function of angle of attack. This model is originally developed for axial and steady inflow conditions, assuming 2D flow without radial interaction between the annuli. Moreover, the sectional aerodynamic coefficients rely on 2D and steady assumptions. Since many of the underlying model assumptions are violated in real life applications, several engineering extensions exist to attempt to overcome these shortcomings, see e.g. [Schepers, 2012]. This reference also explains that the role of calculation time for wind energy calculations is much more crucial than it is for most other areas of technology and that the number of time steps for design calculations is more than 5 million. This makes the use of BEM with engineering methods still inevitable.

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