Blade Element Momentum Method

A blade element momentum (BEM) method is available in Bladed to compute the aerodynamic loading on the turbine rotor. The loading is computed by discretising the blades into multiple aerodynamic blade elements. When the turbine is operating, flow induction is computed using BEM theory and dynamic wake at each blade station. Further corrections due to dynamic stall are also applied to correct the lift and drag force as well as twist moment.

The BEM aerodynamic loading model is essentially a collection of sub-models. The computation of induction is completed using momentum theory and corrections for tip losses, and yawed inflow (skew wake). The user can also switch off the induction calculation based on when the tip speed ratio falls below a user-defined threshold. This is not only important for idling and parked simulations but also in cases where the turbine experiences a startup or shutdown. For unsteady changes in the wind speed and also structural motion a dynamic wake model is required to compute the change in induction on the turbine rotor. Finally, dynamic stall model is used to capture delays in lift and drag force as well as twist moment due to flow unsteadiness at the aerofoil level. The stall model is also used to capture the effect of stall hysteresis at high angles of attack. By default, the dynamic stall model is active during loading analysis of a turbine during parked or idling conditions.

The results of the BEM method in Bladed have been compared to other calculation methods implemented in a range of other software packages including: blade element momentum, vortex wake and computational fluid dynamic methods. Comparisons have also been made to measurement data. Notable studies that demonstrate the BEM method giving good results when compare to other calculation methods and also measurement data are (Bangga, 2023), (Boorsma, 2020), and (Boorsma, 2024).

Warning

Some model options are labelled or described as a "pre-4.8" compatibility setting. This is a reference to an older version of Bladed that used a different BEM method. The purpose of a pre-4.8 model option is for backwards compatibility testing only and it is not recommended as part of running loading analysis.

Last updated 25-09-2024