Abstract
Abstract. We show that the upscaling of wind turbines from rotor diameters of 15–20 m to presently large rotors of 150–200 m has changed the requirements for the aerodynamic blade element momentum (BEM) models in the aeroelastic codes. This is because the typical scales in the inflow turbulence are now comparable with the rotor diameter of the large turbines. Therefore, the spectrum of the incoming turbulence relative to the rotating blade has increased energy content on 1P, 2P, …, nP, and the annular mean induction approach in a classical BEM implementation might no longer be a good approximation for large rotors. We present a complete BEM implementation on a polar grid that models the induction response to the considerable 1P, 2P, …, nP inflow variations, including models for yawed inflow, dynamic inflow and radial induction. At each time step, in an aeroelastic simulation, the induction derived from a local BEM approach is updated at all the stationary grid points covering the swept area so the model can be characterized as an engineering actuator disk (AD) solution. The induction at each grid point varies slowly in time due to the dynamic inflow filter but the rotating blade now samples the induction field; as a result, the induction seen from the blade is highly unsteady and has a spectrum with distinct 1P, 2P, …, nP peaks. The load impact mechanism from this unsteady induction is analyzed and it is found that the load impact strongly depends on the turbine design and operating conditions. For operation at low to medium thrust coefficients (conventional turbines at above rated wind speed or low induction turbines in the whole operating range), it is found that the grid BEM gives typically 8 %–10 % lower 1 Hz blade root flapwise fatigue loads than the classical annular mean BEM approach. At high thrust coefficients that can occur at low wind speeds, the grid BEM can give slightly increased fatigue loads. In the paper, the implementation of the grid-based BEM is described in detail, and finally several validation cases are presented. Comparisons with blade loads from full rotor CFD, wind tunnel experiments and a field experiment show that the model can predict the aerodynamic forces in half-wake, yawed flow, dynamic inflow and turbulent inflow conditions.
Highlights
The blade element momentum (BEM) model (Glauert, 1935) is used extensively within the wind energy research community as well as by the wind turbine industry for simulating the aerodynamic rotor characteristics such as blade aerodynamic loads, rotor power and rotor thrust
We show that the upscaling of wind turbines from rotor diameters of 15–20 m to presently large rotors of 150–200 m has changed the requirements for the aerodynamic blade element momentum (BEM) models in the aeroelastic codes
In the second validation reference, the aerodynamic model in HAWC2 is alternated between the grid BEM and full 3-D CFD, which enables a detailed validation of the grid BEM results
Summary
The blade element momentum (BEM) model (Glauert, 1935) is used extensively within the wind energy research community as well as by the wind turbine industry for simulating the aerodynamic rotor characteristics such as blade aerodynamic loads, rotor power and rotor thrust. The BEM model is the aerodynamic engine in most aeroelastic models used today (FLEX5 (Flex4), Øye, 1996; FAST, Jonkman et al, 2016; BLADED, Bossanyi, 2003; GAST, Riziotis and Voustinas, 1997; Cp-Lambda, Botasso and Croce, 2006–2013; FOCUS, WMC, 2019; HAWC2, Larsen and Hansen, 2007) by the industry for the detailed aeroelastic simulations that are the basis for the certification of wind turbines (Hansen et al, 2015; IEC, 2005) This comprises a significant amount of simulations at normal operating conditions with turbulent inflow and at fault modes of the turbines such as a large yaw error. The purpose of the present article is to present in detail a complete unsteady BEM induction model for non-uniform inflow and loading that can be readily implemented
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