Dynamic Aeroelastic Response of Wind Turbine Rotors Under Active Flow Control
Authors: Menon Muraleekrishnan , Ponta Fernando L. 
Published: November 2019

The significance of wind power and the associated relevance of utility-scale wind turbines are becoming more prominent in tapping renewable sources for power. Operational wind turbines today rated at 8 MW have rotor diameters of 164 m. Economies-of-scale factor suggest a sustained growth in rotor size, forecasting the use of longer and heavier blades. This has led to an increased emphasis on studies related to improvements and innovations in aerodynamic load-control methodologies. Among several approaches to controlling the stochastic aerodynamics loads on wind turbine rotors, most popular is the pitch control. Widely used in operational wind turbines, conventional pitch control is an effective approach for long-term load variations. However, their application to mitigate short-term fluctuations have limitations that present a bottleneck for growth in rotor size. Sporadic changes occurring within short time scales near the turbine rotor have significant impact on the aeroelastic behavior of the blades, power generation, with long-term effects on the rotor life-span. Cyclic variations occurring within few seconds emphasize the need for swift response of control methods that counter the resulting adverse effects.

Current study revolves around the need to evaluate innovative active load control techniques that can swiftly handle high frequency oscillations in dynamic loading of turbine rotors. This may result from sudden changes in wind conditions due to gusts, environmental effects like atmospheric boundary layer and uneven terrain, or from turbine design features and operating conditions such as tower shadow effects. The upward surge in rotor size is linked with a down-side for existing techniques in rotor control that now need to account for heavier blades and the associated inertia. For example, the pitching operation rotates the entire blade around its longitudinal axis to regulate angle of wind at specific blade sections, involving huge inertial loads associated with the entire blade. On the other hand, active flow-control devices (FCDs) have the potential to alleviate load variations through rapid aerodynamic trimming. Trailing-edge flaps are light weight attachments on blades that have gradually gained relevance in studies focused on wind turbine aerodynamics and active load control.

This computational study presents an aeroelastic assessment of a benchmark wind turbine based on the NREL 5-MW Reference Wind Turbine (RWT), with added trailing-edge flaps for rapid load control. The standard blades used on the NREL 5-MW RWT rotor are aerodynamically modified to equip them with actively controllable fractional-chord trailing-edge flaps, along a selected span. The numerical code used in the study handles the complex multi-physics dynamics of a wind turbine based on a self-adaptive ODE algorithm that integrates the dynamics of the control system in to the coupled response of aerodynamics and structural deformations of the rotor.

Using the 5-MW RWT as a reference, the blades are modified to add trailing-edge flaps with Clark Y profile and constant chord. Attached at chosen sections of the blade, these devices have a specific range of operational actuation angles. Numerical experiments cover scenarios relevant to the aeroelastic response of a rotor with such adapted blades under operating conditions observed in utility-scale wind turbines. These fractional-chord devices attached along short spans of the blades make them light weight devices that can be easily controlled using low power of actuation. This overcomes the bottleneck in active aerodynamic load control, giving flexibility to study a wider ranged of control strategies for utility-scale wind turbines of the future. Preliminary outcomes suggest that rapid active flow control has high potential in shaping the future of aerodynamic load control in wind turbines.