Dynamic Aeroelastic Response of Stall-Controlled Wind Turbine Rotors in Turbulent Wind Conditions

Abstract

With the current global trend of the wind turbines to be commissioned, the next generation of state-of-the-art turbines will have a generating capacity of 20 MW with rotor diameters of 250 m or larger. This systematic increase in rotor size is prompted by economies-of-scale factors, thereby resulting in a continuously decreasing cost per kWh generated. However, such large rotors have larger masses associated with them and necessitate studies in order to better understand their dynamics. The present work regarding the aeroelastic behavior of stall-controlled rotors involves the study of the frequency content and time evolution of their oscillatory behavior. A wide range of experiments were conducted to assess the effects of rapid variations on the rotor’s operational conditions. Various gust conditions were tested at different wind speeds, which are represented by pulses of different intensities, occurring suddenly in an otherwise constant wind regime. This allowed us to observe the pure aero-elasto-inertial dynamics of the rotor’s response. A reduced-order characterization of the rotor’s dynamics as an oscillatory system was obtained on the basis of energy-transfer principles. This is of fundamental interest for researchers and engineers working on developing optimized control strategies for wind turbines. It allows for the critical elements of the rotor’s dynamic behavior to be described as a reduced-order model that can be solved in real time, an essential requirement for determining predictive control actions.

Characterization of Oscillatory Response of Light-Weight Wind Turbine Rotors under Controlled Gust Pulses

“…Another finding of the ROC study presented in the work of Jalal et al [18] was that all the curves of flap-wise vibration, centered around the value of the mean blade deflection for a certain steady wind speed, collapse into a unique signal when they are normalized by the kinetic energy content of their respective pulses. That is, the time evolution of the blade deflection scales with the time evolution of the rotor's elastic vibrational energy, which, in turn, scales with the content of kinetic energy density of the pulse, PulseEner, measured in Joules/m 2 .…”

“…Pulses in the time signals of wind speed registered in the anemometry data: These have been typified and classified, according to amplitude and duration, in the previous work by the authors presented in Jalal et al [18]. Section 3.1 of the present study, which covers the dynamics of the SNL-NRT rotor and its flexible variations, adopts the same pulse classification.…”

“…In previous works [18], the authors conducted a wide range of experiments on the DU-5MW-RWT [19], a stall-controlled alternative to the NREL-5MW benchmark turbine [2]. This was performed to assess the effects of rapid wind fluctuations on the turbine's operational conditions in terms of the frequency content and time evolution of the rotor's oscillatory aeroelastic response.…”

“…To complete this sample, Figure 6b shows the same time evolution of U hx for WS = 11.11 m/s, TSR = 6.197 and Pitch = 0 • , which correspond to the operational conditions at the rated wind speed of the SNL-NRT design; and Figure 6c shows the case for WS = 15 m/s, TSR = 4.59, Pitch = 11.651 • , which correspond to the conditions at the NRT's cut-off wind speed. In the previous work by the authors reported in the work of Jalal et al [18], it was found that the time evolution of the blade's oscillatory response was characterized by an exponential decay associated with aerodynamic damping, whose exponent was determined exclusively by the speed of the background wind. A novel aspect observed as a common factor in the current tests on the SNL-NRT rotor, where several flexible variations of the same blade geometry are involved, is that the rate of decay of the dominant mode of the oscillatory response increases as the flexibility of the blade increases.…”

“…The three flexible variations that were selected were "60%SpSh", "40%SpSh" and "20%SpSh". One of the essential steps in the ROC study presented in the work of Jalal et al [18] was to design controlled gust pulses that represented the speed fluctuations found in the real-world environment. One way of doing this was to identify and quantify pulses with a characteristic time-span and amplitude that represented typical oscillations in wind speed in a statistical sense.…”