Galih Bangga

The present study is intended to address important aspects in wind turbine loads assessment to improve the accuracy of state-of-the-art prediction tools. Aeroelastic instability has been a great concern in wind turbine design in the last decades due to an ever-increasing rotor size. One of the hardest tasks in turbine instability predictions associated with stall induced vibrations (SIV) is to obtain the correct aerodynamic damping. This stems from the fact that dynamic stall plays a major role in blade post stall development which generates complex damping mechanisms on the blade and predicting this behavior is an extremely difficult task. Despite the effort of dynamic stall models validation campaigns, at present there is no available study which compares the performance of these models against field measurement data of wind turbines experiencing rotor instability. The present study is specifically aimed at filling this gap by investigating the importance of accurate dynamic stall modeling for instability prediction. The findings obtained from the present studies show that employing an appropriate dynamic stall model is vital for large wind turbine designs.

The present study is intended to address important aspects in wind turbine loads assessment and to improve the accuracy of state-of-the-art prediction tools. Wind farm optimization has been a great interest in the recent years to maximize the energy extraction from the wind for a specific site. There is also a clear interest to include the wind farm effects in time domain calculations in order to obtain a correct estimation of the wind turbine loads and performance. Previous studies show that fatigue loads predicted by vortex wake are generally smaller than BEM, and the results from vortex wake match closer with higher fidelity methods. There is currently lack of evidence if the same conclusion still holds when the turbine is exposed to upstream wind turbine wake, where the velocity can be close to zero or even locally negative. This work explores the feasibility of using the vortex wake approach to model wind turbine loads when wake effects are present. Occurrences of partial wake conditions are identified, and the corresponding induced velocities are assessed. The study provides an initial step toward integrating these methods in the loads analysis.

The present work entails investigation of the unsteady aerodynamic characteristics of a wind turbine airfoil operating in four stall-induced vibration (SIV) regimes, including normal (SIV-1) and reverse (SIV-2) flow conditions. We conducted computational fluid dynamics simulations using OpenFOAM (OF) over the full angle-of-attack range (180° to +180°) and compared the results with those of semi-empirical dynamic stall models obtained using BLADED software, including the Øye, Beddoes–Leishman (BL), and IAG models. Numerical verifications confirmed that both conformal and non-conformal meshes could reproduce the key aerodynamic trends with limited sensitivity, except near the stall onset. In the SIV-1 regime, the OF simulations accurately captured hysteresis loops compared to wind tunnel data; the IAG model demonstrated superior performance among the engineering models, while the Øye and BL models significantly underestimated the hysteresis characteristics. In the SIV-2 regime representing reverse flow conditions relevant to parked or idling wind turbines, pronounced asymmetries were observed between the positive and negative ranges owing to directional vortex evolution and geometric effects of the airfoil. The OF results revealed early separation, irregular vortex shedding, and wide hysteresis loops that could not be reproduced by the existing semi-empirical models without additional treatments. Flow-field analysis confirmed that the reverse flow aerodynamics is dominated by coherent and directionally dependent vortical structures, suggesting that conventional definitions of the stall onset are inadequate in the SIV-2 regime. We observed that setting the critical stall normal force to zero improved the prediction of dynamic stall for the IAG model, although the strong amplitudes of the unsteady loads remained underrepresented. Overall, our findings highlight the need for advanced modeling frameworks capable of capturing the asymmetric vortex-driven phenomena in reverse flow regimes that are critical to standstill load assessments.

The utilization of wind energy across various sectors has increased significantly in recent decades

Present studies are specifically aimed at investigating the sensitivity of different dynamic stall models when exposed to various excitation frequencies. The investigations are targeted at blade edgewise vibrations. This is carried out on a modified version of the IEA 15 MW reference wind turbine employing a wind turbine design tool, DNV Bladed. State-of-the-art dynamic stall models for wind turbine applications, such as the Øye model, Beddoes–Leishman (BL) model and the newly developed IAG model, are evaluated. The beginning of the research work starts by evaluating different dynamic stall models on rigid blade section forces against known airfoil datasets. Then, the blade flexibility is considered to enable systematic evaluations of the blade flexibility influences in comparison to the rigid blade cases. It is observed that the range of the angle of attack grows depending on the excitation frequency and the adopted dynamic stall model. The critical excitation frequency range and the effects of twist distribution are then identified from the studies, which can be useful as a rough guidance when designing wind turbine blades.

The present studies are specifically aimed at investigating the sensitivity of different dynamic stall models when exposed to various excitation frequencies. The investigations are targeted at the blade edgewise vibrations. This is done on a modified version of the IEA 15 MW reference wind turbine employing a wind turbine design tool Bladed. The state-of-the-art dynamic stall models for wind turbine applications such as the Oye model, Beddoes-Leishman (BL) model and the newly developed IAG model are evaluated. The beginning of the research work starts by evaluating different dynamic stall models on rigid blade section forces against known airfoil datasets. Then, the blade flexibility is considered to enable systematic evaluations of the blade flexibility influences in comparison to the rigid blade cases. It is observed that the range of the angle of attack grows depending on the excitation frequency and the adopted dynamic stall model. The critical excitation frequency range and the effects of twist distribution are then identified from the studies, which can be useful as a rough guidance when designing wind turbine blades.

This paper presents comprehensive investigations into idling instability occurring on the IEA 15 MW reference turbine. The systematic studies are carried out by means of Blade Element Momentum (BEM) and free wake vortex (Vortexline) methods. Two state-of-the-art dynamic stall models are tested in the present investigations, namely the Beddoes-Leishman and the IAG dynamic stall models, implemented into a development version of the wind turbine design code Bladed. The studies highlight the importance of unsteady aerodynamic modeling to predict idling instabilities and emphasize the characteristics of each modeling strategy. It is demonstrated that the IAG dynamic stall model predicts a more physically reasonable idling instabilities. Furthermore, Vortexline enables the calculations of the induced velocities even under idling conditions in contrast to BEM. The combination of the Vortexline method and the IAG model is considered to provide the most reasonable turbine response. The studies will be helpful for load engineers to select appropriate modeling strategies and shed some light into future engineering modeling improvements of wind turbines.

Considering the dynamic stall effects in engineering calculations is essential for correcting the aerodynamic loads acting on wind turbines, both during power production and stand-still cases, and impacts significantly the turbine aeroelastic stability. The employed dynamic stall model needs to be accurate and robust for a wide range of airfoils and range of angle of attack. The present studies are intended to demonstrate the performance of a recently implemented “IAG dynamic stall” model in a wind turbine design tool Bladed. The model is transformed from the indicial type of formulation into a state-space representation. The new model is validated against measurement data and other dynamic stall models in Bladed for various flow conditions and airfoils. It is demonstrated that the new model is able to reproduce the measured dynamic polar accurately without airfoil specific parameter calibration and has a superior performance compared to the incompressible Beddoes–Leishman model and the Øye model in Bladed.

Within the framework of the fourth phase of the International Energy Agency (IEA) Wind Task 29, a large comparison exercise between measurements and aeroelastic simulations has been carried out featuring three simulation cases in axial, sheared and yawed inflow conditions. Results were obtained from more than 19 simulation tools originating from 12 institutes, ranging in fidelity from blade element momentum (BEM) to computational fluid dynamics (CFDs) and compared to state-of-the-art field measurements from the 2 MW DanAero turbine. More than 15 different variable types ranging from lifting-line variables to blade surface pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. The result is a unique insight into the current status and accuracy of rotor aerodynamic modeling. For axial flow conditions, a good agreement was found between the various code types, where a dedicated grid sensitivity study was necessary for the CFD simulations. However, compared to wind tunnel experiments on rotors featuring controlled conditions, it remains a challenge to achieve good agreement of absolute levels between simulations and measurements in the field. For sheared inflow conditions, uncertainties due to rotational and unsteady effects on airfoil data result in the CFD predictions standing out above the codes that need input of sectional airfoil data. However, it was demonstrated that using CFD-synthesized airfoil data is an effective means to bypass this shortcoming. For yawed flow conditions, it was observed that modeling of the skewed wake effect is still problematic for BEM codes where CFD and free vortex wake codes inherently model the underlying physics correctly. The next step is a comparison in turbulent inflow conditions, which is featured in IEA Wind Task 47. Doing this analysis in cooperation under the auspices of the IEA Wind Technology Collaboration Program (TCP) has led to many mutual benefits for the participants. The large size of the consortium brought ample manpower for the analysis where the learning process by combining several complementary experiences and modeling techniques gave valuable insights that could not be found when the analysis is carried out individually.