Modeling and Control of Dynamic Stall Loads on Smart Airfoil
Date
2020-08-26
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Abstract
Wind turbines operate under challenging inflow conditions, which can affect the turbine loads greatly. The turbine blades subjected to these excessive, fluctuating aerodynamic loads can go into dynamic stall, during which the flow separation is delayed and intense vortex structures are developed. There is no closed-form solution for a dynamic stall, and the present state-of-the-art still shows limitations in modeling and controlling the dynamic stall loads. The present work aims to improve the modeling of dynamic stall loads and to provide simulation tools for mitigating the variation in these loads with the assistance of a trailing edge flap (TEF). A modified version of the extended ONERA dynamic stall model is proposed for predicting unsteady forces, with an emphasis on modeling the effects of the dynamic stall vortex (DSV). The modifications include modeling the chord-axis forces instead of the wind-axis forces that the model was originally developed for. A novel approach for defining the onset of a dynamic stall in the time-marching solution is proposed based on the events taking place during a dynamic stall in the axial force without correlating them empirically. An extensive validation has been implemented on experimental data of different airfoils relevant to wind turbine applications. The results show an excellent correlation with the experimental data, particularly in deep dynamic stall scenarios, during which large fluctuations in the aerodynamic loads appear. The present work included direct, unsteady force measurements on a NACA 643-618 airfoil equipped with TEF (smart airfoil) in both a wind tunnel and water channel. Measurements in the wind tunnel have been used to expand the application of the new dynamic stall model in terms of predicting and controlling the unsteady loads on smart airfoils. The experiment utilizing the water channel investigated the impact of controlling the TEF hinge moment on unsteady loads. In addition, particle image velocimetry (PIV) measurements were carried out to explore the influence of the TEF actuation on the flow field around the smart airfoil. The modified model demonstrates an excellent correlation to the unsteady loads and the associated fluctuation on the smart airfoil despite the change in the effective angle of attack and apparent camber in response to the TEF actuation. The measurements also show that the TEF has a remarkable ability to reduce load variations despite the excessive effort required to control it in the presence of a substantial laminar separation bubble (LSB) and the development of DSVs. Additionally, the results indicate that the TEF hinge moment can be utilized as a localized sensor for unsteady loads and DSV shedding on the smart rotor. Controlling the hinge moment of TEF provides a promising reduction in the variation in the unsteady normal force.
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Keywords
Dynamic Stall, Smart Airfoil, Trailing Edge Flap, Vortex, Unsteady Aerodynamics, Direct Force Measurements, Wind Tunnel, Water Channel, PIV
Citation
Ayman, A. S. A. (2020). Modeling and Control of Dynamic Stall Loads on Smart Airfoil (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.