Preset angle, aspect ratio, and stall representations effects on the aeroelastic responses of small-scale wind turbine blades

Abstract

This work provides an inclusive nonlinear aeroelastic study on the effects of horizontal axis Small-Scale Wind Turbines (SSWTs)’s preset angle of attack as well as aspect ratio on its aerodynamic stability. For this purpose, an enhanced nonlinear reduced-order model is established and a state–space formulation for the aerodynamics of a comprehensive wing-based aeroelastic system is constructed. This representation captures the influence of the preset angle of attack where the blade no longer displays an oscillatory motion around its zero-equilibrium point in the post-flutter region, instead, a new equilibrium point is introduced to the system through the incorporation of the preset angle. The effect of the preset angle on the blade stability is investigated by jointly performing linear analyses that provide insights on this parameter’s influence on the onset of flutter, accompanied with nonlinear analyses that evaluate the latter design parameter’s effect on the blade bending and torsion amplitudes in the post-flutter regime. Additionally, the Duhamel’s principle specific to low angles of attack is extended to represent unsteady flows with non-conventional lift curves of low aspect ratio blades while considering different representations of the aerodynamic stall. Results show the significant effect of the preset angle on both the onset of flutter as well as the post flutter amplitudes where the higher the preset angle, the higher the flutter speed. Furthermore, this research effort also demonstrates the importance of accurate lift modeling since ill-evaluated lift characteristics leads to a potential over- or under-estimation of the blade’s torsional and bending amplitudes post-flutter, and the conventional 2π slope assumption frequently used in various reduced-order-models within the literature seldom predicts erroneous onset of flutter values as well as inaccurate amplitudes in the post-flutter regime especially for the case of low aspect-ratio systems.