Unsteady Aerodynamics of Static and Oscillating Airfoils at Large Angles of Attack

Abstract

In a world with an urgent demand for sustainable energy, the wind energy industry plays a key role in accelerating this transition. Over the past decades, wind turbines have evolved from expensive, relatively inefficient machines into increasingly cost-effective and highly efficient technologies, supported and promoted by many countries worldwide. However, this rapid progress has come at a cost: larger and more efficient turbines require substantial investments in raw materials, research and development, component standardization, and supply chain optimization.

In recent years, the curtailment of wind power in Europe has increased, largely due to insufficient grid capacity and limited energy storage. As a result, wind turbines are more frequently operated in parked conditions, with their rotors brought to a standstill. Under these circumstances, one of the key challenges in scaling up turbine size is the risk of vortex-induced vibrations (VIV) in the blades. In parked conditions, the blades are often pitched to very high angles of attack (close to 90°) to cut out of the wind. If the vortex shedding frequency approaches the blade’s natural frequency, a lock-in phenomenon may occur, leading to strong vibrations. This vibration in the long term can contribute to the overall fatigue load of the wind turbine and reduce the structural life.

Although increasing attention has been given to VIV in wind turbine blades, significant gaps remain in understanding the fundamental flow physics that govern these vibrations, specifically the unsteady aerodynamics of airfoils at high angles of attack. This dissertation therefore investigates the unsteady aerodynamics of both static and oscillating airfoils under such conditions, with the aim of building a detailed physical understanding of VIV from an aerodynamic perspective.

The research was carried out through a series of wind tunnel measurements. First, a campaign on a static airfoil examined unsteady aerodynamics across a wide range of angles of attack (up to 310°). Aerodynamic forces, vortex shedding patterns, and shedding frequencies were compared between forward flow (leading edge upwind) and reverse flow (trailing edge upwind) conditions. Although reverse flow is uncommon in normal operation, it can occur during parked or installation phases; the insights gained in this research therefore form a critical foundation for subsequent studies on oscillating airfoils.

The main focus of the dissertation is the unsteady aerodynamics of oscillating airfoils, studied using the forced motion method to mimic VIV. Three motion types, namely surging, plunging, and pitching, were investigated. Particle Image Velocimetry (PIV) was employed to capture the flow fields, while surface pressure measurements provided aerodynamic forces. By correlating vortex dynamics with force responses, the study reveals how the mean angle of attack and motion parameters (such as frequency and amplitude) influence the overall unsteady aerodynamics of the airfoil and how lock-in is triggered under different motion kinematics. Comparisons between forward and reverse flow conditions further enrich the findings, where the reverse flow dynamic stall was thoroughly discussed—from vortex dynamics and aerodynamic forces to a newly proposed dynamic stall vortex and trailing edge vortex onset determination method.

Overall, the comprehensive experimental dataset and resulting conclusions advance the fundamental understanding of unsteady airfoil aerodynamics at large angles of attack. These findings not only clarify the underlying mechanisms causing VIV from the perspective of vortex dynamics and aerodynamic forces, but also provide a valuable basis for future aeroelastic VIV studies and the development of engineering models.