Investigations of aerodynamic drag forces during structural blade testing using high-fidelity fluid–structure interaction

Abstract

Aerodynamic loads need to be known for planning and defining test loads beforehand for wind turbine blades that are tested for fatigue certifications. It is known that the aerodynamic forces, especially drag, are different for tests and operation, due to the entirely different flow conditions. In test facilities, a vibrating blade will move in and out of its own wake, increasing the drag forces on the blade. This is not the case in operation. To study this special aerodynamic condition present during experimental tests, numerical simulations of a wind turbine blade during pull–release tests were conducted. High-fidelity three-dimensional computational fluid dynamics methods were used throughout the simulations. In this way, the fluid mechanisms and their impact on the moving blade are clarified, and through the coupling with a structural solver, the fluid–structure interaction is studied. Results are compared to actual measurements from experimental tests, verifying the approach. It is found that the blade experiences a high drag due to its motion towards its own whirling wake, resulting in an effective drag coefficient of approximately 5.3 for the 90∘ angle of attack. This large drag coefficient was implemented in a fatigue test load simulation, resulting in a significant decrease in bending moment along the blade, leading to less load being applied than intended. The confinement from the test facility did not impact this specific test setup, but simulations with longer blades could possibly yield different conclusions. To the knowledge of the authors, this investigation, including three-dimensional effects, structural coupling and confinement, is the first of its kind.