Experimental Investigation of the Rotor–Tower Interaction of a Modern Multi–Megawatt Wind Turbine

Abstract. The increasing demand for a reduction in energy costs per kilowatt hour is leading to larger wind turbines, resulting in longer rotor blades that are also more slender, lighter, and more flexible. These structures are more dynamically responsive and more sensitive to excitations. When a rotor blade passes the tower, an aerodynamic interaction occurs between the blade and the tower, known as rotor–tower interaction. This interaction induces fluctuating loads on both the blade and the tower. Understanding the fluctuating loads on both blade and tower is essential for the design of larger and therefore more dynamically active wind turbines.

To assess how and to what extent the influence of the rotor–tower interaction impact the structure of modern multi–megawatt wind turbines, in this study, the rotor–tower interaction was investigated by means of pressure measurements on the tower of a modern 4.26 MW upwind wind turbine. For the measurements, a pressure belt, equipped with 36 differential pressure sensors was mounted on the tower at mid-rotor height. The measurements were conducted over two months with the aim to measure transient surface pressure fluctuations induced by the passing rotor blades. The blade root bending moments recorded by the wind turbine were also examined for selected operating points.

The results show a clear periodic fluctuation of the aerodynamic loading of the tower at the 3P blade-passing frequency. Aerodynamic phenomena at the tower, such as velocity excess, stagnation point displacement, and synchronized vortex shedding, which had been predicted in earlier numerical studies, are confirmed by these measurements. The maximum dynamic loads on the tower occur when the turbine reaches its rated power, where the aerodynamic load on the blades is at its highest. The Investigation of the blade root bending moment shows that the blade is also influenced by the tower. A fluctuation in the flapwise bending moment of approximately 1 % of the maximum flapwise bending moment is observed when the blade passes the tower. These findings show that the effect of rotor–tower interaction occur in modern multi-megawatt wind turbines and can be measured, even if it is only minor in this particular wind turbine type due to the large blade–tower clearance.