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Abstract
Blade-tower interaction (BTI) is the interaction that occurs each time a rotor blade passes its support structure, causing transient changes to the induced flow and forces acting on both the blade and the tower. BTI has been identified as a likely source of impulsive noise emanating from wind turbines with an upwind configuration. As this is the most popular configuration for wind turbines, it is important to understand the mechanism of BTI noise production in order to prevent annoyance of nearby residents. BTI noise is also important for other applications, such as helicopter-fuselage and propeller-pylon interactions.
To obtain a better understanding of BTI, numerical simulations have been performed and validated against experimental data.
The study was performed by simulating the flow about a rotor-rig, which consists of a three-bladed rotor that is supported by a vertical cylindrical tower. As the existing numerical models were deemed insufficient to capture the full interaction between the blades and the tower, a novel numerical model was used in the simulations. In this model, computational fluid dynamics (CFD) was used to obtain the near-field flow changes, with the sliding mesh method used to simulate the rotation of the blades. Curle's acoustic analogy was used to predict the noise generated from the simulated flow data.
Intense force fluctuations occur during BTI on both the tower and the passing blade. These unsteady forces are the primary sources of BTI noise, which manifest themselves as three acoustic pulses during one rotation cycle. The numerical model successfully predicts the acoustic waveforms and spectra, which are in very good agreement with experimental measurements. The results thus support the notion that BTI generates impulsive noise emanating from upwind turbines. The levels of BTI noise radiated along the plane of rotation are comparable to those along the axis of rotation, which is contrary to the assumption that it has a dipole directivity. More importantly, the contribution of the tower, which previously had been ignored, was revealed to be more than or comparable to the noise created by the blades.
BTI noise is also important for other applications, such as helicopter-fuselage and propeller-pylon interactions.