This paper studies the use of blade tip sensors for load reductions and blade-tower clearance control. Typically, modern blade tip sensors measure flapwise tip deflection distances at a high sampling rate, and such measurements can be utilised as feedback signals for control operations. Thus, this paper proposes a novel blade pitch control design based on the tip deflection measurements and individual pitch control (IPC). Firstly, an IPC system design is presented, using the tip deflection measurements to alleviate turbine fatigue loads caused by differential loads such as wind shear, yaw misalignment and turbulence. Secondly, a novel implementation of IPC with tip trajectory tracking feature is proposed where the blade tips are guided along a fixed trajectory to maximise blade-tower clearance. The motivation of this implementation is to reduce the chance of blade-tower interactions for large and flexible rotors. The presented controller is implemented in HAWC2, and high fidelity load measurements are produced using the DTU10MW reference wind turbine. The simulation results showed that the fatigue damage reduction on key turbine components and the improved blade-tower clearance can be achieved simultaneously. Lifetime equivalent load reductions were seen in both rotating and fixed frame components under the normal operating conditions.

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In this paper we report the results of a workshop organised by the Delft University of Technology in 2014,
aiming at the comparison between different state-of-the-art numerical models for the simulation of
wind turbine wakes. The chosen benchmark case is a wind tunnel measurement, where stereoscopic
Particle Image Velocimetry was employed to obtain the velocity field and turbulence statistics in the near
wake of a two-bladed wind turbine model and of a porous disc, which mimics the numerical actuator
used in the simulations. Researchers have been invited to simulate the experimental case based on the
disc drag coefficient and the inflow characteristics. Four large eddy simulation (LES) codes from different
institutions and a vortex model are part of the comparison. The purpose of this benchmark is to validate
the numerical predictions of the flow field statistics in the near wake of an actuator disc, a case that is
highly relevant for full wind farm applications. The comparison has shown that, despite its extreme
simplicity, the vortex model is capable of reproducing the wake expansion and the centreline velocity
with very high accuracy. Also all tested LES models are able to predict the velocity deficit in the very near
wake well, contrary to what was expected from previous literature. However, the resolved velocity
fluctuations in the LES are below the experimentally measured values.@en