An experimental and numerical investigation about the pitch imbalance effect on a wind turbine model is performed. The characterization of the power losses and loads generated on a small-scale model and the validation of an analytical framework for the performance of unbalanced rotors are proposed. Starting from the optimal collective pitch assessment (performed to identify the condition with the maximum power coefficient), the pitch of just one blade was systematically changed: it is seen that the presence of a pitch misalignment is associated with a degradation of the turbine performance, visible both from experiments and from Blade Element Momentum (BEM) calculations (modified to account for the load asymmetry). Up to 30% power losses and a 15% thrust increase are achievable when an imbalanced rotor operates at tip speed ratios around five, clearly highlighting the importance of avoiding this phenomenon when dealing with industrial applications. The numerical model predicts this result within 5% accuracy. Additional numerical simulations showed that, away from the optimal collective pitch, the blade imbalance can provide a power increase or a power decrease with respect to the balanced case, suggesting how an operator can maximise the production of an unbalanced rotor. An analysis of the axial and lateral forces showed a sensitivity of the loads' standard deviation when imbalance is present. An increase of the lateral loads was observed in all unbalanced cases.

In this work we study the wake of a yawed wind-turbine model immersed in an atmospheric boundary layer (ABL). The ABL is replicated in the wind tunnel by means of a barrier-spires and distributed roughness configuration and is representative of a rural terrain. We quantify the properties of the wake in the horizontal plane at hub height and compare the predictions of available wake models to our data for different yaw angles. It is found that the model based on lifting-line theory performs best in predicting the velocity deficit without the need of tuning the parameters to the current setup. However, the wake deflection is slightly underestimated, most notably at the transition between near and far wake. Furthermore, a comparison with the turbine in a uniform incoming flow highlights the enhanced downward deflection of the wake which results from its interaction with the ABL.

Wind tunnel experiments were performed to investigate the response of a wind turbine model immersed in a replicated atmospheric boundary layer to dynamic changes in the yaw angle. Both the flow field in the wake and the operating properties of the turbine, namely its thrust force, torque, and angular velocity, were monitored during repeated yaw maneuvers for a variety of yaw angles. It was observed that the characteristic time scale of the transient experienced by the turbine scalar quantities was one order of magnitude larger than that of the yaw actuation and depended primarily on the inertia of the rotor and the generator. Furthermore, a Morlet wavelet analysis of the thrust signal showed a strong peak at the rotation frequency of the turbine, with the transient emergence of high activity at a lower frequency during the yaw maneuver. The insights provided by the proper orthogonal decomposition analysis performed on the wake velocity data enabled the development of a simple reduced-order model for the transient in the flow field based on the stationary states before and after the yaw maneuver. This model was then further improved to require only the final state, extending its applicability to any arbitrary wind farm as a dynamical surrogate of the farm behavior.

Wake-steering control has the potential of improving the power production of wind farms by deflecting the wakes of upstream turbines away from the downstream ones, thereby increasing the velocity impinging on the latter by sacrificing the performance of the former. In this work, a wide range of wake-steering control strategies are systematically applied to a 3 × 3 wind farm in a series of wind-tunnel experiments. When each streamwise column is operated identically to the others, the maximum measured power gain is approximately 5.3%. It is observed that the columns respond differently to a given yaw configuration, with the central one generally improving to a smaller degree than the lateral ones. Nevertheless, our data indicate that tuning each column independently of the others does not result in further power improvements. Furthermore, we show that increasing the free-stream velocity enhances the baseline power production of the model farm but reduces the scope for improvement achievable with wake-steering control.

The efficacy of wake-steering control strategies aimed at maximizing a wind-farm power output is investigated by means of wind-tunnel experiments involving a 20-turbine array. The rotors are arranged in five streamwise-aligned columns of four rows, with uniform yaw control applied to turbines within each row. The maximum power improvements are between 2.7% and 4%, depending on the analysis method. Performance gains exhibit a column-dependent variability and are predominantly observed when the upstream turbines are yawed in the positive direction, i.e. counter-clockwise when observed from above. Laser Doppler Velocimetry (LDV) measurements of two-component velocity fields within and above the farm reveal a clear correlation between the mean streamwise velocity and the fluctuations of the vertical component. The data further indicate that the increase in streamwise velocity due to the wake deflections is accompanied by a reduction in turbulent transport, which may reduce the potential power improvements for farms consisting of a large number of rows.