An experimental study of wind farm blockage has been performed to quantify the velocity decrease that the first row of a wind farm experiences due to the presence of the other turbines downstream. The general perception has been that turbines downstream of the first row are only influenced by the wakes from upstream turbines without any upstream effect. In the present study, an attempt is made to demonstrate the existence of a two-way coupling between individual turbines and turbines in the wind farm. Several staggered layouts were tested in the wind tunnel experiments by changing the spacing between rows, spacing between turbines in the rows, and the amount of wind turbines involved. The experiments focused on turbines located in the center of the first row as well as the two turbines located in the row edges, usually believed to experience a speedup. The present results show that no speedup is present and that all the turbines in the first row are subjected to a reduced wind speed. This phenomenon has been considered to be due to "global blockage."An empirical correlation formula between spacing, number of rows, and velocity decrease is proposed to quantify such effect for the center turbine as well as for the turbines at the edges. KEYWORDSwind farm blockage ;23:120-128.wileyonlinelibrary.com/journal/we 4D (with the lowest bound preferably at 2.5D) upstream of the first row of wind turbines. All these considerations concern a single isolated wind turbine. Since the distance between rows of turbines is larger than 3D, it could be expected that a two-way coupling between the turbines should be negligible, namely, that the downstream turbines should not influence the upstream ones (a "one-way coupling" model). Similarly, one could say that the wind should start "feeling" the presence of a wind farm only when the distance is less than 3D from the first row, regardless of the size of the farm, the spacing between the turbines, the number of turbines involved, and the thrust coefficient of the turbines. Since many wind farm assessments were and are still performed with wake models, any upstream effect is invisible. Indeed, this perception that only wakes influence the farm became part of the general understanding of wind farm aerodynamics.A similar idea could be proposed for turbines placed in a single row perpendicular to the wind direction: The effects of side turbines should be negligible, and every turbine should experience the free-stream velocity. However, recent simulations and experiments 9,10 indicated a small pos-
A fast and reasonably accurate numerical three‐dimensional wake model able to predict the flow behaviour of a wind farm over a flat terrain has been developed. The model is based on the boundary‐layer approximation of the Navier–Stokes equations, linearized around the incoming atmospheric boundary layer, with the assumption that the wind turbines provide a small perturbation to the velocity field. The linearization of the actuator‐disc theory brought additional insights that could be used to understand the behaviour, as well as the limitations, of a flow model based on linear methods: for instance, it is shown that an adjustment of the turbine's thrust coefficient is necessary in order to obtain the same wake velocity field provided by the actuator disc theory within the used linear framework. The model is here validated against two independent wind‐tunnel campaigns with a small and a large wind farm aimed at the characterization of the flow above and upstream of the farms, respectively. The developed model is, in contrary to current engineering wake models, able to account for effects occurring in the upstream flow region, thereby including more physical mechanisms than other simplified approaches. The conducted simulations (in agreement with the measurement results) show that the presence of a wind farm affects the approaching flow far more upstream than generally expected and definitely beyond the current industrial standards. Despite the model assumptions, several velocity statistics above wind farms have been properly estimated providing an insight into the transfer of momentum inside the turbine rows. Copyright © 2016 John Wiley & Sons, Ltd.
An experimental study of wind farm blockage has been performed to quantify the velocity decrease that the first row of a wind farm experiences due to the presence of the other turbines downstream. The general perception has been that turbines downstream of the first row are only influenced by the wakes from upstream turbines without any upstream effect. In the present study, an attempt is made to demonstrate the existence of a two‐way coupling between individual turbines and turbines in the wind farm. Several staggered layouts were tested in the wind tunnel experiments by changing the spacing between rows, spacing between turbines in the rows, and the amount of wind turbines involved. The experiments focused on turbines located in the center of the first row as well as the two turbines located in the row edges, usually believed to experience a speedup. The present results show that no speedup is present and that all the turbines in the first row are subjected to a reduced wind speed. This phenomenon has been considered to be due to “global blockage.” An empirical correlation formula between spacing, number of rows, and velocity decrease is proposed to quantify such effect for the center turbine as well as for the turbines at the edges.
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