Average power losses due to wind turbine wakes are of the order of 10 to 20% of total power output in large offshore wind farms. Accurately quantifying power losses due to wakes is, therefore, an important part of overall wind farm economics. The focus of this research is to compare different types of models from computational fluid dynamics (CFD) to wind farm models in terms of how accurately they represent wake losses when compared with measurements from offshore wind farms. The ultimate objective is to improve modelling of flow for large wind farms in order to optimize wind farm layouts to reduce power losses due to wakes and loads.The research presented is part of the EC-funded UpWind project, which aims to radically improve wind turbine and wind farm models in order to continue to improve the costs of wind energy. Reducing wake losses, or even reduce uncertainties in predicting power losses from wakes, contributes to the overall goal of reduced costs. Here, we assess the state of the art in wake and flow modelling for offshore wind farms, the focus so far has been cases at the Horns Rev wind farm, which indicate that wind farm models require modification to reduce under-prediction of wake losses while CFD models typically over-predict wake losses. Further investigation is underway to determine the causes of these discrepancies.

Computational fluid dynamic (CFD) methods are used in this paper to predict the power production from entire wind farms in complex terrain and to shed some light into the wake flow patterns. Two full three‐dimensional Navier–Stokes solvers for incompressible fluid flow, employing k − ϵ and k − ω turbulence closures, are used. The wind turbines are modeled as momentum absorbers by means of their thrust coefficient through the actuator disk approach. Alternative methods for estimating the reference wind speed in the calculation of the thrust are tested. The work presented in this paper is part of the work being undertaken within the UpWind Integrated Project that aims to develop the design tools for next generation of large wind turbines. In this part of UpWind, the performance of wind farm and wake models is being examined in complex terrain environment where there are few pre‐existing relevant measurements. The focus of the work being carried out is to evaluate the performance of CFD models in large wind farm applications in complex terrain and to examine the development of the wakes in a complex terrain environment. Copyright © 2011 John Wiley & Sons, Ltd.

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The modelling of wind turbine wakes is investigated in this paper using a Navier-Stokes solver employing the k-ω turbulence model appropriately modifi ed for atmospheric fl ows. It is common knowledge that even single-wind turbine wake predictions with computational fl uid dynamic methods underestimate the near wake defi cit, directly contributing to the overestimation of the power of the downstream turbines. For a single-wind turbine, alternative modelling enhancements under neutral and stable atmospheric conditions are tested in this paper to account for and eventually correct the turbulence overestimation that is responsible for the faster fl ow recovery that appears in the numerical predictions. Their effect on the power predictions is evaluated with comparison with existing wake measurements. A second issue addressed in this paper concerns multi-wake predictions in wind farms, where the estimation of the reference wind speed that is required for the thrust calculation of a turbine located in the wake(s) of other turbines is not obvious. This is overcome by utilizing an induction factor-based concept: According to it, the defi nition of the induction factor and its relationship with the thrust coeffi cient are employed to provide an average wind speed value across the rotor disk for the estimation of the axial force. Application is made on the case of fi ve wind turbines in a row.Modelling of wind turbine wakes J. M. Prospathopoulos et al.

Aeroelastic stability is a key issue in the design process of wind turbines towards both enchanced stability and increased fatigue life. The theory and models behind the state-ofthe-art aeroelastic stability tools developed for the analysis of the complete wind turbine at the Centre for Renewable Energy Sources and the National Technical University of Athens are presented in this article. Application examples of stability calculations for a pitch, variable speed and a stall-regulated wind turbine are also presented.for the edgewise blade mode shapes. On the other hand, Rasmussen et al. 3 developed a dynamic stall model taking variations in both angle of attack and flow velocity into account, while Chaviaropoulos 4 used eigenvalue analysis combined with unsteady aerodynamics, in the form of the engineering-type ONERA model, 5 to investigate in a linearized sense the flap/lag stability properties of an isolated blade section. The conclusions drawn from the latter work, which are still valid, were the following.'It is seen that in the absence of structural damping the system is neutrally stable when viscous effects are neglected. As soon as viscous effects are taken into account, the system response becomes unstable, especially at the lower reduced frequency regime corresponding to the blade near-tip area. Parametric studies have shown that instability is amplified when (i) the density ratio R f (the air density divided by the normalized linear density of the blade) takes higher values, (ii) the blade natural frequencies become lower and (iii) the lift loss and/or the lift loss and drag derivatives of the aerofoil polar curves take higher values. Performing polar curve computations for five NACA 63-2XX profiles of different thickness, it appears that thicker profiles have more stable behaviour. This indicates once again that instabilities, when present, are triggered from the outer part of the blade. Structural damping has a dramatic stabilizing effect. Even a small amount of structural damping in the edge direction can drastically suppress the range and strength of the unstable region. Low ambient temperature favours aeroelastic instabilities, reducing the structural damping and increasing the density factor. Disregarding Reynolds number effects on the aerodynamic performance, it appears that the flap/lead-lag instability is independent of the blade size for aerodynamically (tip speed is maintained) and structurally similar blades.'The solution of the typical section stability problem with an engineering-type aerodynamic model provided important knowledge at the qualitative level but also significant uncertainty at the quantitative level. Along these lines the problem of the typical section was revisited in the framework of the VISCEL European project, using this time an unsteady Navier-Stokes treatment of the aerodynamics and working, inevitably, in the time domain (see References 6 and 7 for details). The exercise confirmed an earlier finding that linear models appear more conservative in evaluating instabilities...