No abstract

The phenomenon of wake meandering is long known empirically, but has so far not been treated in a satisfactory manner on the wind turbine load modelling side. We present a consistent, physically based theory for wake meandering, which we consider of crucial importance for the overall description of wind turbine loadings in wind farms. In its present version, the model is confined to single wake situations-including a simple heuristic description of wake interaction with a reflecting surface. Contrary to previous attempts to model wind turbine wake loading, the present approach opens for a unifying description in the sense that turbine power and load aspects can be treated simultaneously. This capability is a direct and attractive consequence of the model being based on the underlying physical process, and it potentially opens for optimization of wind farm topology, wind farm operation, as well as control strategies for the individual turbine.The application of the proposed dynamic wake meandering methodology with existing aeroelastic codes is straightforward and does not involve any code modifications. The strategy is simply to embed the combined effect of atmospheric turbulence, added wake turbulence and the intermittent 'turbulence contribution', caused by wake meandering, in files replacing the traditional turbulence file input to aeroelastic computations. Copyright wake profi le is typically assumed Gaussian, 4 and the centreline defi cit decays monotonically with a rate strongly dependent on the ambient turbulence, but also on the turbulence generated by the velocity defi cit profi le itself and the turbulence generated by the mechanical mixing process in the rotor plane. The development of the far wake was modelled with an eddy viscosity model by Ainslie 4 taking into account the ambient turbulence as well as the defi cit-shear-generated turbulence.The problems and uncertainties, by comparing model results with full-scale measurements, were noticed by Taylor et al., 5 based on an investigation where model results were compared with wake measurements on the Nibe 630 kW turbines. He describes that the variations in on-site wind direction shift the wake across the downstream rotor disc, and this will increase the average power output from the downstream turbine measured over some time-a mechanism not taken into account in the modelling. Ainslie 6 discusses the subject in more detail and mentions that wake meandering effects can have considerable infl uence on measured wake defi cits, in particular under non-stable atmospheric conditions. It seems that Ainslie 6 is the fi rst to model the effect from wake meandering on wake defi cits by correlating the wake meandering to the variability in the wind direction. In this way, he computes the averaging of wake defi cits for two full-scale experiments, and the infl uence from the meandering is signifi cant in reducing the depth of the defi cits. 6 Further comparisons of model and experimental results, including the correction for meandering for a number of different ...

As the major part of new wind turbines are installed in clusters or wind farms, there is a strong need for reliable and accurate tools for predicting the increased loadings due to wake operation and the associated reduced power production. The dynamic wake meandering (DWM) model has been developed on this background, and the basic physical mechanisms in the wake—i.e., the velocity deficit, the meandering of the deficit, and the added turbulence—are modeled as simply as possible in order to make fast computations. In the present paper, the DWM model is presented in a version suitable for full integration in an aeroelastic model. Calibration and validation of the different parts of the model is carried out by comparisons with actuator disk and actuator line (ACL) computations as well as with inflow measurements on a full-scale 2 MW turbine. It is shown that the load generating part of the increased turbulence in the wake is due almost exclusively to meandering of the velocity deficit, which causes “apparent” turbulence when measuring the flow in a fixed point in the wake. Added turbulence, originating mainly from breakdown of tip vortices and from the shear of the velocity deficit, has only a minor contribution to the total turbulence and with a small length scale in the range of 10–25% of the ambient turbulence length scale. Comparisons of the calibrated DWM model with ACL results for different downstream positions and ambient turbulence levels show good correlation for both wake deficits and turbulence levels. Finally, added turbulence characteristics are compared with correlation results from literature.

This paper investigates wake effects on load and power production by using the dynamic wake meander (DWM) model implemented in the aeroelastic code HAWC2. The instationary wind farm flow characteristics are modeled by treating the wind turbine wakes as passive tracers transported downstream using a meandering process driven by the low frequent cross-wind turbulence components. The model complex is validated by comparing simulated and measured loads for the Dutch Egmond aan Zee wind farm consisting of 36 Vestas V90 turbine located outside the coast of the Netherlands. Loads and production are compared for two distinct wind directions-a free wind situation from the dominating southwest and a full wake situation from northwest, where the observed turbine is operating in wake from five turbines in a row with 7D spacing. The measurements have a very high quality, allowing for detailed comparison of both fatigue and min-mean-max loads for blade root flap, tower yaw and tower bottom bending moments, respectively. Since the observed turbine is located deep inside a row of turbines, a new method on how to handle multiple wakes interaction is proposed. The agreement between measurements and simulations is excellent regarding power production in both free and wake sector, and a very good agreement is seen for the load comparisons too. This enables the conclusion that wake meandering, caused by large scale ambient turbulence, is indeed an important contribution to wake loading in wind farms.

The phenomenon of wake interaction between two wind turbines was analysed using the actuator line technique and full unsteady Navier-Stokes computations. Results are presented for varying mutual distances between the two turbines and both full wake and half wake situations were considered. Furthermore, simulations were carried out at different degrees of ambient turbulence intensity representing laminar, offshore and onshore conditions. From the simulations, the main characteristics of the interacting wakes were extracted including the averaged velocity and turbulence fi elds as well as the development of wake generated vortex structures. Moreover, the infl uence of the wake of the upstream turbine on the external aerodynamic loading on the blades of the downstream turbine was studied.

A new load-reducing control strategy for individual blade control of large pitch-controlled wind turbines is presented. This control concept is based on local blade inflow measurements and offers the possibility of larger load reductions, without loss of power production, than seen in other state-of-the-art load-reducing concepts. Since the new flow-based concept deviates significantly from previous published load-reducing strategies, a comparison of the performance based on aeroelastic simulations is included. Advantages and drawbacks of the systems are discussed.

No abstract

A comprehensive investigation of the Blade Element Momentum (BEM) model using detailed numerical simulations with an axis symmetric actuator disc (AD) model has been carried out. The present implementation of the BEM model is in a version where exactly the same input in the form of non-dimensional axial and tangential load coeffi cients can be used for the BEM model as for the numerical AD model. At a rotor disc loading corresponding to maximum power coeffi cient, we found close correlation between the AD and BEM model as concerns the integral value of the power coeffi cient. However, locally along the blade radius, we found considerable deviations with the general tendency, that the BEM model underestimates the power coeffi cient on the inboard part of the rotor and overestimates the coeffi cient on the outboard part. A closer investigation of the deviations showed that underestimation of the power coeffi cient on the inboard part could be ascribed to the pressure variation in the rotating wake not taken into account in the BEM model. We further found that the overestimation of the power coeffi cient on the outboard part of the rotor is due to the expansion of the fl ow causing a non-uniform induction although the loading is uniform. Based on the fi ndings we derived two small engineering sub-models to be included in the BEM model to account for the physical mechanisms causing the deviations. Finally, the infl uence of using the corrected BEM model, BEM cor on two rotor designs is presented.