Wake meandering: a pragmatic approach

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 ...

Simulation of wind turbine wakes using the actuator line technique

“…Therefore, only three selected modes are shown and not the orthogonal counterparts. The combination of the first two POD modes governs the very large scale motions, which translates the entire wake and is often collectively referred to as wake meandering [7]. However, it is also important to note that not all POD modes can necessarily be attributed to actual large coherent structures, but for higher POD modes the spatial distribution becomes a mere result of lumping TKE together in an optimal sense.…”

“…However, they do not capture the actual turbulence characteristics in the wake, which may deviate significantly from that of the undisturbed flow field, and in most cases the models need to be calibrated against measurements in order to capture the local power production correctly. Furthermore, they disregard the important effect of wake meandering, which has been shown to have a major impact on wind turbine loading [7]. A more sophisticated type of simulation tool is based on the parabolized Reynolds-averaged Navier-Stokes equations [8,9].…”

TOPFARM: Multi‐fidelity optimization of wind farms

“…The instationary DWM model, [18][19][20][21][22][23] combined with the aero-elastic wind turbine simulation tool HAWC2, 24 is used to estimate the 10-min averaged power production and equivalent fatigue of various wind turbine components under various inflow wake conditions . The stationary model 19 is used to estimate the normative mean wind speed at the position of each wind turbine in the wind farm ('normative' is defined here as the equivalent undisturbed free-stream wind speed that the wind turbine would be operating under to produce a similar induced velocity at the rotor position).…”

“…21,25 It appears as an intermittent phenomenon, where winds at downwind positions may be undisturbed for part of the time but interrupted by episodes of intense turbulence and reduced mean velocity as the wake hits the observation point.…”

“…Detailed descriptions of the various submodels and their implementation in the framework of the aero-elastic code HAWC2 can be found in the following references. [20][21][22]24 …”