11:30 - 13:00 Wakes: LiDAR measurements, layout optimization and modelling uncertainties
The focus of this session is on wind turbine wakes. Two studies deal with measurement of the wake wind field using LiDARs, and subsequently verifying them against reference instruments. One study deals with wind farm layout optimization in a complex terrain, where the wake wind field is generated using CFD tools. One study deals with providing a framework for evaluating wake models uncertainties, and subsequently their influence on the predicted wind turbine power in wakes.
- The importance of evaluating the uncertainty of the estimated wind speed in wakes using measurements from a Doppler LiDAR, with an example of how to do it under different atmospheric conditions
- How a 3D wake wind field looks like from LiDAR measurements, potentially paving the way for its subsequent use in wake tracking and model parameterization
- How CFD wake results in complex terrain can be combined with layout optimization techniques, potentially being applicable to wind farm design
- A framework for wind farm flow model validation, where it is demonstrated how uncertainties in input variables propagate into resulting uncertainties in predicted wind turbine power in wakes
Lead Session Chair:
Ameya Sathe, DTU Wind Energy, Denmark
Davide Trabucchi (1) F P Marjin vanDooren (1) Martin Kuehn (1)
(1) ForWind, Oldenburg, Germany
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Presenter's biographyBiographies are supplied directly by presenters at EWEA 2015 and are published here unedited
Mr. Beck has been researching for two and a half years in wind energy as a research assistant and Ph.D student at ForWind Oldenburg, Germany. He studied aeronautical and aerospace technology at the University of Applied Science in Bremen and advanced this with a degree in Engineering Physics. His research is focused on wake behavior of wind energy turbines and LiDAR scanning technology.
Volumetric wind field measurements of wind turbine wakes with long range lidar
Scanning Doppler lidars are attracting the industry’s attention since they offer new insights in full scale wake and resource assessment measurements. The capabilities of one-dimensional scans are very versatile and go far beyond conventional met mast based measurements. Instationary changes in the wind direction, wakes meandering as well as atmospheric convection and advection are causing dynamic wake displacements which lead to uncertainties in wind speed deficit classification since it is a priori unknown where single planar vertical (RHI) or horizontal (PPI) scans are cutting the wake trajectory. To overcome such uncertainties more sophisticated scan modes are proposed for detailed volumetric measurements of the wake. Such a three-dimensional mapping can provide a deeper understanding of atmospheric effects on the wake flow behaviour and can improve available campaign resources in terms of measurement time, measurement efficiency and by this measurement costs.
The research introduces a method to process multiple planar one-dimensional lidar measurements for reconstruction of the full-scale far wake wind field. The influence of the spatial and temporal combination of conventional vertical and horizontal slices and its effect regarding the reconstruction of dynamic and static wind fields is studied and will be presented with both simulation tests and full-field measurements.
Main body of abstract
Firstly, the lidar simulator LiXIM  is applied on high resolved LES wake simulations for different atmospheric conditions to generate detailed flow information. Selected scan parameters as scanning speed and angular resolution are systematically examined under consideration of different scan frequencies and scan geometries. Secondly, full scale multi-lidar measurements from a ground-based offshore campaign at ‘alpha ventus’ and a nacelle-based onshore measurement campaign are evaluated for the purpose of volumetric wind field reconstruction.
This simulated and measured lidar data provide line-of-sight velocities which are processed using filtering and projection methods to obtain the longitudinal wind component u. A three-dimensional wind field propagation matching the single scans is used under the assumption of mass conservation and flow equation to estimate the spatial and temporal distribution of wind speed between each scan to form a full scale wind turbine wake wind field. Those merged wind fields are compared afterwards with established static and dynamic wake models and LES simulations to assess the quality of the reconstruction
Combination of several vertical and horizontal scans can be used to reconstruct volumetric measurements of wind turbines wakes to compute static or dynamic wake wind fields. The three dimensional wind fields can be used for further investigation of wake tracking, wake model parameterisation and aero-elastic load simulations.
Scanning long-range lidar systems provide new opportunities to perform volumetric measurements for longitudinal spatial wind field reconstruction suitable for various research or industrial applications in wake and possibly resource assessment studies.
 D. Trabucchi, J. J. Trujillo, G. Steinfeld, J. Schneemann, M. Machtaa, J P. Cariou, and M. Kühn,
“Numerical assessment of performance of lidar windscanners for wake measurements”, in EWEA
Annual event (2011).