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Delegates are invited to meet and discuss with the poster presenters during the poster presentation sessions between 10:30-11:30 and 16:00-17:00 on Thursday, 19 November 2015.

Lead Session Chair:
Stephan Barth, ForWind - Center for Wind Energy Research, Germany
John Amund Lund Meventus AS, Norway
Co-authors:
John Amund Lund (1) F Chi-Yao Chang (2) Rolf-Erik Keck (3) Klaus-Ole Vogstad (1)
(1) Meventus AS, Kristiansand, Norway (2) Fraunhofer IWES, Oldenburg, Germany (3) Statkraft AS, Oslo, Norway

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2015 and are published here unedited

John Amund has been in charge of detailed wind flow assessments for more than 70 complex wind farm sites in Scandinavia, ranging from early phase screening to detail planning and Due Dilligence work. He is an experienced user of several tools for modeling of atmospheric flow, and has contributed to the development of ParkDesign/ParkOptimizer, a design tool for wind farms in complex terrain. His work on comparative analyses of wind tunnel experiments and CFD tools used for testing of new wind turbine designs has been published at conferences and in peer-reviewed journals.


Poster

Poster Download poster (13.66 MB)

Abstract

Predicting large-scale recirculation using a hybrid lidar-LES approach

Introduction

Flow recirculation and other complex flow phenomena occur frequently in extremely complex terrain. These phenomena cause high turbulence, and can impose high loading and maintenance costs to exposed wind turbines. It is known that most flow modelling tools fails to predict the presence or behavior of these phenomena. Approximations such as the Reynolds Averaged Navier-Stokes approach used in most commercial CFD tools actually suppress these large scale turbulent fluctuations.

Traditional measurement campaigns can be used to predict the presence of highly turbulent flows, but they cannot detect the source of the turbulence. As the turbulence generated by large scale recirculation is advected downwind, a proper description of the phenomena requires measurements at several locations with a spatial and temporal resolution capable of describing the main length scales of the flow.

Approach

In order to better describe and predict recirculation and high-turbulent spots, a hybrid lidar-LES approach has been developed. The method has been tested in a full scale experiment in extremely complex terrain in a planned wind farm in Norway.

Initially a Large Eddy Simulation (LES) flow model was used to describe the turbulent flow in regions where it was expected that flow recirculation could occur. The flow model was set up to identify how the turbulence generated would be transported downwind and eventually dissipated. In a location where recirculation was expected to occur, a scanning lidar unit was deployed and programmed to measure the flow downwind of the expected recirculation zone.

As the lidar can only measure along its Line of Sight (LoS), it can only provide one-dimensional wind velocity along a two-dimensional line. However, when validating the flow model using the measured wind data, a three dimensional description of the turbulent flow can be obtained.

Main body of abstract

The hybrid approach used in this study is made possible by the recent development of the StreamLine XR lidar. An initial benchmark study against measurements from a 3D sonic anemometer located at a distance of 350 meter showed that the lidar was capable of measuring at 1 Hz with a spatial resolution of 9-18 meters without lack of accuracy or data availability. Excellent correlation at 1 Hz was obtained, even in periods where LoS velocities were close to zero in average. Comparisons of turbulent spectra show that the spatial averaging effect introduced by the lidar is reduced as the range gate is shortened. The reduction in range gate length makes measurement of smaller scales of turbulence possible.

The full scale experiment was conducted downwind of a 150 meter tall cliff and inflow wind speeds of approximately 10 m/s. Measurements were made at 1Hz while retaining a constant elevation for periods of 10 minutes in order to describe the temporal changes in the flow field. The measurements confirmed the presence of large recirculation described by the LES model.

Comparisons of the turbulent spectra clearly identified that the LES-model suffers from a larger spatial averaging effect. However, the large-scale turbulent structures and their behavior is well predicted, as well as the peak of the turbulent spectrum.

Conclusion

The results show that both high resolution lidar measurements and LES flow modes can be used to describe large scale recirculation in complex terrain. In regions where the scales of the main turbulent fluctuations are significantly larger than the spatial resolution of the lidar and the flow model, accurate descriptions of the flow behavior can be made.


Learning objectives
The study presents a framework model that can be used to reduce the risk of wind turbines in complex terrain being exposed to high turbulent flow. Advanced LES models in combination with high resolution scanning lidars provide an efficient toolbox for predicting the presence of recirculation and other complex flow phenomena in extremely complex terrain.