The stability and development of tip and root vortices behind a model wind turbine
When designing new wind farms, one has to rely on models describing the flow field around and inside the farm, since direct numerical simulation is far too computationally expensive. In order to develop better models for power prediction of wind farms, knowledge about the flow field around turbines, the stability of the wakes and the interaction between them is essential. Since the conditions during field measurements are difficult to control, wind tunnel measurements play an important role when studying wakes behind wind turbines. Within the present work, an experimental methodology has been developed to study the evolution and stability of the tip vortices shed from the rotor blades of a small-scale turbine model. The stability of the tip vortices was studied by introducing a periodic disturbance to the flow, which is shown to have a clear effect on the development of the vortices. Prior to the vortex breakdown, clear signs of vortex pairing were also observed. A parameter study was performed by varying the amplitude and frequency of the forced disturbance, and the effect on the tip vortices was evaluated. This experiment is one of the first where the influence of a periodic disturbance on a wind turbine wake is studied, something that previously has been performed in a number of numerical studies.
This work is part of the Nordic consortium for optimization and control of wind power farms and was financed by the Swedish Energy Agency. The turbine blades were designed with the help of Dr. Robert Mikkelsen and manufactured in the workshop at Technical University of Denmark. The turbine model support, strain gauge, generator and electrical equipment were provided by Jan-Å ke Dahlberg. Dr. Antonio Segalini is gratefully acknowledged for his valuable comments on the manuscript.
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