Study of wind turbine wake aerodynamics through vortex flow visualization
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In this master thesis, a flow visualization experiment and a blade tracking experiment have been conducted. The aim of the thesis has been to assess the performance of the new blade tow tank rig at the Fluid Mechanics Lab at NTNU. The blade tracking experiment involved reconstructing the trajectory of the blade and analyzing the blade's vibrations. The flow visualization experiment involved analyzing the tip vortices produced in the wake of the blade. This experiment is conducted by injecting dye into the tank near the tip of the blade. A LabVIEW program has been created to accurately control the blade in the tank, through sending signals to the two stepper motors controlling the traversing and rotational motion of the blade. The program is designed to allow the user to quickly pick a specific velocity and acceleration of the blade, and thus a specific tip speed ratio ($\lambda$). The blade tracking experiment was able to accurately reconstruct the trajectory of the blade. It also yielded some interesting results regarding the error associated with camera focus (depth of field). The results were, however, inconclusive regarding the nature of the blade vibrations. The flow visualization experiment produced images of the tip vortices at different values of $\lambda$. The expected result that is increasing $\lambda$ decreases the strength of the tip vortices was also found, by comparing the change in intensity for the different scenarios. The overall result of confirming the usability of the rig is important, because it opens up the possibility for many future research projects. By creating procedures for creating tip vortices as well as analyzing the trajectory of the blade, this thesis might prove to be an important step in much research in the years to come. Since the strength of the tip vortices is closely linked to the wake recovery length, the results from this thesis further proves the potential for improving wind farm efficiency. The results also proved the importance of high-quality imagery, as it turned out the cameras' frame rate and depth of field proved detrimental to the result of the blade tracking experiment.