Drag and Wake Measurements on Cylinders and Discs for Wind Turbine Wake Modelling

Abstract

This master thesis presents an experimental study of the mean wake velocities, the drag coefficients and the vortex frequencies behind cylinders, porous discs and circular full discs. The thesis contributes to the development and better understanding of simple wind turbine wake models. The future long term goal is to improve the complex fluid flow models of wind turbines.The experimental trials were executed in two different wind tunnels at the Energy and Process Engineering Department of NTNU. The main trials have been conducted to determine the drag coefficient with two independent methods. Pitot tubes were used to determine the mean wake velocities and the drag coefficients by using the measurement by wake method. Furthermore, the force survey method was established by using a force plate. A broad literature survey revealed a consistent range of drag coefficients and mean wake velocities comparable to the determined results of the current investigation. Additionally it was found, that the measured upstream flow velocity has a major influence on the measurement by wake method. The results of the force survey method gave strong evidence, that the blockage effect has a negative influence.The final experiments were conducted as additional study on wake flows. The method applied was based on hot wire velocity measurements to determine the vortex shedding frequencies. A literature review predicted the vortex shedding frequencies of cylinders and circular full discs in a comparable range of the experimental results shown by the power spectral density analysis. The analysis of the porous biplane disc unfolded no discrete frequency.A comparison of the results of the mean wake flows revealed a significant variation between shape of the cylinder and discs to the model wind turbine. However, the porous monoplane disc achieved the best results and is therefore most promising for future investigations on wind turbine wake modelling.