Session: 08-25-01: Flow-Induced Vibrations of Energy Systems

Paper Number: 168089

Small-Scale Experiments to Observe Flow-Induced Instabilities of Wind Turbine Blades 

Larger wind turbine blades that are suitable for offshore wind turbines could become unstable due to the classical flutter or stall-induced flutter, resulting in limit cycle oscillations of these large structures, which could cause immediate failure of the blade. Recent studies in various engineering sub-fields confirm the inherent multi-disciplinary nature of the problem and its relevance, especially in the context of offshore wind energy systems. We discuss a series of experiments that we have conducted to observe flow-induced instabilities on a small-scale wind turbine blade. We have designed and built these blades, both with a constant and variable cross-section along their length, by using very thin steel shim stock as the spine of the blade. The size of this steel plate and its thickness is selected in such a way that the desired natural frequencies in the flapwise and torsional directions are obtained, since we have shown previously that the ratios of these natural frequencies are critical in observing flow-induced instabilities in blades. In the case that we manufactured, a thickness of 0.203 mm shim stock was chosen in order to ensure that the spine is thick enough such that airfoils could be attached, but thin enough that the third flapwise frequency can be excited at a wind speed attainable at the UMass wind tunnel. With fixed values for the material properties, the beam cross-section is varied until the ratio between the torsional and the third flapwise natural frequencies becomes close to 0.8 (same ratio as in the NREL 5MW full scale model). The tail of the airfoil up to the edge of the spine were filled in with material in order to shift the center of gravity behind the elastic axis in reference to the leading edge. The airfoils were 3D printed using the EOS Formiga P110 Printer and with polyamide 12 to obtain a smooth surface and detailed enough to create the desired aerodynamics and still heavy enough to shift the center of mass from the center of the spine to behind the elastic axis. These experiments were conducted for a non-rotating blade. The fixed blade was clamped at the center of the test section. In each set of experiments, we measured the blade's displacement as well as the flow forces that act on the blade for both increasing and decreasing wind speeds using laser displacement sensors, high speed cameras and force sensors. We used small-scale blades of NREL 5MW, the same design that we will later use in a series of large-scale experiments in order to be able to compare the results obtained at both scales. The parameters that we have measured in these experiments will also be used as input parameters to a predictive model for flow-induced instabilities of wind turbine blades.

Presenting Author: Yahya Modarres-Sadeghi University of Massachusetts Amherst

Presenting Author Biography: Yahya Modarres-Sadeghi is a Professor of Mechanical Engineering and the Associate Dean for Academic Affairs and Operations at the College of Engineering, University of Massachusetts, Amherst. He is a Fellow of the American Society of Mechanical Engineers (ASME), a Fellow of the Radcliffe Institute for Advanced Study at Harvard University, and a Research Affiliate at the Massachusetts Institute of Technology (MIT). His research focuses on Fluid-Structure Interactions (FSI) and Nonlinear Dynamics. He uses experimental, theoretical, and numerical tools to understand different FSI phenomena both from a fundamental point of view and with applications in several fields including wind energy, bioinspired robotics, and biomedical science. He has published close to 80 journal articles and a book, titled Introduction to Fluid-Structure Interactions.

Authors:

Sudhansh Tanneru University of Massachusetts, Amherst
Dominique Habchi University of Massachusetts, Amherst
Yahya Modarres-Sadeghi University of Massachusetts Amherst