Abstract Summary
Wind energy plays an increasingly important role in promoting renewable energy development. Therefore, much focus has been naturally given to wind turbines under working conditions. Meanwhile, a standstill condition is another critical state of a wind turbine. When the turbine is at standstill, the blade is often pitched by a large angle to cut out the wind. In this situation, vortices could form along the blade, creating strong vibrations considering the increasing size of wind turbines. This vibration is called vortex-induced vibration (VIV). VIV has a high potential to increase the fatigue load of the wind turbine blade. Meanwhile, the high wind sometimes discourages the installation of wind turbine blades, and workers have to wait until wind speed drops to a certain limit to guarantee a safe rotor installation. Therefore, more labor cost, time cost, rental cost, and in all, Levelized Energy Cost (LEC) increase. Concerning the load and cost problem induced by VIV in the industry, it is essential to understand the unsteady aerodynamics of a wind turbine blade at different inclination angles, especially under large angles of attack (AOA). This research aims to experimentally study the unsteady aerodynamic characteristics of vortex shedding of flat plates. The campaign was carried out in the W Tunnel at TU delft. Three flat plate models were built with the same length of 50 cm and the same thickness of 3mm. They have different chord lengths of 3 cm, 5 cm, and 10 cm, which brings three different aspect ratios of 16.7, 10, and 5, respectively. The ones with aspect ratios of 5 and 16.7 were made of aluminum and the one with the aspect ratio of 10 was made of steel for higher stiffness. In the first step, a force balance was mounted under the models to measure the streamwise and crossflow load at a wind speed of 6.4 m/s. Seven AOAs (0°, 15°, 30°, 45°, 60°, 75° and, 90°) under five inclination angles (30°, 60°, 90°, 120° and 150°) were tested. In the second step, stereoscope PIV was set up to measure the three component velocity field in different spanwise positions of the steel flat plate with the aspect ratio of 10. The main outcomes of this research will be: 1) The influence of inclination angle on the forces and velocity field. 2) The influence of aspect ratio on the forces and velocity field. 3) The difference between tip vortex and leading/trailing edge vortex in terms of structure, influence on the force, etc. 4) The 3D tip effect at different inclination angles and AOAs. These outcomes will provide a fundamental understanding of the unsteady aerodynamics of the flat wing. In addition, the understanding of unsteady vortex shedding paves the way for further research on vortex-induced vibrations on wind turbine blades.
