Thick Airfoil Dynamic Stall
Vertical axis wind turbines (VAWTs) are receiving increased attention due to their appearance in the built environment and their potential for deep water offshore applications. Their well-known advantages include insensitivity to the wind direction and the proximity of the generator to the ground. Furthermore, blade profile uniformity along the span can significantly reduce manufacturing costs, particularly for large scale utility machines. A drawback of VAWTs is the tendency of their blades to stall dynamically when they are pitched beyond their static stall angle. Dynamic stall is characterized by a strong vortex, the dynamic stall vortex (DSV), which forms near the leading-edge. When the DSV is convected downstream, a rapid drop in lift and severe pitching moment fluctuations result. On VAWTs operating at low tip-speed ratios, dynamic stall occurs periodically throughout the rotation of the blades. This situation is unique in that the blades experience pitch oscillations about zero angle of attack and the flow separates alternately on both sides. This results in a number of undesired effects: On the one hand, the full separation on the suction surface produces a sharp drop in cl and thus rotor torque. On the other hand, the unsteady aerodynamic loads cause fatigue damage to the generator and drive train.
The vast majority of dynamic stall research to date was motivated by the load oscillations on helicopter blades and therefore focuses on thin airfoils at Re>106. In the built environment, however, blades experience Reynolds numbers that are approximately one order of magnitude lower. Moreover, thick blade sections that are employed for structural reasons exhibit a fundamentally different stall mechanism that is exacerbated at low Reynolds numbers. The predominant use of thick blade sections at low Reynolds numbers, combined with the dearth of research, motivated an experiential study of VAWT dynamic stall and its control.
The previous focus of the project was a detailed investigation of the dynamic stall mechanism occurring on thick, symmetric blades at Reynolds number in the range of 250.000<Re<500.000. A dynamic stall vortex located above the rear half of the suction side, never reported before was observed. It manifests as a distinct low pressure region that produces a drop in the moment coefficient prior to the growth of the leading-edge dynamic stall vortex. In the next stage of the project, the possibility of using active flow control methods such as constant blowing and zero mass-flux excitation to delay or prevent dynamic stall will be explored. Provision is made to vary the wind tunnel speed simultaneously with the angle of attack to facilitate a more realistic representation of the flow conditions occurring on VAWT blades.
This project is conducted in close collaboration with Prof. Greenblatt, Technion Israel Institute of Technology. The measurements where carried out in the test-section constructed by the chair of fluid dynamics (see here for details).