טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentHanns Muller-Vahl
SubjectWind Turbine Blade Dynamic Stall and its Control
(The research and dissertation were conducted
under a joint PhD program at Technion
and TU berlin)
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Greenblatt David
Full Thesis textFull thesis text - English Version


Abstract

Dynamic stall is an unsteady flow phenomenon that occurs when an airfoil is rapidly pitched beyond the static stall angle. The transient boundary layer separation process, characterized by the shedding of a strong vortex across the suction surface, produces severe aerodynamic load fluctuations that lead to fatigue damage. This is a major problem on wind turbine rotor blades, which are exposed to highly unsteady inflow conditions. Successful control of this phenomenon could potentially yield significant reductions in the cost of energy.


Even though dynamic stall has been extensively studied before, the associated flow speed variations have rarely been considered. In the present study, experiments were conducted in a unique wind tunnel facility that allows to reproduce the unsteady inflow characteristic of wind turbine rotor blades by simultaneously varying the angle of attack and the wind tunnel speed. All experiments were performed on a NACA 0018 airfoil, typically found on vertical axis machines, that exhibits trailing-edge type stall. Flow field measurements were made using Particle Image Velocimetry and the unsteady surface pressure distributions were recorded with arrays of piezo-resistive transducers. Phase locked measurements revealed the formation of a second dynamic stall vortex across the rear half of the airfoil which produces a drop of the pitching moment prior to the shedding of the leading-edge dynamic stall vortex. The experimental results also revealed that the reduced frequency effectively varies as a function of the freestream velocity, leading to significant differences in the transient variations of the aerodynamic coefficients. The matched pitch rate concept was extended to the case of synchronous incidence oscillations and flow speed variations, providing a framework that permits the prediction of unsteady aerodynamic loads from data obtained at constant flow speeds.


A novel flow control concept termed “adaptive blowing” was successfully tested. While the working principle is in part based on classical steady blowing, the fundamental differences lie in the dynamic variation of the control jet momentum for the purpose of controlling unsteady aerodynamic loads and the use of low-momentum blowing to temporarily reduce lift. Initially, steady blowing was investigated to characterize the effect of control. A comparison of blowing at different chordwise positions revealed that the leading-edge slot provides a far larger control authority, allowing for significant changes in lift over a wide range of angles of attack. Slot blowing at moderate and high momentum coefficients produced a substantial increase in lift and fully suppressed the formation of the dynamic stall vortex, thereby eliminating the associated rapid load excursions. In contrast, low momentum blowing was found to induce boundary layer separation at relatively small angles of attack, yielding a significant lift reduction. Once the effect of steady blowing had been established, the momentum coefficient was varied dynamically to compensate for transient changes of the inflow. An iterative control approach was implemented, which successfully identified the time profiles of the control jet momentum flux required to minimize the lift excursions. This strategy provided an unprecedented control authority during various periodic inflow oscillations, producing virtually constant phase averaged lift.