|M.Sc Student||Schulman Magen|
|Subject||Dynamic Stall Control on a Vertical Axis Wind Turbine Using|
Dielectric Barrier Discharge Plasma Actuators
|Department||Department of Mechanical Engineering||Supervisor||PROF. David Greenblatt|
|Full Thesis text|
Vertical axis wind turbines (VAWTs) advantages include wind-direction insensitivity and quiet operation while location of the generator at ground level can potentially reduce maintenance costs. A major problem, however, is that the blades of VAWTs stall dynamically at low blade tip-speed to wind-speed ratios, resulting in significant power losses and unsteady loads that compromise the drive-train, generator-sizing and system reliability. Due to a combination of the technical difficulties associated with deploying blade-mounted actuators on a rotating system and the unique character of alternating upwind/downwind dynamic stall, active flow control on a VAWT has never been demonstrated. The objective of this thesis was to directly measure the increase in wind turbine performance achievable by controlling dynamic stall on a VAWT using dielectric barrier discharge (DBD) plasma actuators. Initially, calculations were performed using blade-element theory based on static airfoil data that included the effects of DBD plasma actuators. Significant increases in performance predicted by the method served as the motivation to design and build a small, high solidity turbine and test it in a low speed wind tunnel. The turbine constructed was an H-rotor type (height and diameter 0.6m and 0.5m respectively), fitted with two extruded aluminum NACA0015 blades of chord length 0.15m. A custom-designed dynamometer was used to characterize its performance at wind speeds between 4m/s and 7m/s. A parametric study showed that turbine performance improvements resulting from pulsed plasma actuation were independent of duty cycle for the range 1% to 50%, which is consistent with performance improvements observed on static airfoils at post-stall angles-of-attack under steady free-stream conditions in other investigations. However, it was observed that performance improved continuously with increases in the pulse-modulation frequency, whereas data acquired on static airfoils showed peak lift coefficient changes are typically in the reduced frequency range 0.5 to 1. This anomaly resulted from a lack of an enforced phase relationship between the pulse-modulation frequency and the turbine rotational frequency (speed). Overall increases in turbine power coefficient of up to 38% for upwind control and 15% for downwind control were measured. Plasma actuation was seen to be particularly amenable to up-scaling. Based on the data acquired, up-scaling the turbine by a factor of 5 and 10, the percentage of plasma power required to produce comparable improvements was 1.7% and 3.3% respectively. Further work will address phase-locking the plasma pulsation and turbine frequencies, employing combined upwind and downwind actuation and measuring the controlled flow field using particle image velocimetry.