Aerodynamics of Wind Turbines 101

1. Aerodynamics of Wind Turbines 101 Women of Wind, March 8, 2011 Presented by Emil Moroz

2. Acknowledgements Slides and some of the themes behind the slides were borrowed from a variety of sources including, but not limited to: TU Delft Georgia Tech UC Davis Middle East Technical University NREL Bonus (now Siemens) Etc.

3. What is Aerodynamics? • A branch of dynamics – How things move under the action of forces • Aerodynamics is the study of motion of air and how it interacts with a moving or stationary object. • Prandtl (1874-1953), a German Engineer, formulated a large number of major theories related to this subject – one of the most important being Boundary Layer theory in 1904.

4. Advantage of assuming a Boundary Layer • Boundary Layer – simplifies the analysis of flows by separating the “viscous” (sticky) layer close to a body “the boundary layer” from the bulk of the flow passing by the body.

5. Aerodynamics is a Broad Field • The scale may vary from a few millimeters to hundreds of meters. (e.g. flow around a bug to flow around the Earth.) • The speed may vary from a few millimeter/s to hundreds of meters per second. • This presentation focuses on flow around horizontal axis wind turbine blades

6. Wind – The Key Ingredient • Wind energy is created when: • the atmosphere is heated unevenly by the Sun • some patches of air become warmer than others • the warm patches of air rise • other air rushes in to fill the void thus, wind blows WIND Source: www.physicalgeography.net/fundamentals/7n.html

7. When wind blows against a surface it exerts a force – which can be very large U D Wind exerts a large force but you need movement to allow power to be extracted

8. Moving Plate Allows Power To Be Extracted U d Power, P=V x d V Disadvantage of movement in direction of wind is that the wind force on the plate is reduced because it sees a lower relative wind (U-V)

9. Utilization of wind 3000 years ago These are considered drag machines

10. Tilted Flat Plate Creates Lift Component “Angle of Attack” α L U D “Angle of Attack” α ~10 degrees L U D As “Angle of Attack” decreases degrees then lift increases and drag decreases

11. Medieval “Post Mill” ~12th Century U Rotation results in a “relative wind, Vrelativewhich results in higher Lift than from wind U alone L U Vrotation Vrelative L D α In-plane force D Thrust Force Component of Lift Force does useful rotational work “torque” (radius x In-plane force) Same Principles Apply to Modern Wind Turbines

12. Wind Mill Sections / Airfoils Have Evolved ~1270 ~1400 ~1600 ~1650 Early Airfoil Designs - Designers mistakenly believed that these airfoils with sharp leading edges will have low drag. In practice, they stalled (lost lift) quickly, and generated considerable drag.

13. Modern Wind Turbines have Custom Designed Airfoil Families

14. Typical Modern Light Weight /Low Cost Structure

15. What Causes Lift? • Take two pieces of paper and crease in middle hold as in (a) and prepare to blow between the sheets • What do you think will happen? (papers blow apart or come together?) • Alternatively, take one sheet of paper as in (c) and prepare to blow across the top of the sheet – at a shallow angle. • What do you think will happen?

16. Lift is Primarily Due to Bernoulli’s Law • The speed of air blowing between the two sheets in (a,b) or over the curved sheet in (c,d) causes a drop in air pressure. • Figure “b” shows that the two sheets should come together because of the pressure differential. • Figure “d” indicates that the paper should rise because of a similar pressure differential.

17. Example of Air Flow Around an Airfoil

18. Pressure Forces acting on an Airfoil Low Pressure High velocity High Pressure Low velocity Low Pressure High velocity High Pressure Low velocity Bernoulli’s Law says where pressure is high, velocity will be low and vice versa.

19. Have to Subtract off atmospheric Pressure pto get Resulting Pressure Forces on an Airfoil Low p-p  High velocity – “Suction Surface” High p-p  Low velocity “Pressure Surface” Low p-p  High velocity High p-p  Low velocity The quantity p-p  is called the gauge pressure. It will be negative over portions of the airfoil, especially the upper surface. This is because velocity there is high and the pressures can fall below atmospheric pressure.

20. Example of Pressure Variation around a Section of a Wind Turbine Blade

21. Drag Relevant to Wind Turbines is caused by: • Skin Friction - the air molecules try to drag the airfoil with them. This effect is due to viscosity of the air. • Pressure Drag – is due to unequal pressure on the airfoil surfaces facing towards and away from the oncoming flow.

22. Skin Friction Particles away from the airfoil move unhindered. Particles near the airfoil stick to the surface, and try to slow down the nearby particles. A drag/retarding force is felt by the airfoil This region of low speed “viscous” flow is the boundary layer.

23. Lift and Drag Coefficients versus AOA

24. Wind Turbine Design is an Interdisciplinary Problem Structures, Structural Dynamics, Vibrations, Stability, Fatigue Life Transmission, gears, tower, power systems, etc. Aerodynamics Noise, aesthetics Control systems for RPM, Pitch, Yaw Cost

25. Basic Theories used in Blade Design

26. Power Available in the Wind • r = air density • v = wind speed • A = cross sectional area swept by rotor (“swept area”)

27. Power Coefficient • Power coefficient Cp is a measure of the aerodynamic efficiency of the wind turbine • Q = turbine’s aerodynamic torque • W = rotor rotational speed • Betz limit - theoretical maximum

28. Tip-Speed Ratio • Ratio of the linear speed of the tip of the blade to the wind speed • Linear speed of a rotating object is angular speed times distance from center of rotation • l = tip-speed ratio • R = rotor radius • w = angular speed • v = wind speed

29. Characteristics of Optimum versus more Practical Design of a Blade

30. Most Aerodynamically Efficient Rotor on Market: Enercon, Cp(Aero)= 0.56 Questions?

31. The End