# Wind Energy Questions and Answers – Wind Turbine Aerodynamics – 2

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This set of Wind Energy Question Bank focuses on “Wind Turbine Aerodynamics – 2”.

1. How is drag force experienced by the blade reduced in various modern blade designs?
a) Bending and tapering the blade
b) Using heavy metals
d) Increasing the length of the blade

Explanation: Turbine blades experience drag force. They are bent and tapered along their length to reduce the effect of drag force.

2. What is angle of attack?
a) Angle between downwind side and upwind side
b) Angle between the blade and the rotor
c) Angle between the direction of wind and pitch of the blade
d) Angle between the blade and the wind tower

Explanation: Angle of attack is the angle between the direction of the oncoming wind and the pitch of the blade. It is important to note that position of the pitch of the blade is with respect to the oncoming wind.

3. When plotting lift vs angle of attack, which of following best describes the shape of the curve?
a) Exponential
b) Cubic
c) Linear
d) Parabolic

Explanation: The shape of lift vs angle of attack curve is parabolic in nature. The value of lift attains a maxima when the angle is between 15-18 degrees.
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4. There is no ideal angle of attack for best rotation.
a) True
b) False

Explanation: There is an ideal angle of attack that creates best rotation. It can be seen that the value of lift increases as the angle increases and then decreases as the angle crosses 20 degrees.

5. The speed at the tip of the blade is faster than its center.
a) True
b) False

Explanation: The speed of the tip of a rotating blade is faster than its center or root. This ensures a good tip-speed-ratio and provides high efficiency.
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6. What is the angle by which the rotor blades are twisted?
a) 5-10 degrees
b) 15-45 degrees
c) 100-120 degrees
d) 10-20 degrees

Explanation: The rotor blades are twisted by an angle between 10-20 degrees from root to tip. This ensures that the angle of attack decreases from the region where the air is moving slowly near the root to the region where the air is moving much faster at the tip.

7. Which of the following blade designs further increases the efficiency of airfoil blades?
a) Twisting and tapering the blades
b) Reducing wind tower height
c) Increasing wind tower height
d) Increasing rotor speed

Explanation: Twisting and tapering any airfoiled blade further increases its efficiency. The other options may increase efficiency but they are not related to blade designs.

8. Why is a blade twisted along its length rather than its width?
a) Drag reduction
b) Weight reduction
c) Drag reduction and improvement in angle of attack
d) Aesthetic reasons

Explanation: A rotor blade is twisted along its length to improve the angle of attack and reduce drag. It is not done to reduce weight or for aesthetic reasons. Every engineering design has a logical reason.

9. Which of the following blade designs further increases the efficiency of airfoil blades?
a) Reducing wind tower height
b) Twisting and tapering the blades
c) Increasing wind tower height
d) Increasing rotor speed

Explanation: Twisting and tapering any airfoiled blade further increases its efficiency. The other options may increase efficiency but they are not related to blade designs.

10. Why does wind speed increase with height above ground?
a) More slip boundary conditions
b) Less obstructions and no slip boundary conditions
c) More obstructions and slip boundary conditions
d) Gas molecules have less mass

Explanation: Increase in wind speed with increasing height is due to both, less number of obstructions and no slip boundary conditions. While the fluid analysis at ground level assume these conditions, the mathematics for analyzing at a considerable height above ground level is suitable changed.

11. Increased wind tower height and longer blades do not result in an increase in power.
a) True
b) False

Explanation: Increasing the wind tower height and blade length results in increased output power. However, the benefits of increasing the form factor comes at the expense of economics and safety.

a) Longer turbine blades experience greater forces
b) Longer turbine blades experience lesser forces
c) Stronger materials are cheaper
d) Stronger materials are available in abundance

Explanation: Longer turbine blades experience greater aerodynamic forces – lift and drag forces. Hence, they need to be made of stronger materials which are not particularly cheap.

13. Which of the following is best reason for not using longer blades even though they produce greater output power?
a) Longer turbine blades experience greater forces
b) Longer turbine blades experience lesser forces
c) Safety concerns due to greater momentum of longer blades
d) Stronger materials are available in abundance

Explanation: Though longer turbine blades produce greater output power; they are not used commercially because they have more mass. If any of the blades falls off the tower, its net momentum could cause extensively damage the environment.

14. Which theory is used to model the wind turbine?
b) Kinetic gas theory
c) Archimedes principle
d) Einstein’s theory of photoelectric effect

Explanation: Blade momentum theory is used to for aerodynamic modelling due to its simplicity. Kinetic gas theory is related to gas molecules and energy of a gas molecule. Archimedes principle is related to floatation and immersion of a body in a liquid. Einstein’s theory is related to the photoelectric effect which is a quantum mechanical phenomenon.

15. What is blade element momentum theory?
a) It is same as blade element theory
b) It is same as momentum theory
c) It talks about the motion of a body in a given frame of reference
d) It is used to calculate the local forces on the propeller or blade

Explanation: Concepts of both, element theory and blade momentum theory are combined and used in Blade element momentum theory. It is used to aerodynamically model turbine blades and rotors and calculate the respective local forces on them.

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