Wind Energy Multiple Choice Questions on “Wind Turbine Aerodynamics – 1”.
1. What are the two primary aerodynamic forces acting on wind turbine rotors?
a) Lift, drag
b) Drag, gravitational force
c) Gravitational force, lift
d) Gravitational force, electrical force
Answer: a
Clarification: Lift and drag are the two primary aerodynamic forces acting on modern wind turbine rotors. Gravitational force is due to earth’s gravity and is not an aerodynamic force. Similarly, electrical force is due to an electrical potential difference or an electric field and is not an aerodynamic force.
2. Lift is an aerodynamic force acting _______
a) opposite to the direction of wind flow
b) parallel to the direction of wind flow
c) diagonal to the direction of wind flow
d) perpendicular to the direction of wind flow
Answer: d
Clarification: Lift is an aerodynamic force that acts perpendicular to the direction of wind flow. A simplified explanation of lift is that it should travel faster to reach the end of the blade to meet the wind travelling over upwind face of the blade.
3. Drag is an aerodynamic force acting ______
a) perpendicular to the direction of wind flow
b) parallel to the direction of wind flow
c) diagonal to the direction of wind flow
d) opposite to the direction of wind flow
Answer: b
Clarification: Drag is an aerodynamic force acting in the direction parallel to the wind flow. Though it is conceptually simple, it has poor power extraction vs area efficiency.
4. What is an aerodynamic force?
a) Force exerted on the body due to mass of another body nearby
b) Force exerted on the body by liquid
c) Force exerted on the body by air (or any gas) in which the body is immersed
d) Force exerted on the body due to a charged particle
Answer: c
Clarification: Aerodynamic force is the force exerted on a body by air (or any gas) in which the body is immersed. It is due to the relative motion between the body and air. Buoyancy is the force exerted by the liquid in which the body is immersed. Gravitational force on a body is due to mass of another body.
5. What happens on the upwind side of the wind mill blade?
a) No pressure is exerted
b) Low pressure area
c) High pressure area
d) Wind is trapped
Answer: c
Clarification: On the upwind side of the wind mill blade, a region of high pressure is formed. This is due to slow movement of air on the upwind side as compared to the downwind side.
6. The upwind side experiences a force that slows the blade.
a) True
b) False
Answer: a
Clarification: The upwind side experiences a force due to the slow movement of air relative to the downside. This results in a region of high pressure on the upwind side which exerts a force that slows the blade’s movement.
7. What is lift-to-drag ratio?
a) Amount of lift generated by a blade divided by its aerodynamic drag
b) Amount of drag generated by a blade divided by its lift
c) Amount of drag generated by a blade divided by its aerodynamic lift
d) Amount of lift generated by a blade divided by its aerodynamic drag
Answer: a
Clarification: Lift-to-drag ratio is the amount of lift generated by a blade (or a wing/ flying vehicle) divided by its aerodynamic drag. It’s essential to determine the blade’s efficiency and is considered during the design of the wing.
8. A higher lift-to-drag ratio indicates a higher efficiency.
a) False
b) True
Answer: b
Clarification: As the name suggests, lift-to-drag ratio is the ratio of the aerodynamic lift generated by the blade to the aerodynamic drag generated by the blade. It is used to determine the efficiency of the blade. Higher the lift-to-drag ratio, higher the efficiency.
9. Turbine blades use a/an _______ design.
a) bulky
b) airfoil
c) teardrop
d) train’s
Answer: b
Clarification: Airfoil design is the cross-sectional shape of an airplane wing. Turbine blades us an airfoil design. Unlike trains and trucks, they are not bulky. Though teardrop is the most aerodynamic shape, turbine blades don’t use this design.
10. Which of the following is the governing equation for power extraction and calculation? Note that the text in bold denotes a vector quantity.
a) Power = force . velocity
b) Power = force . area
c) Power = velocity . force
d) Power = energy / time
Answer: a
Clarification: In aerodynamics, power = force . velocity is used to extract and calculate power. Though power = energy / time is correct but both energy and time are written in bold which is not true. Energy and time are scalar quantities. The other options are mathematically incorrect.
11. Why is it necessary to have an optimal tip speed ratio (TSR)?
a) To ensure maximum efficiency
b) To ensure good aerodynamics
c) To increase drag
d) To ensure minimum efficiency but good aerodynamics
Answer: a
Clarification: Optimal tip speed ratio is required to ensure maximum efficiency in converting wind energy to mechanical energy. If the propeller blades rotate too slowly, it allows too much wind to pass through undisturbed and does not extract sufficient energy. However, if the blades rotate too quickly, it creates a large amount of drag.
12. What is tip speed ratio (TSR)?
a) Ratio of wind speed to the speed of rotor
b) Ratio of the speed of rotor tip to wind speed
c) Ratio of wind speed to the speed of rotor tip
d) Ratio of wind speed to the speed of the blade’s center
Answer: b
Clarification: Tip speed ratio is the ratio of the speed of the rotor tip to the wind speed. It is used to determine the optimal speed at which the rotor (and blades) are required to rotate to ensure maximum efficiency.
13. What does tip speed ratio (TSR) depend on?
a) Motor material
b) Wind tower material
c) Rotor blade shape profile
d) Different types of gases
Answer: c
Clarification: Tip speed ratio depends on rotor blade shape and its wind profile. It also depends on the number of turbine blades and the design of the wind turbine propeller.
14. Which of the following blade designs is becoming less popular?
a) HAWT
b) Airfoil design
c) Curve design
d) Flat blade design
Answer: d
Clarification: Flat blade design is one of the oldest designs and is becoming less popular due its low efficiency. Airfoil and curve designs deliver much higher efficiencies. Horizontal axis wind turbine (HAWT) is a propeller type and not a blade design.
15. The airfoil/curve blade designs are affected by drag along its length.
a) False
b) True
Answer: b
Clarification: Drag is one of the two important aerodynamic forces. It reduces the blade’s rotating speed. Thus, blades need to be designed in such a way that they experience minimum drag force.