WTT-W3-Slides0.pdf - Fundamentals Assist Prof Mahmut Sami...

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Fundamentals to accompany Wind Turbine Technology , by A Hemami (2012) Chapter 3 Assist. Prof. Mahmut Sami BÜKER
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Fundamentals When wind blows , it exerts some forces on all the objects that are touched by the wind. The size and direction of these forces are evident in some cases, and you can easily observe the effect(s). For example, wind bends the trees and carries light objects However, you cannot see the effect of wind on a solid wall . In this chapter, you will learn about the nature of the two main forces from wind on a stationary object which are lift and drag Lift and drag are the two components of the total force from wind . They determine whether an airplane can fly or a wind turbine can turn. Knowledge about these force components is fundamental to understanding wind turbines.
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When an object is in the path of the wind it is subject to two forces from wind . An object can be of any shape and any size. The simplest case to start with is a plate with a rectangular shape . Figure shows a plate that lies in the path of wind at an angle with the wind direction. The direction of wind in figure is from left to right. When wind blows it pushes the plate. This is due to a force component parallel to the wind direction. On the other hand, as wind passes through , depending on the angle of the plate with the direction of wind, it causes the pressure at the two sides of the plate to be different. The result of this pressure difference is a force that pushes the plate to one side (in figure it pushes the plate upward). This push is from the side with higher pressure to the side with lower pressure . This lower or higher pressure is due to the speed of wind that is disturbed because of the plate. In addition to the above- mentioned two force components, one more effect of the wind is to cause a rotation of the plate . This rotation is due to a torque. Force from wind
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Force from wind Three cases are shown in which the angle of the plate with the wind direction are different. In (a) the general case is shown, where all three components two force components and a torque exist. In (b), the plate is parallel to the direction of wind and, as a result, there is no pressure difference on the two sides of the plate . Consequently, there is no side force and no torque . There is only a small force pushing the plate in the same direction as the wind blows. In the third case, (c), the plate is normal (perpendicular) to the wind direction. In this case, again, there is no side force or torque . There is only a pushing force in the direction of wind.
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Force and Torque When a force is applied on an object, it causes a motion in the same direction as the applied force. In the same way that a (linear) motion is generated by a force, a rotational motion is generated by a torque. For example, a motor generates a torque on its shaft, which causes the shaft to turn.
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