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17. Drag CH EN 374: Fluid Mechanics
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Drag is the force a flowing fluid exerts on a body.
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π π« = π π π¨ Drag comes entirely from viscous shear stress
This is friction drag
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π π« =π«ππ¨ No shear stress hereβwhere does drag come from?
This is form drag (or pressure drag)
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Flow Separation Wakes form when flow separates behind an object
They are low pressure and usually turbulent
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What kind of drag explains why a marble drops more slowly through syrup than vegetable oil?
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Ξπ is usually much larger than π π€
So which kind of drag is usually more significant? Do you think friction drag is more likely to be significant in laminar flow or turbulent?
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Lift π π³ π π« π π· Lift is the component of pressure and shear forces around a body that are normal to flow.
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Angle of Attack Airplane wings (for example) are designed with a shape and angle of attack to maximize lift and minimize drag.
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Why do airplane wings have those retractable flaps?
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Streamlining
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Is there any particular reason to streamline an iron?
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Which of these cars experiences more friction drag?
So why does streamlining decrease drag?
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Flow Separation and Streamlining
Streamlining mitigates flow separation. A non-streamline shape is also called a βbluffβ body
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πΆ π· = πΉ π· π΄ 1 2 π π£ 2 Drag Coefficient
πΆ π· = πΉ π· π΄ π π£ 2 Where A is the projected area of the object.
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πΆ π· = 2 πΉ π· π π£ 2 π΄ Drag Coefficient
πΆ π· = 2 πΉ π· π π£ 2 π΄ Where A is the projected area of the object.
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πΉ π· = 1 2 πΆ π· π π£ 2 π΄ Drag Coefficient
πΉ π· = 1 2 πΆ π· π π£ 2 π΄ Where A is the projected area of the object.
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Frontal Area Example π π΄= π π΄=
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Terminal Velocity π£ π‘πππ = 2π πΆ π· ππ΄ π π« πΎ
A falling object will stop accelerating when: π π« πΎ πΉ π· =π πΉ π· = 1 2 πΆ π· π π£ 2 π΄βπ= 1 2 πΆ π· π π£ π‘πππ 2 π΄ π£ π‘πππ = 2π πΆ π· ππ΄
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How does a parachute work?
How does a parachute decrease terminal velocity? π£ π‘πππ = 2π πΆ π· ππ΄
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Car Evolution
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Finding Drag Coefficient
πΆ π· for most shapes can be assumed constant after π
πβ 10 4 Discuss why you think the πΆ π· curve changes with Re.
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