Civil Engineering Study with Vk
Friction and Pressure Drag on a body moving through fluid
- A body moving through fluid experiences a drag force, which is usually divided into two components:
- frictional drag, and
- pressure drag.
- Frictional drag comes from friction between the fluid and the surfaces over which it is flowing.
- This friction is associated with the development of boundary layers, and it scales with Reynolds number.
- Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body.
- This drag is associated with the formation of a wake, which can be readily seen behind a passing boat, and it is usually less sensitive to Reynolds number than the frictional drag.
- It depends on the shape of the body.
- Frictional drag is important for attached flows (that is, there is no separation), and it is related to the surface area exposed to the flow.
- Pressure drag is important for separated flows, and it is related to the cross-sectional area of the body.
- For streamlined bodies (like a fish, or an airfoil at small angles of attack), frictional drag is the dominant source of air resistance.
- For a bluff body (like a brick, a cylinder, or an airfoil at large angles of attack), the dominant source of drag is pressure drag.
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