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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.