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What is meant by boundary layer? Why does it increase with distance from the upstream edge?

(5 Marks) Dec-2018

Subject Fluid Mechanics 2

Topic Boundary layer theory

Difficulty Medium

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Boundary layer

• In fluid mechanics, thin layer of a flowing gas or liquid in contact with a surface such as that of an airplane wing or of the inside of a pipe. The fluid in the boundary layer is subjected to shearing forces.
• A range of velocities exists across the boundary layer from maximum to zero, provided the fluid is in contact with the surface.
• Boundary layers are thinner at the leading edge of an aircraft wing and thicker toward the trailing edge. The flow in such boundary layers is generally laminar at the leading or upstream portion and turbulent in the trailing or downstream portion.

Boundary layer increase with distance from the upstream edge:

• Let us consider the flow with zero pressure gradient on one side of plate, which is stationary. The velocity of fluid on the surface of the plate should be equal to the velocity of the plate.

• But at a distance away from the plate, the fluid is having certain velocity. Thus the velocity gradient is set up in the fluid near surface of the plate, which develops the shearing resistance, which retards the fluid.

• Thus the fluid with a uniform free stream velocity (U) is retarded in the vinicity of the solid surface of the plate and boundary layer region begins at the sharp leading edge.

• At subsequent points down stream the leading edge, the boundary layer region increases because the retarded fluid is further retarded. This is also referred as the growth of boundary layer.

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