What is a boundary layer flow, how is it different from main stream flow? How does it vary with Reynold's number?

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Boundary layer flow is a thin layer of fluid near a solid surface where velocity changes from zero to the free stream velocity.

Boundary layer flow occurs near a solid surface, such as the surface of an aircraft wing or a pipe.
In boundary layer flow, the velocity of the fluid changes from zero at the surface to the velocity of the free stream away from the surface.
The boundary layer can be laminar or turbulent, depending on the Reynolds number.
Laminar boundary layer flow is smooth and orderly, while turbulent boundary layer flow is chaotic and irregular.
The Reynolds number is a dimensionless parameter that determines the type of flow. It is defined as the ratio of inertial forces to viscous forces.
For low Reynolds numbers, the flow is predominantly laminar, while for high Reynolds numbers, the flow is predominantly turbulent.
The transition from laminar to turbulent flow in the boundary layer is influenced by factors such as surface roughness and pressure gradients.
Boundary layer flow has important implications in various engineering applications, such as aerodynamics, heat transfer, and fluid mechanics.

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