• When flow occurs past a solid surface or object, the fluid layer in contact with the surface must be at rest (described as the no slip condition).

• While the fluid layer just adjacent to this layer flows (shears) with a certain velocity, which increases with each subsequent layer.Thus the velocity of the fluid increases layer by layer as one moves perpendicular to the flow until the velocity of the free stream is attained.

• Thus this narrow region near the surface where velocity changes resulting in a gradient, and high shear stresses is known as a boundary layer.

• Similarly, the boundary layer thickness is the distance from the solid surface normal to the surface at which the velocity of the fluid is 99% of the free stream velocity