Boundary layer separation causes skin friction drag
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Answer:-
In most situations it is inevitable that the boundary layer becomes detached from a solid body. This boundary layer separation results in a large increase in the drag on the body. We can understand this by returning to the flow of a nonviscous fluid around a cylinder. The pressure distribution is the same on the downstream side of the cylinder as on the upstream side; thus, there were no unbalanced forces on the cylinder and therefore no drag (d'Alembert's paradox again). If the flow of a viscous fluid about a body is such that the boundary layer remains attached, then we have almost the same result--we'll just have a small drag due to the skin friction. However, if the boundary layer separates from the cylinder, then the pressure on the downstream side of the cylinder is essentially constant, and equal to the low pressure on the top and bottom points of the cylinder.
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When a boundary layer separates, a drag force is induced as a result of differences in pressure upstream and downstream of the wing. ... skin-friction drag is reduced by laminar flow due to a lower shear stress at the wall, but this increases pressure drag when boundary layer separation occurs.