boundary layer explained

A flow that was uniform over the plate slows to 0 near the wall because of no-slip. Far away the free-stream speed stays U∞, but near the wall the arrows shorten and bend down to 0, and this thin band that changes from 0 to U∞ is the boundary layer, and its thickness is δ. Drag the free speed U∞ and the outer arrows grow whole, but at the wall it is always 0. The outside is free and only the wall is held.

The boundary layer starts at the leading edge and grows thicker downstream, because the wall keeps catching more layers and slowing them. Drag the measurement position x and the edge curve of the boundary layer swells like √x. The front is very thin and it thickens slowly as you go far. The front is thin as a blade and the back swells slowly.

A boundary layer also starts laminar and turns turbulent at some point. Compare them with a button and a laminar boundary layer grows thin and smooth and slow, while a turbulent one mixes lively, is thicker, and is fuller near the wall, and this switch happens once you pass the critical Re_x. Mixing makes the layer thick and full.

Zoom in right next to the wall. The speed is 0 at the wall, but the slope du/dy climbing up from that 0 sets the friction the wall feels, and that is the wall shear stress τ_w = μ (du/dy)|_wall. Drag the free speed U∞ and the slope near the wall gets steeper and the stress grows, and when this skin friction adds up it is one part of the drag on a body.

The thickness of a laminar boundary layer follows a clean formula. δx = 5√Re_x, that is δ ≈ 5x√Re_x, and the larger the local Reynolds number Re_x = U∞ xν the relatively thinner the boundary layer. Drag Re_x and you see δx shrink as 1√Re_x. So the faster and larger the flow, the thinner the layer stuck to the surface, and the design of an airplane wing, a golf ball, and a ship hull all happens on this. The drag and the separation that this boundary layer makes as it leaves follow.