Boundary Layers & Separation Right at a wall the fluid must be still, so a thin sheared layer forms and thickens downstream. Climbing the front of the hump the flow speeds up and the layer stays glued; going down the back it must push against rising pressure, stalls, and peels away — a separation bubble with backward flow beneath. Add wall suction and the layer re-attaches. The journey: Prandtl split the flow into an inviscid outside and a slim viscous skin (1904). Here Thwaites’ integral gives the layer thickness and a shape number; when it crosses the Pohlhausen limit the wall shear hits zero and the marker dye curls back.