Boundary layer
Fluid mechanics

When a fluid moves around a fixed obstacle, the velocity and pressure fields are disturbed by it. If the velocity in the distance is small, the effect of viscosity, which is the fluid's adhesion to the surface, results in weak parietal gradients in velocity, and these gradients are felt slightly but very far from the obstacle. If, on the other hand, the velocity is high, braking at the wall is intense and parietal gradients are high, but viscosity contributes to their rapid decrease with distance from the surface. In the latter case, there is a contiguous zone of fluid called the dynamic boundary layer, in which accommodation occurs between velocity nullity (at the wall) and the full local velocity of the free fluid, where velocity gradients are so moderate that the fluid can be considered, to a first approximation, as viscosity-free