Check out Prandtl's Boundary Layer Theory https://en.wikipedia.org/wiki/Boundary_layer https://en.wikipedia.org/wiki/Boundary_layer It's a set of smart physica

Check out Prandtl's Boundary Layer Theory https://en.wikipedia.org/wiki/Boundary_layer https://en.wikipedia.org/wiki/Boundary_layer It's a set of smart physical observations/assumptions that allow you to find closed form solutions for the Navier Stokes equations in fluid mechanics https://en.wikipedia.org/wiki/Navier–Stokes_equations https://en.wikipedia.org/wiki/Navier–Stokes_equations for the important case of flow close to some body, such as an airfoil.