 Navier slip boundary condition

A Navier slip boundary condition relates fluid velocity relative to a stationary flat wall to the wall-normal tangential velocity gradient, $u_b={\ell }_s{u}_z$, with positive slip length ${\ell }_s$. For a Newtonian fluid, it is equivalent to linear resisting wall traction of magnitude $\mu u_b/{\ell }_s$. The normal points from the wall into the fluid; this fixes the sign convention.