Viscous sublayer ($\delta \sim \nu /u_{\ast }$)

The near-wall region where molecular momentum diffusion dominates turbulent transport. Using the shear velocity $u_{\ast }=\sqrt{{\tau }_d/\rho }$ gives the viscous wall length $\nu /u_{\ast }$. The precise extent depends on the chosen crossover criterion and wall condition. In this region constant stress and no slip give $U\simeq u_{\ast }^{2}z/\nu$; the logarithmic law of the wall applies outside it.