= Constant-stress Reynolds-averaged wall flow
{title2=$\tau_d=\mu U_z-\rho\overline{u'w'},\quad(\tau_d)_z=0$}
For a stationary, horizontally homogeneous mean flow without a horizontal pressure gradient, the averaged horizontal <Navier-Stokes equation> is $\nu U_{zz}-(\overline{u'w'})_z=0$. Integrating gives the displayed physical <shear stress>. The derivative of the Reynolds covariance belongs in the stress-divergence equation, not inside the stress itself. The kinematic stress is $\tau_d/\rho$, which must be used when an <eddy viscosity> with units of kinematic viscosity multiplies the mean gradient.
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