 Solution

Write $\mu$ for dynamic viscosity and $\nu =\mu /\rho$ for kinematic viscosity, consistently with the introductory Navier-Stokes equation. The phrase “dynamic viscosity $\nu$” in this request mixes these conventions. A fully developed plane Poiseuille flow, $\mathbf{u}=(u(z),0,0)$, has zero convective acceleration. The Navier-Stokes equation gives $\mu u^{\prime \prime }=-f$ and $p_z=-\rho g$. Applying the no-slip boundary condition at both stationary plates givesThe material coefficients must be effectively constant and positive. The profile is an exact parallel-flow solution for every $\mu >0$; experimental validity additionally requires that the realized flow remain in the laminar flow regime. A sufficient conservative condition isEquivalently $\nu \gg \sqrt{f{b}^3/(12\rho )}$, or $\mu \gg \sqrt{\rho f{b}^3/12}$. This sufficient bound is stronger than necessary; there is no need to discard convective acceleration by an approximation here because it vanishes identically for the assumed profile.