= Rectilinear flow
A <rectilinear flow> has velocity everywhere parallel to a fixed direction and independent of the coordinate in that direction, for example $U(y,z)\widehat x$. Its convective acceleration vanishes. A steady incompressible viscous rectilinear flow obeys the linear transverse Poisson equation $\nu\Delta U=\rho^{-1}dp/dx$, rather than an inertia-viscosity balance. This explains Reynolds-number independence of the spatial equation with fixed scaled forcing and boundary data.
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