Vertical velocity
= Vertical velocity
{title2=$w=\boldsymbol u\cdot\widehat{\boldsymbol z}$}
The vertical velocity is the component of a fluid <velocity field> along the selected upward vertical direction. In <incompressible flow>, impermeable horizontal boundaries impose $w=0$. Eliminating <pressure> often yields a closed <hydrodynamic stability> equation for this component.