Before revision: If this is so, the above equation can be rewritten as <math="block"> \frac{\partial\psi{\partial t} +} \mathbf u \cdot\nabla\psi =0 </math> In particular, if the flow is steady, then which shows that is constant along a streamline.
After revision: If this is so, the above equation can be rewritten as {\partial t} +} In particular, if the flow is steady, then which shows that is constant along a streamline.