Incompressibility of fluid $\Rightarrow \exists\psi: \mathbf{u}=(u,v)=(\psi_y,-\psi_x)$
Where $\mathbf{u}$ is the velocity of the fluid
Could someone explain why $\mathbf{u} \cdot \nabla \psi=0 \Rightarrow \psi$ is constant on streamlines
Many thanks in advance