Past exam of the mathematics course of the University of Cambridge / 2016 / ib / Paper 1 / 17C / c / i

Write the conservative body force per unit mass as $-\nabla \Phi$. For a fluid of constant density $\rho$, the Euler equations for an inviscid fluid and the incompressible flow condition areUsing part (a), these becomeFor irrotational flow, locally choose a velocity potential with $\mathbf{u}=\nabla \varphi$. Then $\nabla ({\partial }_t\varphi +H)=0$, so the unsteady Bernoulli integral isThe constant is spatially uniform on a connected region where the velocity potential is defined; its time dependence can be absorbed into the additive time-dependent gauge of $\varphi$. A global single-valued velocity potential requires an appropriate topological condition, such as a simply connected domain; without it, the integral is locally valid. In a connected steady irrotational region, $\nabla H=0$ directly gives one spatial Bernoulli constant even without choosing a global potential.