Lagrangian pressure perturbation
= Lagrangian pressure perturbation
{c}
{title2=$\Delta p$}
The <Lagrangian pressure perturbation> is the change in <pressure> experienced by a displaced <fluid element>: $\Delta p=\delta p+\boldsymbol\xi\cdot\nabla p_0$. For a free surface against fixed external <pressure>, $\Delta p=0$, rather than $\delta p=0$ at the original surface.