I read papers and see this equation. Honestly, I am not familiar with fluid dynamics. So I was wondering if this equation is common in fluid dynamics, or it is the special case in this paper. Could someone help me figure it out how it comes out? Thank you! First Question! \begin{equation} \nabla^{2}\phi-c^{-2}\phi_{tt}=\frac{1}{2c^2}\left[\frac{d}{dt}(\nabla\phi)^{2}+\frac{\partial}{\partial t}(\nabla\phi)^{2}\right] \end{equation}
where $\phi$ is the potential function of water