空間速度

$ \bm{u}:(\bm{x},t)\mapsto\left.\frac{\partial \bm{\phi}}{\partial t}\right|_{\bm{X}=\bm{\phi}^{-1}}

性質

$ \bm v=\frac{\mathrm D\bm x}{\mathrm Dt}

$ \bm v=\dot{\bm\phi}(\bm\phi^{-1}(\bm x,t),t)