Fluid flow
Navier-Stokes equations
$$
\begin{align}
u\frac{\partial u}{\partial x} + v\frac{\partial u}{\partial y} &=
-\frac{\partial p}{\partial x} + \frac{1}{Re}
\left(\frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2}\right),\\
u\frac{\partial v}{\partial x} + v\frac{\partial v}{\partial y} &=
-\frac{\partial p}{\partial y} + \frac{1}{Re}
\left(\frac{\partial^2 v}{\partial x^2} + \frac{\partial^2 v}{\partial y^2}\right),\\
\frac{\partial u}{\partial x} + \frac{\partial v}{\partial y} &= 0.
\end{align}
$$