\begin{equation*}
\fontsize{12.0}{12.0}\selectfont
    \left(   \DP{}{t} + U \DP{}{x}  \right)
      \left(   \DP[2]{}{x} + \DP[2]{}{y} \right) \psi
    + \left(\beta - \DD[2]{U}{y}\right) \DP{\psi}{x}
  = 0.
\end{equation*}
