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\begin{document}
\fbox{\begin{minipage}{0.8\textwidth}
\begin{equation} \label{eq:contlowmach}
\frac{\partial\rho}{\partial t}+\frac{\partial\rho u_{i}}{\partial x_{i}}=0
\end{equation}
\begin{equation} \label{eq:momlowmach}
\frac{\partial\rho u_{i}}{\partial t}+\frac{\partial\rho u_{i}u_{j}}{\partial x_{j}}=-\frac{\partial p_{2}}{\partial x_{i}}+\frac{1}{Re}\frac{\partial\tau_{ij}}{\partial x_{j}}-\frac{\rho\delta_{i2}}{Fr^{2}}
\end{equation}
\begin{equation} \label{eq:enerlowmach}
\rho\frac{\partial T}{\partial t}+\rho u_{j}\frac{\partial T}{\partial x_{j}}=\frac{1}{RePr}\frac{\partial}{\partial x_{j}}(\kappa\frac{\partial T}{\partial x_{j}})+\frac{\gamma-1}{\gamma}\frac{dp_{0}}{dt}
\end{equation}
\begin{equation} \label{eq:statelowmach}
p_{0}=\rho T\
\end{equation}
\begin{equation} \label{eq:p0lowmach}
\frac{\partial p_{0}}{\partial x_{i}}=0
\end{equation}
\begin{equation} \label{eq:shearlowmach}
\tau_{ij}=\mu(\frac{\partial u_{i}}{\partial x_{j}}+\frac{\partial u_{j}}{\partial x_{i}}-\frac{2}{3}\delta_{ij}\frac{\partial u_{k}}{\partial x_{k}})
\end{equation}
\end{minipage}}
\end{document}