\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}

\[
  \Pi_{ab} = \begin{cases} \frac{-\alpha \bar{c}_{ab} \mu_{ab} + \beta \mu_{ab}^2}{\bar{\rho}_{ab}} & \vec{v}_{ab} \cdot \vec{r}_{ab} < 0 \\
                                                        0  &  \vec{v}_{ab} \cdot \vec{r}_{ab} \geq 0 \\
                                          \end{cases}
\] 

\end{document}
