\large
$$
Q = C_v A_2 \sqrt {{{2(p_1  - p_2 )} \over {\rho \left[ {1 - \left( {{{d_2 } \mathord{\left/
 {\vphantom {{d_2 } {d_1 }}} \right.
 \kern-\nulldelimiterspace} {d_1 }}} \right)^4 } \right]}}} 
$$