Update for consistency

docs/source/FluidEquations.rst

@@ -25,7 +25,7 @@ Conservation of fluid mass:
 Conservation of fluid momentum:
 
 .. math:: \frac{ \partial (\varepsilon_g \rho_g U)}{\partial t} + \nabla \cdot (\varepsilon_g \rho_g U_g U_g) + \varepsilon_g \nabla p_g = \nabla \cdot \tau
-           + \sum_{part} \beta_p (V_p - U_g) + \rho_g g
+           + \sum_p \beta_p (V_p - U_g) + \rho_g g
 
 where :math:`\sum_p \beta_p (V_p - U_g)` is the drag term in which :math:`V_p` represents the particle velocity and :math:`\beta_p` is the drag coefficient associated with that particle