wfullmer test commit

.. math:: \frac{\partial (\varepsilon_g \rho_g)}{\partial t} + \nabla \cdot (\varepsilon_g \rho_g U_g)  = 0

Assuming the fluid phase is incompressible, :math:`\frac{D \rho_g}{Dt} = 0`, the conservation of fluid mass is equivalent to the conservation of fluid volume:

.. math:: \frac{\partial \varepsilon_g}{\partial t} + \nabla \cdot (\varepsilon_g  U_g)  = 0

Conservation of fluid momentum:

.. math:: \frac{ \partial (\varepsilon_g \rho_g U_g)}{\partial t} + \nabla \cdot (\varepsilon_g \rho_g U_g U_g) + \varepsilon_g \nabla p_g = \nabla \cdot \tau

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

Conservation of fluid volume:

.. math:: \frac{\partial \varepsilon_g}{\partial t} + \nabla \cdot (\varepsilon_g  U_g)  = 0