Put variable definitions in a table

docs/source/FluidEquations.rst

-Fluid Equations
+Fluid Variables
 ===============
 
-We define the following fluid variables:
-
-where :math:`\rho_g =` fluid density (assumed to be constant in the absence of reactions)
-
-where :math:`\varepsilon_g =` volume fraction of fluid (accounts only for displacement of fluid by particle, does not account for the EB walls)
-
-where :math:`U_g =` fluid velocity
-
-where :math:`p_g =` fluid pressure
-
-where :math:`tau =` viscous stress tensor
+   +-----------------------+--------------------------------------------------+
+   | Variable              | Definition                                       |
+   +=======================+==================================================+
+   | :math:`\rho_g`        | Fluid density                                    |
+   +------------+-------------------------------------+-----------------------+
+   | :math:`\varepsilon_g` | Volume fraction of fluid (= 1 if no particles)   |
+   +------------+-------------------------------------+-----------------------+
+   | :math:`U_g`           | Fluid velocity                                   |
+   +------------+-------------------------------------+-----------------------+
+   | :math:`\tau`          | Viscous stress tensor                            |
+   +------------+-------------------------------------+-----------------------+
+   | :math:`g`             | Gravitational acceleration                       |
+   +------------+-------------------------------------+-----------------------+
 
-where :math:`g =` gravitational acceleration 
-
-where :math:`\beta_p =` drag coefficient associated with a particle
-
-where :math:`V_p =` velocity associated with a particle
-
-Below are the governing equations for the fluid:
+Fluid Equations
+===============
 
 Conservation of fluid mass:
 
@@ -30,7 +27,8 @@ 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
 
-where :math:`\sum_p \beta_p (V_p - U_g)` is the drag term in which :math:`V_p` represents the particle velocity.
+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: