Add inputs for virtual mass force (!100)

docs/source_docs/refs.bib

@@ -404,6 +404,16 @@ doi = {https://doi.org/10.1016/j.jcp.2022.111305},
   doi = {10.1063/1.864230},
 }
 
+@article{nijssen20,
+  author = {Tim M.J. Nijssen and Hans A.M. Kuipers and Jan {van der Stel} and Allert T. Adema and Kay A. Buist},
+  title = {Complete liquid-solid momentum coupling for unresolved CFD-DEM simulations},
+  journal = {International Journal of Multiphase Flow},
+  volume = {132},
+  pages = {103425},
+  year = {2020}
+}
+
+
 @article{Nonaka2018,
    author = {Andrew Nonaka and Marcus S. Day and John B. Bell},
    title = {A conservative, thermodynamically consistent numerical approach for low Mach number combustion. Part I: Single-level integration},
@@ -601,3 +611,13 @@ doi = {https://doi.org/10.1016/j.jcp.2022.111305},
   year={2007},
   publisher={Elsevier}
 }
+
+@article{zuber64,
+  author = {N. Zuber},
+  title = {On the dispersed two-phase flow in the laminar flow regime},
+  journal = {Chemical Engineering Science},
+  volume = {19},
+  number = {11},
+  pages = {897-917},
+  year = {1964}
+}

docs/source_docs/user_guide/inputs/model_options.rst

@@ -291,7 +291,14 @@ The following inputs are defined using the ``mfix`` prefix.
 Fluid-particle drag
 -------------------
 
-The following input is defined using the ``mfix`` prefix.
+The following input is defined using the ``mfix.drag`` prefix.
+
+
+.. |VirtualMass_Eq| replace:: :math:`\mathbf{F}_p^{vm} = -C_p^{vm}\rho_f \left( \frac{D\mathbf{u}_f}{Dt} - \frac{d\mathbf{u}_p}{dt}\right)\mathcal{V}_p`
+
+.. |VM_Zuber_Eq| replace:: :math:`C_p^{vm} = \frac{1}{2}\frac{1+2\varepsilon_p}{1-\varepsilon_p}`
+
+.. |VM_Nijssen_Eq| replace:: :math:`\begin{split}C_p^{vm} =& C_0^{vm} \\ &+ \sum_{k=1}^3\left\{ \left[ a_{k,0} + a_{k,1} \ln \left( \frac{\rho_p}{\rho_f} \right) \right] \varepsilon_p^k \right\}\end{split}`
 
 +--------------------------+-----------------------------------------------------------------------+-------------+-----------+
 |                          | Description                                                           |   Type      | Default   |
@@ -312,6 +319,24 @@ The following input is defined using the ``mfix`` prefix.
 |                          | fluid-particle drag force. The force is applied to the fluid by       |             |           |
 |                          | multiplying the shear stress by fluid volume fraction.                |             |           |
 +--------------------------+-----------------------------------------------------------------------+-------------+-----------+
+| virtual_mass             | Include virtual mass force in fluid-particle momentum transfer.       | String      | None      |
+|                          | The virtual mass force is not included by default.                    |             |           |
+|                          |                                                                       |             |           |
+|                          |    |VirtualMass_Eq|                                                   |             |           |
+|                          |                                                                       |             |           |
+|                          | * ``Null`` zero virtual mass coefficient, :math:`C_p^{vm}=0`          |             |           |
+|                          | * ``Constant`` - user-specified constant coefficient                  |             |           |
+|                          | * ``Zuber`` :cite:p:`zuber64`                                         |             |           |
+|                          |                                                                       |             |           |
+|                          |     |VM_Zuber_Eq|                                                     |             |           |
+|                          |                                                                       |             |           |
+|                          | * ``Nijssen`` :cite:p:`nijssen20`                                     |             |           |
+|                          |                                                                       |             |           |
+|                          |     |VM_Nijssen_Eq|                                                   |             |           |
+|                          |                                                                       |             |           |
++--------------------------+-----------------------------------------------------------------------+-------------+-----------+
+| virtual_mass.constant    | Constant virtual-mass coefficient.                                    | Real        | 0.5       |
++--------------------------+-----------------------------------------------------------------------+-------------+-----------+
 
 
 .. note::