Merge branch 'GRCBC' of https://github.com/anandrdbz/MFC into GRCBC

Author: Anand Radhakrishnan

docs/documentation/case.md

@@ -805,6 +805,23 @@ The boundary condition supported by the MFC are listed in table [Boundary Condit
 Their number (`#`) corresponds to the input value in `input.py` labeled `bc_[x,y,z]%[beg,end]` (see table [Simulation Algorithm Parameters](#5-simulation-algorithm)).
 The entries labeled "Characteristic." are characteristic boundary conditions based on [Thompson (1987)](references.md#Thompson87) and [Thompson (1990)](references.md#Thompson90).
 
+### Generalized Characteristic Boundary conditions
+
+| Parameter                     | Type    | Description |
+| ---:                          | :----:  | :--- |
+| `bc_[x,y,z]%grcbc_in`         | Logical | Enable grcbc for subsonic inflow |
+| `bc_[x,y,z]%grcbc_out`        | Logical | Enable grcbc for subsonic outflow (pressure)|
+| `bc_[x,y,z]%grcbc_vel_out`    | Logical | Enable grcbc for subsonic outflow (pressure + normal velocity) |
+| `bc_[x,y,z]%[u, v, w]_in`     | Real    | Inflow velocities in x, y and z directions |
+| `bc_[x,y,z]%[u, v, w]_out`    | Real    | Outflow velocities in x, y and z directions |
+| `bc_[x,y,z]%pres_in`          | Real    | Inflow pressure |
+| `bc_[x,y,z]%pres_out`         | Real    | Outflow pressure |
+| `bc_[x,y,z]%alpha_rho_in`     | Real Array | Inflow density |
+| `bc_[x,y,z]%alpha_in`         | Real Array | Inflow void fraction |
+
+*: This boundary condition can be used for subsonic inflow (`bc_[x,y,z]%[beg,end]` = -7) and subsonic outflow (`bc_[x,y,z]%[beg,end]` = -8) characteristic boundary conditions. These are based on [Pirozzoli (2013)](references.md#Pirozzoli13). This enables to provide inflow and outflow conditions outside the computational domain. The parameters associated with this feature are listed in table [Generalized Characteristic Boundary conditions](#generalized-characteristic-boundary-conditions).
+
+
 ### Patch types
 
 | #    | Name               | Dim.  | Smooth | Description |

docs/documentation/references.md

@@ -32,6 +32,8 @@
 
 - <a id="Meng16">Meng, J. C. C. (2016). Numerical simulations of droplet aerobreakup. PhD thesis, California Institute of Technology.</a>
 
+- <a id="Pirozzoli13">Pirozzoli, S., and Colonius, T. (2013). Generalized characteristic relaxation boundary conditions for unsteady compressible flow simulations. Journal of Computational Physics, 248:109-126.</a>
+
 - <a id="Preston07">Preston, A., Colonius, T., and Brennen, C. (2007). A reduced-order model of diffusive effects on the dynamics of bubbles. Physics of Fluids, 19(12):123302.</a>
 
 - <a id="Saurel09">Saurel, R., Petitpas, F., and Berry, R. A. (2009). Simple and efficient relaxation methods for interfaces separating compressible fluids, cavitating flows and shocks in multiphase mixtures. journal of Computational Physics, 228(5):1678–1712</a>