Handy-test-cases.md

 Many test cases are given to help users to understand how to use the different features offered by the MigFlow Software. Test cases are in the ~/MigFlow/testcases directory. They are given to provide users basic tools in order to build computation scripts for their own applications. Movies and figures of some applications are available on the [MigFlow](https://www.migflow.be) Project website
 
-### Test Cases Structure
+### Test Cases Basic Structure
 
 Let us illustrate the structure of the executable python files with a two dimensional deposit of grains in a fluid. This test case is provided in the [testcases/depot-2d/depot.py](https://git.immc.ucl.ac.be/fluidparticles/migflow/blob/master/testcases/depot-2d/depot.py) file.
 
@@ -157,6 +157,17 @@ def iterate(fluid, particles, dt, min_nsub=1, contact_tol=1e-8, external_particl
 ```
    The loop also contains instruction to write output files. It is important to note that after moving the grains in the computational loop you have to use the _set_particles()_ function to search the new location of the grains in the mesh.
 
+#### Optional Options
+
+Optional options can be specified during the fluid initialization
+
+  * _sigma_: surface tension (only when there are two fluids see specific case in the [Main Features Section](https://git.immc.ucl.ac.be/fluidparticles/migflow/-/wikis/main-features#two-fluids-grains))
+  * _volume_drag_: coefficient of a linear volume drag force on the entire fluid domain. It can be used to simulate the front and rear shear stress in two-dimensional simulations
+  * _quadratic_drag_: coefficient of a quadratic volume drag force on the entire fluid domain. It can be used to simulate the front and rear shear stress in two-dimensional simulations
+  * _drag_in_stab_: states if the drag force is in the stabilization term (SUPG and PSPG). If set to _False_, it stabilizes the computation by adding extra diffusivity.
+  * _drag_coefficient_factor_: factor multiplying the drag coefficient
+  * _temporal_: enables time derivative (_False_ = steady equations)
+  * _advection_: enables advective terms (_False_ = Stokes equations, _True_ = Navier-Stokes equations)
 
 ### Handy Test Cases List