use_custom_prob.md

@@ -10,7 +10,7 @@ The fluid domain is set using the class dart::Fluid,
 ```python
 pbl.set(dart.Fluid(msh, 'fld', minf, dim, alpha, beta=0))
 ```
-where `'fld'` is the name of the physical group of the mesh containing the element belonging to the fluid domain. If `minf = 0`, the flow will be considered as incompressible, otherwise it will be considered compressible. Note that, `beta` can be omitted if it is zero.
+where `'fld'` is the name of the physical group of the mesh containing the element belonging to the fluid domain. If `minf = 0`, the flow will be considered as incompressible, otherwise it will be considered compressible. Adding the fluid will automatically add C++ functions computing the density, the Mach number and the pressure coefficient. To avoid singular behavior, these functions are limited for high values of the velocity. Original non-limited function can also be used, but the code must be recompiled (see the class dart::Fluid). Note that, `beta` can be omitted if it is zero.
 
 **2.2 Adding the initial and boundary conditions**  
 The intial condition is added using the dart::Initial class as,