Docu for concentration depenendent density.

Documentation/ProjectFile/material/fluid/density/ConcentrationDependent/c_ConcentrationDependent.md

+\copydoc MaterialLib::Fluid::LinearConcentrationDependentDensity

Documentation/ProjectFile/material/fluid/density/ConcentrationDependent/t_fluid_density_difference_ratio.md

+\f$ \bar \alpha\f$ is the fluid density difference ratio

Documentation/ProjectFile/material/fluid/density/ConcentrationDependent/t_fluid_density_reference_concentration.md

+\f$ C_{\text{ref}}\f$ is the reference concentration

Documentation/ProjectFile/material/fluid/density/ConcentrationDependent/t_fluid_density_reference_density.md

+\f$ \varrho_{\text{ref}}\f$ is the reference density

MaterialLib/Fluid/Density/LinearConcentrationDependentDensity.h

@@ -20,6 +20,12 @@ namespace MaterialLib
 namespace Fluid
 {
 /// Linear concentration dependent density model.
+/// \f[ \varrho = \varrho_{\text{ref}}
+/// (1 + \bar \alpha (C - C_{\text{ref}})) \f]
+/// where
+/// - \f$ \varrho_{\text{ref}}\f$ is the reference density
+/// - \f$ \bar \alpha\f$ is the fluid density difference ratio
+/// - \f$ C_{\text{ref}}\f$ is the reference concentration
 class LinearConcentrationDependentDensity final : public FluidProperty
 {
 public: