Update INSFVTurbulentViscosityWallFunction.md idaholab#27800 idaholab…

…#27887 idaholab#27888

modules/navier_stokes/doc/content/source/fvbcs/INSFVTurbulentViscosityWallFunction.md

@@ -20,9 +20,18 @@ without the need of fully resolving the gradients at the near wall region.
     \tau_w = /frac{ \mu_w u_p}{y_p}
 \end{equation}
 
-To obtain the relationship between the wall shear stress and the dimensionless wall distance,
-four different formulations are supportedas defined by the 
-[!param](/FVBCs/INSFVTurbulentViscosityWallFunction/wall_treatment) parameter.
+where:
+
+- $\mu_w = \mu + \mu_t$  is the total viscosity evaluated at the wall face
+- $\mu_t$ is the turbulent viscosity, evaluated at the wall for the purpose of this boundary condition
+- $\mu$ is the kinematic viscosity, evaluated at the wall for the purpose of this boundary condition
+- $\tau_w$ is the wall-shear stress
+- $u_p$ is the wall-parallel velocity at the centroid
+- $y_p$ is the wall normal distance to the centroid
+
+To impose a correct boundary condition for $\mu_t$, as seen in the Equation above, we need to compute $\tau_w$ using analytical 
+relationships between the wall shear stress and the dimensionless wall distance $y^+$. For this purpose, four different
+formulations are supported as defined by the [!param](/FVBCs/INSFVTurbulentViscosityWallFunction/wall_treatment) parameter.
 
 To set the grid spacing for the first cell near the wall in your mesh, we recommend using the Auxiliary Kernel
 [RANSYPlusAux.md] 
@@ -46,6 +55,7 @@ for the turbulent viscosity.
 where:
 
 - $\rho$ is the density
+- $\mu$ is the kinematic viscosity
 - $u_{\tau} = \sqrt{\frac{\tau_w}{\rho}}$ is the friction velocity and $\tau_w$ is the wall friction
 - $y_p$ is the distance from the boundary to the center of the near-wall cell
 - $u_p$ is the parallel velocity to the boundary computed at the center of the near-wall cell