docstring and comments

src/equations/compressible_navier_stokes.jl

@@ -63,6 +63,16 @@ Under `GradientVariablesEntropy`, the Navier-Stokes discretization is provably e
 struct GradientVariablesPrimitive end
 struct GradientVariablesEntropy end
 
+"""
+    dynamic_viscosity(u, equations)
+
+    Wrapper for the dynamic viscosity that calls
+    `dynamic_viscosity(u, equations.mu, equations)` which dispatches on the type of 
+    `equations.mu`. 
+    For constant `equations.mu`, i.e., `equations.mu` is of `Real`-type it is returned directly.
+    In the opposite case, `equations.mu` is assumed to be a function with arguments
+    `u` and `equations` and is called with these arguments.
+"""
 dynamic_viscosity(u, equations) = dynamic_viscosity(u, equations.mu, equations)
 dynamic_viscosity(u, mu::Real, equations) = mu
 dynamic_viscosity(u, mu::T, equations) where {T} = mu(u, equations)

src/equations/compressible_navier_stokes_1d.jl

@@ -166,6 +166,8 @@ function flux(u, gradients, orientation::Integer,
     # In the simplest cases, the user passed in `mu` or `mu()` 
     # (which returns just a constant) but
     # more complex functions like Sutherland's law are possible.
+    # `dynamic_viscosity` is a helper function that handles both cases
+    # by dispatching on the type of `equations.mu`.
     mu = dynamic_viscosity(u, equations)
 
     # viscous flux components in the x-direction

src/equations/compressible_navier_stokes_2d.jl

@@ -175,6 +175,8 @@ function flux(u, gradients, orientation::Integer,
     # In the simplest cases, the user passed in `mu` or `mu()` 
     # (which returns just a constant) but
     # more complex functions like Sutherland's law are possible.
+    # `dynamic_viscosity` is a helper function that handles both cases
+    # by dispatching on the type of `equations.mu`.
     mu = dynamic_viscosity(u, equations)
 
     if orientation == 1

src/equations/compressible_navier_stokes_3d.jl

@@ -188,6 +188,8 @@ function flux(u, gradients, orientation::Integer,
     # In the simplest cases, the user passed in `mu` or `mu()` 
     # (which returns just a constant) but
     # more complex functions like Sutherland's law are possible.
+    # `dynamic_viscosity` is a helper function that handles both cases
+    # by dispatching on the type of `equations.mu`.
     mu = dynamic_viscosity(u, equations)
 
     if orientation == 1