Created "viscosity" with addParam() instead, since the nonzero requir…

…ement will be manually enforced in Step 8 for demonstration purposes. Also declared _permeability and _viscosity as reference variables

(refs idaholab#15232)

tutorials/tutorial01_app_development/step06_input_params/include/kernels/DarcyPressure.h

@@ -17,7 +17,7 @@ class DarcyPressure : public ADKernelGrad
   /// ADKernel objects must override precomputeQpResidual
   virtual ADRealVectorValue precomputeQpResidual() override;
 
-  /// The variables which hold the value for K and mu
-  const Real _permeability;
-  const Real _viscosity;
+  /// The reference variables which hold the value for K and mu
+  const Real & _permeability;
+  const Real & _viscosity;
 };

tutorials/tutorial01_app_development/step06_input_params/src/kernels/DarcyPressure.C

@@ -12,11 +12,10 @@ DarcyPressure::validParams()
   // Add a required parameter. If this isn't provided in the input file MOOSE will error.
   params.addRequiredParam<Real>("permeability", "The isotropic permeability ($K$) of the medium.");
 
-  // Add an optional parameter that can be any real number, except zero, and set its default value.
-  params.addRangeCheckedParam<Real>(
+  // Add an optional parameter and set its default value.
+  params.addParam<Real>(
       "viscosity",
       7.98e-04,
-      "viscosity != 0",
       "The dynamic viscosity ($\\mu$) of the fluid, the default value is that of water at 30 "
       "degrees Celcius (7.98e-04 Pa-s).");