ENH: extrapolateInternal() method for patch fields

- provides a more succinct way of writing

      {fa,fv}PatchField<Type>::patchInternalField(*this)

  as well as a consistent naming that can be used for patches derived
  from valuePointPatchField

ENH: readGradientEntry helper method for fixedGradient conditions

- simplifies coding and logic.
- support different read construct modes for fixedGradient

applications/solvers/stressAnalysis/solidDisplacementFoam/tractionDisplacement/tractionDisplacementFvPatchVectorField.C

@@ -47,7 +47,7 @@ tractionDisplacementFvPatchVectorField
     traction_(p.size(), Zero),
     pressure_(p.size(), Zero)
 {
-    fvPatchField<vector>::patchInternalField(*this);
+    extrapolateInternal();
     gradient() = Zero;
 }
 
@@ -79,7 +79,7 @@ tractionDisplacementFvPatchVectorField
     traction_("traction", dict, p.size()),
     pressure_("pressure", dict, p.size())
 {
-    fvPatchField<vector>::patchInternalField(*this);
+    extrapolateInternal();
     gradient() = Zero;
 }
 

applications/solvers/stressAnalysis/solidEquilibriumDisplacementFoam/tractionDisplacementCorrection/tractionDisplacementCorrectionFvPatchVectorField.C

@@ -47,7 +47,7 @@ tractionDisplacementCorrectionFvPatchVectorField
     traction_(p.size(), Zero),
     pressure_(p.size(), Zero)
 {
-    fvPatchField<vector>::patchInternalField(*this);
+    extrapolateInternal();
     gradient() = Zero;
 }
 
@@ -79,7 +79,7 @@ tractionDisplacementCorrectionFvPatchVectorField
     traction_("traction", dict, p.size()),
     pressure_("pressure", dict, p.size())
 {
-    fvPatchField<vector>::patchInternalField(*this);
+    extrapolateInternal();
     gradient() = Zero;
 }
 

etc/codeTemplates/dynamicCode/fixedValuePointPatchFieldTemplate.C

@@ -125,10 +125,10 @@ ${typeName}FixedValuePointPatch${FieldType}
     const pointPatch& p,
     const DimensionedField<${TemplateType}, pointMesh>& iF,
     const dictionary& dict,
-    const bool valueRequired
+    IOobjectOption::readOption requireValue
 )
 :
-    parent_bctype(p, iF, dict, valueRequired)
+    parent_bctype(p, iF, dict, requireValue)
 {
     if (${verbose:-false})
     {

etc/codeTemplates/dynamicCode/fixedValuePointPatchFieldTemplate.H

@@ -89,8 +89,8 @@ public:
         (
             const pointPatch&,
             const DimensionedField<${TemplateType}, pointMesh>&,
-            const dictionary&,
+            const dictionary& dict,
-            const bool valueRequired=true
+            IOobjectOption::readOption requireValue = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping a copy onto a new patch

src/OpenFOAM/fields/pointPatchFields/basic/value/valuePointPatchField.C

@@ -62,6 +62,23 @@ bool Foam::valuePointPatchField<Type>::readValueEntry
 }
 
 
+template<class Type>
+void Foam::valuePointPatchField<Type>::extrapolateInternal()
+{
+    const labelUList& meshPoints = pointPatchFieldBase::patch().meshPoints();
+
+    const Field<Type>& iF = this->primitiveField();
+    Field<Type>& pfld = *this;
+
+    pfld.resize_nocopy(meshPoints.size());  // In general this is a no-op
+
+    forAll(meshPoints, i)
+    {
+        pfld[i] = iF[meshPoints[i]];
+    }
+}
+
+
 // * * * * * * * * * * * * * * * * Constructors * * * * * * * * * * * * * * * //
 
 template<class Type>

src/OpenFOAM/fields/pointPatchFields/basic/value/valuePointPatchField.H

@@ -74,6 +74,9 @@ protected:
             Field<Type>::writeEntry("value", os);
         }
 
+        //- Assign the patch field from the internal field
+        void extrapolateInternal();
+
 
 public:
 

src/atmosphericModels/derivedFvPatchFields/atmTurbulentHeatFluxTemperature/atmTurbulentHeatFluxTemperatureFvPatchScalarField.C

@@ -106,15 +106,9 @@ atmTurbulentHeatFluxTemperatureFvPatchScalarField
     Cp0_(Function1<scalar>::New("Cp0", dict, &db())),
     q_(PatchFunction1<scalar>::New(p.patch(), "q", dict))
 {
-    const auto* hasGrad = dict.findEntry("gradient", keyType::LITERAL);
+    if (!this->readGradientEntry(dict) || !this->readValueEntry(dict))
-
-    if (hasGrad && this->readValueEntry(dict))
     {
-        gradient().assign(*hasGrad, p.size());
+        extrapolateInternal();
-    }
-    else
-    {
-        fvPatchField<scalar>::patchInternalField(*this);
         gradient() = Zero;
     }
 }

src/finiteArea/fields/faPatchFields/basic/calculated/calculatedFaPatchField.C

@@ -48,10 +48,10 @@ Foam::calculatedFaPatchField<Type>::calculatedFaPatchField
     const faPatch& p,
     const DimensionedField<Type, areaMesh>& iF,
     const dictionary& dict,
-    IOobjectOption::readOption valueRequired
+    IOobjectOption::readOption requireValue
 )
 :
-    faPatchField<Type>(p, iF, dict, valueRequired)
+    faPatchField<Type>(p, iF, dict, requireValue)
 {}
 
 

src/finiteArea/fields/faPatchFields/basic/calculated/calculatedFaPatchField.H

@@ -79,7 +79,7 @@ public:
             const DimensionedField<Type, areaMesh>&,
             const dictionary& dict,
             //! The "value" entry (default: optional)
-            IOobjectOption::readOption valueRequired = IOobjectOption::LAZY_READ
+            IOobjectOption::readOption requireValue = IOobjectOption::LAZY_READ
         );
 
         //- Construct by mapping given patch field onto a new patch

src/finiteArea/fields/faPatchFields/basic/coupled/coupledFaPatchField.C

@@ -74,11 +74,11 @@ Foam::coupledFaPatchField<Type>::coupledFaPatchField
     const faPatch& p,
     const DimensionedField<Type, areaMesh>& iF,
     const dictionary& dict,
-    IOobjectOption::readOption valueRequired
+    IOobjectOption::readOption requireValue
 )
 :
     lduInterfaceField(refCast<const lduInterface>(p, dict)),
-    faPatchField<Type>(p, iF, dict, valueRequired)
+    faPatchField<Type>(p, iF, dict, requireValue)
 {}
 
 

src/finiteArea/fields/faPatchFields/basic/coupled/coupledFaPatchField.H

@@ -90,7 +90,7 @@ public:
             const DimensionedField<Type, areaMesh>&,
             const dictionary& dict,
             //! The "value" entry (default: mandatory)
-            IOobjectOption::readOption valueRequired = IOobjectOption::MUST_READ
+            IOobjectOption::readOption requireValue = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping the given coupledFaPatchField onto a new patch

src/finiteArea/fields/faPatchFields/basic/extrapolatedCalculated/extrapolatedCalculatedFaPatchField.C

@@ -54,8 +54,7 @@ extrapolatedCalculatedFaPatchField
 :
     calculatedFaPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ)
 {
-    // Set to the internal field
+    faPatchField<Type>::extrapolateInternal();  // Zero-gradient patch values
-    faPatchField<Type>::patchInternalField(*this);
 }
 
 
@@ -109,8 +108,7 @@ void Foam::extrapolatedCalculatedFaPatchField<Type>::evaluate
         this->updateCoeffs();
     }
 
-    // Set to the internal field
+    faPatchField<Type>::extrapolateInternal();  // Zero-gradient patch values
-    faPatchField<Type>::patchInternalField(*this);
     calculatedFaPatchField<Type>::evaluate();
 }
 

src/finiteArea/fields/faPatchFields/basic/fixedGradient/fixedGradientFaPatchField.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2016-2017 Wikki Ltd
+    Copyright (C) 2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -28,6 +29,39 @@ License
 #include "fixedGradientFaPatchField.H"
 #include "dictionary.H"
 
+// * * * * * * * * * * * * Protected Member Functions  * * * * * * * * * * * //
+
+template<class Type>
+bool Foam::fixedGradientFaPatchField<Type>::readGradientEntry
+(
+    const dictionary& dict,
+    IOobjectOption::readOption readOpt
+)
+{
+    if (!IOobjectOption::isAnyRead(readOpt)) return false;
+    const auto& p = faPatchFieldBase::patch();
+
+
+    const auto* eptr = dict.findEntry("gradient", keyType::LITERAL);
+
+    if (eptr)
+    {
+        gradient_.assign(*eptr, p.size());
+        return true;
+    }
+
+    if (IOobjectOption::isReadRequired(readOpt))
+    {
+        FatalIOErrorInFunction(dict)
+            << "Required entry 'gradient' : missing for patch " << p.name()
+            << " in dictionary " << dict.relativeName() << nl
+            << exit(FatalIOError);
+    }
+
+    return false;
+}
+
+
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
 template<class Type>
@@ -42,6 +76,32 @@ Foam::fixedGradientFaPatchField<Type>::fixedGradientFaPatchField
 {}
 
 
+template<class Type>
+Foam::fixedGradientFaPatchField<Type>::fixedGradientFaPatchField
+(
+    const faPatch& p,
+    const DimensionedField<Type, areaMesh>& iF,
+    const dictionary& dict,
+    IOobjectOption::readOption requireGrad
+)
+:
+    faPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ),
+    gradient_(p.size())
+{
+    if (readGradientEntry(dict, requireGrad))
+    {
+        evaluate();
+    }
+    else
+    {
+        // Not read (eg, optional and missing):
+        // - treat as zero-gradient, do not evaluate
+        faPatchField<Type>::extrapolateInternal();
+        gradient_ = Zero;
+    }
+}
+
+
 template<class Type>
 Foam::fixedGradientFaPatchField<Type>::fixedGradientFaPatchField
 (
@@ -56,21 +116,6 @@ Foam::fixedGradientFaPatchField<Type>::fixedGradientFaPatchField
 {}
 
 
-template<class Type>
-Foam::fixedGradientFaPatchField<Type>::fixedGradientFaPatchField
-(
-    const faPatch& p,
-    const DimensionedField<Type, areaMesh>& iF,
-    const dictionary& dict
-)
-:
-    faPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ),
-    gradient_("gradient", dict, p.size())
-{
-    evaluate();
-}
-
-
 template<class Type>
 Foam::fixedGradientFaPatchField<Type>::fixedGradientFaPatchField
 (

src/finiteArea/fields/faPatchFields/basic/fixedGradient/fixedGradientFaPatchField.H

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2016-2017 Wikki Ltd
+    Copyright (C) 2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -27,6 +28,35 @@ Class
     Foam::fixedGradientFaPatchField
 
 Description
+    This boundary condition supplies a fixed gradient condition, such that
+    the patch values are calculated using:
+
+        \f[
+            x_p = x_c + \frac{\nabla(x)}{\Delta}
+        \f]
+
+    where
+    \vartable
+        x_p      | patch values
+        x_c      | internal field values
+        \nabla(x)| gradient (user-specified)
+        \Delta   | inverse distance from patch face centre to cell centre
+    \endvartable
+
+Usage
+    \table
+        Property     | Description             | Required    | Default value
+        gradient     | gradient                | yes         |
+    \endtable
+
+    Example of the boundary condition specification:
+    \verbatim
+    <patchName>
+    {
+        type            fixedGradient;
+        gradient        uniform 0;
+    }
+    \endverbatim
 
 Author
     Zeljko Tukovic, FMENA
@@ -37,8 +67,8 @@ SourceFiles
 
 \*---------------------------------------------------------------------------*/
 
-#ifndef fixedGradientFaPatchField_H
+#ifndef Foam_fixedGradientFaPatchField_H
-#define fixedGradientFaPatchField_H
+#define Foam_fixedGradientFaPatchField_H
 
 #include "faPatchField.H"
 
@@ -56,10 +86,22 @@ class fixedGradientFaPatchField
 :
     public faPatchField<Type>
 {
-    // Private data
+    // Private Data
 
         Field<Type> gradient_;
 
+protected:
+
+    // Protected Member Functions
+
+        //- Read the "gradient" entry into corresponding member
+        //  The reading can be optional (default), mandatory etc.
+        //  \returns True on success
+        bool readGradientEntry
+        (
+            const dictionary& dict,
+            IOobjectOption::readOption readOpt = IOobjectOption::LAZY_READ
+        );
 
 public:
 
@@ -76,12 +118,14 @@ public:
             const DimensionedField<Type, areaMesh>&
         );
 
-        //- Construct from patch, internal field and dictionary
+        //- Construct from patch, internal field and dictionary.
         fixedGradientFaPatchField
         (
             const faPatch&,
             const DimensionedField<Type, areaMesh>&,
-            const dictionary&
+            const dictionary& dict,
+            //! The "gradient" entry (default: mandatory)
+            IOobjectOption::readOption requireGrad = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping the given fixedGradient patch field

src/finiteArea/fields/faPatchFields/basic/fixedValue/fixedValueFaPatchField.C

@@ -58,10 +58,10 @@ Foam::fixedValueFaPatchField<Type>::fixedValueFaPatchField
     const faPatch& p,
     const DimensionedField<Type, areaMesh>& iF,
     const dictionary& dict,
-    IOobjectOption::readOption valueRequired
+    IOobjectOption::readOption requireValue
 )
 :
-    faPatchField<Type>(p, iF, dict, valueRequired)
+    faPatchField<Type>(p, iF, dict, requireValue)
 {}
 
 

src/finiteArea/fields/faPatchFields/basic/fixedValue/fixedValueFaPatchField.H

@@ -87,7 +87,7 @@ public:
             const DimensionedField<Type, areaMesh>&,
             const dictionary&,
             //! The "value" entry (default: mandatory)
-            IOobjectOption::readOption valueRequired = IOobjectOption::MUST_READ
+            IOobjectOption::readOption requireValue = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping the given fixedValue patch field

src/finiteArea/fields/faPatchFields/basic/zeroGradient/zeroGradientFaPatchField.C

@@ -64,8 +64,7 @@ Foam::zeroGradientFaPatchField<Type>::zeroGradientFaPatchField
 :
     faPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ)
 {
-    // Set to the internal field
+    faPatchField<Type>::extrapolateInternal();  // Zero-gradient patch values
-    faPatchField<Type>::patchInternalField(*this);
 }
 
 
@@ -100,8 +99,7 @@ void Foam::zeroGradientFaPatchField<Type>::evaluate(const Pstream::commsTypes)
         this->updateCoeffs();
     }
 
-    // Set to the internal field
+    faPatchField<Type>::extrapolateInternal();  // Zero-gradient patch values
-    faPatchField<Type>::patchInternalField(*this);
     faPatchField<Type>::evaluate();
 }
 

src/finiteArea/fields/faPatchFields/constraint/cyclic/cyclicFaPatchField.C

@@ -73,7 +73,7 @@ Foam::cyclicFaPatchField<Type>::cyclicFaPatchField
     const faPatch& p,
     const DimensionedField<Type, areaMesh>& iF,
     const dictionary& dict,
-    IOobjectOption::readOption valueRequired
+    IOobjectOption::readOption requireValue
 )
 :
     coupledFaPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ),
@@ -90,7 +90,7 @@ Foam::cyclicFaPatchField<Type>::cyclicFaPatchField
             << exit(FatalIOError);
     }
 
-    if (IOobjectOption::isReadRequired(valueRequired))
+    if (IOobjectOption::isReadRequired(requireValue))
     {
         this->evaluate(Pstream::commsTypes::blocking);
     }

src/finiteArea/fields/faPatchFields/constraint/cyclic/cyclicFaPatchField.H

@@ -100,7 +100,7 @@ public:
             const DimensionedField<Type, areaMesh>&,
             const dictionary& dict,
             //! Evaluate (default: mandatory)
-            IOobjectOption::readOption valueRequired = IOobjectOption::MUST_READ
+            IOobjectOption::readOption requireValue = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping given cyclicFaPatchField onto a new patch

src/finiteArea/fields/faPatchFields/faPatchField/faPatchField.C

@@ -63,6 +63,13 @@ bool Foam::faPatchField<Type>::readValueEntry
 }
 
 
+template<class Type>
+void Foam::faPatchField<Type>::extrapolateInternal()
+{
+    faPatchFieldBase::patch().patchInternalField(internalField_, *this);
+}
+
+
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
 template<class Type>

src/finiteArea/fields/faPatchFields/faPatchField/faPatchField.H

@@ -273,6 +273,9 @@ protected:
             Field<Type>::writeEntry("value", os);
         }
 
+        //- Assign the patch field from the internal field
+        void extrapolateInternal();
+
 
 public:
 

src/finiteArea/fields/faePatchFields/basic/calculated/calculatedFaePatchField.C

@@ -61,10 +61,10 @@ Foam::calculatedFaePatchField<Type>::calculatedFaePatchField
     const faPatch& p,
     const DimensionedField<Type, edgeMesh>& iF,
     const dictionary& dict,
-    IOobjectOption::readOption valueRequired
+    IOobjectOption::readOption requireValue
 )
 :
-    faePatchField<Type>(p, iF, dict, valueRequired)
+    faePatchField<Type>(p, iF, dict, requireValue)
 {}
 
 

src/finiteArea/fields/faePatchFields/basic/calculated/calculatedFaePatchField.H

@@ -79,7 +79,7 @@ public:
             const DimensionedField<Type, edgeMesh>&,
             const dictionary& dict,
             //! The "value" entry (default: mandatory)
-            IOobjectOption::readOption valueRequired = IOobjectOption::MUST_READ
+            IOobjectOption::readOption requireValue = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping given patch field onto a new patch

src/finiteArea/fields/faePatchFields/basic/coupled/coupledFaePatchField.C

@@ -71,10 +71,10 @@ Foam::coupledFaePatchField<Type>::coupledFaePatchField
     const faPatch& p,
     const DimensionedField<Type, edgeMesh>& iF,
     const dictionary& dict,
-    IOobjectOption::readOption valueRequired
+    IOobjectOption::readOption requireValue
 )
 :
-    faePatchField<Type>(p, iF, dict, valueRequired)
+    faePatchField<Type>(p, iF, dict, requireValue)
 {}
 
 

src/finiteArea/fields/faePatchFields/basic/coupled/coupledFaePatchField.H

@@ -88,7 +88,7 @@ public:
             const DimensionedField<Type, edgeMesh>&,
             const dictionary& dict,
             //! The "value" entry (default: optional)
-            IOobjectOption::readOption valueRequired = IOobjectOption::LAZY_READ
+            IOobjectOption::readOption requireValue = IOobjectOption::LAZY_READ
         );
 
         //- Construct by mapping the given coupledFaePatchField onto a new patch

src/finiteArea/fields/faePatchFields/basic/fixedValue/fixedValueFaePatchField.C

@@ -59,10 +59,10 @@ Foam::fixedValueFaePatchField<Type>::fixedValueFaePatchField
     const faPatch& p,
     const DimensionedField<Type, edgeMesh>& iF,
     const dictionary& dict,
-    IOobjectOption::readOption valueRequired
+    IOobjectOption::readOption requireValue
 )
 :
-    faePatchField<Type>(p, iF, dict, valueRequired)
+    faePatchField<Type>(p, iF, dict, requireValue)
 {}
 
 

src/finiteArea/fields/faePatchFields/basic/fixedValue/fixedValueFaePatchField.H

@@ -87,7 +87,7 @@ public:
             const DimensionedField<Type, edgeMesh>&,
             const dictionary& dict,
             //! The "value" entry (default: mandatory)
-            IOobjectOption::readOption valueRequired = IOobjectOption::MUST_READ
+            IOobjectOption::readOption requireValue = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping the given fixedValue patch field

src/finiteVolume/expressions/fields/fvPatchFields/exprFixedValueFvPatchField.C

@@ -82,7 +82,7 @@ Foam::exprFixedValueFvPatchField<Type>::exprFixedValueFvPatchField
     const fvPatch& p,
     const DimensionedField<Type, volMesh>& iF,
     const dictionary& dict,
-    const bool valueRequired
+    IOobjectOption::readOption requireValue  // (ignored)
 )
 :
     parent_bctype(p, iF), // bypass dictionary constructor

src/finiteVolume/expressions/fields/fvPatchFields/exprFixedValueFvPatchField.H

@@ -110,7 +110,7 @@ public:
             const fvPatch&,
             const DimensionedField<Type, volMesh>&,
             const dictionary& dict,
-            const bool valueRequired=true
+            IOobjectOption::readOption requireValue = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping onto a new patch

src/finiteVolume/fields/fvPatchFields/basic/extrapolatedCalculated/extrapolatedCalculatedFvPatchField.C

@@ -54,8 +54,7 @@ extrapolatedCalculatedFvPatchField
 :
     calculatedFvPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ)
 {
-    // Set to the internal field
+    fvPatchField<Type>::extrapolateInternal();  // Zero-gradient patch values
-    fvPatchField<Type>::patchInternalField(*this);
 }
 
 
@@ -109,8 +108,7 @@ void Foam::extrapolatedCalculatedFvPatchField<Type>::evaluate
         this->updateCoeffs();
     }
 
-    // Set to the internal field
+    fvPatchField<Type>::extrapolateInternal();  // Zero-gradient patch values
-    fvPatchField<Type>::patchInternalField(*this);
     calculatedFvPatchField<Type>::evaluate();
 }
 

src/finiteVolume/fields/fvPatchFields/basic/fixedGradient/fixedGradientFvPatchField.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
+    Copyright (C) 2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -28,6 +29,39 @@ License
 #include "fixedGradientFvPatchField.H"
 #include "dictionary.H"
 
+// * * * * * * * * * * * * Protected Member Functions  * * * * * * * * * * * //
+
+template<class Type>
+bool Foam::fixedGradientFvPatchField<Type>::readGradientEntry
+(
+    const dictionary& dict,
+    IOobjectOption::readOption readOpt
+)
+{
+    if (!IOobjectOption::isAnyRead(readOpt)) return false;
+    const auto& p = fvPatchFieldBase::patch();
+
+
+    const auto* eptr = dict.findEntry("gradient", keyType::LITERAL);
+
+    if (eptr)
+    {
+        gradient_.assign(*eptr, p.size());
+        return true;
+    }
+
+    if (IOobjectOption::isReadRequired(readOpt))
+    {
+        FatalIOErrorInFunction(dict)
+            << "Required entry 'gradient' : missing for patch " << p.name()
+            << " in dictionary " << dict.relativeName() << nl
+            << exit(FatalIOError);
+    }
+
+    return false;
+}
+
+
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
 template<class Type>
@@ -47,13 +81,24 @@ Foam::fixedGradientFvPatchField<Type>::fixedGradientFvPatchField
 (
     const fvPatch& p,
     const DimensionedField<Type, volMesh>& iF,
-    const dictionary& dict
+    const dictionary& dict,
+    IOobjectOption::readOption requireGrad
 )
 :
     fvPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ),
-    gradient_("gradient", dict, p.size())
+    gradient_(p.size())
 {
-    evaluate();
+    if (readGradientEntry(dict, requireGrad))
+    {
+        evaluate();
+    }
+    else
+    {
+        // Not read (eg, optional and missing):
+        // - treat as zero-gradient, do not evaluate
+        fvPatchField<Type>::extrapolateInternal();
+        gradient_ = Zero;
+    }
 }
 
 

src/finiteVolume/fields/fvPatchFields/basic/fixedGradient/fixedGradientFvPatchField.H

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
+    Copyright (C) 2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -65,8 +66,8 @@ SourceFiles
 
 \*---------------------------------------------------------------------------*/
 
-#ifndef fixedGradientFvPatchField_H
+#ifndef Foam_fixedGradientFvPatchField_H
-#define fixedGradientFvPatchField_H
+#define Foam_fixedGradientFvPatchField_H
 
 #include "fvPatchField.H"
 
@@ -84,10 +85,22 @@ class fixedGradientFvPatchField
 :
     public fvPatchField<Type>
 {
-    // Private data
+    // Private Data
 
         Field<Type> gradient_;
 
+protected:
+
+    // Protected Member Functions
+
+        //- Read the "gradient" entry into corresponding member
+        //  The reading can be optional (default), mandatory etc.
+        //  \returns True on success
+        bool readGradientEntry
+        (
+            const dictionary& dict,
+            IOobjectOption::readOption readOpt = IOobjectOption::LAZY_READ
+        );
 
 public:
 
@@ -104,12 +117,14 @@ public:
             const DimensionedField<Type, volMesh>&
         );
 
-        //- Construct from patch, internal field and dictionary
+        //- Construct from patch, internal field and dictionary.
         fixedGradientFvPatchField
         (
             const fvPatch&,
             const DimensionedField<Type, volMesh>&,
-            const dictionary&
+            const dictionary& dict,
+            //! The "gradient" entry (default: mandatory)
+            IOobjectOption::readOption requireGrad = IOobjectOption::MUST_READ
         );
 
         //- Construct by mapping the given fixedGradientFvPatchField

src/finiteVolume/fields/fvPatchFields/basic/zeroGradient/zeroGradientFvPatchField.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
+    Copyright (C) 2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -51,8 +52,7 @@ Foam::zeroGradientFvPatchField<Type>::zeroGradientFvPatchField
 :
     fvPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ)
 {
-    // Set to the internal field
+    fvPatchField<Type>::extrapolateInternal();  // Zero-gradient patch values
-    fvPatchField<Type>::patchInternalField(*this);
 }
 
 
@@ -100,8 +100,7 @@ void Foam::zeroGradientFvPatchField<Type>::evaluate(const Pstream::commsTypes)
         this->updateCoeffs();
     }
 
-    // Set to the internal field
+    fvPatchField<Type>::extrapolateInternal();  // Zero-gradient patch values
-    fvPatchField<Type>::patchInternalField(*this);
     fvPatchField<Type>::evaluate();
 }
 

src/finiteVolume/fields/fvPatchFields/derived/fixedFluxPressure/fixedFluxPressureFvPatchScalarField.C

@@ -57,15 +57,9 @@ Foam::fixedFluxPressureFvPatchScalarField::fixedFluxPressureFvPatchScalarField
 {
     fvPatchFieldBase::readDict(dict);
 
-    const auto* hasGrad = dict.findEntry("gradient", keyType::LITERAL);
+    if (!this->readGradientEntry(dict) || !this->readValueEntry(dict))
-
-    if (hasGrad && this->readValueEntry(dict))
     {
-        gradient().assign(*hasGrad, p.size());
+        extrapolateInternal();
-    }
-    else
-    {
-        fvPatchField<scalar>::patchInternalField(*this);
         gradient() = Zero;
     }
 }
@@ -85,7 +79,7 @@ Foam::fixedFluxPressureFvPatchScalarField::fixedFluxPressureFvPatchScalarField
     patchType() = ptf.patchType();
 
     // Map gradient. Set unmapped values and overwrite with mapped ptf
-    gradient() = 0.0;
+    gradient() = Zero;
     gradient().map(ptf.gradient(), mapper);
 
     // Evaluate the value field from the gradient if the internal field is valid

src/finiteVolume/fields/fvPatchFields/fvPatchField/fvPatchField.C

@@ -65,6 +65,13 @@ bool Foam::fvPatchField<Type>::readValueEntry
 }
 
 
+template<class Type>
+void Foam::fvPatchField<Type>::extrapolateInternal()
+{
+    fvPatchFieldBase::patch().patchInternalField(internalField_, *this);
+}
+
+
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
 template<class Type>

src/finiteVolume/fields/fvPatchFields/fvPatchField/fvPatchField.H

@@ -302,6 +302,9 @@ protected:
             Field<Type>::writeEntry("value", os);
         }
 
+        //- Assign the patch field from the internal field
+        void extrapolateInternal();
+
 
 public:
 

src/thermoTools/derivedFvPatchFields/fixedIncidentRadiation/fixedIncidentRadiationFvPatchScalarField.C

@@ -77,16 +77,9 @@ fixedIncidentRadiationFvPatchScalarField
     temperatureCoupledBase(patch(), dict),
     qrIncident_("qrIncident", dict, p.size())
 {
-    const auto* hasGrad = dict.findEntry("gradient", keyType::LITERAL);
+    if (!this->readGradientEntry(dict) || !this->readValueEntry(dict))
-
-    if (hasGrad && this->readValueEntry(dict))
     {
-        gradient().assign(*hasGrad, p.size());
+        extrapolateInternal();
-    }
-    else
-    {
-        // Still reading so cannot yet evaluate. Make up a value.
-        fvPatchField<scalar>::patchInternalField(*this);
         gradient() = Zero;
     }
 }

src/thermoTools/derivedFvPatchFields/wallFunctions/sorptionWallFunction/sorptionWallFunctionFvPatchScalarField.C

@@ -303,16 +303,9 @@ sorptionWallFunctionFvPatchScalarField::sorptionWallFunctionFvPatchScalarField
             << exit(FatalIOError);
     }
 
-    const auto* hasGrad = dict.findEntry("gradient", keyType::LITERAL);
+    if (!this->readGradientEntry(dict) || !this->readValueEntry(dict))
-
-    if (hasGrad && this->readValueEntry(dict))
     {
-        gradient().assign(*hasGrad, p.size());
+        extrapolateInternal();
-    }
-    else
-    {
-        // Fallback: set to zero-gradient
-        fvPatchField<scalar>::patchInternalField(*this);
         gradient() = Zero;
     }
 }

src/transportModels/interfaceProperties/alphaContactAngle/alphaContactAngleTwoPhaseFvPatchScalarField.C

@@ -70,18 +70,15 @@ alphaContactAngleTwoPhaseFvPatchScalarField
     fixedGradientFvPatchScalarField(p, iF),  // Bypass dictionary constructor
     limit_(limitControlNames_.get("limit", dict))
 {
-    const auto* hasGrad = dict.findEntry("gradient", keyType::LITERAL);
+    if (this->readGradientEntry(dict))
-
-    if (hasGrad)
     {
-        gradient().assign(*hasGrad, p.size());
         fixedGradientFvPatchScalarField::updateCoeffs();
         fixedGradientFvPatchScalarField::evaluate();
     }
     else
     {
         // Fallback: set to zero-gradient
-        fvPatchField<scalar>::patchInternalField(*this);
+        extrapolateInternal();
         gradient() = Zero;
     }
 }
@@ -142,7 +139,7 @@ void Foam::alphaContactAngleTwoPhaseFvPatchScalarField::evaluate
     }
     else if (limit_ == lcZeroGradient)
     {
-        gradient() = 0.0;
+        gradient() = Zero;
     }
 
     fixedGradientFvPatchScalarField::evaluate();