ENH: enforce snGradTransformDiag() as zero for rotational-invariant

- the snGradTransformDiag() is used in this type of code:

  ```
  if constexpr (!is_rotational_vectorspace_v<Type>)
  {
      // Rotational-invariant type
      return tmp<Field<Type>>::New(this->size(), pTraits<Type>::one);
  }
  else
  {
      return pTraits<Type>::one - snGradTransformDiag();
  }
  ```

  This implies that a rotational-invariant form should return 0
  and not 1 like the pow(..., 0) does.
  This is a backstop for any code that may inadvertently hit it,
  but also allows the possibility of having integer indicator fields
  without upsetting the compiler.

STYLE: doTransform() logic now uses is_rotational_vectorspace

src/OpenFOAM/fields/pointPatchFields/constraint/cyclic/cyclicPointPatchField.H

@@ -117,7 +117,7 @@ public:
 
     // Member functions
 
-        //- Constraint handling
+        // Constraint handling
 
             //- Return the constraint type this pointPatchField implements
             virtual const word& constraintType() const
@@ -125,12 +125,17 @@ public:
                 return cyclicPointPatch::typeName;
             }
 
-        //- Cyclic coupled interface functions
+
+        // Cyclic coupled interface functions
 
             //- Does the patch field perform the transformation
             virtual bool doTransform() const
             {
-                return (pTraits<Type>::rank && !cyclicPatch_.parallel());
+                return
+                (
+                    is_rotational_vectorspace_v<Type>
+                 && !cyclicPatch_.parallel()
+                );
             }
 
             //- Return face transformation tensor

src/OpenFOAM/fields/pointPatchFields/constraint/processor/processorPointPatchField.H

@@ -125,7 +125,7 @@ public:
             //- Return true if running parallel
             virtual bool coupled() const
             {
-                return Pstream::parRun();
+                return UPstream::parRun();
             }
 
             //- Does the patch field perform the transformation
@@ -133,7 +133,7 @@ public:
             {
                 return
                 (
-                    pTraits<Type>::rank
+                    is_rotational_vectorspace_v<Type>
                  && !procPatch_.procPolyPatch().parallel()
                 );
             }

src/OpenFOAM/fields/pointPatchFields/constraint/processorCyclic/processorCyclicPointPatchField.H

@@ -133,7 +133,7 @@ public:
             //- Return true if running parallel
             virtual bool coupled() const
             {
-                return Pstream::parRun();
+                return UPstream::parRun();
             }
 
             //- Does the patch field perform the transformation
@@ -141,7 +141,7 @@ public:
             {
                 return
                 (
-                    pTraits<Type>::rank
+                    is_rotational_vectorspace_v<Type>
                  && !procPatch_.procPolyPatch().parallel()
                 );
             }

src/finiteArea/fields/faPatchFields/basic/basicSymmetry/basicSymmetryFaPatchField.C

@@ -140,11 +140,22 @@ void Foam::basicSymmetryFaPatchField<Type>::evaluate(const Pstream::commsTypes)
 template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::basicSymmetryFaPatchField<Type>::snGradTransformDiag() const
+{
+    if constexpr (!is_rotational_vectorspace_v<Type>)
+    {
+        // Rotational-invariant type
+        // FatalErrorInFunction
+        //     << "Should not be called for this type"
+        //     << ::Foam::abort(FatalError);
+        return tmp<Field<Type>>::New(this->size(), Foam::zero{});
+    }
+    else
     {
         tmp<vectorField> diag(cmptMag(this->patch().edgeNormals()));
 
         return transformFieldMask<Type>(pow<vector, pTraits<Type>::rank>(diag));
     }
+}
 
 
 // ************************************************************************* //

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

@@ -231,6 +231,9 @@ public:
                 const Pstream::commsTypes commsType
             ) const = 0;
 
+
+    // Other
+
         //- Write
         virtual void write(Ostream&) const;
 };

src/finiteArea/fields/faPatchFields/basic/transform/transformFaPatchField.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2016-2017 Wikki Ltd
+    Copyright (C) 2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -86,9 +87,9 @@ Foam::transformFaPatchField<Type>::valueInternalCoeffs
     const tmp<scalarField>&
 ) const
 {
-    // OR (!is_rotational_vectorspace_v<Type>)
-    if constexpr (std::is_arithmetic_v<Type>)
+    if constexpr (!is_rotational_vectorspace_v<Type>)
     {
+        // Rotational-invariant type
         return tmp<Field<Type>>::New(this->size(), pTraits<Type>::one);
     }
     else
@@ -119,9 +120,9 @@ template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::transformFaPatchField<Type>::gradientInternalCoeffs() const
 {
-    // OR (!is_rotational_vectorspace_v<Type>)
-    if constexpr (std::is_arithmetic_v<Type>)
+    if constexpr (!is_rotational_vectorspace_v<Type>)
     {
+        // Rotational-invariant type
         return tmp<Field<Type>>::New(this->size(), Foam::zero{});
     }
     else

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

@@ -183,7 +183,11 @@ public:
             //- Does the patch field perform the transformation
             virtual bool doTransform() const
             {
-                return (pTraits<Type>::rank && !cyclicPatch_.parallel());
+                return
+                (
+                    is_rotational_vectorspace_v<Type>
+                 && !cyclicPatch_.parallel()
+                );
             }
 
             //- Return face transformation tensor

src/finiteArea/fields/faPatchFields/constraint/processor/processorFaPatchField.H

@@ -287,7 +287,11 @@ public:
         //- Does the patch field perform the transformation
         virtual bool doTransform() const
         {
-            return (pTraits<Type>::rank && !procPatch_.parallel());
+            return
+            (
+                is_rotational_vectorspace_v<Type>
+             && !procPatch_.parallel()
+            );
         }
 
         //- Return face transformation tensor

src/finiteArea/fields/faPatchFields/constraint/wedge/wedgeFaPatchField.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2016-2017 Wikki Ltd
+    Copyright (C) 2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -154,10 +155,20 @@ void Foam::wedgeFaPatchField<Type>::evaluate(const Pstream::commsTypes)
 template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::wedgeFaPatchField<Type>::snGradTransformDiag() const
+{
+    if constexpr (!is_rotational_vectorspace_v<Type>)
+    {
+        // Rotational-invariant type
+        // FatalErrorInFunction
+        //     << "Should not be called for this type"
+        //     << ::Foam::abort(FatalError);
+        return tmp<Field<Type>>::New(this->size(), Foam::zero{});
+    }
+    else
     {
         const auto& rot = refCast<const wedgeFaPatch>(this->patch()).faceT();
 
-    const vector diagV = 0.5*(I - rot).diag();
+        const vector diag = 0.5*(I - rot).diag();
 
         return tmp<Field<Type>>::New
         (
@@ -166,7 +177,7 @@ Foam::wedgeFaPatchField<Type>::snGradTransformDiag() const
             (
                 pow
                 (
-                diagV,
+                    diag,
                     pTraits
                     <
                         typename powProduct<vector, pTraits<Type>::rank>::type
@@ -175,6 +186,7 @@ Foam::wedgeFaPatchField<Type>::snGradTransformDiag() const
             )
         );
     }
+}
 
 
 // ************************************************************************* //

src/finiteArea/fields/faPatchFields/constraint/wedge/wedgeFaPatchField.H

@@ -114,9 +114,7 @@ public:
         }
 
 
-    // Member functions
-
-        // Evaluation functions
+    // Member Functions
 
         //- Return gradient at boundary
         virtual tmp<Field<Type>> snGrad() const;
@@ -127,7 +125,6 @@ public:
             const Pstream::commsTypes commsType=Pstream::commsTypes::buffered
         );
 
-
         //- Return face-gradient transform diagonal
         virtual tmp<Field<Type>> snGradTransformDiag() const;
 };

src/finiteVolume/fields/fvPatchFields/basic/basicSymmetry/basicSymmetryFvPatchField.C

@@ -140,11 +140,22 @@ void Foam::basicSymmetryFvPatchField<Type>::evaluate(const Pstream::commsTypes)
 template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::basicSymmetryFvPatchField<Type>::snGradTransformDiag() const
+{
+    if constexpr (!is_rotational_vectorspace_v<Type>)
+    {
+        // Rotational-invariant type
+        // FatalErrorInFunction
+        //     << "Should not be called for this type"
+        //     << ::Foam::abort(FatalError);
+        return tmp<Field<Type>>::New(this->size(), Foam::zero{});
+    }
+    else
     {
         tmp<vectorField> diag(cmptMag(this->patch().nf()));
 
         return transformFieldMask<Type>(pow<vector, pTraits<Type>::rank>(diag));
     }
+}
 
 
 // ************************************************************************* //

src/finiteVolume/fields/fvPatchFields/basic/directionMixed/directionMixedFvPatchField.C

@@ -178,27 +178,35 @@ void Foam::directionMixedFvPatchField<Type>::evaluate(const Pstream::commsTypes)
 template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::directionMixedFvPatchField<Type>::snGradTransformDiag() const
+{
+    // static_assert(!std::is_arithmetic_v<T>, "direction-mixed with scalar??");
+    // if constexpr (!is_rotational_vectorspace_v<Type>)
+    // {
+    //     // Rotational-invariant type
+    //     return tmp<Field<Type>>::New(this->size(), Foam::zero{});
+    // }
+
     {
         vectorField diag(valueFraction_.size());
 
-    diag.replace
+        std::transform
         (
-        vector::X,
-        sqrt(mag(valueFraction_.component(symmTensor::XX)))
+            valueFraction_.cbegin(),
+            valueFraction_.cend(),
+            diag.begin(),
+
+            [](const symmTensor& t)
+            {
+                return vector
+                (
+                    sqrt(mag(t.xx())), sqrt(mag(t.yy())), sqrt(mag(t.zz()))
                 );
-    diag.replace
-    (
-        vector::Y,
-        sqrt(mag(valueFraction_.component(symmTensor::YY)))
-    );
-    diag.replace
-    (
-        vector::Z,
-        sqrt(mag(valueFraction_.component(symmTensor::ZZ)))
+            }
         );
 
         return transformFieldMask<Type>(pow<vector, pTraits<Type>::rank>(diag));
     }
+}
 
 
 template<class Type>

src/finiteVolume/fields/fvPatchFields/basic/transform/transformFvPatchField.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
+    Copyright (C) 2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -97,9 +98,9 @@ Foam::transformFvPatchField<Type>::valueInternalCoeffs
     const tmp<scalarField>&
 ) const
 {
-    // OR (!is_rotational_vectorspace_v<Type>)
-    if constexpr (std::is_arithmetic_v<Type>)
+    if constexpr (!is_rotational_vectorspace_v<Type>)
     {
+        // Rotational-invariant type
         return tmp<Field<Type>>::New(this->size(), pTraits<Type>::one);
     }
     else
@@ -130,9 +131,9 @@ template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::transformFvPatchField<Type>::gradientInternalCoeffs() const
 {
-    // OR (!is_rotational_vectorspace_v<Type>)
-    if constexpr (std::is_arithmetic_v<Type>)
+    if constexpr (!is_rotational_vectorspace_v<Type>)
     {
+        // Rotational-invariant type
         return tmp<Field<Type>>::New(this->size(), Foam::zero{});
     }
     else

src/finiteVolume/fields/fvPatchFields/constraint/cyclic/cyclicFvPatchField.H

@@ -209,7 +209,11 @@ public:
             //- Does the patch field perform the transformation
             virtual bool doTransform() const
             {
-                return (pTraits<Type>::rank && !cyclicPatch_.parallel());
+                return
+                (
+                    is_rotational_vectorspace_v<Type>
+                 && !cyclicPatch_.parallel()
+                );
             }
 
             //- Return face transformation tensor

src/finiteVolume/fields/fvPatchFields/constraint/cyclicACMI/cyclicACMIFvPatchField.H

@@ -333,7 +333,11 @@ public:
             //- Does the patch field perform the transformation
             virtual bool doTransform() const
             {
-                return (pTraits<Type>::rank && !cyclicACMIPatch_.parallel());
+                return
+                (
+                    is_rotational_vectorspace_v<Type>
+                 && !cyclicACMIPatch_.parallel()
+                );
             }
 
             //- Return face transformation tensor

src/finiteVolume/fields/fvPatchFields/constraint/cyclicAMI/cyclicAMIFvPatchField.H

@@ -339,7 +339,11 @@ public:
             //- Does the patch field perform the transformation
             virtual bool doTransform() const
             {
-                return (pTraits<Type>::rank && !cyclicAMIPatch_.parallel());
+                return
+                (
+                    is_rotational_vectorspace_v<Type>
+                 && !cyclicAMIPatch_.parallel()
+                );
             }
 
             //- Return face transformation tensor

src/finiteVolume/fields/fvPatchFields/constraint/processor/processorFvPatchField.H

@@ -298,7 +298,11 @@ public:
         //- Does the patch field perform the transformation
         virtual bool doTransform() const
         {
-            return (pTraits<Type>::rank && !procPatch_.parallel());
+            return
+            (
+                is_rotational_vectorspace_v<Type>
+             && !procPatch_.parallel()
+            );
         }
 
         //- Return face transformation tensor

src/finiteVolume/fields/fvPatchFields/constraint/processorCyclic/processorCyclicFvPatchField.H

@@ -151,12 +151,14 @@ public:
 
     // Member Functions
 
-        // Access
-
         //- Does the patch field perform the transformation
         virtual bool doTransform() const
         {
-                return (pTraits<Type>::rank && !procPatch_.parallel());
+            return
+            (
+                is_rotational_vectorspace_v<Type>
+             && !procPatch_.parallel()
+            );
         }
 
         //- Return face transformation tensor

src/finiteVolume/fields/fvPatchFields/constraint/symmetryPlane/symmetryPlaneFvPatchField.C

@@ -171,6 +171,16 @@ void Foam::symmetryPlaneFvPatchField<Type>::evaluate(const Pstream::commsTypes)
 template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::symmetryPlaneFvPatchField<Type>::snGradTransformDiag() const
+{
+    if constexpr (!is_rotational_vectorspace_v<Type>)
+    {
+        // Rotational-invariant type
+        // FatalErrorInFunction
+        //     << "Should not be called for this type"
+        //     << ::Foam::abort(FatalError);
+        return tmp<Field<Type>>::New(this->size(), Foam::zero{});
+    }
+    else
     {
         const vector diag(cmptMag(symmetryPlanePatch_.n()));
 
@@ -191,6 +201,7 @@ Foam::symmetryPlaneFvPatchField<Type>::snGradTransformDiag() const
             )
         );
     }
+}
 
 
 // ************************************************************************* //

src/finiteVolume/fields/fvPatchFields/constraint/wedge/wedgeFvPatchField.C

@@ -169,10 +169,20 @@ void Foam::wedgeFvPatchField<Type>::evaluate(const Pstream::commsTypes)
 template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::wedgeFvPatchField<Type>::snGradTransformDiag() const
+{
+    if constexpr (!is_rotational_vectorspace_v<Type>)
+    {
+        // Rotational-invariant type
+        // FatalErrorInFunction
+        //     << "Should not be called for this type"
+        //     << ::Foam::abort(FatalError);
+        return tmp<Field<Type>>::New(this->size(), Foam::zero{});
+    }
+    else
     {
         const auto& rot = refCast<const wedgeFvPatch>(this->patch()).cellT();
 
-    const vector diagV = 0.5*(I - rot).diag();
+        const vector diag = 0.5*(I - rot).diag();
 
         return tmp<Field<Type>>::New
         (
@@ -181,7 +191,7 @@ Foam::wedgeFvPatchField<Type>::snGradTransformDiag() const
              (
                  pow
                  (
-                diagV,
+                     diag,
                      pTraits
                      <
                          typename powProduct<vector, pTraits<Type>::rank>::type
@@ -190,6 +200,7 @@ Foam::wedgeFvPatchField<Type>::snGradTransformDiag() const
              )
         );
     }
+}
 
 
 // ************************************************************************* //

src/finiteVolume/fields/fvPatchFields/derived/fixedNormalSlip/fixedNormalSlipFvPatchField.C

@@ -135,6 +135,12 @@ template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::fixedNormalSlipFvPatchField<Type>::snGrad() const
 {
+    static_assert
+    (
+        is_rotational_vectorspace_v<Type>,
+        "normal-slip with vector, tensor only!"
+    );
+
     const vectorField nHat(this->patch().nf());
     const Field<Type> pif(this->patchInternalField());
 
@@ -171,11 +177,24 @@ void Foam::fixedNormalSlipFvPatchField<Type>::evaluate
 template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::fixedNormalSlipFvPatchField<Type>::snGradTransformDiag() const
+{
+    static_assert
+    (
+        is_rotational_vectorspace_v<Type>,
+        "normal-slip with vector, tensor only!"
+    );
+    // if constexpr (!is_rotational_vectorspace_v<Type>)
+    // {
+    //     // Rotational-invariant type
+    //     return tmp<Field<Type>>::New(this->size(), Foam::zero{});
+    // }
+
     {
         tmp<vectorField> diag(cmptMag(this->patch().nf()));
 
         return transformFieldMask<Type>(pow<vector, pTraits<Type>::rank>(diag));
     }
+}
 
 
 template<class Type>

src/finiteVolume/fields/fvPatchFields/derived/partialSlip/partialSlipFvPatchField.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2021 OpenCFD Ltd.
+    Copyright (C) 2017-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -142,17 +142,24 @@ template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::partialSlipFvPatchField<Type>::snGrad() const
 {
-    tmp<vectorField> nHat = this->patch().nf();
     const Field<Type> pif(this->patchInternalField());
 
+    tmp<Field<Type>> rotated;
+
+    if constexpr (!is_rotational_vectorspace_v<Type>)
+    {
+        // Rotational-invariant type
+        rotated.cref(pif);
+    }
+    else
+    {
+        tmp<vectorField> nHat(this->patch().nf());
+        rotated = transform(I - sqr(nHat), pif);
+    }
+
     return
     (
-        lerp
-        (
-            transform(I - sqr(nHat), pif),
-            refValue_,
-            valueFraction_
-        ) - pif
+        lerp(rotated, refValue_, valueFraction_) - pif
     )*this->patch().deltaCoeffs();
 }
 
@@ -168,16 +175,23 @@ void Foam::partialSlipFvPatchField<Type>::evaluate
         this->updateCoeffs();
     }
 
-    tmp<vectorField> nHat = this->patch().nf();
+    tmp<Field<Type>> rotated;
+
+    if constexpr (!is_rotational_vectorspace_v<Type>)
+    {
+        // Rotational-invariant type
+        rotated = this->patchInternalField();
+    }
+    else
+    {
+        tmp<vectorField> nHat(this->patch().nf());
+        rotated = transform(I - sqr(nHat), this->patchInternalField());
+    }
+
 
     Field<Type>::operator=
     (
-        lerp
-        (
-            transform(I - sqr(nHat), this->patchInternalField()),
-            refValue_,
-            valueFraction_
-        )
+        lerp(rotated, refValue_, valueFraction_)
     );
 
     parent_bctype::evaluate();
@@ -187,13 +201,24 @@ void Foam::partialSlipFvPatchField<Type>::evaluate
 template<class Type>
 Foam::tmp<Foam::Field<Type>>
 Foam::partialSlipFvPatchField<Type>::snGradTransformDiag() const
+{
+    // static_assert(!std::is_arithmetic_v<T>, "partial-slip with scalar??");
+    // if constexpr (!is_rotational_vectorspace_v<Type>)
+    // {
+    //     // Rotational-invariant type
+    //     return tmp<Field<Type>>::New(this->size(), Foam::zero{});
+    // }
+
     {
         tmp<vectorField> diag(cmptMag(this->patch().nf()));
 
         return
+        (
             valueFraction_*pTraits<Type>::one
           + (1.0 - valueFraction_)
-       *transformFieldMask<Type>(pow<vector, pTraits<Type>::rank>(diag));
+          * transformFieldMask<Type>(pow<vector, pTraits<Type>::rank>(diag))
+        );
+    }
 }
 
 

src/meshTools/AMIInterpolation/patches/cyclicACMI/cyclicACMIPointPatchField/cyclicACMIPointPatchField.H

@@ -177,7 +177,11 @@ public:
             //- Does the patch field perform the transformation
             virtual bool doTransform() const
             {
-                return (pTraits<Type>::rank && !cyclicACMIPatch_.parallel());
+                return
+                (
+                    is_rotational_vectorspace_v<Type>
+                 && !cyclicACMIPatch_.parallel()
+                );
             }
 
             //- Return face transformation tensor

src/meshTools/AMIInterpolation/patches/cyclicAMI/cyclicAMIPointPatchField/cyclicAMIPointPatchField.H

@@ -161,7 +161,7 @@ public:
         }
 
 
-    // Member functions
+    // Member Functions
 
         // Constraint handling
 
@@ -177,7 +177,11 @@ public:
             //- Does the patch field perform the transformation
             virtual bool doTransform() const
             {
-                return (pTraits<Type>::rank && !cyclicAMIPatch_.parallel());
+                return
+                (
+                    is_rotational_vectorspace_v<Type>
+                 && !cyclicAMIPatch_.parallel()
+                );
             }
 
             //- Return face transformation tensor