functionObjects::subtract: From the first field subtract the remaining fields in the list

The operation can be applied to any volume or surface fields generating a
    volume or surface scalar field.

    Example of function object specification:
    \verbatim
    Tdiff
    {
        type            subtract;
        libs            ("libfieldFunctionObjects.so");
        fields          (T Tmean);
        result          Tdiff;
        executeControl  writeTime;
        writeControl    writeTime;
    }
    \endverbatim

src/functionObjects/field/subtract/*grep*

@@ -0,0 +1,9 @@
+-*- mode: grep; default-directory: "~/OpenFOAM/OpenFOAM-dev/src/functionObjects/field/subtract/" -*-
+Grep started at Fri Nov 18 21:41:56
+
+grep --color -nH -e nitude *
+subtract.H:31:    Calculates the subtract of a field.
+subtract.H:66:        //- Calculate the subtract of the field and register the result
+subtract.H:70:        //- Calculate the subtract of the field and return true if successful
+
+Grep finished (matches found) at Fri Nov 18 21:41:56

src/functionObjects/field/subtract/subtract.C

@@ -0,0 +1,86 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2016 OpenFOAM Foundation
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+\*---------------------------------------------------------------------------*/
+
+#include "subtract.H"
+#include "addToRunTimeSelectionTable.H"
+
+// * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
+
+namespace Foam
+{
+namespace functionObjects
+{
+    defineTypeNameAndDebug(subtract, 0);
+    addToRunTimeSelectionTable(functionObject, subtract, dictionary);
+}
+}
+
+
+// * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
+
+bool Foam::functionObjects::subtract::calc()
+{
+    bool processed = false;
+
+    processed = processed || calcSubtract<scalar>();
+    processed = processed || calcSubtract<vector>();
+    processed = processed || calcSubtract<sphericalTensor>();
+    processed = processed || calcSubtract<symmTensor>();
+    processed = processed || calcSubtract<tensor>();
+
+    return processed;
+}
+
+
+// * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
+
+Foam::functionObjects::subtract::subtract
+(
+    const word& name,
+    const Time& runTime,
+    const dictionary& dict
+)
+:
+    fieldsExpression(name, runTime, dict)
+{
+    read(dict);
+
+    if (fieldNames_.size() < 2)
+    {
+        FatalIOErrorInFunction(dict)
+            << type() << " requires at least 2 fields only "
+            << fieldNames_.size() << " provided: " << fieldNames_
+            << exit(FatalIOError);
+    }
+}
+
+
+// * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //
+
+Foam::functionObjects::subtract::~subtract()
+{}
+
+
+// ************************************************************************* //

src/functionObjects/field/subtract/subtract.H

@@ -0,0 +1,130 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2016 OpenFOAM Foundation
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Class
+    Foam::functionObjects::subtract
+
+Group
+    grpFieldFunctionObjects
+
+Description
+    From the first field subtract the remaining fields in the list.
+
+    The operation can be applied to any volume or surface fields generating a
+    volume or surface scalar field.
+
+    Example of function object specification:
+    \verbatim
+    Tdiff
+    {
+        type            subtract;
+        libs            ("libfieldFunctionObjects.so");
+        fields          (T Tmean);
+        result          Tdiff;
+        executeControl  writeTime;
+        writeControl    writeTime;
+    }
+    \endverbatim
+
+See also
+    Foam::functionObjects::fieldsExpression
+    Foam::functionObjects::fvMeshFunctionObject
+
+SourceFiles
+    subtract.C
+
+\*---------------------------------------------------------------------------*/
+
+#ifndef functionObjects_subtract_H
+#define functionObjects_subtract_H
+
+#include "fieldsExpression.H"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+namespace functionObjects
+{
+
+/*---------------------------------------------------------------------------*\
+                           Class subtract Declaration
+\*---------------------------------------------------------------------------*/
+
+class subtract
+:
+    public fieldsExpression
+{
+    // Private Member Functions
+
+        //- Subtract the list of fields of the specified type
+        //  and return the result
+        template<class GeoFieldType>
+        tmp<GeoFieldType> subtractFields() const;
+
+        //- Subtract the list of fields and register the result
+        template<class Type>
+        bool calcSubtract();
+
+        //- Subtract the list of fields and return true if successful
+        virtual bool calc();
+
+
+public:
+
+    //- Runtime type information
+    TypeName("subtract");
+
+
+    // Constructors
+
+        //- Construct from Time and dictionary
+        subtract
+        (
+            const word& name,
+            const Time& runTime,
+            const dictionary& dict
+        );
+
+
+    //- Destructor
+    virtual ~subtract();
+};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+} // End namespace functionObjects
+} // End namespace Foam
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+#ifdef NoRepository
+    #include "subtractTemplates.C"
+#endif
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+#endif
+
+// ************************************************************************* //

src/functionObjects/field/subtract/subtractTemplates.C

@@ -0,0 +1,79 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2016 OpenFOAM Foundation
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+\*---------------------------------------------------------------------------*/
+
+#include "volFields.H"
+#include "surfaceFields.H"
+
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+template<class GeoFieldType>
+Foam::tmp<GeoFieldType>
+Foam::functionObjects::subtract::subtractFields() const
+{
+    tmp<GeoFieldType> tresult
+    (
+        lookupObject<GeoFieldType>(fieldNames_[0])
+      - lookupObject<GeoFieldType>(fieldNames_[1])
+    );
+
+    for (label i=2; i<fieldNames_.size(); i++)
+    {
+        tresult = tresult - lookupObject<GeoFieldType>(fieldNames_[i]);
+    }
+
+    return tresult;
+}
+
+
+template<class Type>
+bool Foam::functionObjects::subtract::calcSubtract()
+{
+    typedef GeometricField<Type, fvPatchField, volMesh> VolFieldType;
+    typedef GeometricField<Type, fvsPatchField, surfaceMesh> SurfaceFieldType;
+
+    if (foundObject<VolFieldType>(fieldNames_[0]))
+    {
+        return store
+        (
+            resultName_,
+            subtractFields<VolFieldType>()
+        );
+    }
+    else if (foundObject<SurfaceFieldType>(fieldNames_[0]))
+    {
+        return store
+        (
+            resultName_,
+            subtractFields<SurfaceFieldType>()
+        );
+    }
+    else
+    {
+        return false;
+    }
+}
+
+
+// ************************************************************************* //