Add support for the two-schemes localBlended discretization scheme. (#577)

* Add support for the localBlended scheme. Fix #575.

    - The `UBlend` volScalarField should be exist in the 0 folder.
      This field can be initialized using setFields or funkySetFields.

    - Although we only support the localBlended scheme for div(phi,U),
      it can be easily extent to other variables.

    - Reference: 
       https://www.cfd-online.com/Forums/openfoam-programming-development/105843-localblended.html#post544480

* Fix errors in createLocalBlend.

Author: TimoLin

applications/solvers/dfLowMachFoam/createFields.H

@@ -115,6 +115,8 @@ rho = thermo.rho();
 Info<< "Creating field kinetic energy K\n" << endl;
 volScalarField K("K", 0.5*magSqr(U));
 
+#include "createLocalBlend.H"
+
 multivariateSurfaceInterpolationScheme<scalar>::fieldTable fields;
 
 #include "createMRF.H"
@@ -206,4 +208,4 @@ const Switch splitting = CanteraTorchProperties.lookupOrDefault("splittingStrate
 #ifdef USE_LIBTORCH
     const Switch log_ = CanteraTorchProperties.subDict("TorchSettings").lookupOrDefault("log", false);
     const Switch torch_ = CanteraTorchProperties.subDict("TorchSettings").lookupOrDefault("torch", false);
-#endif
+#endif

applications/solvers/dfLowMachFoam/createLocalBlend.H

@@ -0,0 +1,73 @@
+/*---------------------------------------------------------------------------*\
+This function is designed to read the UBlendingFactor for localBlended  
+schemes used by the divSchemes, i.e.:
+```
+divSchemes
+{
+    div(phi,U)        Gauss localBlended linear upwind;
+}
+```
+\*---------------------------------------------------------------------------*/
+
+// Find and parse the divSchemes sub-dictionary
+const dictionary& divSchemesDict = mesh.schemesDict().subDict("divSchemes");
+
+Switch foundLocalBlended = false;
+
+// Parse the divSchemes tokens
+wordList keys = divSchemesDict.toc();
+forAll(keys, i)
+{
+    const word& key = keys[i];
+    // Convert to string
+    ITstream& is = divSchemesDict.lookup(key);
+    OStringStream os;
+    os << is;
+    word schemeStr = os.str();
+
+    if (schemeStr.find("localBlended") != word::npos)
+    {
+        foundLocalBlended = true;
+    }
+}
+
+// Declare pointer for UBlendingFactor
+autoPtr<surfaceScalarField> UBlendingFactorPtr;
+
+// Create the localBlendingFactor field only if localBlended scheme is found
+if (foundLocalBlended)
+{
+    Info << " Creating UBlendingFactor field for localBlended schemes " << endl;
+
+    // Read the scalarField UBlend 
+    volScalarField UBlend
+    (
+        IOobject
+        (
+            "UBlend",
+            mesh.time().timeName(),
+            mesh,
+            IOobject::MUST_READ,
+            IOobject::NO_WRITE
+        ),
+        mesh
+    );
+
+    // Construct the surfaceScalarField UBlendingFactor by interpolating UBlend to cell faces
+    UBlendingFactorPtr.reset
+    (
+        new surfaceScalarField
+        (
+            IOobject
+            (
+                "UBlendingFactor",
+                mesh.time().timeName(),
+                mesh,
+                IOobject::READ_IF_PRESENT,
+                IOobject::NO_WRITE,
+                true
+            ),
+            fvc::interpolate(UBlend)
+        )
+    );
+}