fvOptions: New buoyancyForce and buoyancyEnergy

Provides run-time selection of buoyancy sources for compressible solvers

Replaces the built-in buoyancy sources in XiFoam, reactingFoam and
rhoReactingFoam.

e.g. in constant/fvOptions specify

momentumSource
{
    type            buoyancyForce;

    buoyancyForceCoeffs
    {
        fieldNames      (U);
    }
}

and optionally specify the buoyancy energy source in the enthalpy
equation:

energySource
{
    type            buoyancyEnergy;

    buoyancyEnergyCoeffs
    {
        fieldNames      (h);
    }
}

or internal energy equation

energySource
{
    type            buoyancyEnergy;

    buoyancyEnergyCoeffs
    {
        fieldNames      (e);
    }
}

applications/solvers/combustion/XiFoam/UEqn.H

@@ -6,8 +6,7 @@
       + MRF.DDt(rho, U)
       + turbulence->divDevRhoReff(U)
      ==
-        rho*g
-      + fvOptions(rho, U)
+        fvOptions(rho, U)
     );
 
     UEqn.relax();

applications/solvers/combustion/XiFoam/XiFoam.C

@@ -70,7 +70,6 @@ int main(int argc, char *argv[])
     pimpleControl pimple(mesh);
 
     #include "readCombustionProperties.H"
-    #include "readGravitationalAcceleration.H"
     #include "createFields.H"
     #include "createMRF.H"
     #include "createFvOptions.H"

applications/solvers/combustion/reactingFoam/EEqn.H

@@ -17,8 +17,7 @@
         )
       - fvm::laplacian(turbulence->alphaEff(), he)
      ==
-        rho*(U&g)
-      + reaction->Sh()
+        reaction->Sh()
       + fvOptions(rho, he)
     );
 

applications/solvers/combustion/reactingFoam/UEqn.H

@@ -8,8 +8,7 @@ tmp<fvVectorMatrix> UEqn
   + MRF.DDt(rho, U)
   + turbulence->divDevRhoReff(U)
  ==
-    rho*g
-  + fvOptions(rho, U)
+    fvOptions(rho, U)
 );
 
 UEqn().relax();

applications/solvers/combustion/reactingFoam/reactingFoam.C

@@ -51,7 +51,6 @@ int main(int argc, char *argv[])
     #include "createTimeControls.H"
     #include "createRDeltaT.H"
     #include "initContinuityErrs.H"
-    #include "readGravitationalAcceleration.H"
     #include "createFields.H"
     #include "createMRF.H"
     #include "createFvOptions.H"

applications/solvers/combustion/reactingFoam/rhoReactingFoam/rhoReactingFoam.C

@@ -52,7 +52,6 @@ int main(int argc, char *argv[])
     #include "createTimeControls.H"
     #include "createRDeltaT.H"
     #include "initContinuityErrs.H"
-    #include "readGravitationalAcceleration.H"
     #include "createFields.H"
     #include "createMRF.H"
     #include "createFvOptions.H"