XiFoam: Added relaxation statements to Su and Xi equations

This makes the solver more robust when the "Courant number" of the flame
propagation fluctuates.

applications/solvers/combustion/XiFoam/bEqn.H

@@ -134,7 +134,7 @@ if (ign.ignited())
             (sigmas*SuInf*(Su0 - SuInf) + sqr(SuMin)*sigmaExt)
            /(sqr(Su0 - SuInf) + sqr(SuMin));
 
-        solve
+        fvScalarMatrix SuEqn
         (
             fvm::ddt(rho, Su)
           + fvm::div(phi + phiXi, Su, "div(phiXi,Su)")
@@ -144,6 +144,9 @@ if (ign.ignited())
           - fvm::SuSp(rho*(sigmas + Rc), Su)
         );
 
+        SuEqn.relax();
+        SuEqn.solve();
+
         // Limit the maximum Su
         // ~~~~~~~~~~~~~~~~~~~~
         Su.min(SuMax);
@@ -196,7 +199,7 @@ if (ign.ignited())
 
         // Solve for the flame wrinkling
         // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-        solve
+        fvScalarMatrix XiEqn
         (
             fvm::ddt(rho, Xi)
           + fvm::div(phi + phiXi, Xi, "div(phiXi,Xi)")
@@ -215,6 +218,8 @@ if (ign.ignited())
             )
         );
 
+        XiEqn.relax();
+        XiEqn.solve();
 
         // Correct boundedness of Xi
         // ~~~~~~~~~~~~~~~~~~~~~~~~~

tutorials/combustion/XiFoam/les/pitzDaily3D/system/fvSolution

@@ -48,5 +48,9 @@ PISO
     nNonOrthogonalCorrectors 0;
 }
 
+relaxationFactors
+{
+    "(Xi|Su)"     1;
+}
 
 // ************************************************************************* //