Merge branch 'feature/less-pow-in-thermal-conductivity-laws' into 'master'

Half number of std::pow calls in thermal conductivity laws

I'd like to back-port this to 2.10 if it proofs itself.

See merge request !226

dumux/material/fluidmatrixinteractions/2p/thermalconductivityjohansen.hh

@@ -128,7 +128,8 @@ public:
 
         const Scalar kappa = 15.6; // fitted to medium quartz sand
         const Scalar rhoBulk = rhoSolid*porosity;
-        const Scalar lSat = std::pow(lambdaSolid, (1.0 - porosity)) * std::pow(lambdaW, porosity);
+        // lambdaSolid^(1-porosity) * lambdaW^porosity =
+        const Scalar lSat = lambdaSolid * std::pow(lambdaW / lambdaSolid, porosity);
         const Scalar lDry = (0.135*rhoBulk + 64.7)/(rhoSolid - 0.947*rhoBulk);
         const Scalar Ke = (kappa*satW)/(1+(kappa-1)*satW);// Kersten number, equation 13
 

dumux/material/fluidmatrixinteractions/2p/thermalconductivitysomerton.hh

@@ -119,9 +119,9 @@ public:
                                                const Scalar rhoSolid = 0.0 /*unused*/)
     {
         const Scalar satW = std::max<Scalar>(0.0, sw);
-        // geometric mean
-        const Scalar lSat = std::pow(lambdaSolid, (1.0 - porosity)) * std::pow(lambdaW, porosity);
-        const Scalar lDry = std::pow(lambdaSolid, (1.0 - porosity)) * std::pow(lambdaN, porosity);
+        // geometric mean, using ls^(1-p)*l^p = ls*(l/ls)^p
+        const Scalar lSat = lambdaSolid * std::pow(lambdaW / lambdaSolid, porosity);
+        const Scalar lDry = lambdaSolid * std::pow(lambdaN / lambdaSolid, porosity);
 
         return lDry + std::sqrt(satW) * (lSat - lDry);
     }