Merge branch 'feature/speedup-h2oair-viscosity' into 'master'

[h2oair] Use sqrt instead of pow to speedup viscosity evaluation

test_stokes2c with refined grid speeds up by 2.7%.

See merge request !192

dumux/material/fluidsystems/h2oair.hh

@@ -493,11 +493,9 @@ public:
                     Scalar divisor = 0;
                     for (int j = 0; j < numComponents; ++j)
                     {
-                        Scalar phiIJ = 1 + sqrt(mu[i]/mu[j]) * // 1 + (mu[i]/mu[j]^1/2
-                            pow(M[j]/M[i], 1./4.0);   // (M[i]/M[j])^1/4
-
-                        phiIJ *= phiIJ;
-                        phiIJ /= sqrt(8*(1 + M[i]/M[j]));
+                        // 1 + (mu[i]/mu[j]^1/2 * (M[i]/M[j])^1/4)
+                        Scalar phiIJ = 1 + sqrt(mu[i]/mu[j] * sqrt(M[j]/M[i]));
+                        phiIJ = phiIJ * phiIJ / sqrt(8*(1 + M[i]/M[j]));
                         divisor += fluidState.moleFraction(phaseIdx, j)*phiIJ;
                     }
                     muResult += fluidState.moleFraction(phaseIdx, i)*mu[i] / divisor;