Bug fix: s -> eta*s

dune/tectonic/nicefunction.hh

@@ -51,18 +51,26 @@ class RuinaFunction : public NiceFunction {
     y *= coefficient;
   }
 
+  /*
+    (leaving some terms aside): with s > rho
+
+    1/eta d/dx [ a * (s log s - s) + mu s ] where s = eta x
+    = 1/eta [ a * (log (eta x) * eta) + eta mu ]
+    = a * log(eta x) + mu
+  */
   double virtual leftDifferential(double s) const {
     if (eta * s <= rho)
       return 0;
 
-    return coefficient * normalStress * (a * std::log(s) + mu);
+    return coefficient * normalStress * (a * std::log(eta * s) + mu);
   }
 
+  /* see above */
   double virtual rightDifferential(double s) const {
     if (eta * s <= rho)
       return 0;
 
-    return coefficient * normalStress * (a * std::log(s) + mu);
+    return coefficient * normalStress * (a * std::log(eta * s) + mu);
   }
 
 private: