[Cleanup] Fix sign of normal stress

dune/tectonic/frictionpotential.hh

@@ -31,7 +31,7 @@ class FrictionPotentialWrapper {
 class FrictionPotential : public FrictionPotentialWrapper {
 public:
   FrictionPotential(double coefficient, FrictionData const &fd)
-      : fd(fd), weightTimesNormalStress(coefficient * fd.normalStress) {}
+      : fd(fd), weightTimesNormalStress(coefficient * (-fd.normalStress)) {}
 
   // log(V/V_m)  if V >= V_0
   // 0           otherwise

src/one-body-sample.cc

@@ -261,7 +261,7 @@ int main(int argc, char *argv[]) {
       // volume * gravity * density / area    = normal stress
       // V      * g       * rho     / A       = sigma_n
       // m^d    * N/kg    * kg/m^d  / m^(d-1) = N/m^(d-1)
-      normalStress = volume * gravity * density / area;
+      normalStress = -volume * gravity * density / area;
     }
     FrictionData const frictionData(parset.sub("boundary.friction"),
                                     normalStress);