consistent bnd adv

fluid/fluid_problem.c

@@ -228,8 +228,8 @@ static void f_boundary(WeakBoundary *wbnd, FluidProblem *problem,const double *n
    else if(vid>0) unbnd = (unold+unext)/2;
    if (unbnd<0) {
      for (int id = 0; id < D; ++id) {
-       f0[U+id] += unbnd*((vid<0 ? 0 : data[vid+id])-u[id])*rho/c;
-       f00[(U+id)*n_fields+U+id] -= unbnd*rho/c;
+       f0[U+id] += (unbnd*(vid<0?0:data[vid+id])-unold*u[id])*rho/c;
+       f00[(U+id)*n_fields+U+id] -= unold*rho/c;
      }
    }
 }