raise gravity term interpolation degree by one

LDDsimulation/domainPatch.py

@@ -1165,12 +1165,14 @@ class DomainPatch(df.SubDomain):
 
         for phase in has_phases:
             rho = df.Constant(self.densities[phase])
+            # V = self.pressure[phase]
+            # element = V.ufl_element()
             self.gravity_term.update(
                 {phase: df.Expression('g*rho*x[1]',
                                       domain=self.mesh, #
-                                      degree=self.interpolation_degree,
-                                      g=g,
-                                      rho=rho
+                                      degree=self.interpolation_degree+1,
+                                      g=self.gravity_acceleration,
+                                      rho=self.densities[phase]
                                       )}
             )