fix occasional NaNs in Oldroyd-B test

lbmpy_tests/test_oldroydb.py

@@ -132,6 +132,7 @@ def test_oldroydb():
         assert len(bh._boundary_object_to_boundary_info) == 1, "Restart kernel to clear boundaries"
 
     def init():
+        dh.fill(ρ.name, np.nan, ghost_layers=True, inner_ghost_layers=True)
         dh.fill(ρ.name, 1)
         dh.fill(u.name, np.nan, ghost_layers=True, inner_ghost_layers=True)
         dh.fill(u.name, 0)
@@ -139,9 +140,12 @@ def test_oldroydb():
         dh.fill(tau.name, 0)
         dh.fill(tauflux.name, np.nan, ghost_layers=True, inner_ghost_layers=True)
         dh.fill(tauface.name, np.nan, ghost_layers=True, inner_ghost_layers=True)
+        dh.fill(F.name, np.nan, ghost_layers=True, inner_ghost_layers=True)
+        dh.fill(F.name, 0) # needed for LB initialization
     
         sync_tau() # force calculation inside the initialization needs neighbor taus
         dh.run_kernel(init_kernel)
+        dh.fill(F.name, np.nan)
 
 
     # In[7]: