add reference

bfps/DNS.py

@@ -319,6 +319,9 @@ class DNS(_code):
         return None
     def compute_Reynolds_stress_invariants(
             self):
+        """
+        see Choi and Lumley, JFM v436 p59 (2001)
+        """
         Rij = self.statistics['R_ij(t)']
         Rij /= (2*self.statistics['energy(t)'][:, None, None])
         Rij[:, 0, 0] -= 1./3