Verfication problem in cd/cl - advice required

CH

hello,

i'm trying to simulate 2d pitching/heaving airfoil. I've verified my code with oscillating cylinder and it works.

next i tried to verify my code against some published results on pitching/heaving airfoil. the problem is that although the cl compares relatively well, the cd does not. some are close but others are quite far off by 50-100%.

i wonder what could be the problem. i've tried to reduce the time step as well as refine my grid but the result does not change much.

at this moment, i wonder how i should proceed. can anyone give me some advice?

thank you.