negative drag monitored, but positive in post pro
 

Have anybody encountered the same strange problem in CFX 11.0? When I was monitoring the drag force of a flapping airfoil, the drag was negative, however I stoped the calculation to postprocess it, the drag turned out to be positive.How come? The lift coefficient and the moment coefficient were alright and reasonable. Can anyone give me some advice how could this happen? The other problem is when I used cel areaInt(Wall Shear X)to obtain the wall shear, the error: ERROR The following unrecognised name was referenced: areaInt.. popped up. Is there something wrong with the expression? But I used it before and it was ok.

Re: negative drag monitored, but positive in post
 

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No one knows as to how could it happen? I am simulating a pithcing airfoil with CFX, but the monitored drag sometimes is negative and not reasonable. I get lift and moment well except drag. When the drag is negative I stop the simulation to post process it, surprisingly it is positive! Can anyone tell me?