steady solution, the lift fluctuate,but unsteady method,the lift is convergent
 

case: flow over a naca0012 airfoil, re=20000, cfx, sst,AOA=10deg. In my simulation, when using steady solution, the monitor lift is fluctuating,I think it is right. But when I switch it in unsteady, and want to capture the wortex shedding, unfortunatly, the lift seems convergent. Y+ <1, time step is 0.0001, I can't find the reason. Can anyone help me? Thankyou.

FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

This sounds a case where you need to make sure your time discretisation schemes are accurate. Ensure your time step size is suitable (by a sensitivity check) and you are using second order time differencing.

ghorrocks,Thankyou for your reply.
The transient scheme is second order backward euler. The time step 0.001,0.0001,0.00005,0.00001 have already tried,but there is still no vortex shedding.

A naca0012 airfoil at Re=20000 will have a large amount of laminar boundary layer on it. Are you using the turbulence transition model?

Also: at that Re the transient behaviour is probably a laminar separation bubble, not vortex shedding. This means it is not shedding vorticies into the flow but the separation bubble jiggling about.

Yes, I use Gamma model transitional turbulence.It is really a laminar separation bubble.As you say, the bubble jigging, so the lift can also jiggling. but i can not observe that in my simulation.( why steady the lift is fluctuating? i still puzzled)

Have you checked the drag? It is probably moving more.

How much do you expect the lift to move around by?

The drag is the same with lift. The flowfield is almost the same between 1.0s and 1.5s. Actually, the lift change very very small, for example, the lift is 0.6472 during more than 0.5s, only the fifth digit after the decimal point is changing. In steady solution ,the flowfield changing obviously.

So you are modelling the motion of the laminar separation bubble. That is what you wanted to do isn't it?

Yes, but using transient solution, the laminar separation bubble is steady. I think there is must something wrong this my solution.

Yes, I agree. My suggestions are in my post #2.