i want to know how can a moving cylinder in a stationary fluid can be modeled by fluent and what is boundary condition?

if u mean rotating cylinder on its axis ..you can use sliding mesh approach ...

In my opinion you just give on cylinder wall(boundary condition)the rotational velocity. and that's all.

thanks for your reply, i simulated a flow passes over a cylinder and the answers from fluent were correct, but the answers frome a cylinder that moves in a stationary flow were not equale with later model, how can i model a cylinder that moves to one direction and what are boundary condition?

You should use the dynamic mesh to make your cilinder obscillating. You can impose the movement of the wall using a UDF. It's quite easy especially bacause you consider the cilinder undeformable.Luca

The source of the discrepancy is most likely due to the fact that Fluent throws out the normal component of wall motion when you give it a moving wall boundary condition. Plot contours of the velocity component in the direction of the cylinder's motion to see that this is true. So the only options are to solve for relative motion, as you have done (good for "steady state" solution) or use the deformble mesh approach - forces unsteady solution, although you can let it achieve a constant solution to obtain a steady result.