I try to simulate the flow over a plunge NACA0012 using the SIMPLE algorithm and rigid moving grid with the airfoil. but it is hard to converge and the Cl & Cd are much larger than experiment data. who has experience about this problem?

ok as u said convergence problems it could be worked out with the underrelaxtion factors (decreasing them too much can lead large computational time) but did u check all other conditions whether given properly even if mesh is too much refine this convergence problems come into field so try better with coarse grid to start off,u havent mentioned the flow solver u used.

Is it ur problem is unsteady with moving mesh can u define ur problem more clearly .

if Cl ,Cd are monitoring parameters than they would finally settle down to correct values intially though they may fluctuate .

Have you validate your code!

Thanks! I use a in-house code that was validate many times before calculation this case.