transient analysis diverges

[neri alamanni]

Hi all
i setup my first CFD transient run. The sif file is read and processed correctely, but after some iterations it diverges abruptly giving wrong values for velocity and pressure. The time dependence is very easy: a sinusoid (sin(2*pi*tx)). The step size is small (0.02 sec) in comparison with the time scale. Probably there is an error in the solver setup (Solver method, rather preconditioning). Attached you can find the model.
Thank you in advance
best regards
neri alamanni