Unsteady computation using sliding mesh
 

In solver Fluent 5.3 I am trying to use the multiple rotating frames of reference or the sliding mesh. In my rotor-draft tube interaction the back flow passes through the rotating-stationary interface. If I computed this task as a steady, no problem occurred, convergence facilities were good. When I switch on the unsteady computation, the solution start to diverge after five time steps. The behavior is independent on the time step size. This unsteady computation converged well in turbine operation point, where the back flow did not appear in the rotor-draft tube interface. Is there any solution or advice?