It should be emphasized that the mixing plane model is a reasonable approximation so long as there is not significant reverse flow in the vicinity of the mixing plane. If significant reverse flow occurs, the mixing plane will not be a satisfactory model of the actual flow. In a numerical simulation, reverse flow often occurs during the early stages of the computation even though the flow at convergence is not reversed. Therefore, it is helpful in these situations to first obtain a provisional solution using fixed conditions at the rotor-stator interface. The mixing plane model can then be enabled and the solution run to convergence.
Under-relaxing the changes in the mixing plane boundary values can also help in troublesome situations. In many cases, setting the under-relaxation factor to a value less than 1 can be helpful. Once the flow field is established, you can gradually increase the under-relaxation factor.