How to deal with BC with discontinuity?

HT

Hi, does anyboy have experience using boundary conditions with discontinuity? In my case, non-slip BC is imposed, but as a certain criterion is met, the BC will switch to slip BC. The switch between the 2 kinds of BCs is discontineous regard to time (actually, also regard to space, since a non-slip node may have a slip node).

The simulaiton shows the flow to be unsteady in an oscillating way. But the oscillations seem to be random and their frequecy cannot be controled.

Any one has such experience or an idea? Should the discontinuity (a sigularty?) be smoothed away from the BC?

Thanks a lot for your information?

HT