UPSTREAM SEPARATION IN A ROTATING CHANNEL FLOW

The question of upstream separation of a current of dense rotating flu id, below a stagnant layer of lower density, that is critically contro lled by a weir is addressed in laboratory experiments that follow from theoretical calculations. For a constant potential vorticity flow in a gradually varying channel, it is shown that a range of parameters le ad to upstream separation. In all cases the flow reattaches before rea ching the sill crest, so that a closed gyre is produced. In laboratory experiments separation and reattachment was observed in numerous case s where the theory predicts closed gyres. Although the shape and size of the gyre are in accordance with theory, the circulation within the gyre differs markedly from theoretical predictions. The mechanism that feeds energy into the gyre appears to be an important unsolved proble m. The results indicate that upstream separation is a fact and that th e dynamics of the gyre is significant, so the gyre dynamics may modify the volume flux of fluid over the weir.