Free-surface flows with near-critical flow conditions

Open channel flow situations with near-critical flow conditions are often characterised by the development of free-surface instabilities (i.e. undulations). The paper develops a review of several near-critical flow situations. Experimental results are compared with ideal-fluid flow calculations. The analysis is completed by a series of new experiments. The results indicate that, for Froude numbers slightly above unity, the free-surface characteristics are very similar. However, with increasing Froude numbers, distinctive flow patterns develop.