EXPERIMENTS ON WAVE BREAKING IN STRATIFIED FLOW OVER OBSTACLES

Towing-tank experiments on linearly stratified flow over three-dimensi onal obstacles of various shapes are described. Particular emphasis is given to the parameter regimes which lead to wave breaking aloft, the most important of which is the Froude number defined by F(h) = U/Nh, where U, N and h are the flow speed, the Brunt-Vaisala frequency and t he hill height, respectively. The effects of other parameters, princip ally K (= ND/piU, where D is the fluid depth) and the spanwise and lon gitudinal aspect ratios of the hill, on wave breaking are also demonst rated. It is shown that the Froude-number range over which wave breaki ng occurs is generally much more restricted than the predictions of li near (hydrostatic) theories would suggest; nonlinear (Long's model) th eories are in somewhat closer agreement with experiment. The results a lso show that a breaking wave aloft can exist separately from a furthe r recirculating region downstream of the hill under the second lee wav e, but that under certain circumstances these can interact to form a m assive turbulent zone whose height is much greater than h. Previous th eories only give estimates for the upper critical F(h), below which br eaking occurs; the experiments also reveal lower critical values, belo w which there is no wave breaking.