Some Aspects of the Flow of Stratified Fluids: II. Experiments with a Two-Fluid System

A description is given of the flow of two superimposed layers of fluid over a barrier. This represents a partial experimental investigation of a problem considered theoretically in Part I. In general three regimes of motion are possible: If the velocities of the fluids are sufficiently small the interface is little disturbed except for a slight depression over the barrier. If the velocities are sufficiently high the interface swells symmetrically over the obstacle. At intermediate speeds a hydraulic jump occurs in the lee of the barrier and the lower layer increases in depth upstream. Two occurrences do not fit into the above description: If the obstacle is small compared to the depth of the lower layer, weak lee waves appear at low speeds, increasing in amplitude as the approach velocity of the fluid is increased. This seems to be the only case in which perturbation theory provides an adequate prediction of the flow. The second anomalous occurrence is the appearance of a “jump down” or hydraulic “drop” in the lee when the speed of the fluid is moderately high, the obstacle large, and the upper fluid relatively thin. The description of the experiments is supplemented by a theoretical discussion, employing the assumption of a hydrostatic pressure distribution. In general this theory provides a satisfactory explanation of the observed behavior. The paper concludes with a discussion of meteorological implications.