*VARIATION OF AIR CONCENTRATION IN CLASSICAL HYDRAULIC JUMP

Abstract

The sudden transition from a high-velocity, supercritical flow into slow-moving subcritical flow is a hydraulic jump. In high velocity flow, air is entrained at free surface flow. Such a flow is .characterized by a sudden rise of the free surface, with air entrainment and energy dissipation, waves and spray and presence of air bubble within the flow increases substantially the amount of air-water interface zone and enhance the transfer of air and water between the flow and atmosphere. Cavitation pitting may be reduced or eliminated by protecting the surface either by streamlining the boundaries, or improving the surface finishes, or using cavitations-resistant materials. Cavitation pitting can also be prevented by providing an air concentration of 5% to 10% near the spillway face. An experimental study of variation of air concentration along a hydraulic jump has been carried out for approaching flows with Froude number ranging from 2.54 to 7.26. The experimental study includes the calibration of conductivity probe and its use for measuring air concentration in hydraulic jump formed in a rectangular channel. The observed pattern of air entrainment shows that air is entrained rapidly in the initial portion of the jump on the surface and transported to the other region by the turbulent fluctuations and finally released gradually in the latter portion of the jump. A relationship has been established between the Froude number and maximum mean air concentration. The effect of depth of the water level along the length of jump and the corresponding maximum mean air concentration has been studied. Analysis of the variations of air concentration, the ratio of air discharge to water discharge and the mean air concentration distribution along the length of the jump has been carried out. Based on this analysis, a relationship has been proposed between the maximum mean air concentration and Froude number