Hydraulic characteristics of flow at grid drop-type dissipators with the upstream supercritical approach flow

Abstract

Through installing a grid drop-type dissipator at the edge of the vertical drops with an upstream supercritical approach flow, the present study investigates hydraulic characteristics of flow up- and downstream of the drop, based on model experimentation. Experiments were conducted on three different drop heights, considering eight different grid drop-type dissipators with certain grids dimensions. Results demonstrate that assembling this type of dissipator at the edge of a vertical drop increases the rate of energy dissipation up to about 70%, compared to a plain vertical drop. Owing to the increase in flow momentum and the falling jets’ collision with bottom of the downstream channel, for a given flow discharge, while the porosity of the grid drop-type dissipator is increased, the wetted length of the dissipator and the energy dissipation decrease; however, the length and depth of the pool at the drop base increase. The relative length of the pool is significantly affected by air entrainment, caused by the spaces between the separated falling jets. By increasing flow discharge, pressure fluctuations decrease and trend of the variation of average pressure becomes smoother. Although the former researchers reported a hydraulic jump downstream of the grid drop-type dissipators with an upstream subcritical approach flow, it was not the case in some tests of the present study with an upstream supercritical approach flow. Finally, based on statistical and error analyses, empirical relationships were derived to predict the flow hydraulic characteristics, to be used by design engineers in practice.