Physical and numerical model studies for lower and upper nappe profiles of sharp-edged large orifice

Abstract

Large orifices are generally used in the dam to pass the surplus water from upstream to downstream as well as for flushing of sediments from the reservoirs. The structure with large orifice in a dam is known as orifice spillway. Literature available on the profiles of orifice spillway is scanty. Several attempts have been made to study coefficient of discharge and flow upstream of orifices. On the contrary, little efforts have been made to study the lower as well as upper nappe profiles of jet through large orifices. The studies of profiles may be useful for deriving an equation for design of the roof and bottom profiles of an orifice spillway. Hence, basic research has been conducted to examine the lower and the upper profiles of a sharp-edged large orifice. Number of physical and numerical model studies were conducted for various orifice heights and operating heads. The numerical model was verified in terms of grid convergence and selection of turbulence models. The experimental data were used to validate the numerical model results. The grid size of 0.004 m and RNG k–ε turbulence model is found to be suitable in predicting flow through sharp-edged large orifice in respect of coefficient of discharge as well as upper and lower nappe profiles. The present study confirms the coefficient of discharge of large orifice available in literature. The results of present study indicate that the lower nappe water surface profile obtained may be useful for determining the bottom profile of orifice spillway. However, results in respect of upper nappe profile indicated the importance of solid spillway bottom profile in determining the roof profile of an orifice spillway.