Investigating Discharge Behaviour of a Sharp-Edged Circular Orifice Flow under Low Head from Unsteady Experimental Procedure and Computational Fluid Dynamics

Abstract

A circular sharp-edged orifice is a hydraulic flow device used to measure and control the outflow from channels, detention ponds, reservoirs and tanks. In this study experimentation was carried out at the Soil and Water Conservation Laboratory, University of Ibadan to investigate the discharge coefficient Cd of circular sharp-edged orifice at low head. The experiment was conducted using four different orifice diameters (4.5, 6.0, 7.5, 9.0cm). Flow characteristics in Cd for each orifice diameter were observed. A relationship between Cd and likely independent variables that could affect Cd was determined from dimensional analysis. Simple and multiple regression models of these variables were established. The simulation experimentation of the flow was carried out using COMSOL Multiphysics with Navier-Stokes equation as the governing equation. Data were analysed using ANOVA at α0.05.It was found that the Cd increased as head reduced, and higher increase was observed at lower head. Coefficient of determination (R2) showed that 99.3% variability in Cd is due to Reynolds number-Re and Froude’s number-Fr but only significant at 4.5cm orifice diameter (P < 0.05). Simulation results were similar in trend to experimental results; however, 4.5cm orifice diameter had the highest Root Mean Square Error (RMSE) of 1.9877 for Cd and 0.029 for Fr and minimal errors observed as the diameter increased. This study demonstrated that Cd cannot be taken as a constant value when dealing with low head; however, the simulation was able to forecast the trend but could not predict the exact values.Keywords: Orifice, Computational Fluid Dynamics, Navier-Stokes equations, Discharge Coefficient, Open Channel flow.