عنوان مقاله [English]
Vertical drops reduce the flow velocity and increase the energy loss in steep-slope areas. In subsurface networks, the upstream channels in vertical drops are generally circular and the flow regime is supercritical; thus, the flow characteristics are significantly different from those of the rectangular vertical drops with upstream and downstream rectangular sections and subcritical flow. The present investigation aims to numerically and experimentally model a vertical drop with an upstream pipe and a downstream rectangular channel. A vertical drop with the height 0.345 m was built at the Hydraulic Laboratory, Department of Civil Engineering, Isfahan University of Technology, Isfahan, Iran. The diameter of the upstream pipe was 0.19 m and the width of the downstream channel was 0.4 m. A jet box was installed on the upstream pipe to create free surface flow with different Froude numbers and to make the upstream flow fully developed. The flow was also numerically simulated using OpenFOAM software. The interFOAM solver was utilized to solve the two-phase flow. This solver is a two-phase algorithm based on the Volume of Fluid (VOF) method. The brink, pool, and downstream depths and energy dissipation in the supercritical flow with Froude numbers ranging from 1 to 3.8 and relative discharges from 0.25 to 0.5 were investigated. In the numerical model, the fixed velocity was taken into account for the inlet flow at the upstream boundary. For the bottom and sidewalls of the channel, the non-slip
boundary condition was considered. At the upper boundary of the simulation
domain, the boundary condition of the atmospheric pressure was taken into
account. The downstream boundary condition was set to zero gradients for all
parameters. The free surface was supposed to be an isosurface with a volume
fraction of 0.5. Given that a number of parameters were calculated in numerical
simulations, a code was written in the Octave programming language to facilitate calculations. The results showed that the brink depth was approximately 80% of the upstream depth and the difference increased as the Froude number increased. The pool depth was less than that of the rectangular vertical drop. Moreover, by increasing the relative discharge, both pool and downstream depths would increase. The relative head loss of the present model ranged from 50 t0 70% and was approximately 50% higher than that of the rectangular vertical drop. The numerical simulation results are in good agreement with the laboratory observations.