Numerical and Experimental Study of Trajectory for Free-Falling Jets

The water flow over a dam spillway has a lot of energy, and if this energy is not dissipated, the flowing water can cause damage to the dam and downstream structures. One way to dissipate this extra energy is to expel water jets into a plunging pool. In the present study, the trajectories of free-falling jets are investigated both numerically and experimentally. Ansys Fluent software is used for numerical simulation, and laboratory work is carried out in the hydraulic laboratory of University of Tabriz, Iran. The results showed that a free-falling jet in a laboratory experiment has a shorter trajectory than its calculated value using projectile equations and simulations using Ansys Fluent software; this difference is because of air resistance. By reducing the outlet cross section of the free-falling jet and increasing the head passing through the dam spillway, the free-falling jet reaches the ground at a greater distance from the dam body. Since none of the equations presented for calculating the trajectory of jets have been obtained using laboratory results and have not considered air resistance, they over estimate by 21%, the trajectory jet. In the present study, equations are provided to calculate the trajectory of a free-falling jet including air resistance. The simulation results also showed that the velocity experiences a 247% increase compared to the velocity at the end of the dam overflow, when hits the ground. This increase in velocity requires more attention in the design of stilling basins at the end of dams.