Hydraulic Properties and Reductions of Force and Vibration of Flow on Drops by Dampers
Mohammad Reza Chamani
In irrigation channels and storm water systems, structures are used to offset the elevation difference between the channel slope and the ground slope. These structures usually lead to a sudden vertical change of channel slope and produce a free falling jet, which impacts the drop floor downstream. The impact force may damage the structures. In addition, extra dynamic force is developed in the downstream floor due to the free falling jet fluctuations. Evaluation of the impact force in the downstream is an essential step in the design of drop structures downstream bed. There are limited studies conducted to investigate the vibration problems of the hydraulic structures and this concept is still a baffling case. If the vibration is not properly dissipated, in addition to the structural instability, it may produce environmental pollutions to the neighboring human communities.
The aim of the present study is to evaluate the dynamic force of the falling jet in a vertical drop. This study consists of two parts; i.e., experimental and analytical studies. With subcritical flow in the upstream channel, two physical models with drop heights of 0.226 m and 0.376 m are built in the Hydraulic Lab, Department of Civil Engineering, Isfahan University of Isfahan. For verification, the hydraulic properties of the flow on vertical drop are measured and the results are compared to previous studies. Introducing a SDOF system for the downstream floor, passive dampers were used to control the dynamic forces acting on the drop floor downstream. The drop floor is allowed to have vertical degree of freedom and the absorbing system is designed to reduce the forces on the drop floor. Three types of dampers with various stiffness and damping constants have been used based on the range of jet frequencies. An instrumentation system including several load cells was used to record the fluctuating force time histories.
The hydraulic properties of flow on drops including brink depth, pool depth, length of pool surface, flow depth at the downstream channel, jet width at the pool level, inclination and position of falling jet impact at the downstream channel and downstream floor pressure have been measured. The results agreed well with those of the previous experimental data and analytical predictions. The results of drops with dampers indicated that the vibrations could be lower than those in drop without dampers. The results showed that damper type 3 has the best efficiency in terms of force and amplitude reductions. Using damper type 3, the force amplitude and the mean value of dynamic force are reduced as much as 81% and 25%, respectively.
In the analytical part, a method is presented to estimate the dynamic force of falling jet on the downstream floor. The results of the proposed method showed that, if the sinusoidal variable pressure amplitude and steady pressure variance between two sides of the falling jet are known, the proper value of dynamic force using the proposed method can be evaluated.