Study Of U-shaped Channel Free Overfall Hydraulic Performance And Energy Dissipation Structures

Free overfall can be used as a flow-measuring device with a single measurement of flow depth at or upstream of the brink of an open channel.It has the ability of flow measurement without any additional structure.U-shaped channel has an effective hydraulic section,and free overfall is,in steep slope geographic area,an unavoidable device which also has the ability to estimate flowrate passed through it.The present study is to investigate the hydraulic performance of the U-shaped channel free overfall.Sets of laboratory experiments were conducted in the subcritical flow regime with two different sized U-shaped channels.The main research contents and conclusions are as follows.?1?The data from experiments was used to perform dimensional analysis,and to verify satisfying other two different theoretical relationships of end-depth ratio?EDR?and end-depth discharge?EDD?that were proposed in this study.Those relationships were derived by applying the momentum equation based on Boussinesq approximation and weir flow theory.The value of EDR was found to be around 0.74 up to dimensionless end depth?^*₀)=0.287where both?^*₀)and dimensionless critical depth?^*_{?8? are within the circular arc zone before it decreases with?^*0}.?2?The experimental data was also used to calibrate and validate the numerical simulations which were performed in a computational fluid dynamic?CFD?computer software called FLOW-3D.With TruVOF method and k-?turbulence model,FLOW-3D was used to simulation the U-shaped free overfall in both subcritical and supercritical flow regime.Then,the simulations yielded essential hydraulic parameters including Froude number,flow depth,velocity and pressure which represent the flow behavior as well as the performance of the U-shaped free overfall.Opposite to subcritical flow regime,Froude number upstream is greater than unity in supercritical flow regime;the velocity along the channel is also faster,and flow depth is shallower too.?3?The simulations also demonstrated the impacts of the nappe flow on the downstream channel.The characteristics of impacted pressure distribution on the downstream channel were analyzed.When the drop height is taller,the high pressure pushed area is sharper with greater pressure value.The relationships between maximum impacted pressure value and position with drop height and discharge were established.Finally,two commonly practical free overfall downstream structures,counter-weir and stepped structure,were tested by the CFD simulations.The stilling pool made by the counter-weir does not diminish the pushing pressure from the nappe flow,however,the weir causes the flow downstream slower and deeper.The stepped structure dissipates the pressure impacted on the downstream channel very noticeably,but the continued flow after the structure does not make the flow deep as well as slow as counter weir does.The U-shaped channel free overfall is satisfied to applicated with its accurate flow measurement and great hydraulic efficiency.