جستجوی مقالات مرتبط با کلیدواژه « skimming wall » در نشریات گروه « آب و خاک »

Taking water from rivers is one of the most important topics in hydraulic engineering. One of the problems associated with most intakes is the accumulation of sediments in intake entrances. Failure to control the sediment's entering the intakes results in its transfer into irrigation channels and their establishment that creates many problems due to sediment transport or its settlement in various parts. Due to the development of computing systems as well as unmeasurable complexities of water flow and sediment in laboratory models, using numerical simulations can be very effective and substantial in hydraulic investigation of such flows. Flow passing through the lateral intakes and channel junctions is usually turbulent. Turbulence is one of the most important features of the flow pattern in a bend which influences a lot of processes occurred in rivers including erosion, sediment transport, bed morphology, and shape of natural channels. Investigating the kinetic energy of flow turbulence in open channels due to the maximum shear flow of the bed and the scouring of the floor is very important and can be considered for prediction of bed topography. The accumulation and sedimentation in water intake and reduced intake efficiency is one of the problems that arise in most intakes.

Therefore, in the present study, the sediment controlling structure, a skimming wall, was used in front of the intake. Then, the three-dimensional flow in sedimentary bed around the intake entrance was simulated by Flow3D model and results were compared with experimental model. In this research to increase intake efficiency and control the amount of sediment entering the intak, two structural skimming wall in front of the intak and spur dike on the opposite shore it is used. In the research using three-dimensional flow field numerical model FLOW3D around 60 ° lateral intake located on the direct path was solved numerically and counters velocity and turbulence kinetic energy is drawn. The experiments conducted and results were compared with the numerical model. Flow hydraulic and dynamics in front and inside the intake is studied. Velocity vectors inside the intake, in both longitudinal and lateral directions were compared with experimental results.

In the absence of structures, inside the main channel, the flow separation width at levels close to the bed is broader than the higher levels. However, by installing skimming wall in front of the intake, the flow separation close to the substrates carrying more sediment is reduced and it is increased on the surface that has less sediment. It also allows less sediment into the intake. In the absence of structures, the surface flow lines in the intake tend to the right wall and the bottom flow lines tend to the left wall. The width of separation zone on the surface is broader than the one on the floor. In presence of skimming wall structure or both skimming wall and spur dike, the flow lines near the bottom tends to the channel center. In addition, the zone with stagnant flow on the left side of intake is broader at the bottom than on the surface. In all three cases, the maximum longitudinal velocity and the maximum resultant velocity have occurred at the beginning of the intake inlet in its left corner. The maximum transverse velocity has occurred in the intake center. In these figures, there are areas with secondary vortex flow on downstream and the left of the intake.

The results showed that by adding the structure of the spur dike in the main channel, the velocity in the main channel is 1.5 times compared with the other two cases. and the area inside the intake also affects. Also, the tip of the axis of the velocity vectors is displaced to the intake. As a result, In the back area of the spur dike, the longitudinal velocity decreases and there deposition. Comparison of the distribution of the maximum kinetic energy of flow turbulence at two different depths indicates a 50% increase in the maximum kinetic energy of turbulence in the upper layer compared to the lower layer.

The mechanism of flow and sediment transport in channel bend is much more complex when the outer bank of the bend is employed for lateral diversion. Due to the secondary current, sediment moves away from outer bank and therefore outer bank of the bend is one of the best locations for lateral diversion. In this experimental investigation, three structures including Sill, Dike and skimming wall were used to control the amount of bed sediment at lateral intake with three ratio of diverted flow discharge of 0.17, 0.21 and 0.26. Results showed that to have a fix percentage of flow diversion in all experiments, these structures have been effective on water level compared to the control conditions and they had an effect on input Froude number. The combination of skimming wall of 14 ̊, dike and sill had the most controlled bed sediment in different percentages and 92 percent of sediment movement to the lateral intake decreased compared to the control experiment. In the sill experiment, by increasing the percentage of flow diversion, the effect of this structure reduced, the least percentage of reduction of sediment diversion at discharge ratio of 26 percent has been 65 percent. Changes in the strength of secondary flow were investigated quantitatively at the intake mouth and it was revealed that the process of changes in sill and control experiments is different from other experiments. Investigating shear stress of the bed channel, it was shown that, firstly, the amount of shear stress in outer bend is more than inner bend and secondly, by increasing the percentage of flow diversion, shear stress increases along the intake. Using skimming wall of 14̊, the local pattern of shear stress in front of the intake changes such that the amount of shear stress near the intake decreases, the maximum shear stress is moved towards the center of the channel and back of the structure, which leads to decreased amount of erosion and diversion of sediment to the intake