عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Prediction of scour hole characteristics downstream of structures and water surface regulators is of the most important and most difficult steps in designing the foundation of structures. Considering the hole dimension, the design should be a way to reduce the probability of failure or reversal of the structure. Collision of a water jet to the bed causes an increase of near bed shear stress and leads to the separation of the sediments. At the beginning of this phenomenon, while tail water depth is at minimum level, falling jet penetrated to the vicinity of the alluvial bed then vortices are created. These vortexes penetrate into the pores of the bed particles and cause separation of the particles. By the passage of time and dissipation of flow energy, the local depth of flow will increase and erosion will be reduced. In this case, the sediment particles are carried to downstream as and the suspended load deposit in a relatively short distance from the location of the jet collision. As a result, hill deposit to be formed downstream of the scour hole. The height of this hill is a function of the tail water. Experience has shown that by reducing the tail water depth, the height of the hill would be decreased. The experimental data also show that by changing the nozzle diameter for a constant drop height and tail water depth, the dimensions of the hole will change. Many studies are carried out by researchers to determine characteristics of scour holes formed around hydraulic structures and in most cases, the empirical equations have been presented.
The aim of this study was to evaluate the effect of hydraulic parameters of submerging jets, tail water depth and angle of jet, on the dimension of scour hole and sedimentation hill in the submerged pool. The innovation of this study is the use of non-cohesive alluvial materials with uniform particle size in the range of medium gravel. It is worth noting that the diameter of the materials used in the previous researches was in the range of fine sand and fine gravel. The physical model was made in the Hydraulic Models laboratory of Jundi-Shapur University of Technology in order to conduct the tests. This model has the ability to change the angle of the nozzle, to change in the nozzle installation to the upper levels of sediment, and it is capable of changing the amount of tail water depth. For this purpose, a special chariot was made for establishing the nozzle which is capable to shift horizontally, vertically and rotationally. In this research, the nozzle was fixed at angles of 45 and 60 degrees. Nozzle diameter is constant in all stages of the experiments. By installation of this nozzle on top of a metal tank with a length of 2 m, width of 1 m and a height of 1 m, the necessary condition was provided to evaluate the scour depth in the submerged pool. The nozzle edge was installed as submerged for removing the effect of the concentration of the incoming air around the jet. An electromagnetic flow meter device was used to control the rate of flow with an internal diameter of 100 mm and with an accuracy of two percent of full scale (2 % FS) that is made in Megab-Iranmedar company. A Leica Disto-D8laser model was used to record the bed profile. Effective parameters in this study are presented in the equation (1).
where is the outflow of the nozzle, is inlet discharge of air to the water jet, is the nozzle diameter, is the outlet jet velocity from nozzle, is the width of the pool, is nozzle angle to the horizontal, is tail water depth, is average diameter of particles, is accelerate of gravity, is viscosity of fluid, is the density of the fluid, is density of sediments and is defining characteristics of the scour hole.
In this research, in addition to the variation of the angle of the jet, tail water effect on the scour hole has been also evaluated. The results showed that with increasing depth of tail water, form and deposit style of sedimentation in the scour hole downstream are different. At low depths of tail water, deposit style of sediments is distributed with constant height, approximately. However, with increasing tail water depth, erosion parameter (Ec) reduced and this makes the pattern of turbulence be slow in fluid layers near the bed. The particles which have moved from their places with any form, they are deposited and lose kinetic energy quickly. Experience has shown that in the mentioned condition, the sediments are deposited on the ridge area to form a peak. The results show that by increasing the tail water depth, the erosion parameter value was decreased, and this action reduces the relative scour depth in both of the output angle of the jet. In addition, it is evident that a change in the angle of the jet, a limit of the threshold will be set up on the trend procedure of the relative depth of scour.
Results indicated that the trend procedure of all dependent parameters of the scour phenomenon has a direct relationship with erosion parameter (Froude number of jet). By increasing the depth of tail water, formation and shape of sediment deposition will be changed in the downstream of scour hole. In the low tail water depth, method of depositing sediments is distributed by approximately a constant level. By increasing the depth of tail water, the form of sediment deposits will have a peak form. By increasing the depth of tail water, the amount of erosion will be reduced, and consequently, the relative scour depth will be reduced. This is true for all angles of the outlet jet from thenozzle. For erosion parameters larger than 2.82 (Ec>2.82), the value of relative scour depth with an angle of 45 degrees is higher than the value of 60 degrees. For erosion parameter larger than 1.97 (Ec>1.97), the value of relative ridge of sediments under 60 degrees.