جستجوی مقالات مرتبط با کلیدواژه « Main channel » در نشریات گروه « آب و خاک »

In this study, the flow inside a rectangular composite channel was studied by finite volume numerical method, for which Flow-3d software was used. Comparing the results of the numerical model with the laboratory results, it was concluded that the numerical model can well simulate the flow inside a rectangular composite channel. Performing numerical modeling of suitable boundary conditions, a grid with the desired dimensions and a suitable turbulence model was obtained to describe the hydraulic flow inside the rectangular composite channel. Three turbulence models of the two equations K-ε, RNG K-ε and K-ω and one turbulence model of large vortices LES were used. Also, the effect of geometric parameters and flow on the average velocity in the composite channel was investigated. The results obtained from this study show that with increasing the width of the floodplain at different depths, the flow velocity has decreased, so that in the value of = 0.75 (yf / H), with decreasing the flood width of the right plain from 5.71 to 13.33 cm, the ratio of flow velocity in the section of the main channel to the average velocity has also increased by 51%. It should be noted that with increasing the depth of the floodplain, the flow velocity has decreased. Also, with increasing the height of the main channel wall, the flow velocity decreases.

In the river, velocity parameter is one of the most important hydraulic variables and is effectively used in many river engineering fields like development of stage-discharge curve and sediment transport. In some river engineering schemes, the calculation of average flow velocity is sufficient. However, for some other projects, such as designing hydraulic structures in the river, stable channel design, flood calculations in rivers and floodplains, the lateral and vertical distributions of flow velocity should be calculated. To calculate the two-dimensional distribution of the velocity of flow (in transverse and vertical directions) many mathematical models have been presented with many complexities from practical point of view. In this research, a simple and practical mathematical model of Kean et al in combination with eddy viscosity equation as well as Einstein's law of the wall velocity was used to determine the two-dimensional flow velocity distribution in the smooth and rough compound channels. By numerical solution of this mathematical model, using finite differences method, isovel curves data were calculated for some experimental compound channels with different flow depths and floodplain's roughness coefficients and then they were compared with the experimental data. Also, transverse distributions of the flow velocity were calculated in these channels and compared with the measured profiles. These comparisons showed that the proposed mathematical model with coefficient of determination (R2)= 0.92, root-mean-square error (RMSE)=0.036 and mean absolute percentage error (MAPE)= 2.8% had an acceptable accuracy. The proposed mathematical model was developed with steady and uniform flow assumption, neglecting the secondary flow effect.

The precise prediction of shear stress in open channel is important in many engineering issues such as designing of sustainable channels, calculation of energy losses, and sedimentation in channels. In flood duration, due to difference in depth of flow between the main channel and the flood plains surrounding the flow, the flow velocity is also different, and subsequently the stress and distribution of stress significantly changes. In this paper, it has been shown the performance of the Ansys Fluent three-dimensional numerical model in simulating various hydraulic parameters for a rectangular compound channel with smooth and rough bed and wall. Comparison of shear stress values showed that with decreasing depth in flood plains, the amount and percentage of shear stress in flood walls, and in main channel increased and decreased respectively, and also it was shown that with increasing roughness, shear stress increased. Results also indicated that with increasing flow rate, the power flow increased and velocity and vortices were formed at the intersection of flood plains and main channels. The results of this research can play a role in designing of sustainable channels, especially at the intersection of the main channel and flood walls.

Understanding the conditions of sediment transport has been of great interest by hydraulic engineers. The highest changes in the morphology of the rivers have occurred in flood conditions and the rivers usually appear in the form of compound sections. Hence, the flow hydraulics and sediment transport are quite different compared to those of simple sections. The amount of sediment transport in rivers is a direct function of the flow velocity. Due to the intense variations of the flow velocity in compound sections, the variations of sediment transport in lateral and vertical directions are quite different in flood conditions. Due to this fact, it is better to use point velocities instead of mean velocities for the calculation of the sediment transport capacity of the rivers and then replace these point values in the empirical formulas of sediment transport (e.g. Engelund-Hansen, Ackers-White, etc.). In this research, the two-dimensional distribution of flow velocity is first calculated in the width and depth of a straight laboratory compound section by numerically solving a partial differential equation in the form of Poisson. The obtained 2D velocity distribution is then used for computation of the sediment transport capacity. By the summation of these particle capacities, the total sediment transport capacity is obtained for the canal. For Engelund-Hansen formula at the case of using the mean flow velocity and total bed shear stress, the mean error and the root-mean-square error were obtained to be 58% and 3.63 g/s, respectively. However, by replacing the point velocities and shear stresses, the errors reduced to 15% and 1.71g/s, respectively. For Ackers-White formula, these values of errors were obtained 164% and 9.36 g/s for the case of using the mean flow velocity, while for the case of point velocities, the errors were 30% and 1.86 g/s, respectively.

In open channel junctions development of separation zone at the downstream channels is one of the engineering challenges. Development of separation zone has important role on determination of position of hydraulic structures and also increase of its dimensions caused decreasing of flow area so that leads to more erosion of the bed. As the flow is entered to the channel, flow separated from the wall and a zone with low pressure and vortex is created that is called separation zone. As regards of destructive effects of development of the separation zone, researchers always have focused on this case. The aim of this study is to investigate the effects of factors such as inlet discharge ratio, height of outlets weirs and bed elevation of inlet channels on dimensions of the separation zone in 90 degrees four-branch junction with subcritical flow.
Experiments were carried out in hydraulic Lab of Tabriz University. The setup consists of an intersection of four identical channels which intersect at right angles. Each channel is made from glass with a width 0.4 m and height 0.5 m. Junction was right-angled and bed of the channels was considered horizontal. In this study effects of different geometric and hydraulic parameters such as height of the outlet weirs, bed elevation of the lateral channel and inlet discharge ratio on characteristics of the separation zone in outlet channels in 90 degrees four-branch junction with two inlet flows and two outlet flows was investigated experimentally. In order to investigate the flow pattern were used pigment and sawdust and dimensions of the separation zone were measured by ruler and meter.
The results revealed that decreasing height of outlet weirs increases dimensions of the separation zone so that minimum variations of (ratio of length of the separation zone in main outlet channel to channel width) xo x b L was about 45 percents. As the bed elevation of lateral channels increases dimensions of the separation zone at the main outlet channel increase which variations of xo x b L was between 0.8 to 2.75 but dimensions of the separation zone at the lateral outlet channel decrease and variations of (ratio of length of the separation zone in lateral outlet channel to channel width) xo y b L was at the range of 0.2-2. The results also indicated that as the ratio of outlet weir height of the lateral channel to outlet weir height of the main channel increases, dimensions of the separation zone in both lateral and main outlet channels increase so that variations of xo s b L for both main and lateral channels were at0.2-1.4 and 0.2-2, respectively. Although the dimensions of the separation zone change with respect to different conditions but width to length ratios in lateral and main outlet channels were 0.16 and 0.25, respectively. Finally in order to calculate the dimension of the separation zone, regression equations were presented so that experimental and analytical results had good agreement.
In this research effect of the inlet discharge ratio, height of the outlet weirs and bed elevation of the lateral channels on dimensions of the separation zone in 90 degrees open channel junctions was investigated. Results showed that as increase of the inlet discharge ratio and height of the outlet weirs, dimensions of the separation zone increased. Increase of the bed elevation of the lateral channel increased dimensions of the separation zone in main outlet channel while decreased dimensions of the separation zone in lateral outlet channel. As increased of x y c c dimensions of the separation zone in both outlet channels increased

Most rivers have flood plains that extend laterally away from the main channel at a gentle gradient or in a series of terraces. Multistage channels are deliberately formed in certain cases in order to increase conveyance capacity in large floods، and to have recreational expanses available at other times of the year. Two-stage channels، thus consist typically of a main river channel in which there is some discharge all of the time and flood plains، which are dry for most of the time، yet perform a vital function in times of flood. Since flood alleviation schemes are the focus of much of engineering work، the prediction of the conveyance capacity، velocity distribution and boundary shear stress distribution in such channels is clearly important. The boundary shear stress distribution is a prerequisite for studies on bank protection and sediment transport. Prediction of these parameters in two stage or compound channels is complicated by the lateral exchange of momentum that takes place in the shear layer that forms between the generally faster moving water in the main river channel and the slower moving water on the flood plain. Superposition of high lateral shear on bed-generated turbulence and longitudinal secondary flow structures is a convoluted problem in fluid mechanics. In the context of the river channels with flood plains، the problem is usually further complicated even for moderately straight channels by the complex geometry of the cross-section and the heterogeneous nature of the boundary roughness.

Bridge abutments are usually located in the floodplain zone of rivers where velocity and shear stress are not uniformly distributed. The influence of channel geometry and lateral momentum transfer in compound flow field on the scouring phenomenon has not been fully investigated and understood yet. The impact of lateral momentum transfer on the local scour at abutments terminating in the floodplain of a compound channel is presented in this paper. It is shown that, by accounting for lateral momentum transfer at small floodplain/main channel depth ratios (λa/H<0.3), estimates of maximum local scour depth are increased by up to 30%. Therefore, ignoring the influence of the lateral momentum transfer, in such circumstances, might result in unrealistic estimation of the scour depth. To draw a more general conclusion, more data are required to assess the influence of different parameters affecting the phenomenon in compound flow conditions.