عنوان مقاله [English]
In recent years, the necessity of using numerical multiphase models to separate and simulate bed sediments and suspended loads of rivers has received much attention. Most numerical models of sediment transfer of bed load and suspended load are examined separately and semi-experimental methods are usually used to predict bed load. Although these methods have been widely used, they do not show the details of the mechanisms controlling the sediment transport process well and in turn have problems (Oda 2019). Today, quasi-two-dimensional mathematical models are widely used as an optimal and efficient solution in hydraulic issues of flow and sedimentation of rivers. In these models, velocity changes across rivers are simulated by numerical solution of Navier-Stokes differential equations under steady and uniform flow conditions (Zahiri et al., 2015). Lai et al. (2019) in the study of flow and sediment transfer capacity with a three-dimensional model using the governing equations of flow and suspended sediment, showed that the model has a good agreement between flow and sediment with experimental data.
Ekbatan Dam watershed is located in the southeast of Hamadan city and is below the Qarachai river basin of Hamadan. In this research, the second-order discretization method of the Fluent model to separate the bed and suspended load from Reynolds stress numbers to determine the motion threshold and to analyze the motion pattern and sedimentation of sediment particles in different flow tubes in the laboratory glass channel, from soft Fluent and Acobas software were used. Then, the results of Flont model were evaluated and calibrated with the measured data in the laboratory glass channel. In this way, fine sand and silt particles were placed in the laboratory glass channel in a flow path with a speed of 0.5 m / s and the distance traveled. It was measured by a camera with a very high frame connected to the recording channel and with the output animation of Acobas software. In the next step, the simulated data of Flont model were matched with the data measured in the laboratory and the error percentage was measured. Finally, the appropriate mathematical model for the amount of displacement of different sediment particles was determined based on the particle diameter and the location of sediment particles in the flow path, the distance traveled by different sediment particles in the laboratory flume.
Results and Discussion
The simulation results showed that most of the sediment load of fine sand and silt in the laboratory channel is carried by the Soffman lifting force in the second flow tube. And the performance of this force has caused the transport of most sediment load in the form of suspended load in the Yelfan River. The simulation results showed that after the passage of 0.2 m from the beginning of the flow in the channel, the sedimentation process begins and the accumulation of sediments increases with the continuation of the flow downstream and near the reservoir of the dam. Studies show that due to the presence of isothermal lines and creating a constant flow and velocity in the second flow tube, the suspended sediment load is transported in the riverbed throughout the year to the dam reservoir and the main volume of the reservoir during this process by fine sand sediments and The silt is filled. The study of the measured error percentage indicates the high agreement of the Fluent model with the data measured in the laboratory and shows that the model has a very good ability and accuracy in simulating the movement of sediment particles with different diameters.
The use of multiphase and multiphase models in the analysis of sediment transport by considering suspended particles as a second phase continuous with the fluid phase interaction seems very necessary. Evaluation of the obtained results shows that Flont software with very high and acceptable accuracy has well simulated the transport, transfer and sedimentation of sediments with a range of different ranges. This is important in the sense that the fluent model has high efficiency and accuracy in the study and evaluation of suspended sediments such as fine sand and silt, which in most studies due to the fine grain of these sediment particles, difficult to study and Or not possible. Examination of the percentage of measured error for laboratory and simulated data shows the high agreement of Flont model with data measured in laboratory conditions and the correlation between laboratory results and Fluent model is more than 97%, which is acceptable. Therefore, the combination of Fluent model and laboratory conditions can be a very useful and accurate tool to simulate the movement of sediment particles, determine the location of sedimentation, control erosion and sedimentation and select soil conservation and watershed management projects in river basins.
Keywords: Suspended load, Sedimentation rate, Flow tube, Sediment particles, Ekbatan dam