چكيده به لاتين
Since long ago, rivers have played a very important role in the formation of human civilizations and they have been vital for human life, on the other hand, most of the rivers' paths in nature have always been accompanied by meanders and bends, so knowing the behavior of a meandering river is very important. For this reason, the interaction of vegetation and river bends plays an essential role in the ecosystem and structure of river flow. Vegetation is observed on the bed or the edge of rivers, which have different and complex effects on the structure of the flow and create turbulence. As a result of the current passing through a bent path in an open channel, the water level and flow pattern compared to the straight channel changes due to secondary and spiral currents, which affect the characteristics of the flow, including velocity distribution and turbulence components, to the extent that in This effect is also observed in the flow pattern of the downstream channel. In general, it is assumed that the forces governing the flow in the river bend are centrifugal force and inertia. Investigations show that in all sections inside the bend, the flow velocity increases near the inner wall and decreases near the outer wall, and at the end of the bend, the flow velocity in the region of the inner wall decreases drastically. However, in the presence of vegetation, the flow conditions are completely different from the control bed (without vegetation) so it causes a decrease in the centrifugal force and inertia in the bend and causes a change in the longitudinal and transverse velocity distribution and other hydraulic parameters. Since the changes in reed diameter have a significant effect on the production of secondary currents, horizontal eddies, and changes in the flow structure, it is necessary to investigate the effect of this parameter on the estimation of the resistance coefficient in the presence of vegetation on the sandy bed under non-uniform flow. This research was carried out in the hydraulic laboratory of the Iran University of Science and Technology on a meandering channel with a concrete wall and bed, 8 meters long, with different fineness ratios. To investigate the effect of emergent solid reed vegetation that is observed in rivers in estimating flow resistance, plastic reeds were used on a sandy bed with a slope of 2%. To investigate the turbulent flow structures, the analysis of instantaneous velocity data extracted from the Acoustic-Doppler Velocity (ADV) system has been used. The experiments were conducted in 15 series, 5 series were performed in the control bed (without vegetation) at different water depths and 10 mm series were performed in the cases of cover with a diameter of 5 and 10 mm. Reynolds shear stress, turbulence intensity, turbulence degree, turbulence kinetic energy, drag coefficient, and Manning coefficient were analyzed. The velocity profile in the presence of emergent vegetation in different diameters near the walls of the bend has a different distribution from the control state, in addition, the Reynolds shear stress distribution the degree and intensity of the flow turbulence, and the drag coefficient also show a different trend from their distribution in the control state. Considering that the drag coefficient has a direct relationship with the Reynolds shear stress and the opposite relationship with the second power of the velocity, the distribution of the drag coefficient in the water depth is also non-uniform, and considering a drag coefficient is only based on the bed value along with the error because the value of the drag coefficient varies in the water depth. Is. In the present research, by combining Manning's equation and the relation of the drag coefficient based on the shear velocity, a method is presented that shows that the drag coefficient increases in the presence of vegetation. The results show that at the beginning of the bend in the outer wall and the end of the bend in the inner wall, the witness bed has the highest drag coefficient, and the presence of vegetation in other parts of the channel increases the drag coefficient, and this issue is established at different depths with the same data collection conditions. So the highest drag coefficient is obtained for the bed with vegetation near the inner wall and the center of the channel.
