River engineering
ali dankoo; Hojjat Allah Yonesi; Mojtaba Saneie
Abstract
Introduction With the occurrence of flood, the velocity and depth of the flow in the river increases and the flow enters the flood plains. The velocity difference between the deeper section and the shallow area causes the transfer of momentum between these areas and complicates the flow structure. The ...
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Introduction With the occurrence of flood, the velocity and depth of the flow in the river increases and the flow enters the flood plains. The velocity difference between the deeper section and the shallow area causes the transfer of momentum between these areas and complicates the flow structure. The formation process of secondary flows and its pattern in compound channels have been investigated by researchers such as:Tominaga & Nezu, 1991. The presence of vegetation on flood plains causes complexity in the analysis of hydraulic problems of compound channels. For example, Hamidifar et al. (2012, 2014), using laboratory measurements, showed that the presence of vegetation reduces the flow through the cross section by about 30%. At the same time as the water level rises during the flood, the deck of the bridges will go under water and the current passing under it will be pressurized. In this condition, the flow field is affected by the presence of vegetation, compound channel and pressurized flow. In this research, the laboratory investigation of these complex conditions has been done.MethodologyThe experiments of this research were done with 3 geometric ratios of the compound cross-section, 3 relative depths, 3 vegetation densities, and control experiments in a compound channel with a length of 10 meters and a width of 1.5 meters. The measurement of the flow velocity parameter, the scouring rate of the bridge pier in the conditions of pressurized flow has been done according to the variables mentioned above.Results and Discussion Comparison of depth velocity and logarithmic velocity distribution in the condition without vegetation on the flood plain, the sign shows that in all sections, the distance between the channel bed and the water surface, the difference between the measured velocity values with the logarithmic distribution of the velocity increases. This difference is due to the presence of the bridge deck and the flow retardation. Also, vegetation causes the depth distribution profile of flow velocity to deviate from the curve of logarithmic flow velocity, and the biggest difference will occur in the upstream area between the interface of main channel and flood plain. This phenomenon increases the amount of apparent shear stress between the main channel and the floodplain.With the increase in the density of vegetation, the percentage of floodplain participation in the total discharge is reduced by 20%. The highest participation percentage of floodplain is about 30% in the state without vegetation. In different densities of vegetation with an increase in relative depth from 0.3 to 0.5, the percentage of floodplain participation in the total discharge is less than 10%. With the increase in the density of vegetation, the difference between the percentage of floodplain participation in different cross section widths has decreased.ConclusionsThe findings of recent research to check hydraulic parameters can be summarized as follows:- Increasing the density of vegetation increases the longitudinal velocity in the main channel and decreases it in the floodplain.- Longitudinal velocity and averaged- depth velocity in the main channel in the case without vegetation is lower than the case with vegetation.- Increasing the relative depth increases the percentage of floodplain participation by an average of 5%, and the increase in vegetation density causes a decrease of 17% in the floodplain participation.- With the increase in the vegetation density of the floodplain, the velocity changes in the floodplain decrease compared to the main channel.- Examining the profiles of the depth distribution of the flow shows that due to the presence of the bridge deck and the retardation of the flow, the depth distribution differs greatly from the logarithmic distribution of the velocity . This is despite the fact that in the conditions without the presence of the bridge deck, this amount of difference reaches its minimum.- The presence of the bridge deck and the creation of backwaters reduce the difference in flow velocity in the main channel and floodplain upstream of the bridge, and this in turn reduces the strength of the secondary currents between floodplain and the main channel.- The difference between the global average velocity of the flow and the local velocities increases the slope of the (a-1) and (b-1) lines due to the flow retardation.
minasadat seyedjavad; seyed Taghi Omidnaeeni; mojtaba saneie
Abstract
Introduction The flow passing through a side weir, one of the varieties of water diversion structures, is a variable flow with decreasing flow rate. Labyrinth weir is the basis for piano key weirs. They are often constructed with vertical walls and are much more efficient than the linear weirs. Nevertheless, ...
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Introduction The flow passing through a side weir, one of the varieties of water diversion structures, is a variable flow with decreasing flow rate. Labyrinth weir is the basis for piano key weirs. They are often constructed with vertical walls and are much more efficient than the linear weirs. Nevertheless, the flow, especially the bottom flow, enters this type of weirs and passes through two vertical walls of the side crests. Then it becomes squeezed and therefore the upstream and downstream crests come up with inappropriate hydraulic behavior. Also, the most outstanding disadvantage of this type of weirs is the large foundation area needed to be constructed on the concrete dams. The piano key weirs are a modern type of the nonlinear weirs which have been developed by Hydro Coop Institute of France and the Hydraulic and Environmental laboratory of the Biskara University of Algeria. In general, these weirs comprise of 4 different types, the differences of which lie in the presence or the absence of slopes created for them. Type A is sloped both upstream and downstream, Type B is sloped upstream, Type C is sloped downstream and Type D lacks any slope. The present study conducted show the effective geometrical parameters on the hydraulic performance and discharge coefficient (CM) of the trapezoidal piano key side weirs (TPKSW). The type of flow and its variations in a side weir can be considered as the C_M of the side weir, using simplifications and assumptions suggested by De-Marchi in 1934 to obtain suitable equivalents for side weirs. Methodology All tests have been conducted in a closed-loop rectangular Plexiglas flume in Soil Conservation and Watershed Management Research Institute (SCWMRI), Tehran, Iran. The study canal was10m long, 0.6m wide and 0.6m high. All tests have been carried out on the 0.6m wide canal. To prevent flow turbulence upstream of the canal, tranquilizing racks were used at the upstream. A calibrated triangular weir was also applied to measure the flow at upstream. Also, a calibrated rectangular sharp-crested weir was used downstream. The water surface profiles were measured longitudinally. For this purpose, a digital depth profiler with 0.1 mm precision was used. The profiler accuracy was valid for almost stable water surface but it could be decreased in highly turbulent flows. An electromagnetic velocity meter with 0.001 m/s precision was used to determine velocity components to obtain parallel (𝑉𝑥) and perpendicular (𝑉𝑌)to the side weir. The profiler and the velocity meter could move on a rail in both X and Y directions. Flow rates at the main and the collection canal were measured by a calibrated 90° V-notched and a rectangular weir, respectively. Figure 6 shows a general view of the laboratory. In this research, 16 models of Type-A trapezoidal weirs have been studied in two cases of 1 and 2. The weirs had 3mm thickness made of Plexiglas. The tests were carried out preventing the effects of viscosity and surface tension over the weir and considering the height of more than 3 cm. Results and Discussion In this research, for investigating the effects of a number of inlet cycles, the weirs were tested at two different directions of the side weirs located in the main canal. The results showed that the weirs with 15cm and 20cm had the highest discharge coefficient CM in dimensionless ratios of 0.2> H/P> 0.4 and H/P>0.5 respectively. Having reviewed previous studies, it could be concluded that the trapezoidal piano key side weir was capable of releasing a flow 1.2 times more than that of the linear trapezoidal labyrinth weir with 12 degrees angle and 1.87 times more than the one with 6 degrees angle, and 1.5 times more than that of the triangular labyrinth weir. Conclusions The result of the present study has shown that trapezoidal piano key weir can perform well on the side of trapezoidal and triangular labyrinth weir. In the comparisons made in this study, in comparison with the discharge coefficient of the side weir overflow, the trapezoidal piano key with the lateral overflow of the 12-degree Congress and the 6-degree trapezoidal labyrinth is equal to 1.2 and 1.87, respectively, and 1.5 times the lateral overflow of the triangular labyrinth weir. Although, the trapezoidal piano key weir in both lateral and direct modes is 1.55 times higher than the rectangular overflow of the direct piano key of the discharge coefficient. The trapezoidal piano key side weir in the dimensionless ratio of 0.4> H / P> 0.2 has the highest discharge coefficient flow. Keywords:Discharge, Flow, Water Diversion Structure
Abstract
In recirculating aquaculture systems, inlet flow current usually enters from the inner pyramid of the tank in the form of submerged jets. The hydraulics of jet alignment has an important effect on flow uniformity and solid removal efficiency. This paper examined experimental analysis of flow pattern ...
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In recirculating aquaculture systems, inlet flow current usually enters from the inner pyramid of the tank in the form of submerged jets. The hydraulics of jet alignment has an important effect on flow uniformity and solid removal efficiency. This paper examined experimental analysis of flow pattern in a cylindrical tank by placing a nozzles at each of three depths using four angle options (20°, 40°, 60°, and 80°) between the radius and jet alignments. The results indicate that angles of 40° and 60° created more uniform current in the tank. Changing the jet alignment from 20° to 80° decreased the number of local eddies and increased the average velocity in the tank. Comparison of inlet currents from one nozzle and from three nozzles showed that increasing the number of nozzles increased the jet velocity and the average velocity in the tank, but decreased the ratio of average velocity of the tank to external velocity of the nozzle jet. Paired comparisons of four options (average velocity for angles of 20° (op. 1), 40° (op. 2), 60° (op. 3), and 80° (op. 4)) show that options 2 and 3 had the strongest correlation. Shear stress monitoring in a round tank showed that shear stress increased at the stations of options 2 and 4. Increasing the ratio of h/H, increased the Froude number near the bed of the tank.
