Flow Through Open Channel By Ranga Raju Pdf Download

The design of a channel involves the selection of channel alignment, shape, size, and bottom slope and whether the channel should be lined to reduce seepage and/or to prevent the erosion of channel sides and bottom. Since a lined channel offers less resistance to flow than an unlined channel, the channel size required to convey a specified flow rate at a selected slope is smaller for a lined channel than that if no lining were provided. Therefore, in some cases, a lined channel may be more economical than an unlined channel.

The channel design may be divided into two categories, depending upon whether the channel boundary is erodible or non-erodible. For erodible channels, flow velocities are kept low so that the channel bottom and sides are not eroded. The minimum flow velocity in flows carrying a large amount of sediment should be such that the material being transported is not deposited in the channel.

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A flow control problem at a side-weir intake with a three-chamber settling basin is addressed in the paper. Figure 1 schematically shows the intake structure of a small hydropower plant (SHPP). The intake consists of a weir, fish path with the system for ensuring environmental flow, a settling basin, and a small headpond. The settling basin consists of the common inlet and outlet zones and three settling chambers. In a settling basin, a settling chamber is the most important part, whose geometry is mainly influenced by size distribution, types, and amounts of sediment carried by the installation water flow. Settling chambers usually have trapezoidal bottom and are characterized by their length, width, depth, and water velocity (Garde et al. 1990; Vittal & Raghav 1997; Ranga Raju et al. 1999; Singh et al. 2008). In this settling basin, each chamber has the following features: length of 30.47 m, width of 3.30 m, depth of the transition zone of 2.20 m, whereas the settling zone has the starting depth of 3.39 m with a slope of 2.5 toward the exit. The maximal water velocity in the chamber is 0.259 m/s. At the entrance of each chamber, there is a sluice gate, which is used during the flushing of the settled sediments. Two gates are simultaneously closed during the flushing of the third chamber, whose gate is fully open.

To equalize the flows through the chambers of the settling basin, three sluice gates are used (see Figures 1 and 5). To find their positions, the following methodologies were considered: (1) measurements combined with trial-and-error method (TAE), (2) measurements combined with regression analysis (RA), (3) CFD model combined with TAE, (4) CFD model combined with RA, (5) CFD model supported by a one-dimensional flow model, and (6) CFD model supported with a simple analytical model. he additional models and RA were intended to speed up the solution finding. The first four methodologies from the previous list were excluded because of the inability for proper flow measurements. Namely, the upper surface of the weir is made of reinforced concrete and has only three slits, which are used as the openings for the gates (see Figure 5). Not only do the immersed gates reduce the necessary space for the access of measuring equipment but also the measurements at these positions are not reliable (ISO 748 2007).

The idea was to use the CFD model to find the solution with the help of an as simple as possible analytical model, which would be used as a tugboat that would navigate the CFD model faster toward the solution. The additional value of the analytical model would be in the dynamic flow control by the sluice gates. The development of a model analogous to a one-dimensional pipe flow model was initially tried. Its development was prevented by the inability to find minor pressure losses for a sluice gate at an open channel and for the common inlet zone (see Figures 1 and 4). In addition, the one-dimensional model for the outlet zone that consisted of two T-pipes and a 90 elbow (Idel'chik 1966) (see Figure 4(a)) showed a discrepancy with the CFD data because the real flow has three-dimensional nature. The disruption of one-dimensional flow is caused by: (i) a reinforced concrete beam that submerges the flow streams below the height of the outlet from the reservoir, which (ii) compared with the chambers has a much larger cross-sectional flow area. These were the reasons why the analogy with a one-dimensional flow model was abandoned and a bit more complex ancillary model was developed. The principle behind this analytical model is that the total flow is divided into three parallel streams (see Figure 4(a)). These streams are imaginary in the inlet and outlet zones and real in the chambers, and they have pressure drops equal to the real pressure drop. In the ancillary model, the equations for the minor losses were obtained by uniting simulation results, which were obtained by the CFD model, with RA. In this way, obtained equations for minor pressure losses in the inlet and outlet zones were verified indirectly by the verification of the analytical model. Similarly, the equation obtained for the pressure loss at the sluice gate was verified indirectly and additionally checked by the comparison with the equation for pressure drop at a gate valve installed on a closed channel (Idel'chik 1966).

The model of the water intake, shown in Figure 5, is used to define a fluid domain. This domain has a total volume of 2,721.1 m3 through which the water flows with an average temperature of 12 C and density of 999.45 kg/m3 (The Engineering ToolBox n.d.).

As the water's surface is free (no flow under pressure), the flow through the settling basin is considered as an open-channel flow. For this reason, the flow through the settling basin is measured by means of a current meter. The measurements were conducted according to EN ISO 748: 2007 (ISO 748 2007). The standard specifies methods for determining the velocity and cross-sectional area of water flowing in open channels and for computing the discharge therefrom. The standard deals only with single measurements of the discharge.

Based on the calculated mean velocities in the verticals and the known widths and depths of the water in the chambers, flow rates through the settling basin were calculated. The uncertainty in the flow measurement is 8.48% with a confidence level of 95% and was calculated by Equation (3).

Table 1 shows the relative errors of discrepancies in the chambers for the flow rates obtained by the CFD model in relation to the flow rates obtained by the measurements. The errors for all measurements in all three chambers are less than the calculated measurement uncertainty of 8.48% so that the results of the CFD model can be considered to represent a realistic image of the flow through the chambers of the settling basin.

Table 3 shows the comparison of minor loss coefficients obtained by Equation (7) and by the equation taken from Idel'chik (1966) for closed channel gate valve depending on the relative openness of the gates. In the range from 20 to 80% of the nominal flow rate, these expressions have a good correlation. Compared with the closed channel, the open channel sluice gate produces a slightly larger pressure drop because of the free movement of the water surface in front of the gate.

The comparison of minor loss coefficients obtained by Equation (7) for an open channel and by equation taken from Idel'chik (1966) for closed channel gate valve depending on the relative openness of the gates

The existing sluice gates can be used to control the flows through the settling chambers. Their application equalizes flow rates and average velocities among the chambers but influences the downstream velocity profiles. To create a well-distributed flow without vortices in the chambers, tranquilizing racks should be used just after the gates. In the examined case, their usage completely stops backflow and vortices and reduces maximal velocities in the chambers.

Compared with the pressure losses in the inlet and outlet zones of a multichamber settling basin, the losses in the settling chambers are much smaller. Therefore, in this type of basin, the different widths could not be used to equalize flows through the settling chambers.

Abstract:In the present study, the hydraulic operation of flow through transverse bottom racks screens with conventional triangular wedge wire and alternative circular section wire is experimentally and numerically analyzed. A laboratory prototype is built for experimentation with clean water and sediment. The numerical experiment is developed with the Ansys Fluent software, and a 2D CFD parametric study is designed to evaluate the influence on the captured flow of geometric variables, such as the screen incline, position of the screen along the channel (top, middle, bottom), and shape, wire width, or slot width. The physical model results make it possible to determine the efficiency of the screens in terms of collection capacity and sediment removal. In contrast, numerical experimentation determines the influence of geometric variables on the collection flow. The flows captured by each screen slot are determined numerically. The trends of the total flows captured are determined, and the performance of each studied screen type is assessed using the relevant non-dimensional parameters.Keywords: bottom racks; experimentation; Ansys Fluent; 2D parametric study; CFD

All rivers generally contain sediment loads, depending on the nature of the watershed, including the topography, land cover, land use and soil type. Flow energy is also an essential factor influencing the concentration of sediment loads in rivers. Sedimentation in rivers is a significant problem as rivers comprise large amounts of sediment (Omran & Jaber 2017). The sediment loads in the river increase in rainy season because of the increased flow rate. Rainfall also raises sediment transport through watershed run-off, especially at the onset of the rainy season (Mohammad et al. 2016). Problems related to river use are unavoidable, especially aggradation and degradation. Aggradation reduces the water reservoir in the river leading to floods, whilst degradation causes damage to river structures because of the riverbed decline. Therefore, analysis of river sediment transport is paramount to optimise the river handling (Rafsanjani 2017). 17dc91bb1f