Numerical Investigation of Flow Field in the Skewed Compound Channel

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Dolati Mahtaj Mostafa; REZAEI BAHRAM

Title

Numerical Investigation of Flow Field in the Skewed Compound Channel

Author(s)

Dolati Mahtaj Mostafa | REZAEI BAHRAM | Issue Writer Certificate

Keywords

Numerical Simulation

Velocity Distribution

Boundary Shear Stress

Secondary Currents

RNG Turbulence Model

LES Turbulence Model

Non-prismatic Compound Channel with Skewed Floodplains

Abstract

Introduction: A compound channel consists of one main channel with a deeper flow in the middle and one or two floodplains around the main channel with lower flow depth. The difference between velocity in the main channel and on the floodplains in compound channels creates a strong shear layer at the interface between the main channel and floodplains. Also, because of the three-dimensional (3D) structure of flow, the investigation of flow characteristics in compound channels is completely complicated. In non-prismatic compound channels, due to the mass exchange between subsections, the study of flow is more complex. Therefore, the prediction of flow behavior in the non-prismatic compound channel is an important subject for river and hydraulic engineers. The skewed compound channel is one kind of non-prismatic compound channels. In compound channel with skewed floodplains, one of the floodplains is divergent and the other is convergent. The flow patterns in skewed compound channels have been studied experimentally by many researchers (James and Brown, 1977; Jasem, 1990; Elliott, 1990; Ervine and Jasem, 1995; Chlebek, 2009; Bousmar et al., 2012). However, numerical studies on flow characteristics in skewed compound channels were rarely performed. In this research, the velocity, Boundary Shear Stress distributions, secondary current circulation, and water surface profile in a compound channel with skewed floodplains have been numerically investigated using the Computational Fluid Dynamics (CFD) and two turbulence models of the RNG and LES. Methodology: In the present research, modeled compound channel is similar to the experimental channel used by Chlebek (2009) at the hydraulic laboratory of Birmingham University, Department of Civil Engineering. The experimental studies were performed in a straight flume of 17 m long, 1. 198 m wide, 0. 4 m deep, and with an average bed slope of 2. 003×10-3. The PVC material was used to make compound cross-section. A rectangular main channel of 0. 398 m wide and 0. 05 m deep in the middle, and two floodplains with 0. 4 m wide around the main channel (Fig. 2). The skewed compound channel was made by isolated floodplains using L-shaped aluminum profiles. Experiments were conducted at the skew angle of 3. 81° and four relative depths of 0. 205, 0. 313, 0. 415, and 0. 514. The lateral distributions of depth-averaged velocity and Boundary Shear Stress were measured at six sections along the skewed part of the flume (see Fig. 3), using a Novar Nixon miniature propeller current meter and Preston tube of 4. 77 mm diameter, respectively. For Numerical Simulations of the flow field in the skewed compound channel, the FLOW-3D computational software was used. Also, the renormalization group (RNG) and Large Eddy Simulation (LES) turbulence models were selected as turbulence closure. Two mesh blocks were utilized for gridding, mesh block 1 by coarser mesh size at the upstream of the skewed portion of the channel, and mesh block 2 by smaller mesh size for skewed part (Fig. 5). The flow field is numerically simulated by three computational meshes (fine, medium, and coarse mesh size). Details of gridding for different computational meshes are summarized in Table 2. Finally, the medium mesh by 1653498 cells was selected. For boundary conditions, using volume flow rate condition for inlet, outflow condition for the outlet, symmetry condition for water surface area and the interface of two mesh blocks, and wall condition for lateral boundaries and floor (see Fig. 8 and Table 3). Results and Discussion: The results of the Numerical Simulations show that the RNG Turbulence Model, can predict the depth-averaged velocity and Boundary Shear Stress distributions in the skewed compound channel fairly well (Figs. 9 and 10). In addition, in the skewed compound channel, the mean velocity and Boundary Shear Stress on the diverging floodplain is more than converging floodplain at the same section. The longitudinal discharge distribution on floodplains of the skewed compound channel is linear, and the numerical modeling can compute those values very well (Figs. 11 and 12). By moving along the skewed part of the flume, the regions with higher velocity move toward the diverging floodplain. Also, the position of the maximum velocity, instead of the main channel centerline, move to the interface between the main channel and diverging floodplain (see Figs. 13 and 14). The lateral flow that leaves the converging floodplain, plunging into the main channel flow, creates a secondary flow circulation in the main channel and near the converging floodplain. Also, as moving along the flume and get close to the end of the skewed portion, this secondary flow becomes stronger (Figs. 15 and 16). Regarding the water surface profile in the skewed compound channel, two turbulence models can predict the water depth along the channel fairly well, especially the RNG Turbulence Model (Fig. 17). In addition, the error analysis by using experimental data and numerical results are investigated. For error analysis, Mean Absolute Error (MAE), Mean Absolute Percentage Error (MAPE), Root Mean Square Error (RMSE), and the coefficient of determination (R 2 ) were calculated by using the equations of (12) to (15), respectively. The computational errors between the results of Numerical Simulation and experimental data are presented in Table 5 and are showed in Figs. 18 and 19. Conclusion: In this research, the flow field in a compound channel with skewed floodplains has been numerically simulated. The FLOW-3D software and two turbulence models of the RNG and the LES were used to model the depth-averaged velocity, Boundary Shear Stress distributions, and discharge distribution at different sections along the skewed compound channel. The results of simulations indicated that compared to the LES Turbulence Model, the RNG Turbulence Model are able to predict the velocity and bed shear stress distributions quite well especially at the first half of the skewed portion. Also, by increasing the flow depth, the accuracy of numerical modeling for prediction of the velocity and bed shear stress increase, while for the water surface profile decreases (see Fig. 18).

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APA: Copy

Dolati Mahtaj, Mostafa, & REZAEI, BAHRAM. (2022). Numerical Investigation of Flow Field in the Skewed Compound Channel. JOURNAL OF HYDRAULICS, 17(1 ), 67-87. SID. https://sid.ir/paper/966232/en

Vancouver: Copy

Dolati Mahtaj Mostafa, REZAEI BAHRAM. Numerical Investigation of Flow Field in the Skewed Compound Channel. JOURNAL OF HYDRAULICS[Internet]. 2022;17(1 ):67-87. Available from: https://sid.ir/paper/966232/en

IEEE: Copy

Mostafa Dolati Mahtaj, and BAHRAM REZAEI, “Numerical Investigation of Flow Field in the Skewed Compound Channel,” JOURNAL OF HYDRAULICS, vol. 17, no. 1 , pp. 67–87, 2022, [Online]. Available: https://sid.ir/paper/966232/en

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Scientific Information Database

ISSN: 2588-4824

Journal Paper

Numerical Investigation of Flow Field in the Skewed Compound Channel