Studying the effect of trapezoidal channel side slope on the bed shear stress and hydraulic jump characteristics using Flow3D model

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Ebrahimiyan S.; Hajikandi H.; SHAFAI BEJESTAN M.; JAMALI S.; ASADI E.

Title

Studying the effect of trapezoidal channel side slope on the bed shear stress and hydraulic jump characteristics using Flow3D model

Author(s)

Keywords

Computational fluid dynamicsQ1

Suspended sediment loadQ1

Shear stressQ2

Turbidity flowQ2

Abstract

In this paper, the effect of suspended sediments on the bed Shear stress distribution and hydraulic jump characteristics in a symmetrical trapezoidal channel with 45° side slope is simulated numerically. Observations showed that for lowers sediment concentrations (namely below 8%) both the hydraulic jump and the bed stress distribution became asymmetric, while for the higher sediment concentrations, the flow features were symmetric. It was also observed that the bed Shear stress and relative energy losses increased with the sediment concentration, but the jump length decreased. The results indicated that for the inflow Froude number Fr1=7, by increasing the sediment concentration, the ratio of the hydraulic jump length to initial depth decreased by 18. 1% and sequent depths ratio increased by 17. 2%. Meanwhile, the relative energy loss was increased by 3. 2%. While the hydraulic jump phenomenon has been widely studied by various researchers on open channels, most of the experiments that are presented to calculate the hydraulic jump characteristics have been mainly related to rectangular channels (classic hydraulic jumps) by now. There are very few studies on the trapezoidal channels, although most of the water transfer channels are in trapezoidal shape. Accordingly, in the present study, the effect of trapezoidal channel side slope on Shear stress distribution and jump characteristics (including energy deformation, jump length, and conjugate depths) will be investigated by using Flow3D numerical model. So, at first, model has been calibrated by using an experimental results published for trapezoidal channel of 45 degree side slope. The RGN model was selected from the turbulent models. Then, the model for different hydraulic conditions (Froude number 1. 5 to 12) and two slopes of 60 and 75 degrees has been performed and the results have been obtained. Due to the Shear stress importance, which is usually not investigated in experimental researches, in this study the distribution of bed Shear stress has been calculated in all three channels side slopes, and also the hydraulic jump characteristics. Generally, for Computational fluid dynamics simulating, first it needs to evaluate and calibrate the numerical model with experimental results, then cite it after validating and ensuring its acceptable adaptation with the experimental results. In order to validate the model, the experimental research done by Omid et al. (2007) was used. Omid et al. experiments was done in a horizontal channel with glass sides and smooth bed, rectangular cross section with 0. 50 m wide and 9m long, side walls with three different slopes of 30, 45 and 60 degrees, divergence angle of 3 to 9 degrees and Froude numbers from 3 to 9. 8. With comparison of hydraulic jump length profile in different slide slopes, it can be said that the length and depth of the jump on different slopes have completely different behavior, so that by increasing the side slope, the jump length will decrease and the depth of jump will increase. Investigating the relative energy loss graphs on different side slopes of the trapezoidal channel and comparing it with the USBR results for the rectangular channels shows that in channel of 45-degree side slope with Froude number 7. 9 the maximum amount of energy loss occurs, in comparison to the side slopes of 60 and 75 degrees. In this study, by constant considering of the flume specification, the effect of cross-sectional shape on the Shear stress distribution in the channel has been investigated. Concentration of Shear stress on the left of channel of 45 degree side wall after hydraulic jump is noticeable, while such a situation is not observed in channels of 60 and 75 degrees side slope which their cross-sections are closer to the rectangular channel. This is due to the presence of a separation area on the bed called bottom rollers. In general, after hydraulic jump for Froude number greater than 3, by increasing the side slope, the amount of dimensionless Shear stress is reduced. Comparison of channels with different side slopes indicates that the Shear stress amount decreased after hydraulic jump, which the maximum amount by 48% loss will occur in channel of 45-degree side slope. According to this study, in comparison to trapezoidal channels with different side slopes, the hydraulic jump has a different behavior in channel of 45 degrees side slope. Compared to the well-known hydraulic jump in prismatic rectangular channels (classical jump), the flow configuration of jumps in trapezoidal channels is quite different. Classical jumps are characterized by a high velocity bottom jet and an overlying surface roller. Such jumps are usually referred to as two-dimensional. In trapezoidal channels, inflow is lifted in the air and forms a splashing zone because of the presence of a bottom separation zone, referred to as the bottom roller. Thus, the main stream flows near the surface and slowly diverges to the side walls. This is due to the concentration of the inflow to the center portion of the cross-section, and the resulting depression along the side walls. These are the characteristics of the trapezoidal channels. It is seen that the jump is asymmetric to the left; such asymmetries seem to be typical for relatively Froude numbers greater than 3. The results show that the maximum energy loss associated with the channel of 45 degree side slope and Froude number of 7. 9 by 84. 3%, which shows about 5% and 12% increase in efficiency relative to the channels 60 and 75 degrees, respectively. Comparison of the length of the jump to the initial depth indicates that the highest value is related to the angle of 45 degrees and shows a difference of 20% and 31% relative to the channel 60 and 75 degrees, respectively. Comparison of the graphs shows that with the increase of the side slope, from 45 degrees (slope 1: 1) to 60 degrees (1: 0. 58) and 75 degrees (1: 0. 27), the ratio of the sequent depths decreases.

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APA: Copy

Ebrahimiyan, S., Hajikandi, H., SHAFAI BEJESTAN, M., JAMALI, S., & ASADI, E.. (2020). Studying the effect of trapezoidal channel side slope on the bed shear stress and hydraulic jump characteristics using Flow3D model. IRANIAN WATER RESEARCH JOURNAL, 13(4 (35) ), 103-112. SID. https://sid.ir/paper/159881/en

Vancouver: Copy

Ebrahimiyan S., Hajikandi H., SHAFAI BEJESTAN M., JAMALI S., ASADI E.. Studying the effect of trapezoidal channel side slope on the bed shear stress and hydraulic jump characteristics using Flow3D model. IRANIAN WATER RESEARCH JOURNAL[Internet]. 2020;13(4 (35) ):103-112. Available from: https://sid.ir/paper/159881/en

IEEE: Copy

S. Ebrahimiyan, H. Hajikandi, M. SHAFAI BEJESTAN, S. JAMALI, and E. ASADI, “Studying the effect of trapezoidal channel side slope on the bed shear stress and hydraulic jump characteristics using Flow3D model,” IRANIAN WATER RESEARCH JOURNAL, vol. 13, no. 4 (35) , pp. 103–112, 2020, [Online]. Available: https://sid.ir/paper/159881/en

Scientific Information Database

ISSN: 2588-4824

Journal Paper

Studying the effect of trapezoidal channel side slope on the bed shear stress and hydraulic jump characteristics using Flow3D model