Amirkabir University of TechnologyAmirkabir Journal of Civil Engineering2588-297X54120220321Undular Flow Conditions and Discharge Coefficients in Rectangular Broad-Crested WeirsUndular Flow Conditions and Discharge Coefficients in Rectangular Broad-Crested Weirs125140451210.22060/ceej.2021.18314.6830FABahramNouraniDepartment of Water Engineering, Faculty of Agriculture, University of Tabriz, Tabriz, Iran0000-0001-7921-050XFarzinSalmasiDepartment of Water Engineering, Faculty of Agriculture, University of Tabriz, Tabriz, Iran0000-0002-1627-8598HadiArvanaghiDepartment of Water Engineering, Faculty of Agriculture, University of Tabriz, Tabriz, IranFaezehRezaeiDepartment of Water Engineering, Faculty of Agriculture, University of Tabriz, Tabriz, IranJournal Article20200423<span style="letter-spacing: .05pt;">Weirs are common hydraulic structures that can be used in conveyance water canals for increasing the water depth upstream of turnouts or measurement of flow discharge. In this study, the effect of hydraulic parameters and creation conditions of undular flow in the broad-crested weirs were investigated numerically using the finite volume method and the results were evaluated by the experimental method of other researchers. Results indicated that discharge coefficients (<em>C<sub>d</sub></em>) for experimental data are between 0.321-0.332, whereas the <em>C<sub>d</sub></em> for numerical simulation (using <em>ANSYS FLUENT</em>) is between 0.301-0.354. Over the crest where the minimum water depth (d<sub>min</sub>) happens, when <em>Fr<sub>1</sub> </em>is less than 1.5 (<em>Fr<sub>1</sub></em><1.5), the creation of waves was observed. This type of flow is known as the undular flow. In this situation, measuring water depth over the broad crested weir is not easy and can introduce error for discharge estimation. For preventing of the undular flow, the flow depth cannot be less than a specified value. In this study, this limitation was observed for <em>H/L</em> > 0.1. Thus it can result that long broad-crested weirs (<em>H/L</em><0.1) are more susceptible than the broad-crested weirs (0.1≤<em>H/L</em><0.4) in the creation of the undular flows. Additionally, a regression equation for estimation of the <em>C<sub>d</sub></em> in the broad-crested weirs is proposed with reasonable accuracy.</span><span style="letter-spacing: .05pt;">Weirs are common hydraulic structures that can be used in conveyance water canals for increasing the water depth upstream of turnouts or measurement of flow discharge. In this study, the effect of hydraulic parameters and creation conditions of undular flow in the broad-crested weirs were investigated numerically using the finite volume method and the results were evaluated by the experimental method of other researchers. Results indicated that discharge coefficients (<em>C<sub>d</sub></em>) for experimental data are between 0.321-0.332, whereas the <em>C<sub>d</sub></em> for numerical simulation (using <em>ANSYS FLUENT</em>) is between 0.301-0.354. Over the crest where the minimum water depth (d<sub>min</sub>) happens, when <em>Fr<sub>1</sub> </em>is less than 1.5 (<em>Fr<sub>1</sub></em><1.5), the creation of waves was observed. This type of flow is known as the undular flow. In this situation, measuring water depth over the broad crested weir is not easy and can introduce error for discharge estimation. For preventing of the undular flow, the flow depth cannot be less than a specified value. In this study, this limitation was observed for <em>H/L</em> > 0.1. Thus it can result that long broad-crested weirs (<em>H/L</em><0.1) are more susceptible than the broad-crested weirs (0.1≤<em>H/L</em><0.4) in the creation of the undular flows. Additionally, a regression equation for estimation of the <em>C<sub>d</sub></em> in the broad-crested weirs is proposed with reasonable accuracy.</span>https://ceej.aut.ac.ir/article_4512_9308ce79d389d4864f9eb1c427cb5512.pdf