International Conference on Civil Engineering Architecture & Urban Sustainable Development 27&28 November 2013, Tabriz , Iran 1 Study of Flow over a Triangular Broad-Crested Weir SEYED HOOMAN HOSEINI 1* , ARMIN SHAYEGAN 2 , HOSSEIN AFSHAR 3 1- Department of civil Engineering, Islamic Azad University of Central Tehran Branch, Tehran, Iran. Email: Hooman.Hoseini@yahoo.com 2- Department of civil Engineering, Islamic Azad University of Central Tehran Branch, Tehran, Iran. 3- Department of Mechanical Engineering, Islamic Azad University of East Tehran Branch, Tehran, Iran. Abstract A broad-crested weir is widely used in rates in open channels, rivers, irrigation and drainage systems. In this study, basic experiments were conducted on a triangular broad-crested weir. Also, Computational Fluid Dynamics (CFD) model together with laboratory model were used to determine the free-surface profile of rectangular broad-crested weir. Simulations were performed using the volume of fluid (VOF) free surface model and turbulence model of the RNG k–ε to find the water level profile and results are compared with several experimental data. The structured mesh was used for all domains with high dense mesh near the solid region. A comparison between experimental data and simulations indicates that results obtained from numerical modeling agree well with experimental data and the CFD model can be used to predict the complex flow and water level with high accuracy. Keywords: Broad-crested weir, CFD, Laboratory model, VOF, RNG k–ε. 1. Introduction Broad-crested weirs are defined as structures where the streamlines run parallel to each other over the weir crown, and the crest of the weir is horizontal (Bos, 1976). In most layouts of broad- crested weirs also the hydrostatic pressure is fully accomplished in the middle of the crest. However, in cases where the weir length is too small it might be that the hydrostatic pressure is not fully accomplished (Hager, 1986). The broad-crested weir is in addition to irrigation systems used for highways, railroads and for hydropower structures. Also an application as a simple discharge measurement structure is possible. Study of broad-crested weirs has attracted the attention of many investigators (Ackers 1978, Clemmens 1984 and Montes 1964) performed experiments on broad crested weirs. Woodburn