Abstract
Floodways, where a road embankment is permitted to be overtopped by flood water, are usually designed as broad-crested weirs. Determination of the water level above the floodway is crucial and related to road safety. Hydraulic performance of floodways can be assessed numerically using 1-D modelling or 3-D simulation using computational fluid dynamics (CFD) packages. Turbulence modelling is one of the key elements in CFD simulations. A wide variety of turbulence models are utilized in CFD packages; in order to identify the most relevant turbulence model for the case in question, 96 3-D CFD simulations were conducted using Flow-3D package, for 24 broad-crested weir configurations selected based on experimental data from a previous study. Four turbulence models (one-equation, k-ε, RNG k-ε, and k-ω) ere examined for each configuration. The volume of fluid (VOF) algorithm was adopted for free water surface determination. In addition, 24 1-D simulations using HEC-RAS-1-D were conducted for comparison with CFD results and experimental data. Validation of the simulated water free surface profiles versus the experimental measurements was carried out by the evaluation of the mean absolute error, the mean relative error percentage, and the root mean square error. It was concluded that the minimum error in simulating the full upstream to downstream free surface profile is achieved by using one-equation turbulence model with mixing length equal to 7% of the smallest domain dimension. Nevertheless, for the broad-crested weir upstream section, no significant difference in accuracy was found between all turbulence models and the one-dimensional analysis results, due to the low turbulence intensity at this part. For engineering design purposes, in which the water level is the main concern at the location of the flood way, the one-dimensional analysis has sufficient accuracy to determine the water level.
Highlights
Broad-crested weirs are simple flow control structures widely used in open channels, in addition to their use in flow measurements
Yazdi et al (2010) used the Volume of Fluid (VOF) algorithm and k-ω turbulence model in 3-D simulation of free surface flow around a spur dike, and it was concluded that sufficient accuracy was achieved when the 3-D model results were compared to the flume results
Experimental data were acquired from Aysegul and Mustafa (2016). eir experiment was conducted under steady-state conditions in a rectangular ume 8 m long, 15 cm wide, and 40 cm in height, at the Hydraulics Laboratory of the Civil Engineering Department of Dokuz Eylul University. e rate of discharge was calculated by a 90° V-notch located at the downstream part of the ume, and the ow depth was collected with a ± 0.1 mm accuracy ultrasonic level sensor (USL). e discharge ranged between 1 258 and 3 177 cm3/s due to pump capacity
Summary
Broad-crested weirs are simple flow control structures widely used in open channels, in addition to their use in flow measurements. E coe cient of discharge of broad-crested weirs has been experimentally studied by Fritz and Hager (1998) without considering the e ect of the weir’s upstream-facing slope. It was agreed by all researchers that the coe cient of discharge increases when decreasing the angle of the weir upstream– facing slope It is clearly shown from all previous studies that the trend of using CFD is increasing and that CFD has been progressively developed with regard to the algorithms used, based on the development of computational and data storage resources. As did Prandtl (1945), Kolmogorov selected the turbulence kinetic energy as one of his turbulence parameters, and the second parameter (ω) represents the rate of dissipation of energy per unit of volume and time, where β and σ are the closure coe cients of the model (Eq 7):. E closure coe cients for (RNG k-ε) model are:
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