عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Being aware of such phenomena like dam break may seriously prevent human and environmental disasters. Dams' breakage may lead to devastating waves in downstream valleys, which cause casualties, severe damages to infrastructure and human loss of life. Therefore, leaves huge economic burdens to the governments. Studies related to dam break, especially the flood wave, are necessary for safety management in the downstream of dams. Wave velocity and bed roughness are the two main parameters affecting the qualitative and quantitative changes of flood spreading in downstream. Bed roughness can produce more resistance against flood flow and thus, reduces the wave velocity and increases the depth of flow. It can change the form of wave profile, as well.
The flood wave caused by dam break phenomenon can be simulated experimentally and numerically. In the laboratory, a dam break can be produced by an abrupt opening of sluice gate, which is installed in a flume. In this research study, the dam break has been numerically modeled by using the three- dimensional Flow-3D model. The measured water elevations and velocities of flood wave were provided from another research study, which was carried out in the laboratory of Water Sciences Engineering Faculty, Shahid Chamran University of Ahvaz. Flow-3D model is powerful software in CFD, which has been developed by Flow Science Inc. and is generally used for open channels flow modeling. In this software, five turbulence models are applied and each of them is able to produce a calculation mechanism to quantify the effect of turbulence fluctuations on flow parameters such as the average velocity and the depth. In this study, the geometry model was produced in AutoCAD software and the appropriate grids in upstream and downstream of the sluice gate were developed by the Flow-3D model. The positions of the small triangular pieces used to generate rough bed in downstream were defined as the fixed points in the model. Defining the boundary conditions, the wall conditions were used for the bed and symmetry condition assumed for walls. The initial conditions were defined as constant water elevation in upstream and downstream of the sluice gate.
In this laboratory study, 4 values for downstream water depths (0, 3, 6, 9cm), 3 values for upstream head (24, 28 and 32cm) and 3 types of bed roughness (soft, regular and zigzag patterns of small triangular pieces) were the variables and the water elevation and velocity profiles were provided using the image processing method and a suitable software. In this software, the user was able to specify the wave profile at any time of wave propagation and any place. In this numerical simulation, the upstream head was just 24 cm, so for all 12 water elevations, the profiles were simulated and were compared with the corresponding measured profiles. All five turbulence models were used, to complete the numerical simulation. Comparison of the simulated and measured water elevation profiles showed that the Prandtl and one equation turbulence models were not able to simulate the flow conditions, accurately. The RNG turbulence model showed the best performance among all the proposed turbulence models. Therefore, after verifying the RNG turbulence model, it was possible to print out the water elevation and velocity profiles at any time and for any type of bed roughness and downstream water depths.
Analyzing the simulated results showed that the bed roughness was an important parameter in progressive wave velocity and acted as a resistant factor. It was found that, for the zigzag pattern of bed roughness, the longitudinal decreasing slope of the wave velocity was higher than the regular pattern. And this could be due to the form of ordering the small triangular pieces. The turbulence in the flow, near the bed, was greater for the zigzag pattern in comparison to the regular pattern, due to the interface of pieces and water which caused more resistance, lower velocities and higher depths in longitudinal profile, in the same hydraulic boundaries. Downstream depth was another parameter which affected the form of longitudinal velocity and the water elevation profiles. The average wave velocity decreased with increasing downstream water depth. It was obvious that increasing the downstream water depth amplify the effect of bed roughness in decreasing the velocity. The maximum effectiveness was simulated for the zigzag pattern with 9cm downstream water depth. The other effect of downstream water depth was in the form of the wave break, immediately after the dam break.
According to the results of model predictions and measured values, it was found that the effect of downstream water elevation on reducing wave velocity was much more than the bed roughness. For example, the average of reduction in velocity, soft bed to rough bed, was calculated %24. However, this value for 0 and 9cm downstream depth was %71.