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In this article, two-phase slug flow is simulated numerically in a horizontal duct with rectangular cross-section using Volume Of Fluid (VOF) method. Conservation equations of mass, momentum and advection equation are solved in open source OpenFOAM code accompanying k-ω SST turbulence equations. Simulation is conducted based on the experimental results in the duct with rectangular cross-section. The results shows, due to Kelvin-Helmholtz (K-H) instability criteria slug initiation forms in the air-water interface during three dimensional turbulence modeling. Water level was increased slightly at interface in both numerical simulation and experiment. This level increase satisfies the K-H instability to generate a slug at interface. During slug initiation, the pressure behind slug is increased significantly. Big pressure gradient at the beginning of the slug in compare to the end of it causes the slug length to be increased as propagate along the duct. The numerical simulation of present research is capable of predicting the slug length accurately in accordance with experiment; however, the slug position with 22% inaccuracy was obtained. Comparison of the results with the numerical and experimental results of other researchers confirms higher accuracy of flow prediction in the present work.

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The present study is subjected to analytical, numerical, and experimental simulation of hydraulic characteristics of flow over the streamlined weirs. Numerical simulations were performed using an open source software namely OpenFoam. According to the objectives of the present study, to evaluate the results of numerical modeling, experimental investigation was conducted, studying different models of streamlined weirs, experimentally. The profiles of the experimental models as well as the simulated numerical models were designed based on the Joukowsky transform function. By analyzing the results of different turbulence models including standard k-ε model, realized k-ε model, RNG k-ε model, k-ω SST model and Reynolds stress LRR model, the k-ω SST model was chosen as the most accurate numerical turbulence model for the simulation of flow over the streamlined weirs. The results of the numerical simulations for different flow discharges and different geometrical characteristics, indicated that, increasing the flow discharge and the relative eccentricity in Joukowsky transform function, tends to increase the velocity and consequently decrease the pressure over the weir crest. Therefore, the lowest pressure and the most probable potential of cavitation belongs to the circular-crested weirs with λ = 1 and high flow discharges. Furthermore, the results show that the greatest bed shear stresses and the compressive forces occur at the downstream end of the circular-crested weirs, thus the downstream zone of the circular-crested weirs is responsible to large values of bed erosion. This is partly due to formation of shock waves, reduction of the flow depth and enhanced velocity of flow downstream of the circular-crested weirs. Furthermore, the lowest bed shear stresses occur at the upstream end of the circular-crested weirs. Therefore, potential of sedimentation upstream of the circular-crested weirs increases. Accordingly, by employing streamlined weirs with λ< 1, and an appropriate curvature, unfavorable flow conditions would be improved, leading to a more safe and economic hydraulic structure. The present study is subjected to analytical, numerical, and experimental simulation of hydraulic characteristics of flow over the streamlined weirs. Numerical simulations were performed using an open source software namely OpenFoam. According to the objectives of the present study, to evaluate the results of numerical modeling, experimental investigation was conducted, studying different models of streamlined weirs, experimentally. The profiles of the experimental models as well as the simulated numerical models were designed based on the Joukowsky transform function. By analyzing the results of different turbulence models including standard k-ε model, realized k-ε model, RNG k-ε model, k-ω SST model and Reynolds stress LRR model, the k-ω SST model was chosen as the most accurate numerical turbulence model for the simulation of flow over the streamlined weirs. The results of the numerical simulations for different flow discharges and different geometrical characteristics, indicated that, increasing the flow discharge and the relative eccentricity in Joukowsky transform function, tends to increase the velocity and consequently decrease the pressure over the weir crest. Therefore, the lowest pressure and the most probable potential of cavitation belongs to the circular-crested weirs with λ = 1 and high flow discharges. Furthermore, the results show that the greatest bed shear stresses and the compressive forces occur at the downstream end of the circular-crested weirs, thus the downstream zone of the circular-crested weirs is responsible to large values of bed erosion. This is partly due to formation of shock waves, reduction of the flow depth and enhanced velocity of flow downstream of the circular-crested weirs. Furthermore, the lowest bed shear stresses occur at the upstream end of the circular-crested weirs. Therefore, potential of sedimentation upstream of the circular-crested weirs increases. Accordingly, by employing streamlined weirs with λ< 1, and an appropriate curvature, unfavorable flow conditions would be improved, leading to a more safe and economic hydraulic structure.

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Accurate modeling of the sub-grid scales (SGS) is crucial in determining the accuracy of the large eddy simulations (LES) in turbulent flow analysis. In recent years, new branches of the sub-grid scales models called gradient-based models were developed in computing the sub-grid scales stresses and heat fluxes and used in large eddy simulations. In this work, the modulated gradient model (MGM) equations were implemented in the OpenFOAM package, and pimpleFoam solver was modified to improve the solution accuracy. The modulated gradient model is based on the Taylor-series expansion of the sub-grid scales stress and employs the local equilibrium hypothesis to evaluate the sub-grid scales kinetic energy. To assess the accuracy of the modulated gradient model as well as the improved pimpleFoam solver, turbulent channel flow at a frictional Reynolds number of 395 was simulated via the OpenFOAM package and results were compared with the direct numerical simulation (DNS) data as well as the numerical solution of the Smagorinsky, Dynamic Smagorinsky, Deardorff models. The results show that modulated gradient model evaluates first and second order turbulence parameters with a high-level of accuracy.

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“Minimum-dissipation sub-grid models” are simple alternatives to the Smagorinsky-type approaches to imposing sub-grid scales (SGS)' effects in the large-eddy simulation (LES) approach. Recently, a new model in this family called “anisotropic minimum-dissipation (AMD)” model is represented. AMD is classified as a static type eddy-viscosity sub-grid scale model. The model is more cost effective than the dynamic Smagorinsky model, furthermore; it is not only able to consider the effect of various directions in computing sub-grid stress but also capable of operating for transitional flows from laminar to turbulent. In this study, this sub-grid model has been implemented in the open source package OpenFOAM and its performance is evaluated in the prediction of the flow field inside a channel with a pressure driven air flow. The accuracy of the model has been investigated at different Reynolds numbers including transient and fully turbulent flows and compared with the dynamic Smagorinsky model as well as direct numerical simulation (DNS) solutions. Results reveal that this sub-grid model is quite accurate over a broad range of Reynolds numbers once calculating velocity profiles as well as first and second-order turbulent quantities.

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