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In order to assess the effect of turbulence models in prediction of flow structure with adverse pressure gradient, steady state Reynolds-averaged Navier-Stokes (RANS) equations in an annular axisymmetric diffuser are solved. After selection of the best turbulence model, an approach for the shape optimization of annular diffusers is presented. The goal in our optimization process is to maximize diffuser performance and, in this way, pressure recovery by optimizing the geometry. Our methodology is the optimization through wall contouring of a given two-dimensional diffuser length and area ratio. The developed algorithm uses the CFD software: Fluent for the hydrodynamic analysis and employs surrogate modeling and an expected improvement approach to optimization. The non-uniform rational basic splines (NURBS) are used to represent the shape of diffuser wall with two to ten design variables, respectively. In order to manage solution time, the Kriging surrogate model is employed to predict exact answers. The CFD software and the Kriging model have been combined for a fully automated operation using some special control commands on the Matlab platform. In order to seek a balance between local and global search, an adaptive sample criterion is employed. The optimal design exhibits a reasonable performance improvement compared with the reference design.

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Spillways have long been of practical importance to safety of dams, therefore these structures have to be built strong, reliable and highly efficient. Ski jump dissipator is one the flow energy dissipators which is applicable downstream of spillway chutes with velocity over 20 m/s. Flow over a flip-bucket is a two-phase and strongly turbulent flow. Turbulence modeling is one of the most limiting factors in accurate computer simulation of flows. By fixing the grid resolution and the discretization scheme, the difference of computation time is mainly attributed to the turbulence model. The choice of turbulence model depends on factors such as the physics encompassed in the flow, the level of accuracy required, the available computational resources, and the amount of time available for the simulation. It is a fact that no single turbulence model is universally accepted as being superior for all classes of problems.
The main purpose of the present study is numerical investigation of two-phase turbulent flow over a triangular flip-bucket to evaluate effects of different turbulence models in this type of flow. Hence, using FLUENT® software, two dimensional Reynolds averaged Navier-Stockes equations have been solved in unsteady state. Different turbulence models consist of k-ε, k-ω and RSM; have been used. To simulate two-phase flow, volume of fluid (VOF) method has been applied.
Standard k-ε and stress-omega RSM models with low-Reynolds number modifications have the best performance among the other turbulence models. In standard k-ε model when low-Reynolds number modification was activated, the effects of molecular viscosity were taken into account in near-wall regions. Therefore, in low-Reynolds number k-ε model, maximum dynamic pressure over the bucket was predicted more accurately in comparison with standard k-ε model. Regarding modification in strain-pressure terms in turbulence equations, effects of anisotropic Reynolds stress tensor were taken into account in stress-omega RSM model with low-Reynolds number modifications. Thus, compared to other turbulence models, numerical results of this model are in a better agreement with experimental results. Different k-ε models could not predict the jet trajectory after the bucket very well. Due to blending function in SST k-ω model, this turbulence model effectively blended the robust and accurate formulation of the k-ω model in near-wall regions with the free-stream independence of the k-ε model in the far field. In estimation of maximum dynamic pressure over the bucket, this model had a better performance than standard k-ω model and relatively similar results to k-ε model. In addition, SST k-ω model has shown the best prediction of the jet trajectory among other turbulence models. Eventually, with respect to computation cost and accuracy of results, SST k-ω turbulence model has been introduced as the most suitable turbulence model to predict the flow pattern of a triangular flip-bucket.