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Issue link: http://wateronline.epubxp.com/i/212760
Technology problem. CFD modeling is effective for both wet well and pipe work applications. The development of CFD models can be fraught with uncertainties for the inexperienced — and not so inexperienced — modeler. The simulation results can vary radically depending on assumptions made regarding boundary conditions, model domain, turbulence parameters, etc. The acquisition of valuable experience and engineering judgment comes with observation of relevant experimental data. The old adage of garbage in/garbage out is very relevant to CFD modeling. Therefore, comparing CFD and physical modeling results is essential to instill confidence in application of CFD modeling techniques. About The Open Source CFD Software The OpenFOAM continuum mechanics toolbox can simulate a wide range of problems, including complex fluid flows with chemical reactions and conjugate heat transfer, solid dynamics, multiphase phenomena, acoustics, and even the pricing of options. OpenFOAM includes numerous preconfigured solvers, utilities, and libraries that allow it to be used like conventional simulation tools. The code is, however, open source (GNU General Public License [GPL] v2), which means customers and consultants have full and free access to the source code, allowing unlimited customization and development of new functionality. OpenFOAM is currently the most widely used open source CFD code, and its continued adoption in industry and academia has forced major changes in the CFD landscape, with reduced pricing for high-performance computing licenses being the most noticeable. Straightening Out The Serpentine The Serpentine Road Pumping Station is located in an existing Brisbane Water pipeline in Queensland, Australia. Flow is delivered into the station from 18" and 12" diameter inlets from three existing pumping stations. The Serpentine Road Pumping Station is equipped with two variable-speed, dry-well mounted pump units. The purpose of the physical model study was to verify the pumping station arrangement. Site restrictions placed limitations on the area that the pump station could occupy, leading to a nonstandard arrangement. An OpenFOAM-based CFD model was developed to validate numerical simulation against physically measured data. The HELYX-OS graphical user interface (GUI), designed for use with OpenFOAM, was also employed. The numerical model was able to reproduce all the flow characteristics reported by the physical model. These flow characteristics included: • surface vortices ahead of the pump • preswirl in pump • submerged vortices adjacent to pump • calculated swirl angle. 22 wateronline.com ■ The results presented in the physical modeling report consisted of flow visualization results and quantitative swirl angle calculations. A single-phase solver can often be used in pump stations, assuming the water surface is flat. In this case, the lowest operational water level produces a cascading water surface profile; therefore, a turbulent, transient multiphase flow solver was chosen for the analysis. A dynamic large eddy simulation (LES) turbulence model was chosen for the numerical simulation. Figure 1: CFD model layout The meshing model was constructed with the OpenFOAM in-house meshing tool. In this type of model, uniform distribution of cell grading is required with refinement in areas of interest. A general view of the CFD domain is shown in Figure 1. The surface adjacent to the outlet boundary had a higher refinement level than walls and floor surface regions. More complex geometry based on CAD drawings is uncomplicated using open source CFD software. Wall boundary conditions consist of wall surfaces modeled using turbulent kinematic viscosity, based on turbulent kinetic energy to represent the smooth concrete surface. Inflow and outflow boundary consists of steady mass flow Figure 2: Surface vortex at pump 2, physical model Water Online The Magazine