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Technology rate. Prerotation induced by the pump's rotation element does not affect upstream flow patterns when the pump is operated at design flow; therefore, including a rotating element in the numerical model is not required. Angling For A Solution The preswirl at the floor directly under pump 1 was also captured by the numerical model. The angle is calculated based on the ANSI/HI standards. For the physical model, a rotating vane is used to determine the swirl angle. Intense bidirectional fluttering interspersed by periods of defined rotation was observed, along with a swirl angle of 2 degrees. A plot of the swirl angle from the CFD model is shown in Figure 5. The same bidirectional flow characteristics are observed by the CFD model. The swirl angle is 5 degrees, which is higher than reported for the physical model. This can be attributed to friction loss at the vane, and the prototype flow strength may be slightly underestimated as a result of possible scale effects. It is not uncommon for physical model discharges to be increased by a factor to compensate for these scale effects. Figure 3: Surface vortex at pump 2, CFD model bottom view CFD vs. Physical Modeling Animations of the physical model show that the location of the surface vortex is unstable; it appears submerged and subsurface at various times and tends to periodically disappear. The fully developed form is that of a surface vortex extending to the pump inlet. Figure 5: Swirl angle plot Summary: The Benefits Of CFD Modeling This article briefly describes the complex analysis of pump stations using open source software. CFD modeling can be undertaken relatively inexpensively due to freely available software. Advantages of CFD modeling of pump stations include: • optimization of the design pump stations • identification of adverse hydraulic conditions • reducing probability of costly pump station failure • meeting performance criteria • easier pump station commissioning. Figure 4: Back wall subsurface vortex at pump 2, numerical model Figures 2 and 3 show the same vortex forming at pump 2 with a pool surface elevation of -3.4' (-1.05m). Further refinement is required to fully capture the very fine core of the vortex; this is not necessary for the purpose of this assessment. Submerged vortices adjacent to pump 2 were analyzed during the physical study. The numerical results are shown in Figure 4, which shows agreement with the physical model. wateronline.com Franz Jacobsen has worked widely within the field of civil engineering, including construction and design. His experience includes dams design and other hydraulic structures, as well as river and floodplain modeling. In recent years he has been specializing in hydraulic engineering utilizing complex CFD flow. ■ Water Online The Magazine 23