Water Online

May 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

Issue link: https://wateronline.epubxp.com/i/816402

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massively parallel supercomputers. Alden discussed a one-year modeling effort that compared physical and CFD modeling results using three different commercially available CFD codes (Fluent, CCM+, and FLOW-3D) with simulations conducted by three CFD engineers. Agreement among the experts was unanimous: While CFD is considered a valuable and important tool for enhancing the value of physical model studies, it is still far from being able to replace physical models for pump intake modeling. CFD was unable to predict vortex strength in accordance with the existing HI criteria, and in some cases the direction of rotation was incorrect. A significant shortcoming of using the current HI Standards when interpreting CFD model results is that the measurement techniques and acceptance criteria outlined in ANSI/HI 9.8- 2012 were developed specifically for physical models. The basis for acceptance is measurements that are readily made in a physical model, such as the swirl meter shown in Figure 2. A CFD model cannot report the number of swirl meter rotations because the swirl meter is not included in the model. Therefore, it is recognized that as CFD models improve, it may be necessary to develop standards specific to CFD modeling. Additionally, CFD model results can be influenced by the individual modeler and the CFD code. For instance, different modelers will create different computational meshes, which in turn can affect results. To date there is no standardization for CFD modeling of pump intakes. When CFD is used to model a pump intake, it is important for the owner to realize that no accepted standards for the modeling exist. The Future An interesting topic for discussion is how the use of CFD in pump intake modeling will evolve in the coming years and decades. Crystal balls for predicting the future of CFD are hazy at best, but some information can be surmised with reasonable certainty. CFD models that use two-equation Reynolds Averaged Navier-Stokes (RANS) equation-based turbulence models are unlikely to be able to capture vortexing at pump intakes. The equations include simplifications that should preclude them from consistently predicting vortex formation. For example, the RANS equations assume that turbulent viscosity is isotropic (the same in all directions). While this assumption is valid for many situations, it does not apply to strong swirling flow, such as vortices, where turbulent viscosity is direction-dependent. LES models hold more promise, and while the models are too computationally intensive to be financially viable at this time, computers will continue to improve, and this limitation is expected to gradually diminish. In the near future, LES models will likely remain cumbersome to use and likely require vast amounts of output required to determine if a time-dependent vortex is forming. Despite these challenges, the use of CFD as a component of pump intake studies is expected to increase. Extensive testing and comparisons between CFD results, physical models, and prototype performance are required to confirm that the CFD models can achieve a predictive reliability similar to that of physical models. The HI standards will need to be revised to include performance parameters that can be readily quantified in a CFD model. The original standards were based on many years of research, and it is reasonable that a similar amount of time may be required to develop and test standards written around CFD. Once the requirements for a reliable CFD model are established, it remains to be seen if a CFD model will be less expensive than a physical model. Conclusion While CFD is a useful and important tool in the pump intake designer's toolbox, at present CFD modeling cannot be used to show compliance with the acceptance criteria presented in the testing section of the Hydraulic Institute Standards (ANSI/HI 9.8-2012 and ANSI/HI 9.6.6-2009). A major research effort will be required to conduct the necessary testing required to develop standardized methods for the use of CFD in pump intake modeling. For additional information about the HI Standards, please contact Peter Gaydon (director, technical affairs, Hydraulic Institute, pgaydon@pumps.org). n wateronline.com n Water Innovations Dan Gessler, PE, Ph.D., DWRE, vice president of Alden Research Laboratories, has over 25 years of experience and is responsible for overseeing all of the hydraulic modeling completed at Alden. He provides technical expertise in numeric and physical modeling and also works on projects that combine large CFD and physical modeling efforts. About The Author PUMPSYSTEMS 34 Andy Johansson, director of hydraulic modeling, is responsible for all physical hydraulic model testing performed in Alden Research Laboratories' Holden, MA, office. He provides technical and administrative supervision of Alden's Holden Hydraulic Modeling Department to ensure high technical standards are maintained. Mr. Johansson has been with Alden for over 27 years. Becca Hall, engineer II, is a project manager at Alden who special- izes in conducting physical model tests with an emphasis on pump intake modeling. She has over 10 years of experience in physical modeling of hydraulic structures. Figure 2. Swirl meter instrumentation

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