Calibration of an Air Entrainment Model for CFD Spillway Applications
Air entrained has become one of the main variables in the study of large spillways performance since it can help avoiding cavitation. Moreover, high rates of air concentration produce significant bulking of the flow as well as a water–solid friction reduction, generating flow acceleration and increasing maximum velocities at the inlet of the energy dissipation structure. Air entrained also affects turbulence inside the flow producing different energy dissipation rates. Aerated spillways physical models are affected by scale effects, with Weber and Reynolds numbers being usually too low to adequately reproduce observed flows. Alternatively, simulation of air–water flows can be carried out by means of Computational Fluid Dynamics techniques in 1:1 scale. However, 3D numerical simulations of spillway flows are time expensive and air–water interfaces need fine resolution meshes which would require extensive computing. Thus, the use of a subgrid scale in air entrainment models can be useful to predict the inception point and the air concentration profile of the flow along the spillway. Computational techniques can handle a more accurate momentum distribution over the spillway sections with affordable costs. In this research, FLOW-3D® routine for turbulent air entrainment is used, coupled with variable density evaluation. VOF and κ-ε RNG turbulence model are also employed. Over 200 spillway flow simulations have been carried out to obtain optimal values of the air-entrainment model parameters, which can be used in future spillway simulations. The calibration of the model is carried out employing prototype data. Interesting conclusions are obtained concerning air entrainment model performance.
KeywordsMultiphase flows Air entrainment Flow bulking VOF κ − ε RNG multiphase Aerated flows
The authors thank Dr. Fabian Bombardelli, UC Davis, for his kind attention and advice.
The research described in this paper has been carried out under the research project “Natural and forced air entrainment in dam spillways and potential range of operation enlargement for hydraulic jump energy dissipators”, BIA2011-28756-C03-01, supported by the Spanish Ministry of Economy and Competitiveness and by ERDF funding of the European Union.
- 1.Balachandar, S., & Eaton, J. K. (2010). Turbulent dispersed multiphase flow. Annual Review of Fluid Mechanics, 42, 111–133.Google Scholar
- 2.Bombardelli, F. A. (2012). Computational multi-phase fluid dynamics to address flows past hydraulic structures. In 4th IAHR International Symposium on Hydraulic Structures, February 9–11, 2012, Porto, Portugal. ISBN: 978-989-8509-01-7.Google Scholar
- 5.Brethour, J. M., & Hirt, C. W. (2009). Drift model for two-component flows. FSI-09-TN83Rev. Flow Science, Inc.Google Scholar
- 8.Bureau, O. R. (1977). Design of small dams. Washington, DC: US Department of the Interior.Google Scholar
- 10.Chanson, H. (1996). Air bubble entrainment in free-surface turbulent shear flows. London: Academic Press.Google Scholar
- 17.Falvey, H. T. (1980). Air-water flows in hydraulic structures. USBR Engineering Monograph, No. 41, Denver, CO.Google Scholar
- 18.Falvey, H. T. (1990). Cavitation in chutes and spillways. USBR Engineering Monograph, No. 42, Denver, CO.Google Scholar
- 22.Hirt, C. W. (2003). Modeling turbulent entrainment of air at a free surface. FSI-03-TN61-R. Flow Science, Inc.Google Scholar
- 24.Hirt, C. W., & Sicilian, J. M. (1985). A porosity technique for the definition of obstacles in rectangular cell meshes. In Proceedings of International Conference on Numerical Ship Hydrodynamics (19 p.). Washington, DC: National Academy of Science.Google Scholar
- 25.Jha, S. K., & Bombardelli, F. A. (2010). Toward two‐phase flow modeling of nondilute sediment transport in open channels. Journal of Geophysical Research: Earth Surface, 115(F3), 2003–2012.Google Scholar
- 34.Pope, S. B. (2000). Turbulent flows. Cambridge: Cambridge University Press.Google Scholar
- 35.Versteeg, H.K., & Malalasekera, W. (2007). An introduction to computational fluid dynamics: the finite volume method. Harlow: Pearson Education.Google Scholar
- 36.Vischer, D., Hager, W. H., & Cischer, D. (1998). Dam hydraulics. Chichester, UK: Wiley.Google Scholar
- 37.Wilcox, D. C. (1998). Turbulence modeling for CFD. La Canada, CA: DCW industries.Google Scholar
- 39.Wood, I. R. (1991). Air entrainment in free-surface flows. In IAHR Hydraulic design manual No. 4, hydraulic design considerations. Rotterdam, The Netherlands: Balkema Publications.Google Scholar