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Comparison of physical observations and simulations of air entrainment

Project ID: 9509
Principal Investigator: James Higgs
Research Topic: Water Operation Models and Decision Support Systems
Funded Fiscal Years: 2014
Keywords: air entrainment, air bulking, cfd, numerical modeling, scale effects, uncertaintly, sensitivity

Research Question

Analyses of two-phase flows (air and water mixtures) are necessary for proper design of penstocks, pipelines, spillways and other hydraulic structures. These analyses, which are frequently needed in Reclamation, often require time consuming calculations, very conservative assumptions, and sometimes physical modeling to properly design and size air vents and other system features. The development of a computational tool to estimate air demand would be very valuable for Reclamation.

Computational Fluid Dynamics (CFD) modeling generally costs about one third the cost of a physical hydraulic model. Scale effects of size and water temperature is problematic in physical models, while in CFD air entrainment routines may need calibration or a sensitivity study on input variables.

Questions:

*Can existing CFD software be used to accurately model two-phase flows for Reclamation application?

*Can existing two-phase flow codes and routines written by previous Reclamation researchers (Monograph 42) be accurately applied through more modern software?

*Can flow turbulence and air bulking reactions be simulated in three dimensions (3D) using numerical modeling to adequately plan for air bulking within Reclamationinfrastructure (i.e. sizing air vents on penstocks) for operations and maintenance and for designing new structures?

*What are the issues and uncertainties inherent in determining the potential for CFD modeling of air bulking in turbulent flows?

*Would existing numeric modeling systems, such as FLOW-3D, be robust and accurate enough to simulate air bulking to answer Reclamations needs?

*Do study results provide confidence for CFD modeling of air entrainment cause by turbulence?

*What is the cost/benefit of CFD and WS77? (WS77 is the code used in Monograph 42, Cavitation in Chutes and Spillways, 1990.)

Need and Benefit

The accuracy of the various Computational Fluid Dynamics (CFD) codes used on Reclamation Projects is always a consideration. Errors from small to large may lead to damaging the infrastructure, additional costs due to over design. Reclamation has several needs for needing to model air entrainment in various systems. These include:
• Air content can reduce cavitation damage in hydraulic turbines (>>>renewable energy<<<), pumps, energy dissipators, high velocity spillway flows, and valves.
• Water treatment where air is needed to sustain microorganisms for water purification (Advanced Water Treatment).
• Overtopping of spillways due to a higher water surface caused by air bulking.
• Air entrainment may change effectiveness of energy dissipators.
• CFD simulation of changes in the effective viscosity of air-water mixtures and sublayer thickness can that have a substantial reduction of friction factor (Chanson, Drag reduction in open channel flow by aeration and suspended load, Journal of Hydraulic Research, Jan 2010)
• Design engineers require knowing inlet capacity to pipes and tunnels. A hydraulic jump and associated air demand and bulking of the flow can impact the capacity.

This information was last updated on September 1, 2014
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