Vorticity Considerations for CFD and Scaled Physical Models

Matyas Benke, David Kelsall, Sajid Rafique

Friday 3 july 2015

9:15 - 9:30h at Africa (level 0)

Themes: (T) Special session, (ST) Design of intake stations

Parallel session: 14D. Special session: Design of intake stations

Practical experience has demonstrated that for flowing systems and flow-bearing structures (such as dams, pumping stations, control structures, screening systems and intakes) sustaining long term operations requires a good hydraulic design. Typically, optimal hydraulic design requires steady uniform flows together with minimal head losses through the system. Despite the progress in modern computational methods, design engineers may face considerable challenges in demonstrating the hydraulic effectiveness of the design. There may be a temptation to use computational methods alone to verify the hydraulic design of the flow system. One particular area of practical concern is the formation of vortices. Depending on their strength, attachment points and their mobility, some vortices can induce significant vibration in pumps, turbines and mechanical screens, or even erosion of critical surfaces. Currently it is difficult to rely on CFD to classify the strength or type of any vortex likely to be present due to their transient and random nature. CFD analyses rarely differentiate between a weak and diffuse rotating vortex or, at the other end of the spectrum, a fully developed vortex with an air core. Depending on the skills of the CFD practitioner, the CFD application, the availability of computer resources, and time pressures, vortex activity may be overlooked or underestimated. This may compromise the quality of the design, with increased risk of destabilising structures and/or inefficient operation of mechanical installations. This paper considers the state of the art for vortex characterisation in the context of the industrial application of CFD. For example, to replicate real time behaviour of vortices would require more computer resources than most organisations have, and longer simulation times than projects would allow. Furthermore, by considering the effects of vorticity in various case studies, this paper shows how computational fluid dynamics (CFD) and scaled physical modelling may be used together to ameliorate the effects of vorticity.