Experimental Modelling of Flow in an Open Channel Vortex Chamber

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Sean Mulligan, John Casserly, Richard Sherlock

Thursday 2 july 2015

12:15 - 12:30h at South America (level 0)

Themes: (T) Water engineering, (ST) Hydraulic machinery and industrial flows

Parallel session: 11C. Engineering - Industrial

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The use of strong, free-surface vortex flows has become popular in industry in recent years with applications ranging from energy dissipation in drop shaft structures to novel approaches in hydropower generation. Increasing demand for vortex flow systems necessitates an improved and generalised understanding for the hydraulic characteristics of strong free-surface vortices. In this paper, a comprehensive investigation of the key flow characteristics in an open channel vortex chamber with a subcritical approach flow is presented. The methods of analytical and physical modelling are employed. Particle tracking velocimetry (PTV) and free-surface data from a scaled experimental model are used to validate analytical models and to present information on the dependent variables of the system. Twelve geometries of the vortex chamber casing are examined as well as a single radial flow case. The experimental results indicate that the flow is strongly dependent on three non-dimensional variables related to the approach flow conditions. Two of these variables can be grouped together to create a geometric parameter K1 which directly relates to the circulation number N_ of the vortex. The empirical model can be used to predict the flowrate for a prototype. Errors on the scaled test case using this empirical solution are found to be in the region of 4.5 – 6 %. The free-surface and PTV data also implies that the ideal vortex model provides a good prediction of the tangential velocity and pressure fields with a slight deviation near the vortex core due to the growth of turbulent eddy viscosity or local axial velocities. It is also concluded that the tangential velocity profile is independent of the subsurface depth z/d in the vortex but is diminished close to the tank boundaries and water surface. The radial velocity profiles suggest that the bulk of radial flow is confined to bands at the vessel floor and water surface where the tangential velocity is minimal. The combined outcomes of this paper present simple and effective empirical solutions for approximating discharge and circulation conditions as well as the tangential velocity fields in strong free-surface vortices for relevant industrial applications.