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Numerical simulation of an air-core vortex and its suppression at an intake using OpenFOAM
A common challenge faced by engineers in the hydraulic industry is the formation of free surface vortices at pump and power intakes. This undesirable phenomenon which sometimes entrains air could result in several operational problems: noise, vibration, cavitation, surging, structural damage to turbines and pumps, energy losses, efficiency losses etc. Notwithstanding these problems, free surface vortices have also seen beneficial applications in water vortex hydropower plant system, water treatment, vortex drop shafts in sewer systems, vortex settling basin and vortex confined chambers. This paper investigates the numerical simulation of an experimentally observed air-core vortex at an intake using the LTSInterFoam solver in OpenFOAM. The solver uses local time-stepping integration. In simulating the air-core vortex, the Standard k-ε, Realisable k-ε, Renormalisation Group (RNG) k-ε and the Shear Stress Transport (SST) k-ω models were used. The free surface was modelled using the Volume of Fluid (VOF) model. The simulation was validated using a set of analytical models and experimental data. The SST k-ω model provided the best results compared to the other turbulence models. The study was extended to simulate the effect of installing an anti-vortex device on the formation of the free surface vortex. The application of the local time stepping approach helped to speed up convergence towards steady state conditions. The LTSInterFoam solver proved to be a reliable solver for the steady state simulation of a free surface vortex in OpenFOAM. However, being a steady state solver, it is unable to account for the transient process of evolution of a free surface vortex.