Experimental and Numerical Modelling of Free-Surface Turbulent Flows in Full Air-Core Water Vortices

The results from analytical, numerical and experimental modelling of free-surface vortex flows are presented. Vortex flow is induced in a gravity-driven, open-channel flow chamber with a subcritical approach flow and is simulated using the ANSYS CFX steady Eulerian multiphase flow model in order to determine water surface- and velocity-field characteristics. Solution sensitivity to mesh type, density and various turbulence closure methods is considered. The water surface and tangential velocity profile are also modelled using the Vatistas (n = 2) analytical model. The numerical solution is validated using experiments conducted in a scaled physical model of the chamber which permits the investigation of the air/water interface and determination of the velocity fields using particle tracking velocimetry. The sensitivity analysis carried out presents a case for mesh independence and gives evidence that the baseline Reynolds stress model is most suited in simulating free-surface vortex flows. The predicted shape of the air core is in agreement with the physical model but the location of the resolved free-surface interface is under predicted. Concerning the velocity field, the Reynolds stress model makes a fair to moderate prediction of the tangential velocity field; however, the radial velocity field is typically underpredicted. It is concluded that unsteady flow features inherent in the vortex, namely, free-surface instabilities, are preventing the steady-state model from achieving the required accuracy, thus requiring further transient analysis.