A scale model investigation of free surface vortex with particle tracking velocimetry

Abstract: The occurrence of free surface vortices has been a major source of problems in different kinds of hydraulic intakes. Strong free surface vortices have a tendency to introduce swirl, floating matter and air into the intake, all of which are considered more or less harmful for intake and its subsequent structure operation. Formation and strength of free surface vortices can be minimised by proper design of the intake and/or using vortex suppressors. Intake submergence being the main governing factor in free surface vortex formation, it is chosen as the main design factor in vortex-free intake design.The free surface vortex formation at a hydraulic intake and subsequently its strength and position are, however, also highly influenced by the shape of the surrounding boundaries and approach flow conditions. As these properties always vary with each different installation sites, it is necessary to test the proposed designs thoroughly, even though they had been created according to the design criteria. The testing has to be done with properly scaled hydraulic models, as the geometries at the intake sites tend to be so complex that it is not possible or cost-effective to use computational methods.In this study, the formation of free surface vortex and the affecting factors on its strength were studied at spillway intakes of 1:100 hydraulic scale model of Xiluodu hydropower dam. The findings presented in this thesis reflect the true behaviour of vortices in the prototype. Data collection was done using particle tracking velocimetry (PTV) on the free surface flow field around the spillway intakes of the dam. The surface flow field data produced by the PTV system was interpolated, and the obtained full flow fields were analysed according to the Helmholtz vortex theorems and Rankine combined vortex theory principles: velocities around irregularly shaped vortices were averaged and the circulation strengths were calculated according to the theory.The shape of the intake surroundings was found to be a highly affecting factor in the vortex formation, mainly affecting the local approach flow condition, which is one of the main governing factors in vortex formation, and vortex pinning position, which determines the length of the vortex core and simultaneously affects the strength of the vortex. By changing the discharge inside the spillways with control gates and using only one of the each contiguous spillways, was found to be a way to control vortex formation. During the flood peak conditions it is not possible to avoid harmful vortices, and when compared to vortex-free intake design criteria, it is not possible to avoid harmful vortices even with higher intake submergences. However this kind of conditions occur very seldom, which may not be so harmful for the structures as they have withstand the conditions only for short time periods.