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.