PhD Oral Exam - Rui Zeng, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

In hydraulic engineering, a transition is used to facilitate a change in the direction, slope or cross-section of an open channel or pipeline. This thesis focuses on the behaviours of turbulent flows in transitions where the flows expand in the streamwise direction. Such expanding flows are typically observed in open-channel expansions and hydraulic jumps. This thesis uses wall-resolved Large Eddy Simulation (LES) to study the two-phase turbulent flows in several three-dimensional (3-D) geometries, including non-prismatic open channels and sloping pipes. The LES predictions compare well with corresponding experimental results and benchmark solutions.

First, a turbulent bistable flow (TBF) in a straight-wall channel expansion is studied. For a given set of conditions of flow approaching the expansion, either of two stable flow states can possibly occur, depending on the flow history. The purpose is to reveal the ensemble-average flow characteristics and explore effective ways to control bistability. The velocity field is decomposed into deformation regions and eddy-rotation regions using the Okubo–Weiss parameter. Turbulent eddies initiated by shear instability dominate those associated with sidewall-friction force; this condition is responsible for the occurrence of bistability. Fitting a simple hump at a flat-bottom expansion is an effective way to suppress bistability. The presence of the hump shrinks eddy cores, and breaks the interaction between eddies triggered by instabilities and eddies induced by friction forces. The result is an increase in flow uniformity, and the control of turbulence, flow separation and vortex behaviour.

Next, hydraulic jumps in sloping pipes are investigated to achieve an improved understanding of jump behaviours driven by discharge and slope. Some aspects of the jump behaviours such as free-surface fluctuations and jump-toe oscillations resemble the classical hydraulic jump on horizontal floors. Others like the 3-D distributions of core jet, vorticity and aeration are much more complicated. Depending on the discharge and slope, the resulting jump can be a complete or an incomplete jump. The incomplete hydraulic jump causes flow choking downstream. This has severe consequences on drainage conditions in sewer pipes laid on a sloping terrain. The Okubo-Weiss parameter is a new way to subtly delineate the region of hydraulic jump. It is much more efficient and less ambiguous, compared to traditional visual inspections.

Last, to study turbulent flows in a non-prismatic warped expansion using LES, the thesis discusses rigorous strategies for model setup, parameter selection and parametric value assignment. Mapping mean-velocity distributions from experimental data, combined with the spectral synthesiser approach to velocity fluctuations, gives a satisfactory inlet condition in LES; alternatively, a 1/7th power-law for the mean-velocity, combined with the vortex method for the fluctuations, is acceptable. The predicted turbulence kinetic energy distribution is useful to further study turbulence control and hydraulic efficiency improvement.

Compared to a prismatic channel, a non-prismatic channel exhibits more complicated flow separation, eddy formation and turbulence interaction, differentiating this thesis from previous studies of prismatic open-channel turbulence. This thesis contributes to a systematic assessment of computational strategies and of result visualisation and analysis, with relevance to practical applications.