PhD Oral Exam - Mustafa Al Kayed, 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

The characteristics of turbulence in open-channel transitions have important implications for hydraulic engineering. This study considers a warped transition that links a rectangular channel section with a relatively large trapezoidal channel section. The change in cross-sectional shape as well as area causes an adverse pressure gradient. As a result, the primary flow separates from the transition sidewalls, turbulence eddies form, and losses of flow energy occur; these conditions reduce hydraulic efficiency and are highly undesirable. Previous researchers have made great efforts to reduce energy losses and improve hydraulic efficiency but have not addressed the issues adequately. This study aims to explore the effectiveness of introducing a honeycomb with small open cells to the transition in order to improve the flow characteristics. This study took an experimental approach. Three-dimensional instantaneous velocities were measured using an acoustic Doppler velocimeter. An analysis of the measurements produced cross-sectional distributions of the longitudinal flow velocity, secondary flow, turbulence kinetic energy, and Reynolds shear stress. The energy losses were determined, and they were compared between the case of a plain warped transition (without a honeycomb) and the case with a honeycomb. The presence of the honeycomb in the transition improved the flow characteristics, reducing flow separation, energy losses, and turbulence strength. The honeycomb helped increase flow uniformity and decrease the maximum velocity in the transition and farther downstream. A potential benefit of a decreased maximum velocity is a decreased risk of channel erosion. The honeycomb worked to prevent the formation of large turbulent eddies that are known to be energy-bearing while avoiding excessive frictional losses of flow energy because the contact area of the passing fluids with the open cells is small. The mechanism at work is that the small open cells effectively limit the size of the largest possible turbulent eddies while distributing the approach flow evenly across the width of the transition. For practical application, a honeycomb can easily be installed at the entrance of a new or existing transition for beneficial effects. This study has contributed to improving the design of irrigation, drainage, and hydropower channels, all of which typically need a transition.