PhD Oral Exam - Behzad Lak, 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

This thesis experimentally investigates bistable flow and turbulence in non-prismatic open channels. Bistable flow refers to the possible occurrence of two distinct stable flow states under identical upstream and downstream hydraulic conditions. Such behaviour can reduce hydraulic efficiency and increase erosion risk in erodible open channels. Laboratory experiments at Concordia University combined three-dimensional velocity, water-level, and discharge measurements to examine flow over broad-crested weirs and through a warped lateral expansion (WT). Instantaneous velocities were measured using an acoustic Doppler velocimeter sampling at 120–200 Hz.

Four regimes were identified downstream of the weir: impinging jet, surface jump, stationary non-breaking wave, and surface jet. Regime boundaries were mapped using headwater and tailwater depths above the crest normalised by weir height, and empirical correlations were developed. Within a bistable region, an impinging jet or a surface-flow regime occurred depending on whether tailwater was rising or falling. This hysteresis weakened as headwater submergence increased. Wave amplitude generally decreased with increasing tailwater submergence, while wavelength depended mainly on headwater depth. Measurements revealed distinct recirculation patterns and strong near-bed turbulence beneath impinging jets, suggesting greater bed erosion potential.

In the WT, an entrance honeycomb improved primary-flow uniformity and reduced reversed flow, secondary currents, and turbulence. However, two approximately mirrored asymmetric flow states persisted. Single and double longitudinal beveled vanes with bevel angles of 30° or 45° reduced overall separation and improved downstream flow recovery, despite local increases in separation within the transition. The 30° vanes reduced separated-flow volume by approximately 37% relative to the no-vane case, whereas the 45° vanes produced more symmetric flow within the transition. Double 45° vanes reduced average turbulence kinetic energy by approximately 25% compared with the no-vane case and lowered the peak transverse Reynolds shear stress. The preferred arrangement therefore depends on the hydraulic objective.

These findings improve understanding of bistability in vertical and lateral transitions and demonstrate the benefits and limitations of passive flow control. The experimental datasets support computational fluid dynamics model validation and transition design aimed at reducing erosion risk.