title: Partially-vegetated Compound Channel Flows: Turbulent Flow Structures and a RANS-based modelling
Creators: name: Guan, Yutong
Publisher: name: University of Liverpool Repository
Year: 2029

description: This thesis explores the characteristics of turbulent flow in open compound channels with various vegetation configurations, which include the vegetation grown along the floodplain bank, the vegetation grown along the center of the floodplain, and the vegetation patch grown along the floodplain bank. The scope of this research includes two parts: (a) detailed experimental investigations of turbulent flow structure and (b) a prediction model of lateral distributions of streamwise velocity.
The main aim of the experimental part was to gather high-quality data across a specific range of hydrodynamic conditions and vegetation configurations. To achieve this, three experimental sets were scrutinized in asymmetric compound channels. These experiments aimed to explore flow dynamics across the whole compound channel, with extra focus on the main channel/floodplain interface and the vegetated region. Conducted in a laboratory setting under streamwise uniform flow conditions, these experiments introduced novel configurations with varying vegetation configurations to bridge the data gap in asymmetric partially-vegetated compound open channels. Key measurements were taken to estimate the detailed lateral and vertical distributions of streamwise velocity, the lateral distributions of transverse exchange of streamwise momentum, including the distributions of Reynolds stress, shear stress contributed from the secondary current and the transverse flow, and the coherent structures. This was done to investigate the flow structure in partially vegetated asymmetric compound channels, utilizing both sideward-probe acoustic Doppler velocimeters (ADV) and a propeller.
Based on the detailed experimental data, the thesis develops a predictive model of lateral velocity. This part emphasizes the construction of a novel prediction model tailored explicitly for partially-vegetated compound channels, grounded in the principles of RANS (Reynolds-Averaged Navier-Stokes equations). This section signifies not only a thorough examination of the experimental data but also a crucial change from empirical observations to theoretical modeling. Herein lies the detailed exposition of this section on the predictive model: Despite the numerous studies that have attempted to model the lateral distributions of streamwise velocity in a compound channel, existing models are not universally applicable to asymmetrical channels with partial-vegetation configuration, including field data. This is because most previous studies did not consider the extra mixing layer generated around the vegetated /non-vegetated region interface. In this study, an empirical relationship between the secondary flow term and the gravitational term is established in the main channel. Then, an analytical model was proposed based on the RANS equation and Shiono and Knight method (Shiono and Knight, 1988). The proposed model was then verified and validated using our experimental results and two other experimental datasets. The results suggest that this proposed model can improve prediction accuracy by varying the secondary flow term in each panel, which is suitable to apply to asymmetric, partially-vegetated compound channels.