Effects of lateral inflow on oxygen transfer and hydraulics in open channel flows
Date of Award
Doctor of Philosophy
School of Engineering
The design of channels or hydraulic structures requires the correct prediction of flow properties such as depth of flow. In uniform open channel flows, a one dimensional (1-D) approach which assumes hydrostatic pressure distribution, negligible air entrainment and uniform velocity in the direction of flow is often used. Spatially varied flow (SVF) is a special type of open channel flow in which the discharge increases or decreases along the channel due to lateral inflow or outflow, respectively. As a result, this intricate flow is associated with momentous turbulence and velocity fluctuations in all three directions of flow. Researchers have proven that using the 1-D approach for predicting SVF properties yields erroneous results. This thesis details research conducted (i) to improve the accuracy of the current one-dimensional equation, (ii) to quantify the amount of oxygen transferred by lateral inflow and (iii) to predict turbulence characteristics using three-dimensional turbulence model. First, it is proposed that SVF due to lateral inflow, which is the focus of this study, can be likened to multiple jets in crossflow and open channels with emergent vegetation. In these two cases, the multiple jet and the vegetation stems resemble a solid cylindrical object blocking the crossflow thus effects of the drag force are vital. Similar to open channel with emergent vegetation studies, a new equation accounting for the drag force was developed and tested for different arrangements of SVF. Results indicated significant improvements in predicting water surface profiles. Second, similar to weir flow the amount of oxygen transferred by lateral inflow was measured under different flow conditions and various modes of lateral inflow entry to the channel. The amount of dissolved oxygen (DO) in a body is vital for improving water quality. Results indicated that increasing jet velocity, discharge height and number of jets at optimum water depth in the receiving channel enhances oxygen transfer. Finally, three-dimensional computational fluid dynamics (CFD) analysis of an open channel receiving inflow from multiple jets in tandem issuing from a circular nozzle was conducted using the relatively low cost Reynolds-averaged Navier-Stokes (RANS) models namely the realizable k-ε, shear stress transport (SST) k-ω and the Reynolds stress model (RSM) based on their prominence in jet in crossflow studies. RANS models failed to predict turbulence characteristics within the lateral inflow region although average velocities in the longitudinal direction were acceptable. On the leeward side of the jet, RANS models failed to capture the downward velocity vectors resulting in major deviations in vertical velocity. It can be concluded that standard turbulence models are incapable of predicting the complex characteristics of SVF. However, turbulence models remain superior to the 1-D approach.
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Chipongo, K. (2018). Effects of lateral inflow on oxygen transfer and hydraulics in open channel flows. https://ro.ecu.edu.au/theses/2053