Large eddy simulation of open channel flows for conveyance estimation
PublisherUniversity of Nottingham
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Prediction of conveyance capacity in open channel flows is complex and requires adequate modelling of flow features such as secondary circulation cells and, specifically for over-bank channels, the momentum exchange that occurs at the main channel/floodplain interface. A variety of conveyance estimation methods have been developed with the objective of accurately capturing these flow characteristics through a simplified user-friendly approach. However, these methods usually require calibration of one or more empirical constants. Within this thesis in-bank and over-bank channels have been numerically simulated using Large Eddy Simulation (LES) in order to predict accurate open channel flow behaviour. The LES results are validated against experimental data and then utilised to advise on values of calibration constants f, λ and Γ within a conveyance estimation method, the Shiono and Knight Method (SKM), which has recently been adopted by the Environment Agency (EA) for England and Wales as part of its new Conveyance Estimation System (CES). The LES results are shown to accurately predict the flow features, specifically the distribution of secondary circulations in in-bank channels of aspect ratio as large as 40 and for over-bank channels at varying depth and width ratios. The LES derived f, λ and Γ values are then utilized in the analytical solution of the SKM in order to compute depth averaged velocity profiles for comparison to LES results, producing very good agreement with simulated and experimental profiles. As well as the derivation the calibration constants, the apparent shear stress at the main channel/floodplain interface is investigated and the contributions from both Reynolds stress and secondary circulation terms compared. Also, instantaneous velocity data available from monitor points at the main channel/floodplain interface within over-bank channel simulations is utilized to investigate wave periods of interfacial vortices through spectral analysis. Comparable result to available experimental and stability analysis data are obtained.