A study of turbulence: the unsteady propagation of bores and surges

Abstract

A sudden increase in flow depth induces a positive surge. A tidal bore is a positive surge formed in an estuary under spring tide conditions in a narrow funnelled channel mouth. Albeit the complicated interactions between tidal bores and mankind, the studies of the turbulent characteristics in propagating bores and surges remain limited. The thesis presents a thorough study into the turbulence and hydrodynamic properties of propagating bores and surges. The research work is composed of novel physical experiments, new numerical models using Computational Fluid Dynamic (CFD) simulations, and prototype field measurements.Systematic laboratory experiments over a broad range of flow conditions were conducted in a 19 m long 0.7 m wide rectangular tilting flume in the AEB hydraulic laboratory at the University of Queensland. Unsteady free-surface and velocity measurements were performed using instruments with high temporal resolutions. All experiments were repeated 25 times to obtain ensemble-averaged turbulent properties. Ultra-high-speed video examinations of the breaking bore roller were carried out with frame rates up to 22,700 fps, coupled with a series of two-phase air-water measurements using an array of phase-detection conductivity probes. The experiments documented a rapid longitudinal deceleration with large increase in free-surface and velocity fluctuations in all directions as tidal bores propagated. The Reynolds stresses showed marked increase in all components as the bore passed, indicating large shear between fluid layers. The propagation of breaking bore was a highly turbulent and rapidly fluctuating process, with a three-dimensional breaking roller. The celerity of the roller toe varied rapidly with time, with fluctuations twice as high as the mean celerity. Intensive air bubbles were entrained next to the free-surface at the toe of roller. The turbulent time and length scales suggested that the propagation of tidal bores was an anistropic process, with larger length scales in the longitudinal dimension. The iso-correlation data in the span-wise plane indicated existence of large scale coherent structures underneath the free-surface, resembling the cross-section of a hair-pin vortex. The time and length scales tended to increase during and after the bore passage. Both strain rate and vorticity showed larger values at lower water column near the channel bed, and during the immediate bore passage.Numerical CFD modelling of tidal bores was conducted based upon the experimental flow conditions and validated against the experimental results. The CFD models solved Navier-Stokes equations using a Large Eddy Simulation (LES) coupled with a sub-grid scale model. Both two-dimensional (2D) and three-dimensional (3D) CFD models were conducted, and the results showed some satisfactory agreement with experimental data. The CFD models highlighted large vortical structures underneath the flow, before, during and after the bore arrival, the size of which were comparable to the time and length scales found in laboratory. The 3D model provided results with much better agreement with the experimental data compared to the 2D model. For small inclusions such as air bubbles or water droplets, the current models was still limited both in terms of mesh grid refinement and an appropriate scheme to estimate the energy dissipation in two-phase flows.Two series of field measurements were performed in tidal bore affected river flows in 2015 and 2016. Both series were conducted for one to four consecutive days on the same site: the Arcins channel of Garonne River, France. The field works consisted of a broad range of observations and measurements, encompassing properties of hydrodynamics, turbulence, sedimentology and sediment transport. The tidal bore propagation resulted in an intense flow reversal, large increase in Reynolds shear stresses, and large amount of sediment being suspended and transported. A two-stage bed scour process was observed, highlighting an initial surface erosion followed by delayed mass erosion 5 to 15 minutes after the bore. The study demonstrates the progressive siltation of the river channel sediments following the successive bore events.Overall, the thesis presents a comprehensive study of the tidal bores and positive surges from experimental, numerical and field perspectives, providing a novel and systematic understanding of this complex phenomena. The propagation of bores and surges is three-dimensional anisotropic turbulent process with energetic vortex motions. Future works should investigate further into the air-water interactions in the breaking bore roller, and the air bubble mechanism after entrainment. Collaborations between physical and numerical modellers are needed to advance the prediction of complicated turbulent phenomena.