Modelling Turbulent Mixing and Sediment Process beneath Tidal Bores: Physical and Numerical Investigations

Khezri, Nazanin (2014). Modelling Turbulent Mixing and Sediment Process beneath Tidal Bores: Physical and Numerical Investigations PhD Thesis, School of Civil Engineering, The University of Queensland.

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Author Khezri, Nazanin
Thesis Title Modelling Turbulent Mixing and Sediment Process beneath Tidal Bores: Physical and Numerical Investigations
School, Centre or Institute School of Civil Engineering
Institution The University of Queensland
Publication date 2014
Thesis type PhD Thesis
Open Access Status Other
Supervisor Hubert Chanson
Pierre Lubin
Total pages 267
Total colour pages 167
Total black and white pages 100
Language eng
Subjects 090509 Water Resources Engineering
090599 Civil Engineering not elsewhere classified
040699 Physical Geography and Environmental Geoscience not elsewhere classified
Formatted abstract
A tidal bore is a natural estuarine phenomenon forming a positive surge in a narrow funnelled channel river mouth during the early flood tide under spring tide conditions. The bore may result from a combination of several parameters including (but not limited to) the tidal conditions, bathymetry and river mouth shape. The positive surge propagates upstream into the lower estuarine zone and its arrival induces some enhanced turbulent mixing, with upstream advection of suspended material. In this thesis the flow field and turbulence characteristics of tidal bores propagating over mobile rough bed were measured using both physical and numerical (CFD) modelling. The unsteady turbulent flow analysis was complemented by sediment particle tracking. Combination with some theoretical knowledge led to some new understanding of turbulent velocity field, turbulent mixing process, Reynolds stress tensor, sediment transport, tidal bore hydrodynamics and the forces acting on particles.

The turbulent mixing characteristics of breaking and undular tidal bores were investigated physically including in terms of sediment motion on a movable gravel bed. The experimental study was based upon a Froude dynamic similarity, and some detailed velocity measurements were conducted for a range of Froude numbers to provide some Eulerian description of the turbulent flow field. Some variable interval time average technique (VITA) analysis was performed to characterise the turbulent properties including the Reynolds stress tensor components in breaking and undular tidal bores on fixed and mobile bed. Some sediment tracking under both breaking and undular bores provided some Lagrangian description of transient bed load motion beneath the tidal bore front. The findings showed the dominant role of the longitudinal pressure gradient force on the inception of sediment motion during the passage of the bore, and the effect of shear force in the later sediment motion.

Some numerical (CFD) simulation of the unsteady two-phase undular and breaking bore flow on fixed gravel bed was carried out in the present research. The configurations investigated by DOCHERTY and CHANSON (2012) and KHEZRI and CHANSON (2013) were tested. A numerical CFD tool (Thetis) was used, which was developed by the I2M Laboratory at the University of Bordeaux. Thetis is a CFD model and has been validated for many cases. Some novel method in the numerical tool includes the use of a mesh grid disconnected from the geometry, which allows the fluid domain being treated globally. This study includes further analysis and discussion, including numerical simulations, numerical validation and theoretical considerations.

The numerical simulation of the breaking bore in this study was a “breakthrough”. The simulation of breaking waves has always been a challenge due to the complexities of air entrainment modelling in the flow. In this study the free surface was modelled using the smooth volume of fluid technique (SVOF) and also the turbulence modelling was performed via large eddy simulation technique LES. The data was used to map the velocity and pressure field and resolve some interesting characteristics of the flow. The pressure distribution measured beneath the breaking bore in the numerical model was compared to the experimental estimations. The results were in good agreement and the very large pressure gradient force just beneath the bore toe was also observed in the numerical measurements. The vortical structures were mapped and visualised using the numerical data. The observation of vortical structures and their upward motion beneath the breaking bore in the numerical study, could explain the regular upward particle motion observations during the breaking bore experiments.
Keyword Tidal bores
Physical modelling
numerical simulation
sediment processes
movable bed
unsteady open channel flow
computational fluid dynamics
large eddy simulation

Document type: Thesis
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Created: Tue, 06 May 2014, 18:18:14 EST by Nazanin Khezri on behalf of Scholarly Communication and Digitisation Service