Mixing and transport in inclined gravity currents
File(s)
Author(s)
Dieu, Emily Anna
Type
Thesis
Abstract
Inclined gravity currents are a fundamental type of flow found in a number of applications. These currents are buoyancy-driven flows evolving on a solid boundary making an angle with the horizontal, transporting energy, momentum and density along the sloping boundary. The interface between the current and its environment is unstable to shear, and the nearby environment fluid is entrained into the current. In the present thesis we study inclined gravity currents on a slope characterised by a range of angles and surface roughness.
The current-environment interface is where most studies of inclined gravity currents dynamics have focused their interest, and these dynamics have been defined by parameterising entrainment. However, parameterisations of entrainment throughout the literature have not provided a unified picture of the fundamental dynamics of inclined gravity currents, and it is still challenging to model and understand these dynamics.
Energetics analyses have been used to improve a range of understanding of the fundamental dynamical behaviour of shear- and buoyancy-driven flows, and to give insight into turbulent entrainment and mixing. Energetics frameworks of shear- and buoyancy-driven flows, such as the one proposed in the work of Winters et al. (1995), provide a direct means of evaluating mixing. In the present thesis we adapt the global energetics framework of Winters et al. (1995) to inclined gravity currents.
We use Direct Numerical Simulations to examine the adapted framework and the predictive model of van Reeuwijk et al. (2019) for inclined gravity currents. Data analysis confirms that the adapted framework is useful to characterise inclined gravity currents. We show that the model of van Reeuwijk et al. (2019) has limitations in its physical interpretation of the behaviour of the current, and we develop an alternative parameterisation based on a buoyancy flux defined with the vertical distance of fall of the current.
We analyse how adding and varying roughness at the slope modifies the dynamics and energetics of the current. With an appropriate scaling of the mean velocity and buoyancy profiles above the roughness, the adapted framework can be applied and we find that the dynamics and energetics of the current on a smooth slope are preserved above the roughness.
The current-environment interface is where most studies of inclined gravity currents dynamics have focused their interest, and these dynamics have been defined by parameterising entrainment. However, parameterisations of entrainment throughout the literature have not provided a unified picture of the fundamental dynamics of inclined gravity currents, and it is still challenging to model and understand these dynamics.
Energetics analyses have been used to improve a range of understanding of the fundamental dynamical behaviour of shear- and buoyancy-driven flows, and to give insight into turbulent entrainment and mixing. Energetics frameworks of shear- and buoyancy-driven flows, such as the one proposed in the work of Winters et al. (1995), provide a direct means of evaluating mixing. In the present thesis we adapt the global energetics framework of Winters et al. (1995) to inclined gravity currents.
We use Direct Numerical Simulations to examine the adapted framework and the predictive model of van Reeuwijk et al. (2019) for inclined gravity currents. Data analysis confirms that the adapted framework is useful to characterise inclined gravity currents. We show that the model of van Reeuwijk et al. (2019) has limitations in its physical interpretation of the behaviour of the current, and we develop an alternative parameterisation based on a buoyancy flux defined with the vertical distance of fall of the current.
We analyse how adding and varying roughness at the slope modifies the dynamics and energetics of the current. With an appropriate scaling of the mean velocity and buoyancy profiles above the roughness, the adapted framework can be applied and we find that the dynamics and energetics of the current on a smooth slope are preserved above the roughness.
Version
Open Access
Date Issued
2020-11
Date Awarded
2021-04
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Hughes, Graham
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)