A multi-dimensional two-phase flow modeling framework for sediment transport applications
Date
2016
Authors
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Publisher
University of Delaware
Abstract
Studying coastal processes is essential for the sustainability of human habitat and vibrancy of coastal economy. Coastal morphological evolution is caused by a wide range of coupled cross-shore and alongshore sediment transport processes associated with short waves, infra-gravity waves, and wave-induced currents. One of the key challenges was that the major transport occurs within bottom boundary layers and it is dictated by turbulence-sediment interactions and inter-granular interactions. Therefore, this study focuses on numerical investigations of sediment transport in the bottom wave boundary layers on continental shelves and nearshore zones, with emphasis on both fine sediment (mud) and sand transports.
On the continental shelves, the sea floor is often covered with fine sediments (with settling velocity no more than a few mm/s). Wave-induced resuspension has been identified as one of the major mechanisms in the offshore delivery for fine sediments. A series of turbulence-resolving simulations were carried out to study the role of sediment resuspension/deposition on the bottom sediment transport. Specifically, we focus on how the critical shear stress of erosion and the settling velocity can determine the transport modes. At a given wave intensity associated with more energetic muddy shelves, three transport modes, namely the well-mixed transport (mode I), two-layer like transport with the formation of lutocline (mode II) and laminarized transport (mode III), are obtained by varying the critical shear stress of erosion or the settling velocity. A 2D parametric map is proposed to characterize the transition between transport modes as a function of the critical shear stress and the settling velocity at a fixed wave intensity. In addition, the uncertainties due to hindered settling and particle inertia effects on the transport modes were further studied. Simulation results confirmed that the effect of particle inertia is negligible for fine sediment in typical wave condition on continental shelves. On the other hand, the hindered settling with low gelling concentration can play a key role in sustaining a large amount of suspended sediments and results in the laminarized transport (mode III). Low gelling concentrations can also trigger the occurrence of gelling ignition, a state in which the erosion rate always exceeds the deposition rate. A sufficient condition for the occurrence of gelling ignition is hypothesized for a range of wave intensities as a function of sediment/floc properties and erodibility parameters.
In the more energetic nearshore zones, the sea floor is often covered with sand (with settling velocity exceeds 1 cm/s). Based on the open-source CFD toolbox OpenFOAM, a multi-dimensional Eulerian two-phase modeling framework is developed for sediment transport applications. With closures of particle stresses and fluid-particle interactions, the model is able to resolve full sediment transport profiles without conventional bedload/suspended load assumptions. The turbulence-averaged model is based on a modified k-epsilon closure for the carrier flow turbulence and it was used to study momentary bed failure under sheet flow conditions. Model results revealed that the momentary bed failure and the resulting large transport rate were associated with a large erosion depth, which was triggered by the combination of large bed shear stresses and large horizontal pressure gradients. In order to better resolve turbulence-sediment interactions, the modeling framework was also extended with a 3D turbulence-resolving capability, where most of the turbulence-sediment interactions are directly resolved. The model is validated against a steady sheet flow experiment for coarse light particles. It is found that the drag-induced turbulence damping effect was more significant than the well-known density stratification for the flow condition and grain properties considered. Meanwhile, the turbulence-resolving model is able to reproduce bed intermittency, which was driven by turbulent ejection and sweep motions, similar to the laboratory observation. Finally, simulations for fine sand transport in oscillatory sheet flow demonstrate that the turbulence-resolving model is able to capture the enhanced transport layer thickness for fine sand, which may be related to the burst events near flow reversal. Several future research directions, including further improvements of the present modeling framework and science issues that may be significantly benefited from the present turbulence-resolving sediment transport framework, are recommended.