Direct numerical simulations of particle transport in a model estuary

Abstract

We investigate numerically the mixing of freshwater with ambient saltwater in a model estuary along with associated particle settling processes. We first discuss and specify two numerical setups that consider several relevant features to study the particle settling. The first configuration is a large rectangular basin with a small inlet for the (particle-laden) freshwater; the second is geometrically identical to the first except that the flow is laterally confined to the narrow inlet width. The two flows are computed until a statistically stationary solution is reached. We perform highly resolved direct numerical simulations using a high-order finite difference approach to yield reliable and accurate results. Accordingly, all relevant turbulent scales are resolved and turbulence modeling is not needed. The main target of this study is to describe and illustrate the fluid dynamics and the particle settling processes under the influence of turbulence arising in the freshwater/saltwater-stratified mixing layer. To this end we analyze and compare the two simulations with respect to different aspects of the freshwater/saltwater interaction and to the transport and settling processes of the particles. We investigate the spatial structure as well as the temporal evolution of the flows and the particle suspensions. Generally, we find a qualitatively good agreement of both numerical simulations with pertinent laboratory experiments. Particularly, the results confirm a significant enhancement of the particle settling speeds compared to pure Stokes settling in the presence of turbulence. However, we also demonstrate that the results for the two configurations differ fundamentally in several aspects: stability of the freshwater/saltwater interface, degree of turbulent mixing, particle plume expansion and particle settling enhancement. Using these two simulations we explain the observed settling enhancement, which is not related to the particle inertia (as it is not considered in the numerical model) but rather to the buoyancy velocity of the particle suspension and the geometry of the estuary mouth configuration.

Keywords:

• estuary mouths
• turbulent mixing
• sedimentation
• particle settling
• particle deposition
• numerical simulation
• feedback-coupled particles
• flume experiments

Acknowledgements

We would like to thank D. Obrist for his constant interest in this project and the helpful discussions related to the simulation setup. Support for this project has been obtained through the ETH research grant TH-23/05-2. All computations were performed at the Swiss National Supercomputing Centre (CSCS).