Large Eddy Simulation of Flow Interactions Between a Turbulent Free-Stream and a Permeable Bed

TL;DR

This study develops a high-fidelity Large Eddy Simulation (LES) model to analyze turbulent flow interactions between free-stream and permeable beds, aiding understanding of hyporheic exchange in river systems.

Contribution

It introduces a detailed 3D LES framework for turbulent flow over permeable beds, including mesh assessment, turbulence modeling, and a particle tracking module for dispersion analysis.

Findings

Large turbulent scales influence deeper within the bed.

Accurate resolution of large eddies is essential for interfacial transport.

The particle tracking module enables extraction of Lagrangian dispersion data.

Abstract

Systems comprising a turbulent channel flow overlaying a permeable bed can be found in a variety of industrial and natural applications (e.g. urban planning, fracking, submerged vegetation). One important realization of this system is at the bottom of rivers, where surface waters of the river permeate the deposited sediment and exchange their contents with subsurface waters. Obtaining a complete picture of this process, known as hyporheic exchange, is important for the understanding and maintenance of water quality and river ecology. Given the wide range of length scales (mm to km) and corresponding time scales of interest, reduced-order numerical models (e.g. stochastic particle tracking, advection-dispersion equations) are often used to study the transport of mass and momentum in this system. However, the many scales associated with surface/subsurface mixing make the development of a high-fidelity model for turbulent scalar transport a formidable challenge. In an effort to capture transport behavior at the pore scale, this work is focused on development of a detailed, three-dimensional flow model for the Large Eddy Simulation (LES) of turbulent flow over a permeable bed. A rigorous assessment of the computational mesh, domain size and turbulence model is performed. Concurrently, the double-averaging methodology is used to study the large-scale, persistent flow behavior in the presence of spatio-temporal heterogeneity. It is found that accurate resolution of the largest turbulent scales is necessary to develop a complete picture of interfacial transport, and that these large structures are felt deeper within the bed. Additionally, the development of a particle tracking software module is detailed. This module provides a foundation for the extraction of Lagrangian dispersion information from the LES, which will eventually be used to physically inform reduced-order models.