Pore-resolved investigation of turbulent open channel flow over a randomly packed permeable sediment bed

TL;DR

This study uses pore-resolved DNS to analyze turbulence and flow interactions over permeable sediment beds, revealing how flow characteristics depend on permeability and highlighting the importance of the top layer in sediment transport.

Contribution

It provides detailed pore-scale insights into flow dynamics over permeable beds, including stress distributions and their implications for sediment transport modeling.

Findings

Reynolds and form-induced stresses peak in the top layer of the bed.

Bed stress PDFs collapse across Reynolds numbers, following logarithmic correlations.

Fluctuations in bed stress are mainly driven by the top layer and are well modeled by non-Gaussian fits.

Abstract

Pore-resolved direct numerical simulations (DNS) are performed to investigate the interactions between streamflow turbulence and groundwater flow through a randomly packed porous sediment bed for three permeability Reynolds numbers, $Re_K$, of 2.56, 5.17, and 8.94, representative of natural stream or river systems. Time-space averaging is used to quantify the Reynolds stress, form-induced stress, mean flow and shear penetration depths, and mixing length at the sediment-water interface (SWI). The mean flow and shear penetration depths increase with $Re_K$ and are found to be nonlinear functions of non-dimensional permeability. The peaks and significant values of the Reynolds stresses, form-induced stresses, and pressure variations are shown to occur in the top layer of the bed, which is also confirmed by conducting simulations of just the top layer as roughness elements over an impermeable wall. The probability distribution functions (PDFs) of normalized local bed stress are found to collapse for all Reynolds numbers and their root mean-squared fluctuations are assumed to follow logarithmic correlations. The fluctuations in local bed stress and resultant drag and lift forces on sediment grains are mainly a result of the top layer, their PDFs are symmetric with heavy tails, and can be well represented by a non-Gaussian model fit. The bed stress statistics and the pressure data at the SWI can potentially be used in providing better boundary conditions in modeling of incipient motion and reach-scale transport in the hyporheic zone.