Transport in Turbulent Boundary Layers over Permeable Beds: Pore-resolved Direct Simulations and Macroscale Continuum Modeling

Transient storage, characterized by retention and transport of solutes such as chemicals and pollutants, organic matter, dissolved oxygen, nutrients, and heat across the stream-gravel interface, is one of the most critical concepts in stream ecology with enormous societal value in predicting source of fresh drinking water, nutrient cycling, and sustaining diverse aquatic ecosystems. Lack of an accurate modeling approach that accounts for complex flow physics of turbulence over a permeable bed and coupled transport across the sediment-water interface is one of the key challenges in practical scale modeling of natural aqueous systems.

Typical grain-to-globe prediction models of solute transport are sequential in nature and neglect the direct coupling between turbulent stream flow over a permeable bed and groundwater flow within the bed. It is hypothesized that turbulence at the sediment-water interface can significantly alter the transient storage and solute retention times within the gravel beds.

Specifically, the structure and dynamics of turbulence over a porous bed can be very different than that over an impermeable, rough wall. Quantifying the upwelling and downwelling events at the stream-bed interface and their modulation by the free-stream turbulence, as well as penetration depth of turbulence structures within the porous bed is critical for the development of macroscale, continuum-based modeling of solute transport.

This interdisciplinary research will train two PhD students on direct and large-eddy simulations, volume-averaging techniques, and multiscale analysis.

Fundamental numerical experiments are proposed on turbulent boundary layers over permeable gravel beds representative of stream or river flows. Pore-resolved, direct numerical simulations (DNS), using a solver based on the fictitious domain method developed by the PI, of turbulent flow over stationary, porous beds will be conducted for representative flows in natural streams or rivers.

The turbulent flow physics for a range of permeability Reynolds numbers for mono and polydispersed, randomly packed gravel beds will be investigated and contrasted against that over rough, impermeable wall with similar topology. The detailed, spatiotemporal pressure and velocity fluctuation data at the interface will be used to develop and validate macroscale, continuum models with variable porosity for large-eddy simulation (LES).

The proposed research has the potential to transform the current state-of-the art reach-scale modeling approaches for solute transport used in river management. The pore-resolved DNS data will be used to design a learning module for a graduate level course on `Turbulence Modeling' with a priori and a posteriori analyses of the continuum LES model. This learning module will be freely disseminated on the PI's research site.

The DNS data and macroscale models on turbulence over porous beds will also be valuable for a wide range of fields, such as, noise control in aircraft trailing edges, heat and mass transfer in chemical and nuclear reactors, arterial blood flow and transport in endothelial walls, vegetation canopies; among others.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.