A CFD‐Based Mixing Model for Vegetated Flows

F Sonnenwald,V Stovin,I Guymer

doi:10.1029/2018wr023628

Abstract

AbstractThis paper provides a computational fluid dynamics (CFD)‐based modeling framework for predicting flow field, turbulence, and mixing characteristics within vegetated environments such as ponds and wetlands. The framework has been implemented within a commercial CFD code—ANSYS Fluent 19—via a set of user‐defined functions. Following the approach outlined by King et al. (2012, https://doi.org/10.1017/jfm.2012.113), the standard k‐ε turbulence closure model has been modified to capture the energy transfer at the vegetation/clear flow shear interface and within the vegetation. The implementation assumes that vegetation is vertical, but nonorthogonal flow in the horizontal plane is accounted for. Values for the drag coefficient and the mixing coefficients are estimated based on the vegetation stem diameter and density. Following Tanino and Nepf (2008, https://doi.org/10.1017/S0022112008000505), a switch has been incorporated to account for the fact that the relevant length scale changes from stem diameter to stem spacing as stem density increases. A set of model parameters is proposed, based on a reevaluation of previously published laboratory data and theoretical analysis. Five different experimental data sets are used to demonstrate that the model is able to predict mixing within fully vegetated systems and due to both vertical and horizontal shear layers. The framework was developed to provide a practical prediction tool for engineering purposes, in particular for the estimation of residence time distributions in real partially vegetated stormwater management ponds. Its implementation here within a commercial CFD package potentially facilitates application to complex pond geometries, including patches of different types of vegetation with different bulk stem diameter and density characteristics.

Highlights

Vegetated flows are complex and of interest in both natural and engineered systems, where the presence of vegetation can significantly affect the flow field (Nepf, 2012)
This paper provides a computational fluid dynamics (CFD)‐based modeling framework for predicting flow field, turbulence, and mixing characteristics within vegetated environments such as ponds and wetlands
The objectives of the paper are as follows: 1. to implement the turbulence model outlined by King et al (2012) in 3D; 2. to review the model constants and implement a method for estimating CD from the physical characteristics of the vegetation; 3. to propose a modeling framework suitable for modeling solute transport within vegetation that combines the King et al (2012) model with the approaches outlined in section 2; and 4. to demonstrate the new model's fitness for purpose using a range of laboratory data sets

Summary

Introduction

Vegetated flows are complex and of interest in both natural and engineered systems, where the presence of vegetation can significantly affect the flow field (Nepf, 2012). Vegetation can reduce conveyance and trap pollutants (Darby, 1999). Vegetation can promote short circuiting and reduce efficiency (Sonnenwald, Guymer, et al, 2018). Modeling the presence of vegetation is necessary to predict velocities and other mean flow field effects (Marjoribanks et al, 2017; Nepf, 1999 & Tsavdaris et al, 2013), while solute transport modeling in vegetated systems is necessary to predict mixing and treatment processes

Objectives

Results

Discussion

Conclusion