A CFD‐Based Mixing Model for Vegetated Flows. Issue 3 (22nd March 2019)

• Record Type: Journal Article
• Authors: Sonnenwald, F.; Guymer, I.; Stovin, V.
• Is Part Of: Water resources research. Volume 55:Issue 3(2019)
• Page Start: 2322
• Page End: 2347
• Publication Date: 2019-03-22
• Subjects: Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2018WR023628
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: 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 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. Key Points: This is the first CFD k‐ε‐based approach to model mixing within vegetation, able to incorporate clear water/vegetation interfaces Drag and mixing coefficients are determined from vegetation characteristics of solid volume fraction and stem diameter The approach includes length‐scale switches between stem diameter and stem spacing as a function of solid volume fraction