A CFD-based mixing model for vegetated flows

This paper provides a CFD‐based modelling 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), 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 non‐orthogonal 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), 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 re‐evaluation 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

Computational Fluid Dynamics Modelling Of A Vegetated Stormwater Pond

Stormwater treatment ponds are common, but there are few tools to evaluate their performance. Traditional techniques, i.e. dye traces, take significant amounts of time and are impractical. Computational fluid dynamics is increasingly being used to design and evaluate ponds, but often neglects the effects of vegetation. This study looks at the CFD modelling of a stormwater pond in Lyby, Southern Sweden, both with and without vegetation. The Residence Time Distribution is an established means of evaluating treatment capability of a pond. CFD modelling of solute transport is carried out to obtain RTDs, which are then compared to experimental RTDs obtained from the pond. The capability and benefits of including vegetation in CFD modelling of ponds are examined

Use of drag coefficient to predict dispersion coefficients in emergent vegetation at low velocities

View looking up in the nave, depicting crossing; The wealth of Dijon's middle classes, retail tradesmen to the duke, lawyers and financiers, was spent on fine houses, such as the Hôtel Chambellan, and to the construction of St Michel, where Abbot Richard Chambellan replaced a small Carolingian church with a vast Flamboyant edifice. It was built and vaulted between 1499 and 1513, and it was provided in 1537-1541 with a Renaissance façade with a magnificent Last Judgement carved on the tympanum by Nicolas de la Cour (1550). Source: Grove Art Online; http://www.oxfordartonline.com

Estimating stem-scale mixing coefficients in low velocity flows

Stormwater ponds are SuDS devices intended to moderate the negative environmental impacts of stormwater run-off. A current, joint, research programme is investigating the effects of heterogeneous vegetation distributions in stormwater ponds and developing CFD techniques to simulate 3D solute transport processes in low velocity flows. The aim of the project is to generate a unique dataset that describes the influence of different types and configurations of vegetation on the pond’s fundamental flow – and treatment – characteristics. Better characterisation can then be used to better evaluate existing run-off treatment ponds that may be delivering sub-optimal levels of treatment. This paper presents results from an initial 1D laboratory study, with regular uniform emergent artificial vegetation, from which longitudinal dispersion coefficients have been evaluated over a range of target flow velocities. These have been integrated with stem-scale CFD mean velocity predictions to investigate the ability of a Chikwendu (1986) n-zone type approach to predict transverse dispersion coefficients over a scale suitable for inclusion in future 3D pond models. Assuming that the small stem-scale fluctuations in velocity form a repeating pattern at the patch scale, and that stem-scale transverse dispersion effects integrate, this approach has been successfully applied to predict mixing in a 2D system solely with parameters estimated from a 1D system