Hydrodynamics and Length-Scale Distributions of a Random Cylinder Array

Abstract

In vegetated flows a reliable estimation of flow scales is crucial to understand and model mixing processes. This study presents velocity maps obtained using particle image velocimetry (PIV) within a cylinder array (diameters 4≤d≤20 mm) designed to mimic real emergent vegetation. Tests were undertaken over a comprehensive range of stem Reynolds numbers (100≤Red≤900), intended to characterize time-dependent hydrodynamic features, including their interactions. Time-averaged flow heterogeneities are found to be independent of Red. Vortex dynamics are seen to dominate turbulent fluxes of momentum, and are the relevant coherent structures driving mass transport. The range of characteristic time- and length-scales from these coherent structures was quantified and shown to be determined by the distribution of spaces between cylinders. This is due to: (1) neighboring cylinders forming clusters, leading to larger flow structures, and (2) the maximum size of the flow structures being constrained by the inter-stem space. It is concluded that the Delaunay criterion provides practitioners with a good approximation to the distribution of flow scales in vegetated flows. This study presents the quantification of the scales of flow structures generated in a vegetated flow. These structures play an important role in the quantification of mass transport processes, and estimates of the sizes of these flow structures are necessary to understand the rate of solute transport and therefore provide reliable models for pollutant mixing in vegetated flows. Using state-of-the-art equipment to characterize velocity fields, the present study found that the size of these flow structures is determined by the spacing between cylinders. The authors recommend that for practical applications, engineers quantify the spacing between plant stems using the criteria presented in this paper.