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Spatio-Temporal Characterisation of Hydrodynamics and Mixing Processes in Obstructed Flows Using Optical Techniques

Author(s): Jesus Leonardo Corredor Garcia; Virginia Stovin; Ian Guymer

Linked Author(s): Jesus Leonardo Corredor Garcia, Virginia Stovin, Ian Guymer

Keywords: Emergent Vegetation Particle Image Velocimetry; Laser Induced Fluorescence; Solute Transport; Environmental Fluid Mechanics

Abstract: The presence of obstructions in many flows of interest, e.g. wetlands, reactors, etc., will dictate their hydrodynamics. Understanding the interaction between the obstruction and the fluid is therefore crucial to investigate real life problems such as drag, energy balances, exchange rates and solute transport. This paper presents the results of a novel experimental setup designed to visualise and quantify both the velocity fields and dispersion in a flow containing a randomly positioned array of emergent rigid cylinders of different diameters at a fixed solid volume fraction. Both the diameter and spatial distribution of the cylinders have been proposed as a surrogate for emergent vegetation, based on measurements from winter Typha latifolia. This new experimental study involves the combined use of Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV). LIF was employed to temporally and spatially resolve the concentration field from a pulse injection of fluorescent scalar (Rhodamine 6G) over three consecutive reaches. This spans a distance of 4 metres, along a flume designed to simulate the conditions present in vegetated flows. The pulse injection allowed both longitudinal and transverse dispersion to be quantified from different source locations over a range of flow velocities. In addition to the concentration maps obtained via LIF, velocity maps were obtained using PIV, to study the hydrodynamic processes occurring within the spaces between a comprehensive number of cylinders. This was made possible using glass rods and cylinders over the 0.5 m x 0.3 m area of interest. Calibrations have been applied to account for laser light attenuation in the case of LIF, and illumination heterogeneities due to laser light refraction in the case of PIV, and to obtain a set of concentration and velocity maps. The results are presented at framerates and spatial resolutions high enough to resolve time-averaged quantities, as well as turbulent processes and coherent motions caused by the presence of the obstructions. The flow rates tested, as well as the scalar and vector maps obtained, reveal the effect of hydrodynamics on large scale processes such as mixing. The experiments undertaken for this research are, to the authors’ knowledge, the first ones that integrate velocity and concentration characterisation at this scale; and the results not only inform the physics of obstructed flows for a range of Reynolds numbers, but also serve as a blueprint for future experiments, and validation datasets for numerical models.


Year: 2022

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