Experimental investigation of wake development and mixing behind a multi-scale array of bars

Abstract

This thesis considers the experimental investigation of the near wakes of arrays composed of bars of uniform and non-uniform thickness (i.e. single and multi-scale arrays). The velocity and passive scalar concentration fields are measured by means of Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). The objective is to identify distinctive features associated with the multi-scale turbulence forcing and to characterise their importance for the flow's dynamics and the scalar dispersion.

The velocity fields' inspection reveals shedding modes associated with the particular bars. Additional energetic coherent fluctuations are spotted in the vicinities of wakes’ intersections in the multi-scale array case. In order to better understand these features, an energy budget of the multi-scale flow is considered in its triple-decomposed version where coherent and stochastic fluctuations are treated separately. The latter reveals, amongst other things, that the additional coherence is produced by triadic interactions of the primary shedding fluctuations. The analysis of scalar dispersion shows a specific way of fluid exchange between adjacent wakes, i.e. a quasi-periodic bursting. This phenomenon also appears to be triggered as the wakes’ intersection occurs. Its effects on the quantitative scalar dispersion statistics are carefully studied for the multi-scale array case and related to the underlying velocity field.

A considerable part of this thesis is dedicated to the development of tools needed in the course of the data analysis. Two major outcomes are the PLIF quantification technique and the triple decomposition technique. The former is a major improvement to classical quantification approaches, accounting for a non-linear fluorescent response of the dye, strong attenuation effects and so-called secondary fluorescence. The developed triple decomposition methodology allows distinguishing a number of separate coherent fluctuations coexisting in a flow, e.g. shedding modes of particular bars (as opposed to a singular mode usually considered in the literature).