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Passive scalar dispersion in the near wake of a multi-scale array of rectangular cylinders

Published online by Cambridge University Press:  04 February 2019

Pawel Baj Open the ORCID record for Pawel Baj [Opens in a new window]  and
Oliver R. H. Buxton Open the ORCID record for Oliver R. H. Buxton [Opens in a new window]
Pawel Baj
Affiliation:
Department of Aeronautics, Imperial College London, London SW7 2AZ, UK Department of Energy and Process Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
Oliver R. H. Buxton*
Affiliation:
Department of Aeronautics, Imperial College London, London SW7 2AZ, UK
*
†Email address for correspondence: [email protected]
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Abstract

The near wakes of flows past single- and multi-scale arrays of bars are studied by means of planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV). The aim of this research is to better understand dispersion of passive scalar downstream of the multi-scale turbulence generator. In particular, the focus is on plausible manifestations of the space-scale unfolding (SSU) mechanism, which is often considered in the literature as the reason for the enhancement of the turbulent scalar flux in flows past fractal grids (i.e. specific multi-scale turbulence generators). The analysis of qualitative and quantitative PLIF results, as well as the simultaneously acquired PIV results, confirms the appearance of a physical scenario resembling the SSU mechanism. Unlike the anticipation of the literature, however, this scenario applies to some extent also to the flow past the single-scale obstacle. Application of a triple decomposition technique (which splits the acquired fields into their means, a number of coherent fluctuations and their stochastic parts) and a conditional-averaging technique reveals that the SSU mechanism is active in the vicinity of an intersection point between two adjacent wakes and is driven almost exclusively by coherent fluctuations associated with the larger of the intersecting wakes. This suggests that the SSU mechanism is related to the coherent fluctuations embedded in the flow rather than to the fine-scale turbulence and its underlying integral length scale, as proposed in previous works.

JFM classification

Mixing: Turbulent mixing Wakes/Jets: Wakes
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 864 , 10 April 2019 , pp. 181 - 220
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Copyright
© 2019 Cambridge University Press 

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Baj and Buxton supplementary movie 1

Sequences of consecutive qualitative PLIF images acquired downstream of the multi-scale array (see figure 11a of the manuscript; based on experiment 1).

Download Baj and Buxton supplementary movie 1(Video)
Video 120.7 KB

Baj and Buxton supplementary movie 2

Sequences of consecutive qualitative PLIF images acquired downstream of the single-scale array (suppression period, see figure 11b of the manuscript; based on experiment 1).

Download Baj and Buxton supplementary movie 2(Video)
Video 345.2 KB

Baj and Buxton supplementary movie 3

Sequences of consecutive qualitative PLIF images acquired downstream of the multi-scale array (bursting period, see figure 11c of the manuscript; based on experiment 1).

Download Baj and Buxton supplementary movie 3(Video)
Video 414 KB