Dynamic wake behind a dandelion pappus: PIV and smoke-wire visualization

Dandelion pappus is a special geometry of radial porous disc and exhibits the separated vortex ring wake structure in low Reynolds numbers. In the current study, dynamic wake behind dandelion pappus models were experimentally studied in a high Reynolds number of 8000 by PIV and smoke-wire system. Th...

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Bibliographic Details
Published in:Journal of visualization 2023-08, Vol.26 (4), p.779-794
Main Authors: Xu, Zhihan, Chang, Xu, Meng, Hao, Gao, Donglai
Format: Article
Language:English
Subjects:
Classical and Continuum Physics
Computer Imaging
Engineering
Engineering Fluid Dynamics
Engineering Thermodynamics
Entrainment
Filaments
Fluid flow
Heat and Mass Transfer
High Reynolds number
Pattern Recognition and Graphics
Porosity
Proper Orthogonal Decomposition
Regular Paper
Reynolds number
Shear layers
Smoke
Swirling
Turbulence intensity
Vision
Visualization
Vortex rings
Vortex shedding
Vortex streets
Vortices
Wire
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Summary:Dandelion pappus is a special geometry of radial porous disc and exhibits the separated vortex ring wake structure in low Reynolds numbers. In the current study, dynamic wake behind dandelion pappus models were experimentally studied in a high Reynolds number of 8000 by PIV and smoke-wire system. The effects of porosity and pappus angle were explored. The results revealed that the unsteady vortex shedding occurred in the wake flow field. Due to increased filaments and corresponding reduction in the interspaces for fluids to pass through, the shear layers were developed more fully and expanded, and the entrainment was stronger with the porosity decreased. Therefore, the recirculation zone was decentralized in width and enlarged, shifted further downstream as well. The turbulence intensity was also increased with lower porosity. By proper orthogonal decomposition (POD) and phase-averaged swirling strength λ ci , the wake had two different shedding patterns when taking 83% as an approximate porosity threshold. The pappus angle played a role in concentrating wake, elongating the recirculation zones and reducing kinetic turbulence. In the smoke-wire visualization, multiple Karman vortex streets are observed, which is induced by the filaments. Combined these two regions, we interpreted the flow properties of pappus and revealed the mechanism of dynamic wake. Graphical Abstract
ISSN:1343-8875
1875-8975
DOI:10.1007/s12650-023-00915-5