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. 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: [Figure not available: see fulltext.]