Unsteady large-scale wake structure behind levitated freestream-aligned circular cylinder

TL;DR

This study investigates large-scale wake structures behind a freestream-aligned circular cylinder using wind tunnel tests, modal decomposition, and stereo PIV, revealing relationships between vortex patterns, shear modes, and recirculation bubble dynamics.

Contribution

It identifies and analyzes four characteristic wake structures and their interactions, providing new insights into wake behavior at different vortex states and their causality.

Findings

Four large-scale wake structures identified: recirculation bubble pumping, azimuthal shear mode, vortex shedding, streaks.

Vortex shedding states classified into three patterns: anticlockwise, clockwise, flapping.

Recirculation bubble pumping influenced by vortex position and streak strength, with causality in low-frequency range.

Abstract

The relationships between characteristic large-scale wake structures appearing behind a freestream-aligned circular cylinder are investigated and discussed from the velocity field obtained by wind tunnel tests. The tests were conducted under a supportless condition using a magnetic suspension and balance system and stereo PIV measurements at a Reynolds number of $3.46\times10^4$. The velocity fields were analysed with a modal decomposition combining azimuthal Fourier decomposition and proper orthogonal decomposition. The wake behind the freestream-aligned circular cylinder with three different fineness ratios of 1.0, 1.5 and 2.0 was investigated, and the wake structures in a nonreattaching flow formed by the cylinder at the fineness ratio of 1.0 are mainly discussed in the present study. Four characteristic large-scale wake structures of recirculation bubble pumping, azimuthal shear mode, large-scale vortex shedding and streaks are identified and mainly focused on in the present study. The state of the vortex shedding is classified into three; anticlockwise/clockwise circular and flapping patterns. Each state has a relationship with the azimuthal shear mode and it tends to appear when the state is circular. Furthermore, from the analysis of the relationship between modes, the recirculation bubble pumping is found to be related to the vortex shedding position in the radial direction and the strength of the streaks. Particularly, analysis of causality shows that the recirculation bubble pumping is affected by them in the low-frequency range.