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The relationships between characteristic large-scale wake structures appearing behind a free-stream-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 $3.46\times 10^4$ . The velocity fields were analysed with a modal decomposition combining azimuthal Fourier decomposition and proper orthogonal decomposition. The wake behind the free-stream-aligned circular cylinder with three different fineness ratios of 1.0, 1.5 and 2.0 was investigated, and the wake structures in a non-reattaching flow formed by the cylinder at a fineness ratio of 1.0 are mainly discussed in the present study. Four characteristic large-scale wake structures of the 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.
The relationships between characteristic large-scale wake structures appearing behind a free-stream-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 $3.46\times 10^4$ . The velocity fields were analysed with a modal decomposition combining azimuthal Fourier decomposition and proper orthogonal decomposition. The wake behind the free-stream-aligned circular cylinder with three different fineness ratios of 1.0, 1.5 and 2.0 was investigated, and the wake structures in a non-reattaching flow formed by the cylinder at a fineness ratio of 1.0 are mainly discussed in the present study. Four characteristic large-scale wake structures of the 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.
Base pressure fluctuations associated with the large-scale wake structures behind a freestream-aligned circular cylinder and aerodynamic force fluctuations related to them are experimentally investigated in the wind tunnel tests. Measurements at ReD=6.97×104 and 1.04×105 were conducted using pressure-sensitive paint (PSP) and a magnetic suspension and balance system (MSBS) for creating a supportless condition. The obtained pressure fields were mainly analyzed by a modal decomposition combining azimuthal Fourier decomposition and proper orthogonal decomposition (POD). The pressure fluctuations caused by large-scale vortex shedding were observed from the results of frequency analysis for mode coefficients. The states of the fluctuations were classified into three patterns, which are anticlockwise/clockwise circular and flapping patterns. These patterns have been observed in the previous studies for velocity fluctuations in the wake of a freestream-aligned circular cylinder. The conditional sampling analysis revealed that the trend in the amplitude of the pressure fluctuations is different by the state, and the flapping pattern causes a large pressure difference across the cylinder axis. Furthermore, the relationship between the antisymmetric pressure fluctuations and lift fluctuations, which act in the lateral direction of the cylinder, is confirmed.
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