Passive control of cylinder aeolian tone by surface protrusions at low Reynolds number

The perforated shrouds have been proposed to control cylinder flows, while the effects and mechanisms at different Reynolds numbers (Res) remain unclear. Three-dimensional numerical simulations are conducted in this paper to compare the aerodynamic performance of the flow around a smooth cylinder and a shrouded cylinder at Re of 3900 and 1.4×105. The results indicate that the drag of the perforated shrouded cylinder is reduced by 30.8% at the high Re, while increased by 26% at Re of 3900 compared with the smooth cylinder. Differently, the lift oscillations of the cylinder are greatly weakened by 83.3% at the Re of 3900 and 98.5% at the Re of 1.4×105, which implies the wake oscillations are nearly eliminated at the Re of 1.4×105. Further analysis exhibits the near wake region is elongated along the mainstream, with significantly recovered pressure. Especially, the greater pressure loss owing to outer shrouds even leads to the negative drag of the inner cylinder at the high Re. In addition, the incoming flow is broken up by outer shrouds, and different flow patterns appear in the gap. The discernible vortex pairs occur in the gap at the Re of 3900, while as Re increases to 1.4×105, the quantities of small-scale vortex weaken the impact on the inner cylinder. The shear layer characteristics are elucidated by Lamb vector curl and Kelvin–Helmholtz instabilities. The vorticity stretching and tilting in the shear layer of the shrouded cylinder is much weaker at the high Re. Generally, the energy for shear layer instabilities at low frequencies is diminished with the presence of perforated shrouds. However, the energy at higher frequencies is strengthened at the low Re.

Numerical study on three-dimensional flow around a cylinder with perforated shrouds at different Reynolds numbers

Zhou,

Liu,

Chen

et al. 2023

Three-dimensional shape optimization of a submerged body under wave diffraction

Lu,

Liu,

Guo

2023

This study explores the use of shape optimization to reduce wave forces on a submerged floating body subjected to wave diffraction. To this end, gradient-based shape optimization is adopted, in which dimensionless wave excitation forces are the optimization objective. A shape parameterization method based on the Fourier-series expansion is developed that permits the representation of an arbitrary three-dimensional floating body. The discrete adjoint method is utilized to calculate the gradient of the objective function with respect to the shape parameters. Using three-dimensional shape optimization, taking the initial shape to be a hemisphere, a significant reduction in surge, heave, and pitch wave forces is achieved, with a maximum reduction of 48.40%, 68.43%, and 46.22%, respectively. Furthermore, optimization effectively suppresses wave run-up, with a maximum reduction of 15.62%. A comprehensive analysis of parameters is performed to reveal the effects of wave number, incident angle, and shape parameters on the final optimized shape and wave load characteristics. This study provides a solid guide to the optimization of floating offshore platforms and the development of innovative structural systems.

Aeroacoustic simulation of bluff bodies with protrusions at moderate Reynolds number

Kusano

2024

This paper presents an evaluation of passive control methods that employ surface protrusions to mitigate the aerodynamic sound generated from a cylinder wake flow. Building on previous designs optimized for low Reynolds numbers (Re = 150) through adjoint-based aeroacoustic shape optimization, this study investigated the performance under a moderate Reynolds number (Re = 67 000) condition typical of mechanical engineering applications using aeroacoustic simulations based on the lattice Boltzmann method. Three configurations of surface protrusions were tested, all of which were found to significantly reduce the mean drag by at least 45% compared with that of an unmodified circular cylinder. Designs featuring rear protrusions outperformed the conventional splitter plate in terms of the sound reduction performance, with symmetrical protrusions on both the front and rear surfaces achieving a tonal sound reduction of 13 dB. However, a specific protrusion design increased the low-frequency sound owing to the intensified large-scale flow separation. These findings highlight the effectiveness of rear protrusions in suppressing wake oscillations and dipole sound generation in the subcritical Reynolds number range. Moreover, the study revealed the need to tailor the front protrusion shape to the Reynolds number for performance optimization.