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The near-wake vortex dynamics of a wall-mounted hemisphere measured with tomographic particle image velocimetry

Published online by Cambridge University Press:  30 May 2025

Jiaxin Liu Open the ORCID record for Jiaxin Liu [Opens in a new window] ,
Yichen Zhu Open the ORCID record for Yichen Zhu [Opens in a new window]  and
Jinjun Wang Open the ORCID record for Jinjun Wang [Opens in a new window]
Jiaxin Liu
Affiliation:
Fluid Mechanics Key Laboratory of Education Ministry, Beijing University of Aeronautics and Astronautics, Beijing 100191, PR China
Yichen Zhu
Affiliation:
Fluid Mechanics Key Laboratory of Education Ministry, Beijing University of Aeronautics and Astronautics, Beijing 100191, PR China
Jinjun Wang*
Affiliation:
Fluid Mechanics Key Laboratory of Education Ministry, Beijing University of Aeronautics and Astronautics, Beijing 100191, PR China
*
Corresponding author: Jinjun Wang, jjwang@buaa.edu.cn
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Abstract

The dynamics of flow over an isolated surface-mounted hemisphere are investigated with tomographic particle image velocimetry (PIV). The 10 mm height hemisphere is completely submerged in the laminar boundary layer, and the height-based Reynolds number is 1530. The evolution of typical coherent structures around the hemisphere are discussed, with emphasis on the hairpin vortex (HV) and side hairpin vortex (SHV) formed periodically in the middle and both sides of the wake, respectively. Proper orthogonal decomposition (POD) analysis is conducted to explore the vortex dynamics. The shedding processes of the HV and SHV are each dominated by two different POD modes with correspondingly different characteristic frequencies, which has not been reported before in the literature. Furthermore, the coexistence of symmetric and asymmetric shedding patterns is explored for the first time in the shedding process of the HV at such a low Reynolds number. The asymmetric behaviour is controlled by the asymmetric shedding POD mode, whose dominant frequency is exactly half of the symmetric mode. In addition, SHVs on both sides of the wake are throughout formed and shed alternately, and the streamwise extensions of a horseshoe vortex also oscillate asymmetrically, which are responsible for the formation of the asymmetric shedding pattern of the HV. These findings help to fill the gaps in the related field and contribute to studies on the vortex dynamics of the flow over a hemisphere.

JFM classification

Vortex Flows: Vortex dynamics Vortex Flows: Vortex shedding Wakes/Jets: Wakes

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Type
JFM Papers
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Journal of Fluid Mechanics , Volume 1012 , 10 June 2025 , A7
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© The Author(s), 2025. Published by Cambridge University Press

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