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Tomographic PIV investigation on near-wake structures of a hemisphere immersed in a laminar boundary layer

Published online by Cambridge University Press:  22 September 2023

Han Tu Open the ORCID record for Han Tu [Opens in a new window] ,
Zhongyi Wang ,
Qi Gao Open the ORCID record for Qi Gao [Opens in a new window] ,
Wenxuan She Open the ORCID record for Wenxuan She [Opens in a new window] ,
Fujun Wang ,
Jinjun Wang Open the ORCID record for Jinjun Wang [Opens in a new window]  and
Runjie Wei
Han Tu
Affiliation:
State Key Laboratory of Fluid Power and Mechatronic System, School of Aeronautics and Astronautics, Zhejiang University, Hangzhou 310027, PR China Hubei Provincial Key Laboratory of Chemical Equipment Intensification and Intrinsic Safety, School of Mechanical and Electrical Engineering, Wuhan Institute of Technology, Wuhan 430205, PR China
Zhongyi Wang
Affiliation:
Key Laboratory of Fluid Mechanics, Ministry of Education, Beihang University, Beijing 100191, PR China
Qi Gao*
Affiliation:
State Key Laboratory of Fluid Power and Mechatronic System, School of Aeronautics and Astronautics, Zhejiang University, Hangzhou 310027, PR China
Wenxuan She
Affiliation:
State Key Laboratory of Fluid Power and Mechatronic System, School of Aeronautics and Astronautics, Zhejiang University, Hangzhou 310027, PR China
Fujun Wang
Affiliation:
State Key Laboratory of Fluid Power and Mechatronic System, School of Aeronautics and Astronautics, Zhejiang University, Hangzhou 310027, PR China
Jinjun Wang
Affiliation:
Key Laboratory of Fluid Mechanics, Ministry of Education, Beihang University, Beijing 100191, PR China
Runjie Wei
Affiliation:
MicroVec Inc., Beijing 100191, PR China
*
†Email address for correspondence: qigao@zju.edu.cn
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Abstract

The near wake of a hemisphere immersed in a laminar boundary layer is studied utilizing time-resolved tomographic particle image velocimetry (TPIV). Focus is placed on the three-dimensional vortical structures and the formation details of hairpin vortices before the onset of transition. The three-dimensional instantaneous pressure field of the hemisphere wake is reconstructed for better understanding the flow mechanism. Experiments are carried out with Reynolds number $Re_{r}=1370$, based on the hemisphere radius $R$. Features of periodicity of the near wake are analysed using proper orthogonal decomposition and Fourier transformation. The velocity fluctuation in the wall-normal direction is shown to be crucial to the formation of hairpin vortices in the near wake. By investigating the transport of mass and vorticity, and the correlation between pressure and hairpin vortex strength, the formation mechanism is revealed clearly. Specifically, the main hairpin vortices (MHVs) are formed within the reaction of outer high-speed flow and near-wall flow. The formation of the head portion is followed by the leg portion formation. The shedding of the MHVs is highly correlated with the pressure, as well as the pressure gradient in the wall-normal direction. For the side hairpin vortices (SHVs), the leg portion is formed first, followed by the generation of the head portion thanks to induction of the re-oriented standing vortices. The generation of the SHVs can be regarded as the downstream bridging of the standing vortices, similar to the generation of hairpin vortices due to the connection of streamwise vortices in turbulent boundary layers.

JFM classification

Wakes/Jets: Wakes
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 971 , 25 September 2023 , A36
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Copyright
© The Author(s), 2023. Published by Cambridge University Press

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