Near wake coherent structures of a turbulent axisymmetric bluff body wake
File(s)
Published version
Author(s)
Zhu, Taihang
Rigas, Georgios
Morrison, Jonathan F
Type
Journal Article
Abstract
The coherent structures of a turbulent axisymmetric bluff body wake are investigated based on synchronised
near-wake velocity and base pressure measurements. The proper orthogonal decomposition (POD) analysis
confirms the persistence of the laminar spatio-temporal symmetry breaking instabilities at high Reynolds
numbers (here 𝑅𝑒𝐷 = 1.88 × 105
). The laminar rotational symmetry breaking mode randomly meanders in the
azimuthal direction, and the unsteady laminar unstable eigenmodes manifest as asymmetric unsteady vortex
shedding. Additionally, a coherent streamwise wake pulsation is identified (bubble pumping). Based on the
symmetry-breaking property of the turbulent wake, the vector field 𝐪 is decomposed into two antisymmetric
components 𝐪
+ and 𝐪
− to perform conditional POD (CPOD) with a comparison of 𝐪
′+ and 𝐪
+′, extracting
antisymmetric modes in the stable asymmetric wake states. The asymmetry of the wake due to rotational
symmetry break, quantified using the centre of pressure (CoP), is correlated to the base pressure using
conditional averaging. The most probable symmetry-breaking wake state corresponds to a low-pressure (high
drag) region, and two high-pressure (low drag) regions at the limit of axisymmetric (CoP → 0) and highly
asymmetric (CoP → ∞) wake states are identified. The high-pressure wake state in the highly asymmetric
wake is caused by the backflow, which results in a high-pressure region near the base edge. Conditional
averaging based on the base pressure shows that the transition between high- and low-pressure conditions is
coupled with wake asymmetry.
near-wake velocity and base pressure measurements. The proper orthogonal decomposition (POD) analysis
confirms the persistence of the laminar spatio-temporal symmetry breaking instabilities at high Reynolds
numbers (here 𝑅𝑒𝐷 = 1.88 × 105
). The laminar rotational symmetry breaking mode randomly meanders in the
azimuthal direction, and the unsteady laminar unstable eigenmodes manifest as asymmetric unsteady vortex
shedding. Additionally, a coherent streamwise wake pulsation is identified (bubble pumping). Based on the
symmetry-breaking property of the turbulent wake, the vector field 𝐪 is decomposed into two antisymmetric
components 𝐪
+ and 𝐪
− to perform conditional POD (CPOD) with a comparison of 𝐪
′+ and 𝐪
+′, extracting
antisymmetric modes in the stable asymmetric wake states. The asymmetry of the wake due to rotational
symmetry break, quantified using the centre of pressure (CoP), is correlated to the base pressure using
conditional averaging. The most probable symmetry-breaking wake state corresponds to a low-pressure (high
drag) region, and two high-pressure (low drag) regions at the limit of axisymmetric (CoP → 0) and highly
asymmetric (CoP → ∞) wake states are identified. The high-pressure wake state in the highly asymmetric
wake is caused by the backflow, which results in a high-pressure region near the base edge. Conditional
averaging based on the base pressure shows that the transition between high- and low-pressure conditions is
coupled with wake asymmetry.
Date Issued
2025-03-01
Date Acceptance
2024-11-16
Citation
International Journal of Heat and Fluid Flow, 2025, 112
ISSN
0142-727X
Publisher
Elsevier BV
Journal / Book Title
International Journal of Heat and Fluid Flow
Volume
112
Copyright Statement
© 2024 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
License URL
Identifier
10.1016/j.ijheatfluidflow.2024.109668
Publication Status
Published
Article Number
109668
Date Publish Online
2024-12-11