Spatially coherent uniform momentum zones in accelerating turbulent pipe flow
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Date
2025
Authors
Gunaratne, I.
Guerrero, B.
Lambert, M.
Chin, R.
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Journal article
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Physics of Fluids, 2025; 37(7):075140-1-075140-12
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Isuru Gunaratne, Byron Guerrero, Martin Lambert, and Rey Chin
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Abstract
This paper investigates the transient characteristics of spatially coherent uniform momentum zones (UMZs) in an unsteady flow scenario comprising a rapidly accelerating turbulent pipe flow. The current work is based on high-fidelity direct numerical simulation (DNS) data sets starting from an initial friction Reynolds number (Reτ0)=500 up to a final friction Reynolds number (Reτ1)=670. All instantaneous uniform momentum zones (UMZs) are identified by constructing the probability density functions (PDFs) of the streamwise velocity component of the turbulent flow, as proposed by Adrian et al., “Vortex organization in the outer region of the turbulent boundary layer,” J. Fluid Mech. 422, 1–54 (2000). A rapidly accelerating turbulent pipe flow undergoes four unsteady flow stages: inertial stage, pre-transition stage, transition stage, and finally, core-relaxation stage. The results show that despite the number of flow regions of instantaneous UMZs dropping during the pre-transition stage, more spatially coherent flow regions are seen during the pre-transition stage. Investigation of streamwise length-scales of spatially coherent UMZs reveals that UMZs experience an elongation during the pre-transition stage, followed by a relaxation during the transition and core relaxation stages along the streamwise direction, with UMZs located at different wall-normal locations stretching and relaxing differently. Despite the structural characteristic differences of both instantaneous and spatially coherent UMZs, results indicate that a similar mechanism causes them to either merge or to be annihilated as the flow accelerates, and for them to breakup or reappear as the flow recovers.
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Published Online: 15 July 2025
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© 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/4.0/).