Uniform momentum zone kinematics under rapidly accelerating turbulent pipe flow conditions
Date
2025
Authors
Gunaratne, Isuru Chinthana
Editors
Advisors
Chin, Rey
Lambert, Martin
Guerrero, Byron
Lambert, Martin
Guerrero, Byron
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Thesis
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Abstract
Turbulent fluid flows within pipes in real-life applications are rarely steady. As a result, to properly understand transient turbulent flow dynamics within pipes and to implement them in engineering applications, it is crucial to untangle the complexities incorporated with unsteady wall-bounded turbulence. The presentwork focuses on large-scale coherent structures termed uniformmomentumzones (i.e.UMZs) and the flow kinematics associated with these structures in rapidly accelerating flow conditions between two steady Reynolds numbers. This thesis presents a series of investigations based on direct numerical simulation (DNS) datasets. The first study investigates the initial base flow Reynolds number effects on UMZs between two rapidly accelerating pipe flow cases starting from low and moderate initial Reynolds number base flow. It is shown that these coherent structures show unique responses to the accelerating flow based on the initial base flow Reynolds number, further deviating from their behaviour in steady turbulent pipe flows. The second study investigates the kinematics of instantaneous UMZs in a rapidly accelerating turbulent pipe flow scenario. The results revealed that as the turbulent flow accelerates, UMZs experience a drop in the number of zoneswithin these flowregions by dynamically reconfiguring densely packed flow regions to regions with fewer zones. Towards the end of the core-relaxation stage of the transient flow, UMZs located near the wall are seen to recover first alongside the bulk flow near the wall. However, UMZs located within the core region show the need for extended time scales to recover alongside the bulk flow away from the wall, and this need for extended timescales is shown to be initial steady state base flow Reynolds number dependent. Finally, the third study isolates and investigates spatially coherent UMZs filtered out from all instantaneous UMZs in rapidly accelerating turbulent pipe flow. It is shown that despite the drop in the number of instantaneous UMZs, as revealed through the current work, an increase in the number of spatially coherent UMZs occurs, revealing that the dynamic reconfiguration of densely packed UMZs to flow regions with fewer zones coupled with the annihilation of near- wall flow structures causes UMZs to becomemore spatially coherent, similar to near core UMZs being the most spatially coherent large scale structures as seen in steady turbulent wall-bounded flows. The investigation of individual length scales along the streamwise axis reveals that the spatially coherent flow structures elongate and relax, with faster spatially UMZs elongating the most. In comparison, slower spatially coherent UMZs show an earlier recovery. All the knowledge developed throughout this series of studies was based on furthering our understanding on how large-scale flow structures within flow regions characterised by uniform momentum within the flow behave in unsteady flow conditions caused by a rapid acceleration imposed, filling an essential gap in the literature in the current understanding of UMZs in unsteady wall-bounded flow.
School/Discipline
School of Electrical and Mechanical Engineering
Dissertation Note
Thesis (MPhil) -- University of Adelaide, School of Electrical and Mechanical Engineering, 2025
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This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals