Boulafentis, Theofilos;
(2025)
Elasto-inertial instabilities and turbulence in Taylor-Couette flows.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Elasto-Inertial Turbulence (EIT) is a new turbulent-like flow observed in complex, polymeric fluids, and gaining increasing interest. EIT is a distinct flow state of early turbulence, arising from a combination of inertia and elastic forces due to the polymers (hoop stresses). The transition to EIT is rich in flow intermittent states and characterised by different coherent structures and energy cascades to its Newtonian counterpart that is not fully explored. In the present thesis, Taylor-Couette (TC) flow (the flow between two concentric cylinders) was employed as an exemplar, to experimentally study elastoinertial instabilities and transition to EIT. Boger fluids, namely elastic fluids with constant viscosity, were employed. The rheological properties of these fluids were determined by means of shear and extensional rheometry. The elastoinertial TC flows were characterized using flow visualisation and Particle Image Velocimetry (PIV). Spatiotemporal flow maps produced from the flow visualisation experiments revealed three distinct transitional pathways for the fluid elasticities employed. When elastic forces are negligible the flow transitions are inertia-driven and exhibit an early onset of wavy instabilities with modified spatial and temporal characteristics. For intermediate elasticity, Rotating Standing Waves appear, leading to EIT through the Merging and Splitting mechanism of the Taylor vortices. At high elasticity, the transitional flow states were dominated by flame-pattern/diwhirls during ramp-up/down, respectively. The coherent structures of the flow for high elasticity fluids, termed solitary pairs of vortices, were resolved using PIV. Their stability, interactions, spectral properties and onset mechanism were investigated. The effect of elasticity on the coherent structures and turbulent flow properties was further investigated using PIV measurements for various elasticities at a fixed Reynolds number. Increasing fluid elasticity beyond the flame-pattern regime led to a new flow state, with low velocity fluctuations coexisting with solitary pairs, leading to turbulence suppression through increasing flow intermittency and hibernation. This thesis represents the first experimental effort to quantify EIT in TC flows, providing valuable insights into this new form of turbulence. The findings can be harnessed to enhance transport phenomena and drag reduction in manufacturing and energy applications.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Elasto-inertial instabilities and turbulence in Taylor-Couette flows |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10207569 |
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