Purely elastic turbulence in channel flows of dilute polymer solutions
Solutions of long, flexible polymer molecules are complex fluids that simultaneously exhibit fluid-like and solid-like behaviour. When subjected to an external flow, dilute polymer solutions exhibit elastic turbulence — a unique, chaotic flow state absent in Newtonian fluids, like water. Unlike its...
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| Other Authors: | , , , , , |
| Format: | Doctoral or Postdoctoral Thesis |
| Language: | English |
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The University of Edinburgh
2024
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| Subjects: | |
| Online Access: | https://hdl.handle.net/1842/42248 https://doi.org/10.7488/era/4968 |
| Summary: | Solutions of long, flexible polymer molecules are complex fluids that simultaneously exhibit fluid-like and solid-like behaviour. When subjected to an external flow, dilute polymer solutions exhibit elastic turbulence — a unique, chaotic flow state absent in Newtonian fluids, like water. Unlike its Newtonian counterpart, elastic turbulence is caused by polymer molecules stretching and aligning in the flow, and can occur at vanishing inertia. While experimental realisations of elastic turbulence are well-documented, there is currently no understanding of its mechanism. In the present thesis, following two preparational studies, I present large-scale direct numerical simulations of elastic turbulence in pressuredriven flows through straight channels: After characterising recently found two-dimensional viscoelastic exact coherent structures called narwhal states, I compute their linear stability to three-dimensional perturbations and discover that they are linearly unstable. Then, using initial conditions based on the narwhal states, I demonstrate that the transition to elastic turbulence is subcritical, giving rise to spot-like flow structures that, further away from the transition, eventually spread throughout the domain. Lastly, I provide evidence that elastic turbulence is organised around unstable coherent states that are localised close to the channel midplane. My results show that elastic turbulence is very different from its Newtonian counterpart and they pave the way to understanding its mechanism. |
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