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Author ORCID Identifier

https://orcid.org/0000-0001-9689-9337

AccessType

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Mechanical Engineering

Year Degree Awarded

2020

Month Degree Awarded

September

First Advisor

Jonathan P. Rothstein

Subject Categories

Complex Fluids | Dynamics and Dynamical Systems | Engineering Mechanics | Fluid Dynamics | Mechanical Engineering

Abstract

It is well known that when a flexible or flexibly-mounted structure is placed perpendicular to a Newtonian fluid flow, it can oscillate due to the shedding of vortices at high Reynolds numbers. Unlike Newtonian fluids, viscoelastic fluid flow can become unstable even at infinitesimal Reynolds numbers due to a purely elastic flow instability occurring at large Weissenberg numbers. This thesis focuses on exploring the mechanisms of viscoelastic fluid-structure interactions (VFSI) through experimental investigations on several different combinations of flexible and flexibly-mounted circular cylinders, micro and macro-scale cantilevered beams and viscoelastic fluids such as wormlike micelle solutions and polymer solutions.

VFSI study of a flexible cylinder in a flow of wormlike micelle solution is presented where the fluctuating fluid forces exerted on the structure from the elastic flow instabilities lead to a dynamic coupling between the oscillatory structural motion and fluid flow. The presence of a viscoelastic lock-in is reported, for the first time, through a set of experiments where the frequency of the elastic instabilities and the natural frequency of the flexible structure become equal as the flow velocity was increased. Unlike Newtonian fluid-structure interactions, where lock-in corresponds to the maximum observed amplitude of oscillations, in VFSI, the amplitude of oscillations reached a plateau while in lock-in, but increased with Weissenberg number and reduced velocity before and after lock-in.

Microfluidic VFSI of a polymer solution flowing past a cantilevered beam is investigated for varying beam flexibility and Weissenberg number as the flow field transitioned from a stable detached vortex upstream of the beam to a time-dependent unstable vortex shedding. The oscillations of the beam were observed to show two distinct regimes: a clear single vortex shedding regime, and another characterized by 3D chaotic-like instabilities. Finally, VFSI of a cantilevered beam in a flow of wormlike micelle solution is studied as a function of the beam's tip angle. For beams with small tip angles of 0o and 25o, no oscillation was observed. However, for beams with larger tip angles of 45o and 65o, an oscillatory motion coupled to the flow instability was observed, where the amplitude of the beam oscillations increased with increasing tip angle.

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