2 research outputs found
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Experimental Study on Viscoelastic Fluid-Structure Interactions
It is well known that when a flexible or flexibly-mounted structure is placed perpendicular to the flow of a Newtonian fluid, it can oscillate due to the shedding of separated vortices at high Reynolds numbers. If the same flexible object is placed in non-Newtonian flows, however, the structure\u27s response is still unknown. The main objective of this thesis is to introduce a new field of viscoelastic fluid-structure interactions by showing that the elastic instabilities that occur in the flow of viscoelastic fluids can drive the motion of a flexible structure placed in its path. Unlike Newtonian fluids, the flow of viscoelastic fluids can become unstable at infinitesimal Reynolds numbers due to the onset of a purely elastic flow instability. This instability occurs in the absence of nonlinear effects of fluid inertia and the Reynolds number of the flows studied here are in the order of 10-4. When such an elastic flow instability occurs in the vicinity of a flexible structure, the fluctuating fluid forces exerted on the structure grow large enough to cause a structural instability which in turn feeds back into the fluid resulting in a flow instability. Nonlinear periodic oscillations of the flexible structure are observed which have been found to be coupled to the time-dependent growth and decay of viscoelastic stresses in the wake of the structure. Presented in this thesis are the results of an investigation of the interaction occurring in the flow of a viscoelastic wormlike micelle solution past a flexible rectangular sheet. The structural geometries studied include: flexible sheet inclinations at 20°, 45° and 90° and flexible sheet widths of 5mm and 2.5mm. By varying the flow velocity, the response of the flexible sheet has been characterized in terms of amplitude and frequency of oscillations. Steady and dynamic shear rheology and filament stretching extensional rheology measurements are conducted in order to characterize the viscoelastic wormlike micelle solution. Bright field images show the deformation of the flexible sheet during an unstable oscillation while flow-induced birefringence images highlight the viscoleastic fluid stresses produced in the wake of the flexible sheet
Recommended from our members
Experimental Study of Viscoelastic Fluid-Structure Interactions
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\u27s 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