VORTEX - Grupo de Mecânica dos Fluidos de Escoamentos Complexos


Comportamento Dinâmico de Bolhas em Fluidos Complexos

Autor: Rodrigo Avelino Mesquita dos Santos

Orientador: Francisco Ricardo Cunha

Resumo
The aim of this work is to examine the dynamical response of a spherical os- cillatory bubble immersed in a suspension composed of a viscous fluid and anisotropic particles. These particles can be long fibers or macromolecules of high molecular weight. The bubble is composed of an ideal gas and it is set into oscillatory motion in an acoustic pressure field. We consider an extension of Rayleigh-Plesset equation for this study. The main physical parameters involved in the system are the inertial forces, viscous forces and surface tension forces. Geometrical parameters due to the particles, such as aspect ratio, volumetric concentration of additive and particle orientation, affect the motion stabiliza- tion. We propose a non-dimensionalization of the governing equation to proceed with the study. We found the main physical non-dimensional parameters of the system such as the Reynolds number and the Weber number. Regular asymptotic expansions are used to find second order solution of the non-linear governing equation. The results are com- pared with numerical integration of the non-linear IPV problem by using a Runge-Kutta scheme. Variations in the physical parameters of the system, such as We, Re, pressure amplitude ε and forcing frequency ω can make the bubble have since a linear behavior until a strong nonlinear behavior and sometimes reach the collapse. Numerical results of the bubble response to an acoustic pressure field showed that the addition of a low concentration of additives aligned to the flow undergoing an acoustic pressure field may change significantly the nonlinear motion of collapsing bubbles. A model, that considers initially randomly oriented particles in the flow, shows that the contribution of particles in the stabilization of the movement of the bubble is lesser in this case since the effect of extensional viscosity occurs due to the particle resistance to the movement throughout its longitudinal direction. In addition, a model that take into account, beyond the anisotropic effect of particles, the elastic effect is presented. This viscoelasticity model showed that anisotropy effect are predominant in detriment to the elastic effect in the stabilization of the bubble, mainly for longer additives.

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