| 1. |
- Andric, Jelena, 1979, et al.
(författare)
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Rheological properties of dilute suspensions of rigid and flexible fibers
- 2014
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 212, s. 36-46
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Tidskriftsartikel (refereegranskat)abstract
- Particle-level simulations are used to study the rheology of monodispersed suspensions of rigid and flexible fibers in a creeping, simple shear flow of a Newtonian fluid. We also investigate the influence of different equilibrium shapes (straight and curved) of the fibers on the behavior of the suspension. A parametric study of the impacts of fiber flexural rigidity and morphology on rheology quantifies the effects of these realistic fiber features on the experimentally accessible rheological properties. A fiber is modeled as a chain of rigid cylindrical segments, interacting through a two-way coupling with the fluid described by the incompressible three-dimensional Navier--Stokes equations. The initial fiber configuration is in the flow--gradient plane. We show that, when the shear rate is increased, straight flexible fibers undergo a buckling transition, leading to the development of finite first and second normal stress differences and a reduction of the viscosity. These effects, triggered by shape fluctuations, are dissimilar to the effects induced by the curvature of stiff, curved fibers, for which the viscosity increases with the curvature of the fiber. An analysis of the orbital drift of fibers initially oriented at an angle to the flow--gradient plane provides an estimate for the time-scale within which the prediction of the rheological behavior is valid. The information obtained in this work can be used in the experimental characterization of fiber morphology and mechanics through rheology.
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| 2. |
- Cunha, F. R., et al.
(författare)
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Oscillatory motion of a spherical bubble in a non-Newtonian fluid
- 2013
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 191, s. 35-44
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Tidskriftsartikel (refereegranskat)abstract
- The motion of a spherical bubble in a nonlinear viscoelastic media subjected to an acoustic pressure field is considered. The ambient fluid is composed of a Newtonian liquid in which additives at small volume fraction are diluted. The contribution of the additives with high aspect ratio brings strong anisotropy and is described by an extensional viscosity. The elastic effect is presented by the relaxation time of the additives. A lower convected Maxwell model is adopted to describe the viscoelastic properties, resulting in a modified Rayleigh-Plesset equation. The set of governing equations does not require a numerical solution for the space domain. Non-linear radial oscillations of a single bubble are obtained numerically using a fifth order Runge-Kutta scheme with adaptive time step. The results predict an extra anisotropy for a Deborah number regime De∼. 1, due to stretched additives, which contributes to bubble motion stabilization. Under this condition, the relaxation time is greater than the time scale of the flow, where no interaction between the elastic effect of the additives and the motion of the bubble is found. However, for De∼. 0.1 we observe an increase of vibrational modes on the frequency domain and higher bubble internal pressure, which may lead to collapse occurrence. The decrease in the volume fraction of the additives also shows significant variation of bubble oscillations as the elastic effect has a proportionally larger contribution than the anisotropic effect. Other results and considerations regarding relevant parameters are also discussed.
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| 3. |
- Haque, Simon, et al.
(författare)
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Stability of fluids with shear-dependent viscosity in the lid-driven cavity
- 2012
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 173-174, s. 49-61
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Tidskriftsartikel (refereegranskat)abstract
- The classical problem of the lid-driven cavity extended infinitely in the spanwise direction is considered for non-Newtonian shear-thinning and shear-thickening fluids, where the viscosity is modeled by the Carreau model. Linear stability is used to determine the critical Reynolds number at which the two-dimensional base-flow becomes unstable to three-dimensional spanwise-periodic disturbances. We consider a square cavity, characterized by steady unstable modes, and a shallow cavity of aspect ratio 0.25, where oscillating modes are the first to become unstable for Newtonian fluids. In both cases, the critical Reynolds number first decreases with decreasing power-index n (from shear-thickening to shear-thinning fluids) and then increase again for highly pseudoplastic fluids. In the latter case, this is explained by the thinner boundary layers at the cavity walls and less intense vorticity inside the domain. Interestingly, oscillating modes are found at critical conditions for shear-thickening fluids in a square cavity while the shallow cavity supports a new instability of lower frequency for large enough shear-thinning. Analysis of kinetic energy budgets and structural sensitivity are employed to investigate the physical mechanisms behind the instability.
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