| 1. |
- Gibson, A. G., et al.
(författare)
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Mechanics of the squeeze flow of planar fibre suspensions
- 1999
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - 0377-0257. ; 82:1, s. 1-24
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Tidskriftsartikel (refereegranskat)abstract
- This paper discusses the axisymmetric squeeze flow of concentrated transversely isotropic fibre suspensions in a power-law matrix and relates to the processing of composite materials such as sheet moulding compounds (SMCs) and glass mat thermoplastics (GMTs). A solution to the squeeze flow problem for a transversely isotropic power-law fluid is presented first, followed by a more detailed micromechanical analysis. In the first part of the paper a variational approach is applied to the interpretation of squeeze flow behaviour. This gives a simple expression for the total pressure, which enables the contributions due to extension and shear to be separated. Applying the procedure to GMT data suggests that the dissipation is predominantly extensional, except at very low plate separations. In the second part, a non-local constitutive equation is derived based on a simple drag law for hydrodynamic interactions. This is then used to model the pressure distribution when the effective length of the fibres is comparable to or determined by the dimensions of the squeeze flow plates. The model is shown to describe the observed squeeze flow stresses in both long and short fibre systems and to relate behaviour to the underlying resin flow properties. © 1999 Elsevier Science B.V. All rights reserved.
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| 2. |
- Abdulrazaq, Muhammed, et al.
(författare)
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Numerical modelling of the extensional dynamics in elastoviscoplastic fluids
- 2023
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 318
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Tidskriftsartikel (refereegranskat)abstract
- The extensional dynamics of an elasto-viscoplastic (EVP) fluid is studied by means of numerical simulations modelling an experimental configuration. Specifically, we track the interface between the EVP material and the Newtonian medium using an algebraic volume of fluid method (MTHINC-VOF) and employ a fully Eulerian immersed boundary method (IBM) to model the motion of the piston responsible for the extension of the material. We investigate the role of different values of the yield stress, surface tension at the interface between the EVP material and the surrounding fluid, polymer viscosity ratio, and extension rates on the necking thickness of the material, extensional viscosity, and yielding of the material for two sets of parameter with low and high elasticity. The results of the simulations reveal that when the yield stress of the EVP material is much larger than the viscous stresses, the material undergoes an elastic deformation, regardless of the selected values of the extension rate, interfacial forces, and viscosity ratio. Moreover, by increasing the ratio of the polymeric viscosity to the total viscosity of the system, the EVP material produces stronger strain hardening and reaches the minimum resolvable width sooner. Specific and novel to our study, we show that interfacial forces cannot be ignored when the surface tension coefficient is such that a Capillary number based on the extensional rate is of order 1. For large values of the surface tension coefficient, the EVP material fails sooner, with a clear deviation from the exponential reduction in the neck thickness. Moreover, our results suggest that the role of the yield stress value on the dynamics of the material is more pronounced at lower elasticity.
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| 3. |
- 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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| 4. |
- Bazesefidpar, Kazem, et al.
(författare)
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Numerical simulation of the coalescence-induced polymeric droplet jumping on superhydrophobic surfaces
- 2022
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 307
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Tidskriftsartikel (refereegranskat)abstract
- Self-propelled jumping of two polymeric droplets on superhydrophobic surfaces is investigated by three-dimensional direct numerical simulations. Two identical droplets of a viscoelastic fluid slide, meet and coalesce on a surface with contact angle 180 degrees. The droplets are modelled by the Giesekus constitutive equation, introducing both viscoelasticity and a shear-thinning effects. The Cahn-Hilliard Phase-Field method is used to capture the droplet interface. The simulations capture the spontaneous coalescence and jumping of the droplets. The effect of elasticity and shear-thinning on the coalescence and jumping is investigated at capillary-inertial and viscous regimes. The results reveal that the elasticity of the droplet changes the known capillary-inertial velocity scaling of the Newtonian drops at large Ohnesorge numbers; the resulting viscoelastic droplet jumps from the surface at larger Ohnesorge numbers than a Newtonian drop, when elasticity amplifies visible shape oscillations of the merged droplet. The numerical results show that polymer chains are stretched during the coalescence and prior to the departure of two drops, and the resulting elastic stresses at the interface induce the jumping of the liquid out of the surface. This study shows that viscoelasticity, typical of many biological and industrial applications, affects the droplet behaviour on superhydrophobic and self-cleaning surfaces.
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| 5. |
- Chaparian, Emad, et al.
(författare)
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An adaptive finite element method for elastoviscoplastic fluid flows
- 2019
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 271, s. 104148-
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Tidskriftsartikel (refereegranskat)abstract
- Elastoviscoplastic fluids are a class of yield-stress fluids that behave like neoHookean (or viscoelastic) solids when the imposed stress is less than the yield stress whereas after yielding, their behaviour is described by a viscoplastic fluid with an additional elastic history. This exceptional behaviour has been recently observed by many yield stress fluids in rheometric tests such as waxy crude oil, Carbopol gel, etc. Moreover, interesting phenomena have been evidenced experimentally such as the presence of a negative wake and a loss of fore-aft symmetry about a settling particle which are predominantly related to the elastic behaviour of yield-stress fluids (i.e., coupling of elasticity and plasticity). Here, we present a numerical scheme based on the so-called augmented Lagrangian method for numerical simulation of elastoviscoplastic fluid flows. The method is benchmarked by two rheometric flows: Poiseuille and circular Couette flows for which analytical solutions are derived. Moreover, anisotropic adaptive mesh procedure (which was previously introduced for viscoplastic fluid flows by Saramito and Roquet, Comput. Meth. Appl. Mech. Eng., vol. 190, 2001, pp. 5391-5412) is coupled to obtain a fine resolution of the yield surfaces. Finally, the presented method is applied to study more complex flows: elastoviscoplastic fluid flow in a wavy channel.
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| 6. |
- Chaparian, Emad, et al.
(författare)
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Particle migration in channel flow of an elastoviscoplastic fluid
- 2020
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier B.V.. - 0377-0257 .- 1873-2631. ; 284
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Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics of a neutrally buoyant rigid sphere carried by an elastoviscoplastic fluid in a pressure-driven channel flow numerically. The yielding to flow is marked by the yield stress which splits the flow into two main regions: the core unyielded region and two sheared yielded regions close to the walls. The particles which are initially in the plug region are observed to translate with the same velocity as the plug without any rotation/migration. Keeping the Reynolds number fixed, we study the effect of elasticity (Weissenberg number) and plasticity (Bingham number) of the fluid on the particle migration inside the sheared regions. In the viscoelastic limit, in the range of studied parameters (low elasticity), inertia is dominant and the particle finds its equilibrium position between the centreline and the wall. The same happens in the viscoplastic limit, yet the yield surface plays the role of centreline. However, the combination of elasticity and plasticity of the suspending fluid (elastoviscoplasticity) trigger particle-focusing: in the elastoviscoplastic flow, for a certain range of Weissenberg numbers (≈0.5), isolated particles migrate all the way to the centreline by entering into the core plug region. This behaviour suggests a particle-focusing process for inertial regimes which was not previously found in a viscoelastic or viscoplastic carrying fluid.
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| 7. |
- 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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| 8. |
- De Vita, Francesco, et al.
(författare)
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Elastoviscoplastic flows in porous media
- 2018
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier. - 0377-0257 .- 1873-2631. ; 258, s. 10-21
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the elastoviscoplastic flow through porous media by numerical simulations. We solve the Navier–Stokes equations combined with the elastoviscoplastic model proposed by Saramito for the stress tensor evolution [1]. In this model, the material behaves as a viscoelastic solid when unyielded, and as a viscoelastic Oldroyd-B fluid for stresses higher than the yield stress. The porous media is made of a symmetric array of cylinders, and we solve the flow in one periodic cell. We find that the solution is time-dependent even at low Reynolds numbers as we observe oscillations in time of the unyielded region especially at high Bingham numbers. The volume of the unyielded region slightly decreases with the Reynolds number and strongly increases with the Bingham number; up to 70% of the total volume is unyielded for the highest Bingham numbers considered here. The flow is mainly shear dominated in the yielded region, while shear and elongational flow are equally distributed in the unyielded region. We compute the relation between the pressure drop and the flow rate in the porous medium and present an empirical closure as function of the Bingham and Reynolds numbers. The apparent permeability, normalized with the case of Newtonian fluids, is shown to be greater than 1 at low Bingham numbers, corresponding to lower pressure drops due to the flow elasticity, and smaller than 1 for high Bingham numbers, indicating larger dissipation in the flow owing to the presence of the yielded regions. Finally we investigate the effect of the Weissenberg number on the distribution of the unyielded regions and on the pressure gradient.
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| 9. |
- Djalili-Moghaddam, M., et al.
(författare)
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A model for short-range interactions in fibre suspensions
- 2005
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 132:1-3, s. 73-83
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Tidskriftsartikel (refereegranskat)abstract
- This paper introduces a model for short-range fibre-fibre interaction and combines it with the Shaqfeh-Fredrickson result for long-range hydrodynamic interactions. The short-range interactions are assumed to be localised, such that the interaction forces may be taken to act at points on the fibre axes. Only interactions of viscous Newtonian character are explored in this work; the approach, however, may be used to treat more general nonlinear interactions, such as non-Newtonian vicous forces or friction. To simulate the orientation evolution, a convective discretisation technique combined with a modification of the Folgar-Tucker rotary diffusion model is proposed. The rotational parallel-plate geometry is analysed, and transient as well as steady state responses are computed. Experiments are performed on suspensions of polyamide fibres in silicon oil, using a rotational parallel-plate rheometer. It is demonstrated that the effect of fibre volume fraction and aspect ratio are well captured by this theory. Normal force as well as torque is adequately predicted. Other effects, however, such as shear thinning and effect of absolute particle size, are not predicted by the linear theory.
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| 10. |
- Fallahjoybari, Nima, et al.
(författare)
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Large eddy simulation of turbulent pulp flow in a channel
- 2020
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 285
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Tidskriftsartikel (refereegranskat)abstract
- Large eddy simulation (LES) of turbulent flow of concentrated fiber suspension or pulp is carried out to investigate the flow and turbulence structures in a channel. The simulations are carried out for the turbulent flow of Eucalyptus pulp suspension using OpenFOAM for three fiber concentrations (c = 1.5, 2.0 and 2.5) and six different Reynolds numbers (6 <= Re-s <= 16,600). It is observed that the variation in flow regime is similar in the two lower fiber concentrations while the flow regime is highly affected by fiber concentration for c = 2.5. Visualizations of vortical structures for different Reynolds numbers and fiber concentrations are used to investigate different flow regimes. Variation of apparent viscosity with Reynolds number and fiber concentration is also presented to show its effect on the turbulent properties of fiber suspension flow. It is shown that the deviation of turbulent velocity profile from that of a Newtonian flow increases with an increase in Reynolds number and fiber concentration. Also, the extend of buffer layer increases at higher Re. Using the calculated turbulent velocity profile, the values of constant in logarithmic velocity profile is proposed for fiber suspension. Finally, a discussion is presented on the variation of turbulent intensities and Reynolds stress with Reynolds number and fiber concentration.
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