| 1. |
- Holmvall, Martin, 1976-, et al.
(author)
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Simulation of two-phase flow with moving immersed boundaries
- 2011
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In: International Journal for Numerical Methods in Fluids. - : Wiley. - 0271-2091 .- 1097-0363. ; 67:12, s. 2062-2080
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Journal article (peer-reviewed)abstract
- A two-dimensional model for immiscible binary fluid flow including moving immersed objects is presented. The fluid motion is described by the incompressible Navier-Stokes equation coupled with a phase-field model based on van der Waals’ free energy density and the Cahn-Hilliard equation. The immersed boundary method has been utilised to handle moving immersed objects and the phase-field boundary conditions have been adapted accordingly. Numerical stability and execution time was significantly improved by the use of a new boundary condition which implements minimisation of the free energy in a direct way. Convergence toward the analytical solution was demonstrated for equilibrium contact angle, the Lucas-Washburn theory and Stefan’s problem. The proposed model may be used for two-phase flow problems with moving boundaries of complex geometry, such as the penetration of fluid into a deformable, porous medium.
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| 2. |
- Holmvall, Martin, 1976-, et al.
(author)
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Transfer of a microfluid to a stochastic fibre network
- 2011
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In: Journal of Fluids and Structures. - : Elsevier BV. - 0889-9746 .- 1095-8622. ; 27:7, s. 937-946
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Journal article (peer-reviewed)abstract
- The transfer of a microscopic fluid droplet from a flat surface to a deformable stochastic fibre network is investigated. Fibre networks are generated with different levels of surface roughness, and a two-dimensional, two-phase fluid-structure model is used to simulate the fluid transfer. In simulations, the Navier-Stokes equations and the Cahn-Hilliard phase-field equations are coupled to explicitly include contact line dynamics and free surface dynamics. The compressing fibre network is modelled as moving immersed boundaries. The simulations show that the amount of transferred fluid is approximately proportional to the contact area between the fluid and the fibre network. However, areas where the fluid bridges and never actually makes contact with the substrate must be subtracted.
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| 3. |
- Kulachenko, Artem, et al.
(author)
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Basic Mechanisms of Fluting
- 2006
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In: Annual Meeting of the Pulp and Paper Technical Association of Canada (PAPTAC). - Que., Canada : Pulp and Paper Technical Association of Canada. ; , s. A161-A173, s. 161-173
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Conference paper (peer-reviewed)abstract
- Out-of-plane deformations of paper, such as fluting, significantly deteriorate the quality of a printed product. There are several explanations of fluting presented in the literature but there is no unanimously accepted theory regarding fluting formation consistent with all field observations. The present paper reviews the existing theories and proposes a mechanism that might give an answer to most of the questions regarding the fluting. The fluting formation has been considered as a post-buckling phenomenon which is analysed with the help of the finite element method. Fluting retention has been modelled by introducing an ink layer over the paper surface with ink stiffness estimated from experimental results. The impact of fast drying on fluting has been assessed numerically and experimentally. The result of the study suggests that fluting occurs due to small-scale strain variations, which in turn are caused by the moisture variations created during fast convection drying. The result also showed that ink stiffening alone cannot explain the fluting amplitudes observed in practice, suggesting the presence of other mechanisms of fluting retention.
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| 4. |
- Kulachenko, Artem, et al.
(author)
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Basic mechanisms of fluting formation and retention in paper
- 2007
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In: Mechanics of materials. - : Elsevier BV. - 0167-6636 .- 1872-7743. ; 39:7, s. 643-663
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Journal article (peer-reviewed)abstract
- Out-of-plane deformations of paper, such as fluting, significantly deteriorate the quality of a printed product. There are several explanations of fluting presented in the literature but there is no unanimously accepted theory regarding fluting formation and retention which is consistent with all field observations. This paper first reviews the existing theories and proposes a mechanism that might give an answer to most of the questions regarding fluting. The fluting formation has been considered as a post-buckling phenomenon which has been analysed with the help of the finite element method. Fluting retention has been modelled by introducing an ink layer over the paper surface with the ink stiffness estimated from experimental results. The impact of fast drying on fluting has been assessed numerically and experimentally. The result of the study suggests that fluting occurs due to small-scale hygro-strain variations, which in turn are caused by the moisture variations created during fast convection (through-air) drying. The result also showed that ink stiffening alone cannot explain the fluting amplitudes observed in practice, but that high drying temperatures promote inelastic (irreversible) deformations in paper and this may itself preserve fluting.
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| 5. |
- Kulachenko, Artem, 1978-, et al.
(author)
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Direct simulations of fiber network deformation and failure
- 2012
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In: Mechanics of materials. - : Elsevier BV. - 0167-6636 .- 1872-7743. ; 51, s. 1-14
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Journal article (peer-reviewed)abstract
- A finite element model for 3D random fiber networks was constructed to simulate deformation and failure behavior of networks with dynamic bonding/debonding properties. Such fiber networks are ubiquitous among many living systems, soft matters, bio-materials, and engineering materials (papers and non-woven). A key feature of this new network model is the fiber–fiber interaction model that is based on AFM measurements from our earlier study. A series of simulations have been performed to investigate strain localization behavior, strength statistics, in particular, the variations of strength, strain-to-failure and elastic modulus, and their size dependence. Other variables investigated are fiber geometries. The result showed that, in spite of its disordered structure, strength and elastic modulus of a fiber network varied very little statistically, as long as the average number of fibers in the simulated specimen and the degree of fiber orientation are kept constant. However, strain-to-failure showed very significant statistical variations, and thus more sensitivity to the disordered structures.
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| 6. |
- Kulachenko, Artem, et al.
(author)
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Reinventing mechanics of fibre network
- 2008
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In: Progress in Paper Physics Seminar. - Helsinki : Helsinki University Press. ; , s. 185-193
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Conference paper (other academic/artistic)
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| 7. |
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| 8. |
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| 9. |
- Kulachenko, Artem, et al.
(author)
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Tension wrinkling and fluting in heatset web offset printing process : post buckling analysis
- 2005
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In: Advances in Paper Science and Technology. - : The Pulp and Paper Fundamental Research Society. - 9780954527235 ; , s. 1075-1099
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Conference paper (peer-reviewed)abstract
- Geometrically non-linear, large scale post-buckling analyses were carried out to investigate the influence of different parameters on residual waviness (fluting) after printing in a heat set web offset printing press. Mixed implicit-explicit finite element techniques were used in the analyses. The numerical procedure was verified by experimentally acquired data. Results show that when the paper web is perfectly flat before printing, fluting patterns after drying and moisture recovery generally have higher wavelength than those typically observed in fluted samples. Initial cockles of imprinted sheets were found to have impacts on the fluting patterns and amplitudes. Among the factors investigated, ink thickness and hygroexpansivity had significant influences on fluting: increasing these factors increased fluting amplitudes.
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| 10. |
- Lindström, Stefan B, 1974-, et al.
(author)
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New insights in paper forming from particle-level process simulations
- 2009
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In: Papermaking Research Symposium 2009. - Kuopio, Finland : University of Kuopio. - 9789512710164 ; , s. 38-
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Conference paper (other academic/artistic)abstract
- By virtue of the recent developments in simulation techniques for fibre suspensions flows, it is now possible to directly simulate forming of the paper sheet at a particle level under realistic flow conditions. This opens up a window of opportunity to better understand the microscale development of the paper structure, and to attribute particular features of the structure to different drainage elements.The simulations are based on a particle-level fibre suspension model, in which fibres are represented by chains of cylindrical fibre segments. The fibre model includes curled shapes and the torsion and bending of the fibres. It also captures the two-way interactions between the fibres and the fluid phase. The fluid motion is integrated from the Navier--Stokes equations.To illustrate the usage of the simulation tool, a sample parametric study of the effects of different fibre furnishes on the paper structure and wet strength is presented. Such an investigation could almost as easily have been performed with experiments. Simulations, however, have some advantages: First, the cost is almost nothing as compared to pilot trials. Secondly, the parameters of the simulations can be controlled one at a time, whereas in pilot trials, changing one process parameter will affect the others. Thirdly, every detail of the evolving paper structure is accessible at every instant in the simulations. That is, the forming process needs no longer be considered a "black box". Simulations also have some drawbacks. For instance, it is not possible to include the smallest particles, due to their vast number, while maintaining sufficiently large flow geometry. Therefore, simulations must target paper grades of low fines contents.In this communication, the pros and cons of particle-level simulations are discussed, and put into the context of previous forming and dewatering models in the literature. The development of the paper microstructure predicted in the simulations shows that thickening is the dominant forming mechanism, while filtration only occurs in the most dilute end of the typical range of consistencies used in the industry. This predicted behaviour is compared with the conventional view of dewatering, which holds filtration as the dominant forming mechanism.
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