| 1. |
- Andric, Jelena, 1979, et al.
(författare)
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A particle-level rigid fiber model for high-Reynolds number flow, implemented in a general-purpose CFD code
- 2013
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Ingår i: 8th International Conference on Multiphase Flow ICMF 2013, Korea.
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Konferensbidrag (refereegranskat)abstract
- A particle-level rigid fiber model has been integrated into a general-purpose, open source computational fluid dynamics code to carry out detailed studies of fiber–flow interactions in realistic flow fields. The fibers are modeled as chains of cylindrical segments, and their translational and rotational degrees of freedom are considered. The equations of motion contain the contributions from hydrodynamic forces and torques, and the segment inertia is taken into account. The model is validated for the rotational motion of isolated fibers in simple shear flow, and the computed period of rotation is in good agreement with the one computed using Jeffery’s equation for a prolate spheroid with an equivalent aspect ratio. The model is applied by suspending a number of fibers in the swirling flow of a conical diffuser, resembling one stage in the dry-forming of pulp mats. The Reynolds-averaged Navier–Stokes equations with an eddy-viscosity turbulence model are employed to describe the fluid motion, and a one-way coupling between the fibers and the fluid phase is included. The dependence of the fiber motion on initial position and density is analyzed.
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| 2. |
- Andric, Jelena, 1979, et al.
(författare)
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A study of a flexible fiber model and its behavior in DNS of turbulent channel flow
- 2013
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Ingår i: Acta Mechanica. - : Springer Science and Business Media LLC. - 0001-5970 .- 1619-6937. ; 224:10, s. 2359-2374
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of individual flexible fibers in a turbulent flow field have been analyzed, varying their initial position, density and length. A particlelevel fiber model has been integrated into a general-purpose, open source Computational Fluid Dynamics (CFD) code. The fibers are modeled as chains of cylindrical segments connected by ball and socket joints. The equations of motion of the fibers contain the inertia of the segments, the contributions from hydrodynamic forces and torques, and the connectivity forces at the joints. Direct Numerical Simulation (DNS) of the incompressible Navier–Stokes equations is used to describe the fluid flow in a plane channel and a one-way coupling is considered between the fibers and the fluid phase. We investigate the translational motion of fibers by considering the mean square displacement of their trajectories. We find that the fiber motion is primarily governed by velocity correlations of the flow fluctuations. In addition, we show that there is a clear tendency of the thread-like fibers to evolve into complex geometrical configurations in a turbulent flow field, in fashion similar to random conformations of polymer strands subjected to thermal fluctuations in a suspension. Finally, we show that fiber inertia has a significant impact on reorientation time-scales of fibers suspended in a turbulent flow field.
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| 3. |
- Andric, Jelena, 1979, et al.
(författare)
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Description and validation of a flexible fiber model, implemented in a general-purpose CFD code
- 2013
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Ingår i: Proceedings of the 8th International Conference on Multiphase Flow ICMF 2013, Korea.
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Konferensbidrag (refereegranskat)abstract
- A flexible fiber model has been implemented in a general purpose open-source computational fluid dynamics code. The fibers are modeled as chains of cylindrical segments. Each segment is tracked individually and their equations of motion account for the hydrodynamic forces and torques from the interaction with the fluid, the elastic bending and twisting torques, and the connectivity forces and moments that ensure the fiber integrity. The segment inertia is taken into account and a one-way coupling with the fluid phase is considered. The model is applied to the rotational motion of an isolated fiber in a low segment Reynolds number shear flow. In the case of a stiff fiber, the computed period of rotation is in good agreement with the one computed using Jeffery's equation for an equivalent spheroid aspect ratio. A qualitative comparison is made with experimental data for flexible fibers. These results show that the implemented model can reproduce the known dynamical behavior of rigid and flexible fibers successfully.
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| 4. |
- Andric, Jelena, 1979
(författare)
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Implementation of a flexible fiber model in a general purpose CFD code
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work is related to the process of making pulp mats for use in hygienicproducts. One part of that process is the transportation of flexiblecellulose fibers suspended in flowing air. The fibers should be evenlydistributed on the substrate, and it is thus of high importance to avoidthe formation of fiber flocks during the transportation. The purposeof the present work is to implement a flexible fiber model in a generalpurpose Computational Fluid Dynamics (CFD) code, for detailed studiesof fiber-fiber and fiber-flow interaction in real flow situations. Thefibers are modeled as chains of cylindrical segments, and the translationaland rotational degrees of freedom of each segment are taken intoaccount. Each segment is tracked individually, using Lagrangian ParticleTracking (LPT), and the equations of fibermotion are derived fromthe conservation of momentum for each segment. The segment inertiais taken into account and the one-way coupling with the fluid phase isconsidered. The fiber integrity is ensured through connectivity forcesacting between the adjacent fiber segments. The implemented modelhas been applied both with imposed flow fields, and in a flow field simultaneouslypredicted by the CFD solver. The results show that thefibers are transported by the flow and are deformed due to flow gradients.Further, a generic test case is described and used to validate theenergy conservation and response time of the fiber model concept.This work is the foundation for further improvements of the fibermodelthrough the addition of bending and twisting forces, as well as the inclusionof interaction (e.g. collision) forces between individual fiber segments.These features, together with a two-way coupling with the flow,will lead to a more complete fiber model.
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| 5. |
- Andric, Jelena, 1979, et al.
(författare)
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Numerical investigation of fiber flocculation in the air flow of an asymmetric diffuser
- 2014
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Ingår i: Proceedings of the ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting and 12th International Conference on Nanochannels, Microchannels and Minichannels, FEDSM 201, August 3-7, Chicago, Illinois, USA..
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Konferensbidrag (refereegranskat)abstract
- A particle-level rigid fiber model is used to studyflocculation in an asymmetric planar diffuser with a turbulent Newtonian fluid flow, resembling one stage in dry-forming process of pulp mats. The fibers are modeled as chains of rigid cylindrical segments. The equations of motion incorporatehydrodynamic forces and torques from the interaction with thefluid, and the fiber inertia is taken into account. The flow isgoverned by the Reynolds-averaged Navier ̶ Stokes equationswith the standard k-omega turbulence model. A one-waycoupling between the fibers and the flow is considered. Astochastic model is employed for the flow fluctuations tocapture the fiber dispersion. The fibers are assumed to interactthrough short-range attractive forces, causing them to interlockas the fiber-fiber contacts occur during the flow. It is found thatthe formation of fiber flocs is driven by both the turbulenceinduceddispersion and the gradient of the averaged flow field
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| 6. |
- Andric, Jelena, 1979
(författare)
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Numerical modeling of air-fiber flows
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The dynamics of fiber suspensions are of great importance in applications such as the dry-forming process of pulp mats for use in hygiene products. In this forming process, fibers are transported in flowing air. The fibers interact with the fluid, and may interact with each other and interlock in flocs. The characteristics of the suspension structure are essential for the design and optimization of the forming process, and for improving the quality of the final products. Particularly, it is desired to achieve a uniform fiber distribution in the pulp mats. Thus, it is of high interest to develop tools, which can be used to perform comprehensive studies of the complex phenomenon of fiber suspension flows.This work is concerned with numerical analysis of fiber suspensions, related to the mat-forming process. For that purpose, a particle-level fiber model has been implemented into an open source computational fluid dynamics (CFD) code. A fiber is modeled as a chain of rigid cylindrical segments. The segments interact with the flow through hydrodynamic drag forces, and may interact with each other through short-range attractive forces. The segments are tracked individually using Lagrangian particle tracking (LPT). The implemented model comprises two alternatives, the flexible and the rigid fiber model, respectively. The equations of motion of a flexible fiber represent the application of Euler's second laws for rigid body motion for the fiber segments. The flexible fiber model takes into account all the degrees of freedom necessary to realistically reproduce the fiber dynamics. Connectivity forces act between the adjacent fiber segments to ensure the fiber integrity. The rigid fiber model keeps the relative orientation between the segments fixed. The equations of motion are formulated for the fiber as a whole, while the hydrodynamic contributions are taken into account from the individual segments. The fiber inertia is taken into account in both alternatives of the model. The fiber model has been coupled with imposed flow fields, or with flow fields computed by the CFD solvers.The behavior of the implemented model is compared with analytical and experimental results available in the literature. The simulation results show that the model correctly predicts the dynamics of isolated rigid and flexible fibers in creeping shear flow.The model is used to study the dynamics of flexible and rigid fibers in high Reynolds number flows and in geometries that are representative for the mat-forming process. The effects of fiber properties, such as fiber inertia and fiber length are analyzed.Simulations are carried out to investigate the rheology of suspensions of flexible and curved fibers in creeping shear flow of a Newtonian fluid. The effects of fiber flexibility and fiber curvature on the specific viscosity and the normal stress differences are examined.Finally, aggregation of rod-like fibers is investigated in a turbulent flow of an asymmetric planar diffuser. The influences of the average flow gradient, the fiber inertia and the turbulence dispersion on the aggregation rate are analyzed. The study identifies a darting fiber motion as a mechanism that significantly enhances fiber collisions and aggregation.
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| 7. |
- 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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