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Simulation of deformable objects transported in fluid flow
KTH, School of Engineering Sciences (SCI), Mechanics.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Deformable particles suspended in a viscous fluid can be found in many industrial and biological applications. In this thesis, two different numerical tools have been developed to simulate suspensions of capsules, thin membranes enclosing a second fluid and a rigid nucleus so to work as model for ”Eukaryotic” cells, and flexible slender bodies known as filaments/fibres. Both tools use a semi-implicit fluid flow solver with different approaches for the deformable structure. The capsule membrane is modelled as a thin hyperelastic material and the elasticity equations are solved with an accurate spectral representation of the capsule shape as a truncated number of spherical harmonics. The filaments are considered as one dimensional inextensible slender bodies obeying Euler-Bernoulli beam equations which is solved by a two-step method using finite difference discretisation. The immersed boundary method is exploited to couple the fluid and solid motion using different versions for the two different objects considered. The nucleus inside the capsules is modelled either as a second stiffer capsule or as a rigid particle. In order to avoid membrane-membrane, membrane-wall and membrane-nucleus overlapping, a short range repulsive force is implemented in terms of a potential function of the distance between the approaching objects. For the short range interactions between the filaments, both lubrication correction and collision forces are considered and it is found that the inclusion of the lubrication correction has significant effect on the rheology in shear flow. Both codes are validated against the numerical and experimental data in the literature. We study the capsule behaviour in a simple shear flow created by with two walls moving in opposite directions. The membrane obeys the Neo-Hookean constitutive equations and, in the simulations with a rigid nucleus, its radius is fixed to half the capsule initial radius. The filaments, on the other hand, are studied in 4 different flow configurations: shear flow, channel flow, settling in quiescent fluid and homogeneous isotropic turbulence. The results indicate that for single capsule, the nucleus reduces the membrane deformation significantly and changes the deformed shaped when there is negligible bending resistance of the membrane. The rheological properties of nucleated capsule suspensions result from the competition between the capsule deformation and their orientation angle and similarly to the case of single capsules, the nucleus reduces the mean deformation. By increasing the capsule volume fraction, the relative viscosity increases and capsules become more oriented in the mean flow direction. Filament suspensions in shear flow exhibit shear thinning behaviour with respect to deformability; inertia has a significant effect on the rheological properties of the suspensions as documented here. For the case of settling fibres, we document the formation of columnar structures with higher settling velocity known as streamers, which are more pronounced at higher volume fractions and for flexible fibres. For a single filament in homogeneous isotropic turbulence, two distinct regimes for the filament motion are identified with a sharp transition from one to another at a critical bending stiffness. In turbulent channel flow, we demonstrate how finite-size filaments cause considerable drag reduction, of the order of 30% for volume fractions of the order of 1.5%, and that the main averaged quantities are almost independent of the filament flexibility for the bending rigidities studied here.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 72
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-250721ISBN: 978-91-7873-219-7 (print)OAI: oai:DiVA.org:kth-250721DiVA, id: diva2:1313488
Public defence
2019-06-03, F3 - Sing Sing, Lindstedtsvägen 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20190507

Available from: 2019-05-07 Created: 2019-05-03 Last updated: 2019-05-07Bibliographically approved
List of papers
1. Numerical simulations of elastic capsules with nucleus in shear flow
Open this publication in new window or tab >>Numerical simulations of elastic capsules with nucleus in shear flow
2017 (English)In: EUROPEAN JOURNAL OF COMPUTATIONAL MECHANICS, ISSN 1779-7179, Vol. 26, no 1-2, p. 131-153Article in journal (Refereed) Published
Abstract [en]

The shear-induced deformation of a capsule with a stiff nucleus, a model of eukaryotic cells, is studied numerically. The membrane of the cell and of its nucleus are modelled as a thin elastic material obeying a Neo-Hookean constitutive law. The fluid-structure coupling is obtained using an immersed boundary method. The variations induced by the presence of the nucleus on the cell deformation are investigated when varying the viscosity ratio between the inner and outer fluids, the membrane elasticity and its bending stiffness. The deformation of the eukaryotic cell is smaller than that of the prokaryotic one. The reduction in deformation increases for larger values of the capillary number. The eukaryotic cell remains thicker in itsmiddle part compared to the prokaryotic one, thus making it less flexible to pass through narrow capillaries. For a viscosity ratio of 5, the deformation of the cell is smaller than in the case of uniform viscosity. In addition, for non-zero bending stiffness of the membrane, the deformation decreases and the shape is closer to an ellipsoid. Finally, we compare the results obtained modelling the nucleus as an inner stiffer membrane with those obtained using a rigid particle.

Place, publisher, year, edition, pages
Taylor & Francis, 2017
Keywords
Capsule, nucleus, shear flow, immersed boundary method
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-211628 (URN)10.1080/17797179.2017.1294828 (DOI)000406001800009 ()2-s2.0-85014470144 (Scopus ID)
Funder
EU, European Research Council, ERC-2013-CoG616186Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC), SNIC 2016/10-36
Note

QC 20170809

Available from: 2017-08-09 Created: 2017-08-09 Last updated: 2019-05-07Bibliographically approved
2. Suspensions of nucleated capsules at finite inertia
Open this publication in new window or tab >>Suspensions of nucleated capsules at finite inertia
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-250720 (URN)
Note

QC 20190507

Available from: 2019-05-03 Created: 2019-05-03 Last updated: 2019-05-07Bibliographically approved
3. Numerical study of filament suspensions at finite inertia
Open this publication in new window or tab >>Numerical study of filament suspensions at finite inertia
(English)In: Article in journal (Other academic) Submitted
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-250686 (URN)
Note

QC 20190507

Available from: 2019-05-02 Created: 2019-05-02 Last updated: 2019-06-12Bibliographically approved
4. Numerical study of settling of flexible fiber suspensions
Open this publication in new window or tab >>Numerical study of settling of flexible fiber suspensions
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-250687 (URN)
Note

QC 20190507

Available from: 2019-05-02 Created: 2019-05-02 Last updated: 2019-05-07Bibliographically approved
5. Flexible Fiber Reveals the Two-Point Statistical Properties of Turbulence
Open this publication in new window or tab >>Flexible Fiber Reveals the Two-Point Statistical Properties of Turbulence
2018 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 4, article id 044501Article in journal (Refereed) Published
Abstract [en]

We study the dynamics of a flexible fiber freely moving in a three-dimensional fully developed turbulent field and present a phenomenological theory to describe the interaction between the fiber elasticity and the turbulent flow. This theory leads to the identification of two distinct regimes of flapping, which we validate against direct numerical simulations fully resolving the fiber dynamics. The main result of our analysis is the identification of a flapping regime where the fiber, despite its elasticity, is slaved to the turbulent fluctuations. In this regime the fiber can be used to measure two-point statistical observables of turbulence, including scaling exponents of velocity structure functions, the sign of the energy cascade and the energy flux of turbulence, as well as the characteristic times of the eddies within the inertial range of scales. Our results are expected to have a deep impact on the experimental turbulence research as a new way, accurate and efficient, to measure two-point, and more generally multipoint, statistics of turbulence.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-232886 (URN)10.1103/PhysRevLett.121.044501 (DOI)000439547100006 ()2-s2.0-85050744852 (Scopus ID)
Note

QC 20180809

Available from: 2018-08-09 Created: 2018-08-09 Last updated: 2019-05-07Bibliographically approved
6. Flowing fibers as a proxy of turbulence statistics
Open this publication in new window or tab >>Flowing fibers as a proxy of turbulence statistics
Show others...
(English)In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648Article in journal (Refereed) Accepted
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-250690 (URN)
Note

QC 20190507

Available from: 2019-05-02 Created: 2019-05-02 Last updated: 2019-05-07Bibliographically approved

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