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Numerical study of non-spherical/spherical particles in laminar and turbulent flows
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0003-4328-7921
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The presence of solid rigid particles alters the global transport and rheological properties of the mixture in complex (and often unpredictable) ways. In recent years a few studies have been devoted to investigating the behavior of dense suspensions in the turbulent/inertial regime with the majority of theses analyses limited to mono-disperse rigid neutrally-buoyant spheres. However, one interesting parameter that is rarely studied for particles with high inertia is the particle shape. Spheroidal particles introduce an anisotropy, e.g. a tendency to orient in a certain direction, which can affect the bulk behavior of a suspension in an unexpected ways. The main focus of this study is therefore to investigate the behavior of spheroidal particles and their effect on turbulent/inertial flows.

We perform fully resolved simulations of particulate flows with spherical/spheroidal particles, using an efficient/accurate numerical approach that enables us to simulate thousands of particles with high resolutions in order to capture all the fluid-solid interactions.

Several conclusions are drawn from this study that reveal the importance of particle's shape effect on the behaviour of a suspension e.g. spheroidal particles tend to cluster while sedimenting. This phenomenon is observed in this work for both particles with high inertia, sedimenting in a quiescent fluid and inertialess particles (point-like tracer prolates) settling in homogenous isotropic turbulence. The mechanisms for clustering is indeed different between these two situations, however, it is the shape of particles that governs these mechanisms, as clustering is not observed for spherical particles. Another striking finding of this work is drag reduction in particulate turbulent channel flow with rigid oblate particles. Again this drag reduction is absent for spherical particles, which instead increase the drag with respect to single-phase turbulence. 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. , 31 p.
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-204421ISBN: 978-91-7729-333-0 (print)OAI: oai:DiVA.org:kth-204421DiVA: diva2:1084483
Presentation
2017-04-20, E51, Osquars backe 14, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20170328

Available from: 2017-03-28 Created: 2017-03-24 Last updated: 2017-03-28Bibliographically approved
List of papers
1. Numerical study of the sedimentation of spheroidal particles
Open this publication in new window or tab >>Numerical study of the sedimentation of spheroidal particles
2016 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 87, 16-34 p.Article in journal (Refereed) Published
Abstract [en]

The gravity-driven motion of-rigid particles in a viscous fluid is relevant in many natural and industrial processes, yet this has mainly been investigated for spherical particles. We therefore consider the sedimentation of non-spherical (spheroidal) isolated and particle pairs in a viscous fluid via numerical simulations using the Immersed Boundary Method. The simulations performed here show that the critical Galileo number for the onset of secondary motions decreases as the spheroid aspect ratio departs from 1. Above this critical threshold, oblate particles perform a zigzagging motion whereas prolate particles rotate around, the vertical axis while having their broad side facing the falling direction. Instabilities of the vortices in the wake follow when farther increasing the Galileo number. We also study the drafting kissing-tumbling associated with the settling of particle pairs. We find that the interaction time increases significantly for non-spherical particles and, more interestingly, spheroidal particles are attracted from larger lateral displacements. This has important implications for the estimation of collision kernels and can result its increasing clustering in suspensions of sedimenting spheroids.

Keyword
Non-spherical particles, Sedimentation, Particle pair interactions, Drafting-kissing-tumbling, Numerical modelling
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-196969 (URN)10.1016/j.ijmultiphaseflow.2016.08.005 (DOI)000386645300003 ()2-s2.0-84985916725 (Scopus ID)
Note

QC 20161213

Available from: 2016-12-13 Created: 2016-11-28 Last updated: 2017-03-24Bibliographically approved
2. Drag reduction in turbulent channel flow laden with finite-size oblate spheroids
Open this publication in new window or tab >>Drag reduction in turbulent channel flow laden with finite-size oblate spheroids
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2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 816, 43-70 p.Article in journal (Refereed) Published
Abstract [en]

We study suspensions of oblate rigid particles in a viscous fluid for different values of the particle volume fractions.Direct numerical simulations have been performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle-particle and particle-wall interactions. With respect to the single phase flow, we show that in flows laden with oblate spheroids the drag is reduced and the turbulent fluctuations attenuated.In particular, the turbulence activity decreases to lower values than those obtained by only accounting for the effective suspension viscosity.To explain the observed drag reduction we consider the particle dynamics and the interactions of the particles with the turbulent velocity field and show that the particle wall layer, previously observed and found to be responsible for the increased dissipation in suspensions of spheres, disappears in the case of oblate particles.These rotate significantly slower than spheres near the wall and tend to stay with their major axes parallel to the wall, which leads to a decrease of the Reynolds stresses and turbulence production and so to the overall drag reduction.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-204160 (URN)2-s2.0-85014045322 (Scopus ID)
Note

QC 20170328

Available from: 2017-03-23 Created: 2017-03-23 Last updated: 2017-03-28Bibliographically approved
3. Inertial migration of spherical and oblate particles in straight ducts
Open this publication in new window or tab >>Inertial migration of spherical and oblate particles in straight ducts
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(English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Accepted
Abstract [en]

We study numerically the inertial migration of a single rigid sphere and an oblate spheroid in straight square and rectangular ducts. A highly accurate interface-resolved numerical algorithm is employed to analyse the entire migration dynamics of the oblate particle and compare it with that of the sphere. Similarly to the inertial focusing of spheres, the oblate particle reaches one of the four face-centred equilibrium positions, however they are vertically aligned with the axis of symmetry in the spanwise direction. In addition, the lateral trajectories of spheres and oblates collapse into an equilibrium manifold before ending at the equilibrium positions, with the equilibrium manifold tangential to lines of constant background shear for both sphere and oblate particles. The differences between the migration of the oblate and sphere are also presented, in particular the oblate may focus on the diagonal symmetry line of the duct cross-section, close to one of the corners, if its diameter is larger than a certain threshold. Moreover, we show that the final orientation and rotation of the oblate exhibit a chaotic behaviour for Reynolds numbers beyond a critical value. Finally, we document that the lateral motion of the oblate particle is less uniform than that of the spherical particle due to its evident tumbling motion throughout the migration. In a square duct, the strong tumbling motion of the oblate in the first stage of the migration results in a lower lateral velocity and consequently longer focusing length with respect to that of the spherical particle. The opposite is true in a rectangular duct where the higher lateral velocity of the oblate in the second stage of the migration, with negligible tumbling, gives rise to shorter focusing lengths.These results can help the design of microfluidic systems for bio-applications.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-204162 (URN)10.1017/jfm.2017.189 (DOI)000405373500005 ()2-s2.0-85018317724 (Scopus ID)
Note

QC 20170328

Available from: 2017-03-23 Created: 2017-03-23 Last updated: 2017-08-03Bibliographically approved
4. Heat transfer in laminar Couette flow laden with rigid spherical particles
Open this publication in new window or tab >>Heat transfer in laminar Couette flow laden with rigid spherical particles
(English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Submitted
Abstract [en]

We study heat transfer in plane Couette flow laden with rigid spherical particles by means of direct numerical simulations using a direct-forcing immersed boundary method to account for the dispersed phase. A volume of fluid approach is employed to solve the temperature field inside and outside of the particles. We focus on the variation of the heat transfer with the particle Reynolds number, total volume fraction (number of particles) and the ratio between the particle and fluid thermal diffusivity, quantified in terms of an effective suspension diffusivity. We show that, when inertia at the particle scale is negligible, the heat transfer increases with respect to the unladen case following an empirical correlation recently proposed.In addition, an average composite diffusivity can be used to predict the effective diffusivity of the suspension the inertialess regime when varying the molecular diffusion in the two phases.At finite particle inertia, however, the heat transfer increase is significantly larger, saturating at higher volume fractions. By phase-ensemble averaging we identify the different mechanisms contributing to the total heat transfer and show that the increase of the effective conductivity observed at finite inertia is due to the increase of the transport associated to fluid and particle velocity. We also show that the heat conduction in the solid phase reduces when increasing the particle Reynolds number and so that particles of low thermal diffusivityweakly alter the total heat flux in the suspension at finite particle Reynolds numbers.On the other hand, a higher particle thermal diffusivity significantly increase the total heat transfer.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-204163 (URN)
Note

QC 20170328

Available from: 2017-03-23 Created: 2017-03-23 Last updated: 2017-04-04Bibliographically approved
5. Sedimentation of inertia-less prolate spheroids in homogenous isotropic turbulence with application to non-motile phytoplankton
Open this publication in new window or tab >>Sedimentation of inertia-less prolate spheroids in homogenous isotropic turbulence with application to non-motile phytoplankton
Show others...
(English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Submitted
Abstract [en]

Phytoplankton are the foundation of aquatic food webs. Through photosynthesis, phytoplankton draw down $CO_2$ at magnitudes equivalent to forests and other terrestrial plants and convert it to organic material that is then consumed by other organisms of phytoplankton in higher trophic levels. Mechanisms that affect local concentrations and velocities are of primary significance to many encounter-based processes in the plankton including prey-predator interactions, fertilization and aggregate formation. We report results from simulations of sinking phytoplankton, considered as elongated spheroids, in homogenous isotropic turbulence to answer the question of whether trajectories and velocities of sinking phytoplankton are altered by turbulence. We show in particular that settling spheroids with physical characteristics similar to those of diatoms weakly cluster and preferentially sample regions of down-welling flow, corresponding to an increase of the mean settling speed with respect to the mean settling speed in quiescent fluid.  We explain how different parameters can affect the settling speed and what underlying mechanisms might be involved.  Interestingly, we observe that the increase in the aspect ratio of the prolate spheroids can affect the clustering and the average settling speed of particles by two mechanisms: first is the effect of aspect ratio on the rotation rate of the particles, which saturates faster than the second mechanism of increasing drag anisotropy.   

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-204164 (URN)
Note

QC 20170328

Available from: 2017-03-23 Created: 2017-03-23 Last updated: 2017-04-04Bibliographically approved

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