Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Suspensions of finite-size rigid particles in laminar and turbulent flows
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0003-0418-7864
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Dispersed multiphase flows occur in many biological, engineering and geophysical applications. Understanding the behavior of suspensions is a difficult task. In the present work, we numerically study the behavior of suspensions of finite-size rigid particles in different flows. Firstly, the sedimentation of spherical particles larger than the Taylor microscale in sustained homogeneous isotropic turbulence and quiescent fluid is investigated. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. We also investigate the settling in quiescent fluid of oblate particles. We find that at low volume fractions the mean settling speed of the suspension is substantially larger than the terminal speed of an isolated oblate. Suspensions of finite-size spheres are also studied in turbulent channel flow. First, we change the solid volume and mass fractions, and the solid-to-fluid density ratio in an idealized scenario where gravity is neglected. Then we investigate the effects of polydispersity. It is found that the statistics are substantially altered by changes in volume fraction. We then consider suspensions of solid spheres in turbulent duct flows. We see that particles accumulate mostly at the corners or at the core depending on the volume fraction. Secondary motions are enhanced by increasing the volume fraction, until excluded volume effects are so strong that the turbulence activity is reduced. The inertial migration of spheres in laminar square duct flows is also investigated. We consider semi-dilute suspensions at different bulk Reynolds numbers and duct-to-particle size ratios. The highest particle concentration is found around the focusing points, except at very large volume fractions. Finally we study the rheology of confined dense suspensions of spheres in simple shear flow. We focus on the weakly inertial regime and show that the effective viscosity varies non-monotonically with increasing confinement.

Place, publisher, year, edition, pages
Kungliga Tekniska högskolan, 2017.
Series
TRITA-MEK, ISSN 0348-467X
Keywords [en]
Suspensions, complex fluids, sedimentation, rheology, turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-217812ISBN: 978-91-7729-607-2 (print)OAI: oai:DiVA.org:kth-217812DiVA, id: diva2:1157858
Public defence
2017-12-15, D3, Lindstedtsvägen 5, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
EU, European Research Council, ERC-2013-CoG-616186, TRITOS
Note

QC 20171117

Available from: 2017-11-17 Created: 2017-11-16 Last updated: 2017-11-29Bibliographically approved
List of papers
1. Sedimentation of finite-size spheres in quiescent and turbulent environments
Open this publication in new window or tab >>Sedimentation of finite-size spheres in quiescent and turbulent environments
2016 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 788, p. 640-669Article in journal (Refereed) Published
Abstract [en]

Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yetlittle is known about the behavior of finite-size particles inhomogeneous isotropic turbulence.

To fill this gap, we perform Direct Numerical Simulations of sedimentation in quiescent and turbulent environments using anImmersed Boundary Method to accountfor the dispersed rigid spherical particles. The solid volume fractions considered are 0.5-1%,while the solid to fluid density ratio 1.02.The particle radius is chosen to be approximately 6 Komlogorov lengthscales.

Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behaviour of finite-size particles in homogeneous isotropic turbulence. To fill this gap, we perform direct numerical simulations of sedimentation in quiescent and turbulent environments using an immersed boundary method to account for the dispersed rigid spherical particles. The solid volume fractions considered are phi = 0.5-1%, while the solid to fluid density ratio rho(p)/rho(f) = 1.02. The particle radius is chosen to be approximately six Kolmogorov length scales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reductions with respect to a single particle in quiescent fluid are approximately 12 % and 14% for the two volume fractions investigated. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails arc associated with the intermittent fast sedimentation of particle pairs in drafting kissing tumbling motions. The particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude as a consequence of the highly intermittent behaviour observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulae and show that non-stationary effects, related to vortex shedding, explain the increased reduction in mean settling Velocity in a turbulent environment.

Place, publisher, year, edition, pages
Cambridge University Press, 2016
Keywords
multiphase and particle-laden flows, particle/fluid flow, suspensions
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-177894 (URN)10.1017/jfm.2015.698 (DOI)000368413600019 ()2-s2.0-84997815913 (Scopus ID)
Funder
EU, European Research Council, ERC-2013-CoG-616186
Note

QC 20160220

Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2017-12-01Bibliographically approved
2. Reduced particle settling speed in turbulence
Open this publication in new window or tab >>Reduced particle settling speed in turbulence
2016 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 808, p. 153-167Article in journal (Refereed) Published
Abstract [en]

We study the settling of finite-size rigid spheres in sustained homogeneous isotropic turbulence (1111) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We study semi-dilute suspensions at different Galileo numbers, Ga. The Galileo number is the ratio between buoyancy and viscous forces, and is here varied via the solid-to-fluid density ratio rho(p)/rho(f), The focus is on particles that are slightly heavier than the fluid. We find that in HIT, the mean settling speed is less than that in quiescent fluid; in particular, it reduces by 6 %-60 % with respect to the terminal velocity of an isolated sphere in quiescent fluid as the ratio between the latter and the turbulent velocity fluctuations it is decreased. Analysing the fluid particle relative motion, we find that the mean settling speed is progressively reduced while reducing rho(p)/rho(f) due to the increase of the vertical drag induced by the particle cross-flow velocity. Unsteady effects contribute to the mean overall drag by about 6%-10%. The probability density functions of particle velocities and accelerations reveal that these are closely related to the features of the turbulent flow. The particle mean-square displacement in the settling direction is found to be similar for all Ga if time is scaled by (2a)/u' (where 2a is the particle diameter and a is the turbulence velocity root mean square).

Place, publisher, year, edition, pages
Cambridge University Press, 2016
Keywords
multiphase and particle-laden flows, particle/fluid flow, suspensions
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-198953 (URN)10.1017/jfm.2016.648 (DOI)000387140500010 ()2-s2.0-84992747372 (Scopus ID)
Funder
Swedish e‐Science Research CenterEU, European Research Council, ERC-2013-CoG-616186Swedish Research Council
Note

QC 20170113

Available from: 2017-01-13 Created: 2016-12-22 Last updated: 2017-11-29Bibliographically approved
3. Clustering and increased settling speed of oblate particles at finite Reynolds number
Open this publication in new window or tab >>Clustering and increased settling speed of oblate particles at finite Reynolds number
2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Submitted
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-217630 (URN)
Note

QC 20171116

Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-02-26Bibliographically approved
4. Rheology of extremely confined non-Brownian suspensions
Open this publication in new window or tab >>Rheology of extremely confined non-Brownian suspensions
Show others...
2016 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 116, no 1, article id 018301Article in journal (Other academic) Published
Abstract [en]

We study the rheology of confined suspensions of  neutrally buoyant rigid monodisperse spheres in plane-Couetteflow using Direct Numerical Simulations.We find that if the width of the channel is a (small) integer multiple of the spherediameter, the spheres self-organize into two-dimensional layersthat slide on each other and the effective viscosity of the suspension  issignificantly reduced.  Each two-dimensional layer is found to be structurallyliquid-like but its dynamics is frozen in time.

Place, publisher, year, edition, pages
American Physical Society, 2016
Keywords
IMMERSED BOUNDARY METHOD, DENSE SUSPENSIONS, VISCOSITY, DIMENSIONS, SPHERES
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-177898 (URN)10.1103/PhysRevLett.116.018301 (DOI)000367784300020 ()2-s2.0-84954455170 (Scopus ID)
Funder
EU, European Research Council, ERC-2013-CoG-616186Swedish Research Council, 2011-542Swedish Research Council, 638-2013-9243
Note

QC 20160205

Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2017-12-01Bibliographically approved
5. Inertial migration in dilute and semidilute suspensions of rigid particles in laminar square duct
Open this publication in new window or tab >>Inertial migration in dilute and semidilute suspensions of rigid particles in laminar square duct
2017 (English)In: Physical Review Fluids, ISSN 1943-6947, E-ISSN 1553-0124, Vol. 2, no 8, article id 084301Article in journal (Refereed) Published
Abstract [en]

We study the inertial migration of finite-size neutrally buoyant spherical particles in dilute and semidilute suspensions in laminar square duct flow. We perform several direct numerical simulations using an immersed boundary method to investigate the effects of the bulk Reynolds number Re-b, particle Reynolds number Re-p, and duct to particle size ratio h/a at different solid volume fractions phi, from very dilute conditions to 20%. We show that the bulk Reynolds number Re-b is the key parameter in inertial migration of particles in dilute suspensions. At low solid volume fraction (phi = 0.4%), low bulk Reynolds number (Re-b = 144), and h/a = 9 particles accumulate at the center of the duct walls. As Re-b is increased, the focusing position moves progressively toward the corners of the duct. At higher volume fractions, phi = 5%, 10%, and 20%, and in wider ducts (h/a = 18) with Re-b = 550, particles are found to migrate away from the duct core toward the walls. In particular, for phi = 5% and 10%, particles accumulate preferentially at the corners. At the highest volume fraction considered, phi = 20%, particles sample all the volume of the duct, with a lower concentration at the duct core. For all cases, we find that particles reside longer times at the corners than at the wall centers. In a duct with lower duct to particle size ratio h/a = 9 (i.e., with larger particles), phi = 5%, and high bulk Reynolds number Re-b = 550, we find a particle concentration pattern similar to that in the ducts with h/a = 9 regardless of the solid volume fraction phi. Instead, for lower Bulk Reynolds number Re-b = 144, h/a = 9, and phi = 5%, a different particle distribution is observed in comparison to a dilute suspension phi = 0.4%. Hence, the volume fraction plays a key role in defining the final distribution of particles in semidilute suspensions at low bulk Reynolds number. The presence of particles induces secondary cross-stream motions in the duct cross section, for all phi. The intensity of these secondary flows depends strongly on particle rotation rate, on the maximum concentration of particles in focusing positions, and on the solid volume fraction. We find that the secondary flow intensity increases with the volume fraction up to phi = 5%. However, beyond phi = 5% excluded-volume effects lead to a strong reduction of cross-stream velocities for Re-b = 550 and h/a = 18. Inhibiting particles from rotating also results in a substantial reduction of the secondary flow intensity and in variations of the exact location of the focusing positions.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-217628 (URN)10.1103/PhysRevFluids.2.084301 (DOI)000407101700001 ()2-s2.0-85035330723 (Scopus ID)
Funder
Swedish e‐Science Research CenterEU, Horizon 2020, ERC-2013-CoG-616186Swedish Research Council
Note

QC 20171116

Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-12-11Bibliographically approved
6. The effect of particle density in turbulent channel flow laden with finite size particles in semi-dilute conditions
Open this publication in new window or tab >>The effect of particle density in turbulent channel flow laden with finite size particles in semi-dilute conditions
2016 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 3, article id 033301Article in journal (Refereed) Published
Abstract [en]

We study the effect of varying the mass and volume fraction of a suspension of rigid spheres dispersed in a turbulent channel flow. We performed several direct numerical simulations using an immersed boundary method for finite-size particles changing the solid to fluid density ratio R, the mass fraction χ, and the volume fraction φ. We find that varying the density ratio R between 1 and 10 at constant volume fraction does not alter the flow statistics as much as when varying the volume fraction φ at constant R and at constant mass fraction. Interestingly, the increase in overall drag found when varying the volume fraction is considerably higher than that obtained for increasing density ratios at same volume fraction. The main effect at density ratios R of the order of 10 is a strong shear-induced migration towards the centerline of the channel. When the density ratio R is further increased up to 1000, the particle dynamics decouple from that of the fluid. The solid phase behaves as a dense gas and the fluid and solid phase statistics drastically change. In this regime, the collision rate is high and dominated by the normal relative velocity among particles.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016
Keywords
Low-Reynolds-Number, Numerical-Simulation, Molecular Dimensions, Suspensions, Rheology, Spheres, Fluid, Microstructure, Statistics, Stress
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-187293 (URN)10.1063/1.4942518 (DOI)000373600600023 ()2-s2.0-84959563104 (Scopus ID)
Funder
EU, European Research Council, ERC-2013-CoG-616186Swedish Research Council
Note

QC 20160520

Available from: 2016-05-20 Created: 2016-05-19 Last updated: 2017-11-30Bibliographically approved
7. The effect of polydispersity in a turbulent channel flow laden with finite-size particles
Open this publication in new window or tab >>The effect of polydispersity in a turbulent channel flow laden with finite-size particles
2018 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 67, p. 54-64Article in journal (Refereed) Published
Abstract [en]

We study turbulent channel flows of monodisperse and polydisperse suspensions of finite-size spheres by means of Direct Numerical Simulations using an immersed boundary method to account for the dispersed phase. Suspensions with 3 different Gaussian distributions of particle radii are considered (i.e. 3 different standard deviations). The distributions are centered on the reference particle radius of the monodisperse suspension. In the most extreme case, the radius of the largest particles is 4 times that of the smaller particles. We consider two different solid volume fractions, 2% and 10%. We find that for all polydisperse cases, both fluid and particles statistics are not substantially altered with respect to those of the monodisperse case. Mean streamwise fluid and particle velocity profiles are almost perfectly overlapping. Slightly larger differences are found for particle velocity fluctuations. These increase close to the wall and decrease towards the centerline as the standard deviation of the distribution is increased. Hence, the behavior of the suspension is mostly governed by excluded volume effects regardless of particle size distribution (at least for the radii here studied). Due to turbulent mixing, particles are uniformly distributed across the channel. However, smaller particles can penetrate more into the viscous and buffer layer and velocity fluctuations are therein altered. Non trivial results are presented for particle-pair statistics.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Suspensions, Particle-laden flows, Particle/fluid flow
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-217629 (URN)10.1016/j.euromechflu.2017.08.003 (DOI)000418726900005 ()2-s2.0-85028452609 (Scopus ID)
Funder
Swedish Research CouncilSwedish e‐Science Research Center
Note

QC 20171116

Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-01-11Bibliographically approved
8. Suspensions of finite-size neutrally buoyant spheres in turbulent duct flow
Open this publication in new window or tab >>Suspensions of finite-size neutrally buoyant spheres in turbulent duct flow
2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Submitted
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-217711 (URN)
Note

QC 20171116

Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-02-20Bibliographically approved

Open Access in DiVA

fulltext(2157 kB)231 downloads
File information
File name FULLTEXT02.pdfFile size 2157 kBChecksum SHA-512
f609aef4e30e9188094ee163a64d3b3082f030d078a216c88eddcfbc842dd618024f2a49f0c5ea68d440e3089cd213f635c633be3de15b0bbddf65cbca948129
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Fornari, Walter
By organisation
Mechanics
Fluid Mechanics and Acoustics

Search outside of DiVA

GoogleGoogle Scholar
Total: 231 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 556 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf