Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • 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 spheres in different flow cases
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.ORCID iD: 0000-0003-0418-7864
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Dispersed multiphase flows occur in many biological, engineering and geophysical applications such asfluidized beds, soot particle dispersion and pyroclastic flows. Understanding the behavior of suspensionsis a very difficult task. Indeed particles may differ in size, shape, density and stiffness, theirconcentration varies from one case to another, and the carrier fluid may be quiescent or turbulent.When turbulent flows are considered, the problem is further complicated due to the interactionsbetween particles and eddies of different size, ranging from the smallest dissipative scales up to thelargest integral scales. Most of the investigations on the topic have dealt with heavy small particles (typicallysmaller than the dissipative scale) and in the dilute regime. Less is known regarding the behavior ofsuspensions of finite-size particles (particles that are larger than the smallest lengthscales of the fluid phase).

In the present work, we numerically study the behavior of suspensions of finite-size rigid spheres indifferent flows. In particular, we perform Direct Numerical Simulations using an ImmersedBoundary Method to account for the solid phase. Firstly is investigated the sedimentation of particles slightly larger than theTaylor microscale in sustained homogeneous isotropic turbulence and quiescent fluid. The results show thatthe mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. By estimatingthe mean drag acting on the particles, we find that non stationary effects explain the increased reductionin mean settling velocity in turbulent environments.

We also consider a turbulent channel flow seeded with finite-size spheres. We change the solid volumefraction and solid to fluid density ratio in an idealized scenario where gravity is neglected. The aim isto independently understand the effects of these parameters on both fluid and solid phases statistics.It is found that the statistics are substantially altered by changes in volume fraction, while the main effectof increasing the density ratio is a shear-induced migration toward the centerline. However, at very high density ratios (~100) the two phases decouple and the particles behave as a dense gas.

Finally we study the rheology of confined dense suspensions of spheres in simple shear flow. We focus onthe weakly inertial regime and show that the suspension effective viscosity varies non-monotonically with increasingconfinement. The minima of the effective viscosity occur when the channel width is approximately an integernumber of particle diameters. At these confinements, the particles self-organize into two-dimensional frozen layers thatslide onto each other.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xii, 30 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2015:12
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-177900OAI: oai:DiVA.org:kth-177900DiVA: diva2:874914
Presentation
2015-12-17, D3, Lindstedtsvägen 5, KTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Funder
EU, European Research Council
Note

QC 20151130

Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2015-11-30Bibliographically 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, 640-669 p.Article 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
Keyword
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. 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
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We study the effect of varying the mass and volume fraction of a suspension of rigid spheres dispersedin a turbulent channel flow. We performed several Direct Numerical Simulations using an Immersed Boundary Method forfinite-size particles changing the solid to fluid density ratio R, the mass fraction and the volume fraction. We find that varying the density ratio R between 1 and 10 at constant volume fraction does not alter the flow statisticsas much as when varying the volume fraction at constant R and at constant mass fraction.

Interestingly, the increase in overall drag found when varying the volume fraction is considerablyhigher than that obtained for increasing density ratios at same volume fraction. The main effect atdensity ratios R of the order of 10 is a strong shear-induced migration towards the centerline of the channel. When thedensity ratio R is further increased up to 100 the particle dynamics decouple from that of the fluid. The solid phase behaves as a dense gas andthe fluid and solid phase statistics drastically change. In this regime, the collisionrate is high and dominated by the normal relative velocity among particles.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-177897 (URN)
Funder
EU, European Research Council
Note

QS 2015

Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2015-11-30Bibliographically approved
3. 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, 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
Keyword
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

Open Access in DiVA

Thesis(4855 kB)318 downloads
File information
File name FULLTEXT01.pdfFile size 4855 kBChecksum SHA-512
f079a24fbea396bb4b631d2412b42c9b4b55e30708e0abdc4a41f120a585c853b767b4fea6b4a880a3efbc0b5359a291acf2e25b91167e7420542a96bb6f90e3
Type fulltextMimetype application/pdf

Authority records BETA

Fornari, Walter

Search in DiVA

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

Search outside of DiVA

GoogleGoogle Scholar
Total: 318 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

urn-nbn

Altmetric score

urn-nbn
Total: 692 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • 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