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Suspensions of finite-size rigid spheres in different flow cases
KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik.ORCID-id: 0000-0003-0418-7864
2015 (engelsk)Licentiatavhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Stockholm: KTH Royal Institute of Technology, 2015. , s. xii, 30
Serie
TRITA-MEK, ISSN 0348-467X ; 2015:12
HSV kategori
Forskningsprogram
Teknisk mekanik
Identifikatorer
URN: urn:nbn:se:kth:diva-177900OAI: oai:DiVA.org:kth-177900DiVA, id: diva2:874914
Presentation
2015-12-17, D3, Lindstedtsvägen 5, KTH, Stockholm, 14:00 (engelsk)
Opponent
Veileder
Forskningsfinansiär
EU, European Research Council
Merknad

QC 20151130

Tilgjengelig fra: 2015-11-30 Laget: 2015-11-30 Sist oppdatert: 2015-11-30bibliografisk kontrollert
Delarbeid
1. Sedimentation of finite-size spheres in quiescent and turbulent environments
Åpne denne publikasjonen i ny fane eller vindu >>Sedimentation of finite-size spheres in quiescent and turbulent environments
2016 (engelsk)Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 788, s. 640-669Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
Cambridge University Press, 2016
Emneord
multiphase and particle-laden flows, particle/fluid flow, suspensions
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-177894 (URN)10.1017/jfm.2015.698 (DOI)000368413600019 ()2-s2.0-84997815913 (Scopus ID)
Forskningsfinansiär
EU, European Research Council, ERC-2013-CoG-616186
Merknad

QC 20160220

Tilgjengelig fra: 2015-11-30 Laget: 2015-11-30 Sist oppdatert: 2017-12-01bibliografisk kontrollert
2. The effect of particle density in turbulent channel flow laden with finite-size particles in semi-dilute conditions
Åpne denne publikasjonen i ny fane eller vindu >>The effect of particle density in turbulent channel flow laden with finite-size particles in semi-dilute conditions
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
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.

HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-177897 (URN)
Forskningsfinansiär
EU, European Research Council
Merknad

QS 2015

Tilgjengelig fra: 2015-11-30 Laget: 2015-11-30 Sist oppdatert: 2015-11-30bibliografisk kontrollert
3. Rheology of extremely confined non-Brownian suspensions
Åpne denne publikasjonen i ny fane eller vindu >>Rheology of extremely confined non-Brownian suspensions
Vise andre…
2016 (engelsk)Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 116, nr 1, artikkel-id 018301Artikkel i tidsskrift (Annet vitenskapelig) 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.

sted, utgiver, år, opplag, sider
American Physical Society, 2016
Emneord
IMMERSED BOUNDARY METHOD, DENSE SUSPENSIONS, VISCOSITY, DIMENSIONS, SPHERES
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-177898 (URN)10.1103/PhysRevLett.116.018301 (DOI)000367784300020 ()2-s2.0-84954455170 (Scopus ID)
Forskningsfinansiär
EU, European Research Council, ERC-2013-CoG-616186Swedish Research Council, 2011-542Swedish Research Council, 638-2013-9243
Merknad

QC 20160205

Tilgjengelig fra: 2015-11-30 Laget: 2015-11-30 Sist oppdatert: 2017-12-01bibliografisk kontrollert

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