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Droplets in homogeneous shear turbulence
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.ORCID iD: 0000-0002-9004-2292
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.ORCID iD: 0000-0002-4222-012x
Stanford Univ, Ctr Turbulence Res, Stanford, CA 94305 USA..
Stanford Univ, Ctr Turbulence Res, Stanford, CA 94305 USA..
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2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 876, p. 962-984Article in journal (Refereed) Published
Abstract [en]

We simulate the flow of two immiscible and incompressible fluids separated by an interface in a homogeneous turbulent shear flow at a shear Reynolds number equal to 15 200. The viscosity and density of the two fluids are equal, and various surface tensions and initial droplet diameters are considered in the present study. We show that the two-phase flow reaches a statistically stationary turbulent state sustained by a non-zero mean turbulent production rate due to the presence of the mean shear. Compared to single-phase flow, we find that the resulting steady-state conditions exhibit reduced Taylor-microscale Reynolds numbers owing to the presence of the dispersed phase, which acts as a sink of turbulent kinetic energy for the carrier fluid. At steady state, the mean power of surface tension is zero and the turbulent production rate is in balance with the turbulent dissipation rate, with their values being larger than in the reference single-phase case. The interface modifies the energy spectrum by introducing energy at small scales, with the difference from the single-phase case reducing as the Weber number increases. This is caused by both the number of droplets in the domain and the total surface area increasing monotonically with the Weber number. This reflects also in the droplet size distribution, which changes with the Weber number, with the peak of the distribution moving to smaller sizes as the Weber number increases. We show that the Hinze estimate for the maximum droplet size, obtained considering break-up in homogeneous isotropic turbulence, provides an excellent estimate notwithstanding the action of significant coalescence and the presence of a mean shear.

Place, publisher, year, edition, pages
Cambridge University Press, 2019. Vol. 876, p. 962-984
Keywords [en]
drops, multiphase flow, turbulence simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-257426DOI: 10.1017/jfm.2019.581ISI: 000480242100001Scopus ID: 2-s2.0-85070481433OAI: oai:DiVA.org:kth-257426DiVA, id: diva2:1347705
Note

QC 20190902

Available from: 2019-09-02 Created: 2019-09-02 Last updated: 2020-02-27Bibliographically approved
In thesis
1. On droplet interactions and suspension flow: If you want to follow the dissertation but are not able to do so via zoom, please contact luca@mech.kth.se for further information
Open this publication in new window or tab >>On droplet interactions and suspension flow: If you want to follow the dissertation but are not able to do so via zoom, please contact luca@mech.kth.se for further information
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Droppinteraktioner och suspensionsflöden
Abstract [en]

Micron to millimetre sized droplets, precisely generated or sustained in controlled environment, have great potential in myriads of engineering applications functioning as the basic element to assemble metamaterials, deliver drugs, host surfactant, reduce friction and damp turbulence. The interaction of droplets from pairwise to collective levels is the most important factor in controlling these processes, yet little is known about the detailed mechanisms in various nonideal conditions. The present thesis combines a number of studies aiming to elucidate the physical principles of droplet interactions and suspension flow using both high- and low-fidelity numerical simulations.

We first study flow-assisted droplet assembly in microfluidic channels, seeking to harness the droplet interactions to produce photonic bandgap materials. A novel interface-correction level set/ghost fluid method (ICLS/GFM) is developed to directly simulate liquid droplets under depletion forces. Comparing to previous methods, ICLS/GFM conserves the global mass of each fluid using a simple mass-correction scheme, accurately computes the surface tension and depletion forces under the same framework, and has subsequently been applied to investigate the droplet clustering observed in a microfluidic experiment. Our simulations, supported by theoretical derivations, suggest that the observed fast self-assembly arises from a combination of strong depletion forces, confinement-mediated shear alignments of the droplets, and fine-tuned inflow conditions of the microchannel. However, the interplay of these 3D effects negates a simple droplet interaction model of parametric dependence, rendering the design of microfluidic chips for photonic crystal fabrications difficult in practice.

The next objective of the thesis is the implementation of a minimal hybrid lubrication/granular dynamics (HLGD) model for simulation of dense particle suspensions. The main ingredients of HLGD include (i) a frame-invariant, short-range lubrication model for spherical particles, and (ii) a soft-core, stick/slide frictional contact model activated when particles overlap. Since contact interactions dominate at high particle concentrations, we expect the methodology to be applicable for probing the jamming of non-spherical particles and the rheology of foams as well.

Finally, we include two miscellaneous studies concerning the slippage property of liquid-infused surfaces and droplets statistics in a homogeneous turbulent shear flow. Overall, results of these simulations provide detailed flow visualisations and qualitative dependence of the target functional on various governing parameters, facilitating experimental and theoretical investigations to design more robust drag-reducing surfaces and predict droplet distributions in emulsions.

Abstract [sv]

Mikro- till millimeter stora droppar, exakt genererade eller hållna i en kontrollerad miljö, har stor potential i många olika tekniska tillämpningar. De representerar en grundläggande teknik vid uppbyggnad av metamaterial, transport av läkemedel i kroppen, som bärare av ytaktiva medel, vid minskning av friktion och dämpning av turbulens. Växelverkan mellan droppar från parvis till kollektivnivå är den viktigaste faktorn för att kontrollera dessa processer; ändå är lite känt om de detaljerade mekanismerna vid olika icke-ideala förhållanden. I denna avhandling kombineras ett antal studier som syftar till att belysa de fysikaliska principerna för dropp-växelverkningar och suspensionsflöden med numeriska simuleringar av högre och lägre noggrannhet.

Vi studerar först flödesassisterad droppmontering i mikrofluidkanaler och försöker utnyttja dropp-växelverkningar för att producera fotoniska bandgapmaterial. En ny interface-correction level set/ghost fluid method (ICLS/GFM) är utvecklad för att direkt simulera vätskedroppar under inverkan av utarmningskrafter. Jämfört med tidigare metoder bevarar ICLS/GFM den totala massan för varje fluid med hjälp av ett enkelt masskorrigeringsschema, och beräknar exakt ytspänningen och utarmningskrafterna under samma omständigheter. Detta tillämpas sedan för att undersöka droppklustring, något som observerats i mikrofluidiska experiment. Våra simuleringar, med stöd av teoretiska härledningar, antyder att den observerade snabba klustringen uppstår på grund av en kombination av starka utarmningskrafter och inneslutningsförmedlade skjuvkrafter på dropparna samt finjusterade inflödesförhållanden för mikrokanalen. Men samspelet mellan dessa 3D-effekter omöjliggör en enkel parameterberoende dropp-växelverkningsmodell vilket gör att utformningen av mikrofluidiska chips för fotonisk kristallfabrikation är svår i praktiken.

Nästa fokus i avhandlingen är implementeringen av en minimal hybrid lubrication/granular dynamics (HLGD) för simulering av täta partikelsuspensioner. Två huvudingredienser i modellen är (i) en referensram-invariant smörjmodell med kort räckvidd för sfäriska partiklar, och (ii) en stick/slip friktionskontaktmodell med mjuk kärna som aktiveras när partiklar överlappar varandra. Eftersom kontakt-växelverkningar dominerar fysikaliskt vid höga partikelkoncentrationer, förväntar vi oss att metodologin också är tillämplig för att undersöka inklämning av icke-sfäriska partiklar och reologi för skum.

Slutligen inkluderar vi också två studier rörande glidningsegenskaper hos vätske-bemängda ytor och droppstatistik i ett homogent turbulent skjuvflöde. Sammantaget ger resultaten av dessa simuleringar detaljerade flödesvisualiseringar, kvalitativt beroende av målfunktionen på olika reglerande parametrar, underlättar, experimentellt och teoretiskt, utformningen av mer robusta dragreducerande ytor samt förutsäger droppfördelningar i emulsioner.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. p. 76
Series
TRITA-SCI-FOU ; 2020:04
Keywords
droplets, suspension, multiphase flow, microfluidics, soft matter, rheology, depletion force, level set, ghost fluid., droppar, suspension, flerfasflöde, mikrofluidik, mjukt material, reologi, utarmningskraft, nivåuppsättning, spökvätska.
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-268953 (URN)978-91-7873-456-6 (ISBN)
Public defence
2020-03-27, https://kth-se.zoom.us/j/270282702 och sal F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
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Supervisors
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Soft Matter
Note

QC200304

Available from: 2020-03-04 Created: 2020-02-27 Last updated: 2020-03-26Bibliographically approved

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Rosti, Marco E.Ge, ZhouyangBrandt, Luca

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