Search

Many microorganisms propel themselves through complex media by deforming their flagella. The beat is thought to emerge from interactions between forces of the surrounding fluid, the passive elastic response from deformations of the flagellum and active forces from internal molecular motors. The beat varies in response to changes in the fluid rheology, including elasticity, but there are limited data on how systematic changes in elasticity alter the beat. This work analyses a related problem with fixed-strength driving force: the emergence of beating of an elastic planar filament driven by a follower force at the tip of a viscoelastic fluid. This analysis examines how the onset of oscillations depends on the strength of the force and viscoelastic parameters. Compared to a Newtonian fluid, it takes more force to induce the instability in viscoelastic fluids, and the frequency of the oscillation is higher. The linear analysis predicts that the frequency increases with the fluid relaxation time. Using numerical simulations, the model predictions are compared with experimental data on frequency changes in the bi-flagellated alga Chlamydomonas reinhardtii. The model shows the same trends in response to changes in both fluid viscosity and Deborah number and thus provides a possible mechanistic understanding of the experimental observations.

In this paper the numerical simulation of the interaction between Oldroyd-B viscoelastic fluid flows and hyperelastic solids is approached. The algorithm employed is a classical block-iterative scheme, in which the solid and the fluid mechanics problems are solved sequentially. A Galerkin finite element approach has been employed for the numerical approximation of the solid, while the flow equations are approximated using a stabilized finite element method based on the Variational Multi-Scale approach to overcome the instabilities of the Galerkin method. To be able to deal with flows with dominant elasticity, a log-conformation reformulation of the constitutive equation can be employed; here this approach is extended to Fluid–Structure Interaction problems. Several numerical examples are presented and discussed to assess the robustness of the proposed scheme and its applicability to problems with viscoelastic fluids in which elasticity is dominant interacting with hyperelastic solids., L. Moreno acknowledges the support received from the project Nemesis, LARE_BIRD2020_01 (id: 33593). I. Castañar gratefully acknowledges the support received from the Agència de Gestió d’Ajuts i de Recerca through the predoctoral FI grant 2019-FI-B-00649. R. Codina gratefully acknowledges the support received through the ICREA, Spain Acadèmia Research Program of the Catalan Government. This work was partially funded through the TOP-FSI: RTI2018-098276-B-I00 project of the Spanish Government. CIMNE is a recipient of a “Severo Ochoa Programme for Centers of Excellence in R&D” grant (CEX2018-000797-S) by the Spanish Ministry of Economy and Competitiveness ., Peer Reviewed, Postprint (published version)

We present direct numerical simulations of the Taylor–Couette flow of a dilute polymer solution when only the inner cylinder rotates and the curvature of the system is moderate (¿=0.77 ). The finitely extensible nonlinear elastic-Peterlin closure is used to model the polymer dynamics. The simulations have revealed the existence of a novel elasto-inertial rotating wave characterized by arrow-shaped structures of the polymer stretch field aligned with the streamwise direction. This rotating wave pattern is comprehensively characterized, including an analysis of its dependence on the dimensionless Reynolds and Weissenberg numbers. Other flow states having arrow-shaped structures coexisting with other types of structures have also been identified for the first time in this study and are briefly discussed. This article is part of the theme issue ‘Taylor–Couette and related flows on the centennial of Taylor’s seminal Philosophical Transactions paper (part 2)’., Postprint (author's final draft)

Viscoelastic flows through porous media become unstable and chaotic beyond critical flow conditions, impacting widespread industrial and biological processes such as enhanced oil recovery and drug delivery. Understanding the influence of the pore structure or geometry on the onset of flow instability can lead to fundamental insights into these processes and, potentially, to their optimization. Recently, for viscoelastic flows through porous media modeled by arrays of microscopic posts, Walkama et al. [D. M. Walkama, N. Waisbord, J. S. Guasto, Phys. Rev. Lett. 124, 164501 (2020)] demonstrated that geometric disorder greatly suppressed the strength of the chaotic fluctuations that arose as the flow rate was increased. However, in that work, disorder was only applied to one originally ordered configuration of posts. Here, we demonstrate experimentally that, given a slightly modified ordered array of posts, introducing disorder can also promote chaotic fluctuations. We provide a unifying explanation for these contrasting results by considering the effect of disorder on the occurrence of stagnation points exposed to the flow field, which depends on the nature of the originally ordered post array. This work provides a general understanding of how pore geometry affects the stability of viscoelastic porous media flows., source:https://www.pnas.org/content/118/38/e2111651118

We study the dynamics of a neutrally buoyant rigid sphere carried by an elastoviscoplastic fluid in a pressure-driven channel flow numerically. The yielding to flow is marked by the yield stress which splits the flow into two main regions: the core unyielded region and two sheared yielded regions close to the walls. The particles which are initially in the plug region are observed to translate with the same velocity as the plug without any rotation/migration. Keeping the Reynolds number fixed, we study the effect of elasticity (Weissenberg number) and plasticity (Bingham number) of the fluid on the particle migration inside the sheared regions. In the viscoelastic limit, in the range of studied parameters (low elasticity), inertia is dominant and the particle finds its equilibrium position between the centreline and the wall. The same happens in the viscoplastic limit, yet the yield surface plays the role of centreline. However, the combination of elasticity and plasticity of the suspending fluid (elastoviscoplasticity) trigger particle-focusing: in the elastoviscoplastic flow, for a certain range of Weissenberg numbers (≈0.5), isolated particles migrate all the way to the centreline by entering into the core plug region. This behaviour suggests a particle-focusing process for inertial regimes which was not previously found in a viscoelastic or viscoplastic carrying fluid., QC 20200929

This study investigates viscoelastic fluid, which possesses both viscous and elastics properties, by employing a fractional derivative model to reveal the stress relaxation phenomenon with distance. The spatial-fractional derivative used in the momentum conservation equation is the Riemann–Liouville type derivative. Further, we use non-uniform boundary conditions subject to the boundary layer equations of the fluid flowing through a semi-infinite permeable flat surface. Owing to the fractional derivative model and non-uniform boundary conditions, this problem is complex. Thus, a finite difference scheme is applied after the coupled continuity equation and momentum equation are decoupled and linearized. The accuracy, convergence, and stability of the numerical method are presented. It is shown that non-uniform mass transfer through a permeable surface considerably affects the velocity boundary layer. By illustrating the physical interactions between the velocity fields in the boundary layer and the permeation mode in the surface, this paper predicts the velocity distributions with varying permeable surface, and also provides the possibility of changing the velocity fields by altering the permeable sheet. The results and numerical technique used in this study will help in the understanding of fractional calculus investigation in engineering.

In everyday human life, laminar flow is arguably an exception whereas turbulent flow is the norm. Yet, the former has been much better understood, naturally since laminar flow renders itself to treatment in a relatively easier fashion compared to turbulence with its chaotic dynamics across multiple scales in space and time. A parallel analogy in terms of sophistication of dynamics can be drawn between single phase and multiphase flows; the latter being the norm yet poorly understood due to numerous complexities arising on account of the huge parameter space involved. It is also remarkable that numerical studies are more prevalent in this field and there is a dearth of experimental results, which are important for both validation purposes and as a beacon to navigate research in practically relevant directions. This work has emerged to address the above issues. The attention has been largely directed towards understanding the flow of spherical particles in a square duct at moderately high concentrations using Particle Image Velocimetry (PIV) with refractive-index-matched (RIM) hydrogel particles. Fluids with Newtonian, viscoelastic and elastoviscoplastic rheology have been investigated due to their presence in natural and industrially relevant flows. Experiments and Direct Numerical Simulations (DNS) with spherical particles in a round pipe with turbulent flow of a Newtonian fluid are also conducted to extend and generalise the observations made in the square duct. With the ability to optically interrogate the bulk of the flow at high particle concentrations (20\% in this work), many interesting measurements are made possible, focussing on the turbulent regime. For the Newtonian fluid, the pressure drop or, equivalently, the energy required to pump the fluid-particle mixture is a complex function of particle size and concentration in the duct. This phenomenon arises due to the particle concentration distribution, with a local maxima at the core and the walls, and its, Man kan med visst fog hävda att turbulens är normen och laminär strömning mer ovanlig i vårt dagliga liv. Förståelsen av laminär strömning är å andra sidan bättre, vilket är naturligt eftersom det ofta är enklare att studera laminärt flöde än turbulens med sin kaotiska dynamik som sträcker sig över ett stort intervall av tids- och längdskalor. En parallell analogi vad gäller komplexitet och sofistikerad dynamik kan göras mellan enfas- och flerfasströmning. Det senare fallet är vanligast, men det är inte lika väl förstått och beskrivet på grund av att komplexiteten ökar snabbt när nya parametrar såsom partikelstorlek och koncentration införs. Det bör också nämnas att numeriska simuleringar dominerar i flerfasforskningen, varför det råder brist på detaljerade experimentella resultat, trots att dessa är viktiga både för validering av simuleringar och för att identifiera relevanta forskningsproblem. Det här föreliggande avhandlingsarbetet har ambitionen att bidra med experimentella resultat på flerfasströmning. Fokus har huvudsakligen varit att förstå flöden av suspensioner (blandningar) med sfäriska partiklar i kanaler med kvadratiska tvärsnitt och måttligt hög koncentration av partiklar. Partiklarna som använts är brytningsindexmatchade (refraction-index-matching, RIM) och har studerats med PIV (particle image velocimetry). Fluider med newtonsk, viskoelastisk och elastoviskoplastisk reologi har använts. Sådana fluider förekommer i flöden både i naturen och i industriella tillämpningar. Experiment och direkta numeriska simuleringar (DNS) av turbulent strömning med sfäriska partiklar i ett rör med cirkulärt tvärsnitt har också genomförts för att utöka och generalisera observationerna i den kvadratiska kanalen. Tack vare möjligheten att optiskt studera stora delar av strömningen vid relativt höga partikelkoncentrationer (maximalt 20 procent) har den turbulenta strömningen karakterisats i detalj. Med newtonsk fluid är tryckfallet, eller energin som krävs för att pumpa sus, QC 20190903

In everyday human life, laminar flow is arguably an exception whereas turbulent flow is the norm. Yet, the former has been much better understood, naturally since laminar flow renders itself to treatment in a relatively easier fashion compared to turbulence with its chaotic dynamics across multiple scales in space and time. A parallel analogy in terms of sophistication of dynamics can be drawn between single phase and multiphase flows; the latter being the norm yet poorly understood due to numerous complexities arising on account of the huge parameter space involved. It is also remarkable that numerical studies are more prevalent in this field and there is a dearth of experimental results, which are important for both validation purposes and as a beacon to navigate research in practically relevant directions. This work has emerged to address the above issues. The attention has been largely directed towards understanding the flow of spherical particles in a square duct at moderately high concentrations using Particle Image Velocimetry (PIV) with refractive-index-matched (RIM) hydrogel particles. Fluids with Newtonian, viscoelastic and elastoviscoplastic rheology have been investigated due to their presence in natural and industrially relevant flows. Experiments and Direct Numerical Simulations (DNS) with spherical particles in a round pipe with turbulent flow of a Newtonian fluid are also conducted to extend and generalise the observations made in the square duct. With the ability to optically interrogate the bulk of the flow at high particle concentrations (20\% in this work), many interesting measurements are made possible, focussing on the turbulent regime. For the Newtonian fluid, the pressure drop or, equivalently, the energy required to pump the fluid-particle mixture is a complex function of particle size and concentration in the duct. This phenomenon arises due to the particle concentration distribution, with a local maxima at the core and the walls, and its, Man kan med visst fog hävda att turbulens är normen och laminär strömning mer ovanlig i vårt dagliga liv. Förståelsen av laminär strömning är å andra sidan bättre, vilket är naturligt eftersom det ofta är enklare att studera laminärt flöde än turbulens med sin kaotiska dynamik som sträcker sig över ett stort intervall av tids- och längdskalor. En parallell analogi vad gäller komplexitet och sofistikerad dynamik kan göras mellan enfas- och flerfasströmning. Det senare fallet är vanligast, men det är inte lika väl förstått och beskrivet på grund av att komplexiteten ökar snabbt när nya parametrar såsom partikelstorlek och koncentration införs. Det bör också nämnas att numeriska simuleringar dominerar i flerfasforskningen, varför det råder brist på detaljerade experimentella resultat, trots att dessa är viktiga både för validering av simuleringar och för att identifiera relevanta forskningsproblem. Det här föreliggande avhandlingsarbetet har ambitionen att bidra med experimentella resultat på flerfasströmning. Fokus har huvudsakligen varit att förstå flöden av suspensioner (blandningar) med sfäriska partiklar i kanaler med kvadratiska tvärsnitt och måttligt hög koncentration av partiklar. Partiklarna som använts är brytningsindexmatchade (refraction-index-matching, RIM) och har studerats med PIV (particle image velocimetry). Fluider med newtonsk, viskoelastisk och elastoviskoplastisk reologi har använts. Sådana fluider förekommer i flöden både i naturen och i industriella tillämpningar. Experiment och direkta numeriska simuleringar (DNS) av turbulent strömning med sfäriska partiklar i ett rör med cirkulärt tvärsnitt har också genomförts för att utöka och generalisera observationerna i den kvadratiska kanalen. Tack vare möjligheten att optiskt studera stora delar av strömningen vid relativt höga partikelkoncentrationer (maximalt 20 procent) har den turbulenta strömningen karakterisats i detalj. Med newtonsk fluid är tryckfallet, eller energin som krävs för att pumpa sus, QC 20190903

Depending on the application one needs to either stabilize or destabilize the interfacial properties of an emulsion. An aspect of the dynamics that governs the stability of emulsions in general is the drainage time of the film that is formed when two drops collide. In this work, we study the effect that viscoelasticity of the matrix fluid has on this drainage time of two Newtonian drops that perform a head-on collision under an applied macroscopic extensional rate. For the modeling of the viscoelastic matrix material the Giesekus model is chosen. A cylindrical coordinate system is applied with imposed axisymmetry and the resulting equations are solved using fully resolved numerical simulations employing a finite element discretization. Our results show that viscoelasticity reduces the drainage time, which is a combined effect of three different stages.

Depending on the application one needs to either stabilize or destabilize the interfacial properties of an emulsion. An aspect of the dynamics that governs the stability of emulsions in general is the drainage time of the film that is formed when two drops collide. In this work, we study the effect that viscoelasticity of the matrix fluid has on this drainage time of two Newtonian drops that perform a head-on collision under an applied macroscopic extensional rate. For the modeling of the viscoelastic matrix material the Giesekus model is chosen. A cylindrical coordinate system is applied with imposed axisymmetry and the resulting equations are solved using fully resolved numerical simulations employing a finite element discretization. Our results show that viscoelasticity reduces the drainage time, which is a combined effect of three different stages.

In everyday human life, laminar flow is arguably an exception whereas turbulent flow is the norm. Yet, the former has been much better understood, naturally since laminar flow renders itself to treatment in a relatively easier fashion compared to turbulence with its chaotic dynamics across multiple scales in space and time. A parallel analogy in terms of sophistication of dynamics can be drawn between single phase and multiphase flows; the latter being the norm yet poorly understood due to numerous complexities arising on account of the huge parameter space involved. It is also remarkable that numerical studies are more prevalent in this field and there is a dearth of experimental results, which are important for both validation purposes and as a beacon to navigate research in practically relevant directions. This work has emerged to address the above issues. The attention has been largely directed towards understanding the flow of spherical particles in a square duct at moderately high concentrations using Particle Image Velocimetry (PIV) with refractive-index-matched (RIM) hydrogel particles. Fluids with Newtonian, viscoelastic and elastoviscoplastic rheology have been investigated due to their presence in natural and industrially relevant flows. Experiments and Direct Numerical Simulations (DNS) with spherical particles in a round pipe with turbulent flow of a Newtonian fluid are also conducted to extend and generalise the observations made in the square duct. With the ability to optically interrogate the bulk of the flow at high particle concentrations (20\% in this work), many interesting measurements are made possible, focussing on the turbulent regime. For the Newtonian fluid, the pressure drop or, equivalently, the energy required to pump the fluid-particle mixture is a complex function of particle size and concentration in the duct. This phenomenon arises due to the particle concentration distribution, with a local maxima at the core and the walls, and its, Man kan med visst fog hävda att turbulens är normen och laminär strömning mer ovanlig i vårt dagliga liv. Förståelsen av laminär strömning är å andra sidan bättre, vilket är naturligt eftersom det ofta är enklare att studera laminärt flöde än turbulens med sin kaotiska dynamik som sträcker sig över ett stort intervall av tids- och längdskalor. En parallell analogi vad gäller komplexitet och sofistikerad dynamik kan göras mellan enfas- och flerfasströmning. Det senare fallet är vanligast, men det är inte lika väl förstått och beskrivet på grund av att komplexiteten ökar snabbt när nya parametrar såsom partikelstorlek och koncentration införs. Det bör också nämnas att numeriska simuleringar dominerar i flerfasforskningen, varför det råder brist på detaljerade experimentella resultat, trots att dessa är viktiga både för validering av simuleringar och för att identifiera relevanta forskningsproblem. Det här föreliggande avhandlingsarbetet har ambitionen att bidra med experimentella resultat på flerfasströmning. Fokus har huvudsakligen varit att förstå flöden av suspensioner (blandningar) med sfäriska partiklar i kanaler med kvadratiska tvärsnitt och måttligt hög koncentration av partiklar. Partiklarna som använts är brytningsindexmatchade (refraction-index-matching, RIM) och har studerats med PIV (particle image velocimetry). Fluider med newtonsk, viskoelastisk och elastoviskoplastisk reologi har använts. Sådana fluider förekommer i flöden både i naturen och i industriella tillämpningar. Experiment och direkta numeriska simuleringar (DNS) av turbulent strömning med sfäriska partiklar i ett rör med cirkulärt tvärsnitt har också genomförts för att utöka och generalisera observationerna i den kvadratiska kanalen. Tack vare möjligheten att optiskt studera stora delar av strömningen vid relativt höga partikelkoncentrationer (maximalt 20 procent) har den turbulenta strömningen karakterisats i detalj. Med newtonsk fluid är tryckfallet, eller energin som krävs för att pumpa sus, QC 20190903

In everyday human life, laminar flow is arguably an exception whereas turbulent flow is the norm. Yet, the former has been much better understood, naturally since laminar flow renders itself to treatment in a relatively easier fashion compared to turbulence with its chaotic dynamics across multiple scales in space and time. A parallel analogy in terms of sophistication of dynamics can be drawn between single phase and multiphase flows; the latter being the norm yet poorly understood due to numerous complexities arising on account of the huge parameter space involved. It is also remarkable that numerical studies are more prevalent in this field and there is a dearth of experimental results, which are important for both validation purposes and as a beacon to navigate research in practically relevant directions. This work has emerged to address the above issues. The attention has been largely directed towards understanding the flow of spherical particles in a square duct at moderately high concentrations using Particle Image Velocimetry (PIV) with refractive-index-matched (RIM) hydrogel particles. Fluids with Newtonian, viscoelastic and elastoviscoplastic rheology have been investigated due to their presence in natural and industrially relevant flows. Experiments and Direct Numerical Simulations (DNS) with spherical particles in a round pipe with turbulent flow of a Newtonian fluid are also conducted to extend and generalise the observations made in the square duct. With the ability to optically interrogate the bulk of the flow at high particle concentrations (20\% in this work), many interesting measurements are made possible, focussing on the turbulent regime. For the Newtonian fluid, the pressure drop or, equivalently, the energy required to pump the fluid-particle mixture is a complex function of particle size and concentration in the duct. This phenomenon arises due to the particle concentration distribution, with a local maxima at the core and the walls, and its, Man kan med visst fog hävda att turbulens är normen och laminär strömning mer ovanlig i vårt dagliga liv. Förståelsen av laminär strömning är å andra sidan bättre, vilket är naturligt eftersom det ofta är enklare att studera laminärt flöde än turbulens med sin kaotiska dynamik som sträcker sig över ett stort intervall av tids- och längdskalor. En parallell analogi vad gäller komplexitet och sofistikerad dynamik kan göras mellan enfas- och flerfasströmning. Det senare fallet är vanligast, men det är inte lika väl förstått och beskrivet på grund av att komplexiteten ökar snabbt när nya parametrar såsom partikelstorlek och koncentration införs. Det bör också nämnas att numeriska simuleringar dominerar i flerfasforskningen, varför det råder brist på detaljerade experimentella resultat, trots att dessa är viktiga både för validering av simuleringar och för att identifiera relevanta forskningsproblem. Det här föreliggande avhandlingsarbetet har ambitionen att bidra med experimentella resultat på flerfasströmning. Fokus har huvudsakligen varit att förstå flöden av suspensioner (blandningar) med sfäriska partiklar i kanaler med kvadratiska tvärsnitt och måttligt hög koncentration av partiklar. Partiklarna som använts är brytningsindexmatchade (refraction-index-matching, RIM) och har studerats med PIV (particle image velocimetry). Fluider med newtonsk, viskoelastisk och elastoviskoplastisk reologi har använts. Sådana fluider förekommer i flöden både i naturen och i industriella tillämpningar. Experiment och direkta numeriska simuleringar (DNS) av turbulent strömning med sfäriska partiklar i ett rör med cirkulärt tvärsnitt har också genomförts för att utöka och generalisera observationerna i den kvadratiska kanalen. Tack vare möjligheten att optiskt studera stora delar av strömningen vid relativt höga partikelkoncentrationer (maximalt 20 procent) har den turbulenta strömningen karakterisats i detalj. Med newtonsk fluid är tryckfallet, eller energin som krävs för att pumpa sus, QC 20190903

In everyday human life, laminar flow is arguably an exception whereas turbulent flow is the norm. Yet, the former has been much better understood, naturally since laminar flow renders itself to treatment in a relatively easier fashion compared to turbulence with its chaotic dynamics across multiple scales in space and time. A parallel analogy in terms of sophistication of dynamics can be drawn between single phase and multiphase flows; the latter being the norm yet poorly understood due to numerous complexities arising on account of the huge parameter space involved. It is also remarkable that numerical studies are more prevalent in this field and there is a dearth of experimental results, which are important for both validation purposes and as a beacon to navigate research in practically relevant directions. This work has emerged to address the above issues. The attention has been largely directed towards understanding the flow of spherical particles in a square duct at moderately high concentrations using Particle Image Velocimetry (PIV) with refractive-index-matched (RIM) hydrogel particles. Fluids with Newtonian, viscoelastic and elastoviscoplastic rheology have been investigated due to their presence in natural and industrially relevant flows. Experiments and Direct Numerical Simulations (DNS) with spherical particles in a round pipe with turbulent flow of a Newtonian fluid are also conducted to extend and generalise the observations made in the square duct. With the ability to optically interrogate the bulk of the flow at high particle concentrations (20\% in this work), many interesting measurements are made possible, focussing on the turbulent regime. For the Newtonian fluid, the pressure drop or, equivalently, the energy required to pump the fluid-particle mixture is a complex function of particle size and concentration in the duct. This phenomenon arises due to the particle concentration distribution, with a local maxima at the core and the walls, and its, Man kan med visst fog hävda att turbulens är normen och laminär strömning mer ovanlig i vårt dagliga liv. Förståelsen av laminär strömning är å andra sidan bättre, vilket är naturligt eftersom det ofta är enklare att studera laminärt flöde än turbulens med sin kaotiska dynamik som sträcker sig över ett stort intervall av tids- och längdskalor. En parallell analogi vad gäller komplexitet och sofistikerad dynamik kan göras mellan enfas- och flerfasströmning. Det senare fallet är vanligast, men det är inte lika väl förstått och beskrivet på grund av att komplexiteten ökar snabbt när nya parametrar såsom partikelstorlek och koncentration införs. Det bör också nämnas att numeriska simuleringar dominerar i flerfasforskningen, varför det råder brist på detaljerade experimentella resultat, trots att dessa är viktiga både för validering av simuleringar och för att identifiera relevanta forskningsproblem. Det här föreliggande avhandlingsarbetet har ambitionen att bidra med experimentella resultat på flerfasströmning. Fokus har huvudsakligen varit att förstå flöden av suspensioner (blandningar) med sfäriska partiklar i kanaler med kvadratiska tvärsnitt och måttligt hög koncentration av partiklar. Partiklarna som använts är brytningsindexmatchade (refraction-index-matching, RIM) och har studerats med PIV (particle image velocimetry). Fluider med newtonsk, viskoelastisk och elastoviskoplastisk reologi har använts. Sådana fluider förekommer i flöden både i naturen och i industriella tillämpningar. Experiment och direkta numeriska simuleringar (DNS) av turbulent strömning med sfäriska partiklar i ett rör med cirkulärt tvärsnitt har också genomförts för att utöka och generalisera observationerna i den kvadratiska kanalen. Tack vare möjligheten att optiskt studera stora delar av strömningen vid relativt höga partikelkoncentrationer (maximalt 20 procent) har den turbulenta strömningen karakterisats i detalj. Med newtonsk fluid är tryckfallet, eller energin som krävs för att pumpa sus, QC 20190903

The paper deals with the modeling of the constant-pressure cake filtration of suspensions in viscoelastic liquids. The filtration equations are presented based on the capillary hybrid model for power law fluid flow and comprising correction functions of the influence of dispersing liquid elasticity. The validity of these equations has been verified experimentally. The tests with suspensions of nearly spherical polystyrene particles Krasten in viscoelastic aqueous solutions of polyacrylamides Kerafloc and Praestol were performed at constant pressure in a laboratory cylindrical filtration unit. The rheological properties of liquids have been measured on a rotational rheometer Mars II and an extensional rheometer Caber 1. By analysing the experimental data, the filtration characteristics have been evaluated. The elastic effects manifest themselves predominantly at the early period of the filtration due to a higher filtration velocity which results in an evidently increased filtration resistance. The satisfactory agreement of the experimental dependences of the cumulative filtration volume on the time of filtration has been achieved when compared to those obtained by the numerical solution of the proposed differential equation of filtration. This confirms that the cake filtration from viscoelastic liquids can be predicted with a satisfactory accuracy, thus making the suggested filtration model applicable in practice., Práce se zabývá modelováním koláčové filtrace suspenzí ve viskoelastických kapalinách za konstantního tlaku.

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

The present study analyses the effects of Hall current and heat absorption on a viscous, incompressible, optically thick and electrically conducting viscoelastic fluid flow past an infinite vertical porous plate through porous medium in rotating system with variable suction, thermal radiation and chemical reaction in the presence of uniform magnetic field. The perturbation technique is employed to solve the governing nonlinear partial differential equations to obtain the expressions for velocity, temperature and concentration profile. With the help of graphs and tables, the effects of pertinent flow parameters on the velocity, temperature and concentration fields, shear stress, Nusselt number and Sherwood number within the boundary layer are discussed. The results reveal that the observed parameters in rotating system have a noteworthy influence on the ow, heat and mass transfer.

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

The paper deals with the modeling of the constant-pressure cake filtration of suspensions in viscoelastic liquids. The filtration equations are presented based on the capillary hybrid model for power law fluid flow and comprising correction functions of the influence of dispersing liquid elasticity. The validity of these equations has been verified experimentally. The tests with suspensions of nearly spherical polystyrene particles Krasten in viscoelastic aqueous solutions of polyacrylamides Kerafloc and Praestol were performed at constant pressure in a laboratory cylindrical filtration unit. The rheological properties of liquids have been measured on a rotational rheometer Mars II and an extensional rheometer Caber 1. By analysing the experimental data, the filtration characteristics have been evaluated. The elastic effects manifest themselves predominantly at the early period of the filtration due to a higher filtration velocity which results in an evidently increased filtration resistance. The satisfactory agreement of the experimental dependences of the cumulative filtration volume on the time of filtration has been achieved when compared to those obtained by the numerical solution of the proposed differential equation of filtration. This confirms that the cake filtration from viscoelastic liquids can be predicted with a satisfactory accuracy, thus making the suggested filtration model applicable in practice.

I ABSTRACT Particle migration is very important and essential for focusing, separating, counting, detecting or analysis in numerous biological and chemical applications. A variety of microfluidic devices have been designed to realize particle migration in Newtonian fluids. With the aid of external force fields, specially designed channel structures, or hydrodynamic forces, particles can migrate to one or several equilibrium positions in Newtonian fluids. While extensive research on fundamentals and application based on particle migration in Newtonian fluids has been conducted, there are fewer research on particle migration in non-Newtonian fluids. Actually, non-Newtonian fluids such as blood, cytoplasm, and many other body fluids, are very ubiquitous in our daily life and in real world issues. Therefore, it is important to invest our research focus on particle migration in non-Newtonian fluids to develop deep understanding of cell behaviours in these body fluids. Recently, the research interests on particle migration based on non-Newtonian fluid have been increasing. The increasing attention on particle migration based on non-Newtonian fluids is a result of its interesting intrinsic fluid properties. Compared with particle focusing in a Newtonian fluid, 3D particle focusing in a non-Newtonian fluid can be easily realized in simple channels without the need of any external force fields; and particles with a much smaller size, such as from submicrometer to even nanometer particles can be manipulated. Particle migration in non-Newtonian fluids can break the limitations of requiring extra external components, lower throughput, complex fabrications in Newtonian fluids. Moreover, it can develop simpler, more flexible and versatile particle manipulation methods with lower costs. These advantages of non-Newtonian fluids allow fast growth of microfluidic applications based on viscoelastisity-induced particle migration, enabling the development of various micro-devices for bi

This paper presents an investigation for boundary layer flow of viscoelastic fluids past a flat plate. Fractional-order Blasius equation with spatial fractional Riemann–Liouville derivative is derived firstly by using Lie group transformation. The solution is obtained numerically by the generalized shooting method, employing the shifted Grünwald formula and classical fourth order Runge–Kutta method as the iterative scheme. The effects of the order of fractional derivative and the generalized Reynolds number on the velocity profiles are analyzed and discussed. Numerical results show that the smaller the value of the fractional order derivative leads to the faster velocity of viscoelastic fluids near the plate but not to hold near the outer flow. As the Reynolds number increases, the fluid is moving faster in the whole boundary layer consistently.

[Background] Force generation and the material properties of cells and tissues are central to morphogenesis but remain difficult to measure in vivo. Insight is often limited to the ratios of mechanical properties obtained through disruptive manipulation, and the appropriate models relating stress and strain are unknown. The Drosophila amnioserosa epithelium progressively contracts over 3 hours of dorsal closure, during which cell apices exhibit area fluctuations driven by medial myosin pulses with periods of 1.5–6 min. Linking these two timescales and understanding how pulsatile contractions drive morphogenetic movements is an urgent challenge., [Results] We present a novel framework to measure in a continuous manner the mechanical properties of epithelial cells in the natural context of a tissue undergoing morphogenesis. We show that the relationship between apicomedial myosin fluorescence intensity and strain during fluctuations is consistent with a linear behaviour, although with a lag. We thus used myosin fluorescence intensity as a proxy for active force generation and treated cells as natural experiments of mechanical response under cyclic loading, revealing unambiguous mechanical properties from the hysteresis loop relating stress to strain. Amnioserosa cells can be described as a contractile viscoelastic fluid. We show that their emergent mechanical behaviour can be described by a linear viscoelastic rheology at timescales relevant for tissue morphogenesis. For the first time, we establish relative changes in separate effective mechanical properties in vivo. Over the course of dorsal closure, the tissue solidifies and effective stiffness doubles as net contraction of the tissue commences. Combining our findings with those from previous laser ablation experiments, we show that both apicomedial and junctional stress also increase over time, with the relative increase in apicomedial stress approximately twice that of other obtained measures., [Conclusions] Our results show that in an epithelial tissue undergoing net contraction, stiffness and stress are coupled. Dorsal closure cell apical contraction is driven by the medial region where the relative increase in stress is greater than that of stiffness. At junctions, by contrast, the relative increase in the mechanical properties is the same, so the junctional contribution to tissue deformation is constant over time. An increase in myosin activity is likely to underlie, at least in part, the change in medioapical properties and we suggest that its greater effect on stress relative to stiffness is fundamental to actomyosin systems and confers on tissues the ability to regulate contraction rates in response to changes in external mechanics.

The deformation of a hydrogen microbubble line and/or mesh in a viscoelastic fluid around a rising bubble was tracked from the original static position in order to discuss the mechanism of the typical phenomena such as the negative wake or the cusp shape. This new experimental method is essentially important because of the hysteresis-dependent nature of the viscoelastic fluid. This new method makes this study distinctive from a number of conventional studies of viscoelastic fluids focusing on the non-Newtonian properties and/or the instantaneous flow field. According to our experimental results, the flow mechanism responsible for the negative wake or cusp shape was attributed to the accumulation and release of the shear strain energy. Some residual displacements were observed after the bubble rising, which were almost completely reproduced as the internal dissipations in a Maxwell model modified with a non-linear spring.

In this thesis, simulations are performed to study the motion ofindividual cells in flow, focusing on the hydrodynamics of actively swimming cells likethe self-propelling microorganisms, and of passively advected objects like the red bloodcells. In particular, we develop numerical tools to address the locomotion ofmicroswimmers in viscoelastic fluids and complex geometries, as well as the motion ofdeformable capsules in micro-fluidic flows. For the active movement, the squirmer is used as our model microswimmer. The finiteelement method is employed to study the influence of the viscoelasticity of fluid on theperformance of locomotion. A boundary element method is implemented to study swimmingcells inside a tube. For the passive counterpart, the deformable capsule is chosen as the modelcell. An accelerated boundary integral method code is developed to solve thefluid-structure interaction, and a global spectral method is incorporated to handle theevolving cell surface and its corresponding membrane dynamics. We study the locomotion of a neutral squirmer with anemphasis on the change of swimming kinematics, energetics, and flowdisturbance from Newtonian to viscoelastic fluid. We also examine the dynamics of differentswimming gaits resulting in different patterns of polymer deformation, as well as theirinfluence on the swimming performance. We correlate the change of swimming speed withthe extensional viscosity and that of power consumption with the phase delay of viscoelasticfluids. Moreover, we utilise the boundary element method to simulate the swimming cells in astraight and torus-like bent tube, where the tube radius is a few times the cell radius. Weinvestigate the effect of tube confinement to the swimming speed and power consumption. Weanalyse the motions of squirmers with different gaits, which significantly affect thestability of the motion. Helical trajectories are produced for a neutralsquirmer swimming, in qualitative agreement with experimental observation, QC 20140313

In this thesis, simulations are performed to study the motion ofindividual cells in flow, focusing on the hydrodynamics of actively swimming cells likethe self-propelling microorganisms, and of passively advected objects like the red bloodcells. In particular, we develop numerical tools to address the locomotion ofmicroswimmers in viscoelastic fluids and complex geometries, as well as the motion ofdeformable capsules in micro-fluidic flows. For the active movement, the squirmer is used as our model microswimmer. The finiteelement method is employed to study the influence of the viscoelasticity of fluid on theperformance of locomotion. A boundary element method is implemented to study swimmingcells inside a tube. For the passive counterpart, the deformable capsule is chosen as the modelcell. An accelerated boundary integral method code is developed to solve thefluid-structure interaction, and a global spectral method is incorporated to handle theevolving cell surface and its corresponding membrane dynamics. We study the locomotion of a neutral squirmer with anemphasis on the change of swimming kinematics, energetics, and flowdisturbance from Newtonian to viscoelastic fluid. We also examine the dynamics of differentswimming gaits resulting in different patterns of polymer deformation, as well as theirinfluence on the swimming performance. We correlate the change of swimming speed withthe extensional viscosity and that of power consumption with the phase delay of viscoelasticfluids. Moreover, we utilise the boundary element method to simulate the swimming cells in astraight and torus-like bent tube, where the tube radius is a few times the cell radius. Weinvestigate the effect of tube confinement to the swimming speed and power consumption. Weanalyse the motions of squirmers with different gaits, which significantly affect thestability of the motion. Helical trajectories are produced for a neutralsquirmer swimming, in qualitative agreement with experimental observation, QC 20140313

We present an Immersed Boundary method for interactions between elastic boundaries and mixtures of two fluids. Each fluid has its own velocity field and volume-fraction. A penalty method is used to enforce the condition that both fluids' velocities agree with that of the elastic boundaries. The method is applied to several problems: Taylor's swimming sheet problem for a mixture of two viscous fluids, peristaltic pumping of a mixture of two viscous fluids, with and without immersed particles, and peristaltic pumping of a mixture of a viscous fluid and a viscoelastic fluid. The swimming sheet and peristalsis problems have received much attention recently in the context of a single viscoelastic fluid. Numerical results demonstrate that the method converges and show its capability to handle a number of flow problems of substantial current interest. They illustrate that for each of these problems, the relative motion between the two fluids changes the observed behaviors profoundly compared to the single fluid case.

In this thesis, simulations are performed to study the motion ofindividual cells in flow, focusing on the hydrodynamics of actively swimming cells likethe self-propelling microorganisms, and of passively advected objects like the red bloodcells. In particular, we develop numerical tools to address the locomotion ofmicroswimmers in viscoelastic fluids and complex geometries, as well as the motion ofdeformable capsules in micro-fluidic flows. For the active movement, the squirmer is used as our model microswimmer. The finiteelement method is employed to study the influence of the viscoelasticity of fluid on theperformance of locomotion. A boundary element method is implemented to study swimmingcells inside a tube. For the passive counterpart, the deformable capsule is chosen as the modelcell. An accelerated boundary integral method code is developed to solve thefluid-structure interaction, and a global spectral method is incorporated to handle theevolving cell surface and its corresponding membrane dynamics. We study the locomotion of a neutral squirmer with anemphasis on the change of swimming kinematics, energetics, and flowdisturbance from Newtonian to viscoelastic fluid. We also examine the dynamics of differentswimming gaits resulting in different patterns of polymer deformation, as well as theirinfluence on the swimming performance. We correlate the change of swimming speed withthe extensional viscosity and that of power consumption with the phase delay of viscoelasticfluids. Moreover, we utilise the boundary element method to simulate the swimming cells in astraight and torus-like bent tube, where the tube radius is a few times the cell radius. Weinvestigate the effect of tube confinement to the swimming speed and power consumption. Weanalyse the motions of squirmers with different gaits, which significantly affect thestability of the motion. Helical trajectories are produced for a neutralsquirmer swimming, in qualitative agreement with experimental observation, QC 20140313

In this thesis, simulations are performed to study the motion ofindividual cells in flow, focusing on the hydrodynamics of actively swimming cells likethe self-propelling microorganisms, and of passively advected objects like the red bloodcells. In particular, we develop numerical tools to address the locomotion ofmicroswimmers in viscoelastic fluids and complex geometries, as well as the motion ofdeformable capsules in micro-fluidic flows. For the active movement, the squirmer is used as our model microswimmer. The finiteelement method is employed to study the influence of the viscoelasticity of fluid on theperformance of locomotion. A boundary element method is implemented to study swimmingcells inside a tube. For the passive counterpart, the deformable capsule is chosen as the modelcell. An accelerated boundary integral method code is developed to solve thefluid-structure interaction, and a global spectral method is incorporated to handle theevolving cell surface and its corresponding membrane dynamics. We study the locomotion of a neutral squirmer with anemphasis on the change of swimming kinematics, energetics, and flowdisturbance from Newtonian to viscoelastic fluid. We also examine the dynamics of differentswimming gaits resulting in different patterns of polymer deformation, as well as theirinfluence on the swimming performance. We correlate the change of swimming speed withthe extensional viscosity and that of power consumption with the phase delay of viscoelasticfluids. Moreover, we utilise the boundary element method to simulate the swimming cells in astraight and torus-like bent tube, where the tube radius is a few times the cell radius. Weinvestigate the effect of tube confinement to the swimming speed and power consumption. Weanalyse the motions of squirmers with different gaits, which significantly affect thestability of the motion. Helical trajectories are produced for a neutralsquirmer swimming, in qualitative agreement with experimental observation, QC 20140313

In this thesis, simulations are performed to study the motion ofindividual cells in flow, focusing on the hydrodynamics of actively swimming cells likethe self-propelling microorganisms, and of passively advected objects like the red bloodcells. In particular, we develop numerical tools to address the locomotion ofmicroswimmers in viscoelastic fluids and complex geometries, as well as the motion ofdeformable capsules in micro-fluidic flows. For the active movement, the squirmer is used as our model microswimmer. The finiteelement method is employed to study the influence of the viscoelasticity of fluid on theperformance of locomotion. A boundary element method is implemented to study swimmingcells inside a tube. For the passive counterpart, the deformable capsule is chosen as the modelcell. An accelerated boundary integral method code is developed to solve thefluid-structure interaction, and a global spectral method is incorporated to handle theevolving cell surface and its corresponding membrane dynamics. We study the locomotion of a neutral squirmer with anemphasis on the change of swimming kinematics, energetics, and flowdisturbance from Newtonian to viscoelastic fluid. We also examine the dynamics of differentswimming gaits resulting in different patterns of polymer deformation, as well as theirinfluence on the swimming performance. We correlate the change of swimming speed withthe extensional viscosity and that of power consumption with the phase delay of viscoelasticfluids. Moreover, we utilise the boundary element method to simulate the swimming cells in astraight and torus-like bent tube, where the tube radius is a few times the cell radius. Weinvestigate the effect of tube confinement to the swimming speed and power consumption. Weanalyse the motions of squirmers with different gaits, which significantly affect thestability of the motion. Helical trajectories are produced for a neutralsquirmer swimming, in qualitative agreement with experimental observation, QC 20140313

QC 20120613

In this paper, the problem of steady laminar three-dimensional magnetohydrodynamic (MHD) boundary layer flow and heat transfer over a stretching surface in a viscoelastic fluid is investigated. The equations which govern the flow are coupled nonlinear ordinary differential equations, which are solved numerically using a finite-difference scheme known as the Keller-box method. Various physical governing parameters such as the magnetic parameter M, the material or viscoelastic parameter K and the Prandtl number Pr are considered and the effects of these parameters are investigated. It is found that the material parameter K and the magnetic parameter M present opposite effects on the fluid flow and heat transfer characteristics. The numerical results obtained for the skin friction coefficient and the local Nusselt number are presented in tables. The features and profiles of the flow and heat transfer characteristics are illustrated in the forms of graphs.

In this paper, the problem of steady laminar three-dimensional magnetohydrodynamic (MHD) boundary layer flow and heat transfer over a stretching surface in a viscoelastic fluid is investigated. The equations which govern the flow are coupled nonlinear ordinary differential equations, which are solved numerically using a finite-difference scheme known as the Keller-box method. Various physical governing parameters such as the magnetic parameter M, the material or viscoelastic parameter K and the Prandtl number Pr are considered and the effects of these parameters are investigated. It is found that the material parameter K and the magnetic parameter M present opposite effects on the fluid flow and heat transfer characteristics. The numerical results obtained for the skin friction coefficient and the local Nusselt number are presented in tables. The features and profiles of the flow and heat transfer characteristics are illustrated in the forms of graphs.

In this paper, the problem of steady laminar three-dimensional magnetohydrodynamic (MHD) boundary layer flow and heat transfer over a stretching surface in a viscoelastic fluid is investigated. The equations which govern the flow are coupled nonlinear ordinary differential equations, which are solved numerically using a finite-difference scheme known as the Keller-box method. Various physical governing parameters such as the magnetic parameter M, the material or viscoelastic parameter K and the Prandtl number Pr are considered and the effects of these parameters are investigated. It is found that the material parameter K and the magnetic parameter M present opposite effects on the fluid flow and heat transfer characteristics. The numerical results obtained for the skin friction coefficient and the local Nusselt number are presented in tables. The features and profiles of the flow and heat transfer characteristics are illustrated in the forms of graphs.

A lattice Boltzmann method (LBM) for two-phase viscoelastic fluid flows is proposed. The method is mainly an extension of the LBM for two-phase flows with large density differences proposed by Inamuro et al. [Journal of Computational Physics Vol.198, No.2 (2004), pp.628–644]. The viscoelastic effects are introduced by the constitutive equation based on the Maxwell model, which has a spring and a dashpot connected with each other in series. The method is applied to simulations of a drop under shear flow in viscoelastic fluids and of a bubble rising in viscoelastic fluids. In the simulation of drop deformation under shear flows, the effects of viscoelasticity on the deformation and orientation angle are evaluated. In the simulation of bubble rising in viscoelastic fluids, a cusp configuration at the trailing edge is investigated and compared with the theoretical prediction and other numerical results.

We present a novel Lagrangian finite element method for simulating suspensions of particles in viscoelastic fluids. We solve the flow in a unit cell containing a small number of particles with doubly periodic boundary conditions on a self-replicating two-dimensional lattice to replicate a suspension on an infinite domain. The method uses a Lagrangian finite element grid that deforms with fluid combined with a quotient representation of the periodic lattice. We show that qualitatively different results are obtained for the shear-thinning pompom constitutive equation compared to those obtained using the Oldroyd B fluid. For the pom-pom fluid we show that the changes to shear viscosity with the addition of particles can be obtained by a simple shifting of the shear-rate and shear-stress.

We present a novel Lagrangian finite element method for simulating suspensions of particles in viscoelastic fluids. We solve the flow in a unit cell containing a small number of particles with doubly periodic boundary conditions on a self-replicating two-dimensional lattice to replicate a suspension on an infinite domain. The method uses a Lagrangian finite element grid that deforms with fluid combined with a quotient representation of the periodic lattice. We show that qualitatively different results are obtained for the shear-thinning pompom constitutive equation compared to those obtained using the Oldroyd B fluid. For the pom-pom fluid we show that the changes to shear viscosity with the addition of particles can be obtained by a simple shifting of the shear-rate and shear-stress.

Submitted to Applied Rheology, August 2004, We investigate the dynamics of the capillary thinning and break-up process for low viscosity elastic fluids such as dilute polymer solutions. Standard measurements of the evolution of the midpoint diameter of the necking fluid filament are augmented by high speed digital video images of the break up dynamics. We show that the successful operation of a capillary thinning device is governed by three important time scales (which characterize the relative importance of inertial, viscous and elastic processes), and also by two important length scales (which specify the initial sample size and the total stretch imposed on the sample). By optimizing the ranges of these geometric parameters, we are able to measure characteristic time scales for tensile stress growth as small as 1 millisecond for a number of model dilute and semi-dilute solutions of polyethylene oxide (PEO) in water and glycerin. If the aspect ratio of the sample is too small, or the total axial stretch is too great, measurements are limited, respectively, by inertial oscillations of the liquid bridge or by the development of the well-known beads-on-a-string morphology which disrupt the formation of a uniform necking filament. By considering the magnitudes of the natural time scales associated with viscous flow, elastic stress growth and inertial oscillations it is possible to construct an “operability diagram” characterizing successful operation of a capillary break-up extensional rheometer. For Newtonian fluids, viscosities greater than approximately 70 mPa.s are required; however for dilute solutions of high molecular weight polymer the minimum viscosity is substantially lower due to the additional elastic stresses arising from molecular extension. For PEO of molecular weight 106 g/mol, it is possible to measure relaxation times of order 1 ms in dilute polymer solutions of viscosity 2 – 10 mPa.s., NSF

Thermal convection in binary fluids with Oldroyd viscoelastic properties is studied close to threshold. The role of retardation time, characteristic of viscoelastic fluids, and Soret effect, characteristic of binary fluids, is emphasized. A weakly nonlinear analysis is performed for stationary convection. The coefficients of the corresponding amplitude equation are determined analytically and the nature of the bifurcation is discussed. © 2003 Elsevier B.V. All rights reserved., SCOPUS: cp.j, info:eu-repo/semantics/published

The threshold for oscillatory convection in Rayleigh-Benard experiments with viscoelastic binary fluids is explicitly determined as a function of separation ratio and rheological parameters. In particular, it is shown that the critical oscillation frequency may differ by several orders of magnitude on varying separation ratio and Deborah number. The results suggest that binary fluid aspects may not be discarded when studying thermal convection in polymeric solutions. (C) 2000 Elsevier Science B.V. All rights reserved., SCOPUS: ar.j, info:eu-repo/semantics/published

We use the finitely extensible non-linear elastic dumbbell theory developed by Chilcott and Rallison in order to analyze the flow of an inorganic Boger fluid around a sphere for two ratios of the cylinder to sphere radii. We use four different sets of material parameters for describing the same viscometric behavior of the fluid; we find important differences in the drag dependence upon the Weissenberg number. In particular, we examine the generation of recirculating vortices in the wake of the sphere. We also calculate the effect of the second normal stress difference upon the drag.

The spectral element method is analyzed for the simulation of viscoelastic fluid flows in plane and cylindrical geometries. The implementation of the method and choices of functional spaces is discussed in detail. The method is then applied to the case of the Maxwell-B fluid and compared with the so-called 4 x 4 SUPG finite element method on the perturbed channel flow problem. Analysis of the rate of convergence of the method is provided as well as thorough discussion on the decomposition of the geometry in spectral elements: the choice is between the h- vs. p-refinement. A comparison with other numerical (e.g. pseudo-spectral) methods is performed for the flow of the Maxwell-B fluid in a wavy tube. In each test case, very good agreement is found.

Effects of the stress overshoot on the transmitting force generated in some cases of squeezing flows are analytically investigated using a constitutive equation. A nondimensional factor (O_s) that corresponds to the magnitude of the stress overshoot, and an apparent relaxation time (λ_O) are defined by extending the definition in the simple shear flow. The magnitude and the peak time of overshoot in the force induced in normal or reverse squeezing flow are respectively correlated with the O_s and λ_O. In the case of successive squeezing flows consisting of normal squeezing in the first half and reverse squeezing in the second half and vice versa, the force generated in the second squeezing is only dependent on the normalized time t/λ_O. When the two surfaces are subjected to sinusoidal oscillations in the region of high Deborah number, the amplitude of the force and phase lag become large with an increase in O_s, and separating forces time-averaged during the sinusoidal oscillation decrease with an increase in O_s, especially in the extreme case, the two surfaces being forced to approach with sinusoidal oscillation.