Abstracts from IUTAM Istanbul

Abstracts from IUTAM Istanbul

Fiber Suspension Flow in a Converging Channel
Cyrus K. Aidun
Georgia Institute of Technology

Mehran Parsheh
Univ. of Minnesota

Full text: Not available
Last modified: February 28, 2007
Presentation date: 06/12/2007 9:50 AM in ITU Macka Conference Room
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Abstract
We have investigated the competing influence of extensional strain and turbulence on the development of the orientation distribution of a dilute density matched suspension of stiff fibers in a planar contraction. Application of a Fokker-Planck type equation is shown to accurately model the development of orientation anisotropy. Nearly isotropic homogenous turbulence with uniform mean velocity profile is introduced at the contraction inlet. Measurement of the downstream fiber orientation distribution shows that the rotational diffusion coefficient decays exponentially with local contraction ratio, C, and is dependent on inlet turbulent characteristics. However, the effect of turbulent energy production in the contraction is observed to be negligible. This is attributed to large streamwise rate of strain at C > 2, which offsets the effect of turbulence, and small production of turbulent energy at C < 2 where turbulence closely follows the decay of grid generated turbulence in a rectangular channel. The development of the orientation distribution function implies a rather weak dependence on the channel Reynolds number. Furthermore, the results show that the influence of turbulence on fiber rotation is negligible when the rotational Peclet number, , which is a measure of the relative influence of the mean gradient component and the rotational diffusion, is less than 10.
I will also discuss the new methods developed in my group for direct numerical simulation of deformable fibers and particles in shear flow.

Point-particle simulations of shear-induced self-diffusion in a wall-bounded dilute suspension
Evgeny S. Asmolov
Central Aero-Hydrodynamic Institute, Moscow

Full text: Not available
Last modified: March 12, 2007
Presentation date: 06/14/2007 11:10 AM in ITU Macka Conference Room
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Abstract
Non-Brownian particles migrate randomly across the streamlines of a carrier flow in sheared suspensions at small Reynolds numbers (Eckstein et al. 1977). A random motion is due to particle hydrodynamic interactions and is characterized by the coefficient of shear-induced self-diffusion which is linear in particle volume fraction in a dilute suspension. The phenomenon was explained by irreversibilities of the two-particle interactions of rough spheres (Da Cunha & Hinch 1996), or by the asymmetry of three-particle interactions (Wang et al. 1996). However, the theories do not predict satisfactorily shear-induced diffusive behavior in the dilute limit. The three-particle mechanism requires the diffusivity to be proportional to volume fraction square. The rough-sphere model gives correct linear dependence, but the values of the diffusivity is an order of magnitude less than the experimental ones (Zarraga & Leighton 2002).
In the present work the mechanism of the self-diffusion is studied due to far-field collective hydrodynamic interactions in a wall-bounded Couette cell. The motion of identical particles distributed randomly in a 3D rectangular cell is simulated. Large-scale density fluctuations induce fluid velocity disturbances with the lengthscale compared to the separation between the walls. The flow disturbances are due to particles freely rotating in a shear flow which can be approximated at large distances by symmetric force dipoles. The boundary conditions are the no-slip conditions on the cell walls, and the periodic boundary conditions in the directions of the undisturbed velocity and vorticity. The particle velocities in a dilute suspension are assumed to be the sum of the undisturbed velocity and the large-scale disturbances. The numerical approach neglecting short-range interactions appears suitable for large dilute systems. The similar model has been used recently (Asmolov 2007) for sedimenting particles when the large-scale fluid flow is due to point forces.
The mean-square displacement curves are quadratic in time initially and show the linear behavior corresponding to the diffusive regime after a time of order unity. The self-diffusivity is determined as the time rate of the change of half the displacements. It is evaluated as a function of initial particle position. The calculated diffusivity is linear in the shear rate and volume fraction. The diffusivity is less for particles close to the walls as the large-scale disturbances are zero on the walls. The theoretical value for particles in the median slice is close to the experimental one (Zarraga & Leighton 2002).

Hydrodynamic interaction of two capsules in simple shear flow
Dominique Barthes-Biesel
Universite de Technologie de Compiegne

Etienne Lac
Schlumberger Cambridge Research

Full text: Not available
Last modified: February 7, 2007
Presentation date: 06/12/2007 10:50 AM in ITU Macka Conference Room
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Abstract
We present a numerical model of the hydrodynamic interactions between two identical capsules freely suspended in a simple shear flow (SSF). Each capsule consists of a liquid droplet enclosed by a thin hyper-elastic membrane. The particle Reynolds number is small and a boundary integral formulation is used to represent the fluid motion of the internal and suspending liquids. The particle motion and deformation result from the interaction of viscous deforming stresses balanced by elastic tensions in the membrane. The latter may undergo large shape changes, thus making the problem non-linear. Monitoring the stress level in the membrane is important to predict burst.
Two capsules suspended in SSF usually have different velocities and thus eventually overlap and pass each other. The flow is such that the capsules are not subjected to stress levels leading to burst when they are far apart. However, during crossing, the membranes are submitted to extra strains and stresses that may lead to unexpected break-up. Pair interactions also cause an irreversible cross-flow trajectory shift, indicating a self-diffusion effect in dilute suspensions of capsules.

An experimental study of the regimes of motion of spheres falling or ascending freely in a Newtonian fluid
Arie Biesheuvel
University of Twente

*Christian Veldhuis
University of Twente

Full text: Not available
Last modified: January 5, 2007
Presentation date: 06/14/2007 12:10 PM in ITU Macka Conference Room
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Abstract
This paper presents the results of an experimental investigation aimed at verifying some of the interesting conclusions of the numerical study by Jenny, Dušek & Bouchet (J. Fluid Mech. 508 (2004), 201) concerning the instability and the transition of the motion of solid spheres falling or ascending freely in a Newtonian fluid. The phenomenon is governed by two dimensionless parameters: the Galileo number G, and the ratio of the density of the spheres to that of the surrounding fluid rho_s/rho. Jenny et al. showed that the (G, rho_s/rho) parameter space may be divided into regions with distinct features of the trajectories followed eventually by the spheres after their release from rest. The characteristics of these `regimes of motion' as described by Jenny et al. agree well with what was observed in our experiments. However, flow visualizations of the wakes of the spheres using a Schlieren optics technique, raise doubts about another conclusion of Jenny et al., namely the absence of a bifid wake structure.

Shear Rheology and Aging of Soft Particle Pastes
Roger T. Bonnecaze
The University of Texas at Austin

Jyoti Seth
The University of Texas at Austin

Michel Cloitre
ESPCI, Paris

Full text: Not available
Last modified: March 1, 2007
Presentation date: 06/11/2007 10:20 AM in ITU Macka Conference Room
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Abstract
Soft particle pastes (SPPs) are composed of deformable particles randomly packed into a dense suspension. Examples of the constituent m), polyelectrolytesoft particles include polymer coated colloids (R~100 microgels (R~200 nm) and star polymers (R~10 nm). In spite of the differences in the particle sizes and source of elasticity, all of these materials show similar rheological characteristics and aging behavior. That is, their microstructure and rheological properties change slowly with time without apparent end.
Here we present a numerical simulation of these SPPs to understand the fundamental nature of the observed bulk rheology and aging. The methodology is founded on a Stokesian-dynamics like simulation of the centers of the particles coupled with an additional dynamical equation of the elasto-hydrodynamic contacts between particles. An approximate method is presented for computing the EHD contacts that is accurate and extremely efficient for the simulation of tens of thousands of particles. Predictions of the viscosity and normal stress differences for sheared SPPs of varying concentrations are presented for a variety of packing fractions for mono- and polydisperse soft spheres. The predicted shear viscosity closely matches experimental observations. Further, the normal stress differences observed in the simulation can be used to explain the instabilities observed in coating operations of SPPs.
A pairwise interaction theory is also developed that compares well to the numerical and experimental measurements. As part of this theory we present a methodology to compute a priori the radial distribution function for the quiescent or “equilibrium” glassy SPPs based on free volume considerations and energetic constraints.
Finally, the simulation is used to probe the aging of an initially sheared suspension. It is found that the recovery of the particles after cessation of the flow follows a ballistic path with a power law distribution of recovery times. This observation is seen experimentally in diffuse-wave spectroscopy measurements. The microstructural recovery process is qualitatively related to the aging of the macroscopic properties.

Simulation of Suspensions and Granular Media: Wet vs. Dry
John Brady
California Institute of Technology

Full text: Not available
Last modified: March 1, 2007
Presentation date: 06/12/2007 9:00 AM in ITU Macka Conference Room
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Abstract
Computer simulation of multiphase flows has grown dramatically in the last two decades. Problems as diverse as the Brownian motion of small colloidal particles, the rheology of dense suspensions and emulsions, and the dynamics of bubbly liquids have been addresses by dynamic simulation. This talk will trace the development of the methodologies used to simulate multiphase, particulate systems. A central feature in these systems is properly accounting for the hydrodynamic interactions among particles. Hydrodynamic forces are non-conservative, non-central, couple translational and rotational motion, have distinctive far- and near-field behaviors and are many-body. The incorporation of hydrodynamic interactions has been accomplished rigorously and successfully in the case of small particle Reynolds numbers and in the limit of inviscid flow at high Reynolds numbers. Examples showing how hydrodynamic forces influence suspension structure and determine macroscopic behavior will be given. Of particular interest will be recent work aimed at exploring the similarities and differences between the behavior of low-Reynolds number viscous suspensions and rapid granular flows at high Reynolds numbers: wet vs. dry.

Rheology and structure of concentrated suspensions of non-colloidal particles subject to oscillatory shear
Jason Butler
University of Florida

*Jonathan Bricker
University of Florida

Full text: Not available
Last modified: February 20, 2007
Presentation date: 06/11/2007 11:00 AM in ITU Macka Conference Room
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Abstract
Non-colloidal suspensions undergoing unsteady shear flows demonstrate
unique behaviors not observed under steady flow conditions. To
understand the dynamics of suspensions in unsteady shear flow, the
rheological behavior under oscillatory flow was examined using
experiments and simulations.
Experiments were performed to evaluate the complex viscosity as a
function of the total strain. The complex viscosity decreases with
total strain for high strain amplitudes and increases for low strain
amplitudes. The total strain necessary to attain steady conditions
greatly increases as the amplitude decreases and the minimum in the
complex viscosity occurs at a strain amplitude of one. The results
were independent of the shear cell geometry, suggesting that
shear-induced particle migration is unimportant and that the observed
behavior results from changes in the suspension microstructure.
Simulations of the oscillating suspensions predict the changes in
rheology that occur over large total strains and reproduce a
non-monotonic relationship between strain amplitude and the steady
value of the complex viscosity. As in the experiments, the minimum in
complex viscosity occurs at the strain amplitude of one. The
simulations provide insight into the underlying microstructure that
generates the macroscopic rheology observed in the experiments. Three
distinct phases were identified, with hydroclusters dominating the
large amplitude results and ordered microstructures appearing at
intermediate and low strain amplitudes.

Shock-bubble interaction near a rigid surface
Michael L. Calvisi
School of Mathematics, The University of Birmingham

*Jonathan I. Iloreta
Department of Mechanical Engineering, University of California at Berkeley

*John R. Blake
School of Mathematics, The University of Birmingham

*Andrew J. Szeri
Department of Mechanical Engineering, University of California at Berkeley

Full text: Not available
Last modified: February 23, 2007
Presentation date: 06/13/2007 10:50 AM in ITU Macka Conference Room
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Abstract
In this talk, we present the results of numerical simulations of the nonspherical collapse of bubbles excited by shock waves near a rigid boundary. The waves we consider are representative of those developed by shock wave lithotripsy or shock wave therapy devices. The rigid boundaries we consider are representative of kidney stones and reflective bony tissue. The presence of the boundary causes constructive interference between reflected and incident waves that enhances the expansion and subsequent collapse of bubbles located in a region near the boundary. Quantities such as kinetic energy, Kelvin impulse, and centroid translation are calculated in order to illuminate the physics of the collapse. The main finding is that the dynamics of the bubble collapse depend strongly on the distance of the bubble relative to the wall when reflection is taken into account but much less so when reflection is omitted. The work done by the shock wave on the bubble is shown to predict strongly the maximum bubble volume regardless of the standoff distance and the presence or absence of reflection; furthermore, with appropriate interpretation, these predictions match almost exactly those of a spherical bubble model.

The Role of Particle Shape on Particle-Phase Stress
Jennifer Sinclair Curtis
ChE Dept., University of Florida

Benjamin James
ChE Dept., University of Florida

Full text: Not available
Last modified: March 1, 2007
Presentation date: 06/11/2007 5:00 PM in ITU Macka Conference Room
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Abstract
Virtually all solid handling operations involve particles that are non-spherical in shape. However, most fundamental studies of particulates undertaken to date have involved spherical particles. Hence, there is a current significant disconnect between the model particles which are used in fundamental research studies and the characteristics of real particles dealt with in industry. While industrial practitioners comprehend fully that the influence of particle shape on particle flow behavior is significant, the role of particle shape is not understood.
This presentation will discuss the results of DEM simulations of non-spherical particles in planar shear flow. Particles with large aspect ratios and rough particles are simulated. A few of the key conclusions are:
• Increases in the particle elongation ratio decreases the kinetic contribution to the particle-phase stress and significantly increases the collisional contribution to the particle-phase stress
• There exists a finite maximum collisional stress at a specific degree of particle roughness
• Increasing the particle friction and decreasing the particle elasticity increases the collisional stress for elongated particles due to an increase in low velocity collisions
In addition, we show a relationship between the kinetic stress and the projected particle length which is valid for all particle shapes. This relationship forms the basis for a constitutive model for particle-phase stress that can be employed in two-fluid CFD simulations.

Direct Numerical Simulation of Emulsion Flow Through Porous Media
Robert H. Davis
University of Colorado

Alexander Zinchenko
Department of Chemical & Biological Engineering, University of Colorado

Full text: Not available
Last modified: February 26, 2007
Presentation date: 06/12/2007 11:10 AM in ITU Macka Conference Room
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Abstract
The flow of an emulsion containing drops or bubbles through a porous medium has practical applications in biology, engineering and geology. Of fundamental importance are the relationships between pressure drop and flow rates of the dispersed and continuous phases, and the conditions where the drops or bubbles become trapped within the porous medium. These issues are particularly challenging when the drops or bubbles have sizes close to or larger than the pores, in which case an effective-medium approach fails.
This talk presents direct numerical simulations of emulsion flows through granular media at low Reynolds number, using boundary-integral methods. A model problem is considered first, where a single drop squeezes between two or more solid obstacles. It is shown that the drop becomes trapped when the capillary number (representing the ratio of viscous and interfacial forces) is below a critical value and the drop is not able to deform sufficiently to pass through the constriction. Subsequently, an efficient, multipole-accelerated algorithm was used for dynamical simulations of many nonwetting deformable drops squeezing through a granular medium comprised of randomly distributed fixed spheres in a periodic box. Simulations are made with typically 50-100 drops and 50-100 particles in a periodic box. A large number of typically 5000 or more boundary elements per surface is needed, because of the lubrication sensitivity of drop-solid interactions. Surprisingly, away from the critical condition, the dispersed phase has higher average velocity than the continuous phase. This result is due to steric exclusion of the drops from the slow-moving streamlines near the solid surfaces. Near the critical condition, however, the average velocity of the dispersed phase is reduced as the drops become trapped (or nearly so) in the narrow spaces between the particles comprising the granular medium.

Cylindrical Bubble Dynamics : Exact and DNS Results
Can F Delale
IstanbulTechnicalUniversity