POF Nameplate
  • Volume/Page
  • Keyword
  • DOI
  • Citation
  • Advanced
   
 
 
 

Flickr Twitter iResearch App Facebook

Search Issue | RSS Feeds RSS
Next Issue

Jan 2011

Volume 23, Issue 1, Articles (01xxxx)

Issue Cover Spotlight Figure

Phys. Fluids 23, 011702 (2011); http://dx.doi.org/10.1063/1.3541844 (4 pages)

Hongjie Zhong, Shiyi Chen, and Cunbiao Lee
Enlarge Image | Read More
Page 1 of 3 Pages Return to All Sections Next Page
back to top
RSS Feeds
back to top Biofluid Mechanics

Instability regimes in flowing suspensions of swimming micro-organisms

Amir Alizadeh Pahlavan and David Saintillan

Phys. Fluids 23, 011901 (2011); http://dx.doi.org/10.1063/1.3529411 (18 pages) | Cited 6 times

Online Publication Date: 6 January 2011

Full Text: Read Online (HTML) | Download PDF

multimedia

Show Abstract
The effects of an external shear flow on the dynamics and pattern formation in a dilute suspension of swimming micro-organisms are investigated using a linear stability analysis and three-dimensional numerical simulations, based on the kinetic model previously developed by [ D. Saintillan and M. J. Shelley, Phys. Fluids 20, 123304 (2008) ]. The external shear flow is found to damp the instabilities that occur in these suspensions by controlling the orientation of the particles. We demonstrate in our simulations that the rate of damping is direction-dependent: it is fastest in the flow direction, but slowest in the direction perpendicular to the shear plane. As a result, transitions from three- to two- to one-dimensional instabilities are observed to occur as shear rate increases, and above a certain shear rate the instabilities altogether disappear. The density patterns and complex flows that arise at long time in the suspensions are also analyzed from the numerical simulations using standard techniques from the literature on turbulent flows. The imposed shear flow is found to have an effect on both density patterns and flow structures, which typically align with the extensional axis of the external flow. The disturbance flows in the simulations are shown to exhibit similarities with turbulent flows, and in particular two of the seemingly universal characteristics of turbulent flows also occur, namely: (i) the bias of Q-R plots toward the second and fourth quadrants, corresponding to stable focus/stretching and unstable node/saddle/saddle flow topologies, respectively, and (ii) the alignment of the vorticity vector with the intermediate strain-rate eigenvector. However, the flows described herein also significantly differ from turbulent flows owing to the strong predominance of large scales, as exemplified by the very rapid decay of the kinetic energy spectrum, an effect further enhanced after the transitions to two- and one-dimensional instabilities.
Show PACS
47.20.Ft Instability of shear flows (e.g., Kelvin-Helmholtz)
47.27.-i Turbulent flows
47.32.cd Vortex stability and breakdown
47.57.E- Suspensions
47.63.Gd Swimming microorganisms
47.11.-j Computational methods in fluid dynamics

Passive hydrodynamic synchronization of two-dimensional swimming cells

Gwynn J. Elfring and Eric Lauga

Phys. Fluids 23, 011902 (2011); http://dx.doi.org/10.1063/1.3532954 (19 pages) | Cited 6 times

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Spermatozoa flagella are known to synchronize when swimming in close proximity. We use a model consisting of two-dimensional sheets propagating transverse waves of displacement to demonstrate that fluid forces lead to such synchronization passively. Using two distinct asymptotic descriptions (small amplitude and long wavelength), we derive the synchronizing dynamics analytically for arbitrarily shaped waveforms in Newtonian fluids, and show that phase-locking will always occur for sufficiently asymmetric shapes. We characterize the effect of the geometry of the waveforms and the separation between the swimmers on the synchronizing dynamics, the final stable conformations, and the energy dissipated by the cells. For two closely swimming cells, synchronization always occurs at the in-phase or opposite-phase conformation, depending solely on the geometry of the cells. In contrast, the work done by the swimmers is always minimized at the in-phase conformation. As the swimmers get further apart, additional fixed points arise at intermediate values of the relative phase. In addition, computations for large amplitude waves using the boundary integral method reveal that the two asymptotic limits capture all the relevant physics of the problem. Our results provide a theoretical framework to address other hydrodynamic interactions phenomena relevant to populations of self-propelled organisms.
Show PACS
87.17.Jj Cell locomotion, chemotaxis
87.17.Rt Cell adhesion and cell mechanics
47.63.Gd Swimming microorganisms
back to top Micro- and Nanofluid Mechanics
FREE

Modeling drag reduction and meniscus stability of superhydrophobic surfaces comprised of random roughness

Mohamed A. Samaha, Hooman Vahedi Tafreshi, and Mohamed Gad-el-Hak

Phys. Fluids 23, 012001 (2011); http://dx.doi.org/10.1063/1.3537833 (8 pages) | Cited 15 times

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Previous studies dedicated to modeling drag reduction and stability of the air-water interface on superhydrophobic surfaces were conducted for microfabricated coatings produced by placing hydrophobic microposts/microridges arranged on a flat surface in aligned or staggered configurations. In this paper, we model the performance of superhydrophobic surfaces comprised of randomly distributed roughness (e.g., particles or microposts) that resembles natural superhydrophobic surfaces, or those produced via random deposition of hydrophobic particles. Such fabrication method is far less expensive than microfabrication, making the technology more practical for large submerged bodies such as submarines and ships. The present numerical simulations are aimed at improving our understanding of the drag reduction effect and the stability of the air-water interface in terms of the microstructure parameters. For comparison and validation, we have also simulated the flow over superhydrophobic surfaces made up of aligned or staggered microposts for channel flows as well as streamwise or spanwise ridges configurations for pipe flows. The present results are compared with theoretical and experimental studies reported in the literature. In particular, our simulation results are compared with work of Sbragaglia and Prosperetti, and good agreement has been observed for gas fractions up to about 0.9. The numerical simulations indicate that the random distribution of surface roughness has a favorable effect on drag reduction, as long as the gas fraction is kept the same. This effect peaks at about 30% as the gas fraction increases to 0.98. The stability of the meniscus, however, is strongly influenced by the average spacing between the roughness peaks, which needs to be carefully examined before a surface can be recommended for fabrication. It was found that at a given maximum allowable pressure, surfaces with random post distribution produce less drag reduction than those made up of staggered posts.
Show PACS
47.20.-k Flow instabilities
02.60.-x Numerical approximation and analysis
47.61.Jd Multiphase flows
47.11.-j Computational methods in fluid dynamics
68.35.B- Structure of clean surfaces (and surface reconstruction)

A full analytical solution for the force-driven compressible Poiseuille gas flow based on a nonlinear coupled constitutive relation

R. S. Myong

Phys. Fluids 23, 012002 (2011); http://dx.doi.org/10.1063/1.3540671 (21 pages) | Cited 3 times

Online Publication Date: 24 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The compressible Poiseuille gas flow driven by a uniform force is analytically investigated using a phenomenological nonlinear coupled constitutive relation model. A new fully analytical solution in compact tangent (or hyperbolic tangent in the case of diatomic gases) functional form explains the origin behind the central temperature minimum and a heat transfer from the cold region to the hot region. The solution is not only proven to satisfy the conservation laws exactly but also well-defined for all physical conditions (the Knudsen number and a force-related dimensionless parameter). It is also shown that the non-Fourier law associated with the coupling of force and viscous shear stress in the constitutive relation is responsible for the existence of the central temperature minimum, while a kinematic constraint on viscous shear and normal stresses identified in the velocity shear flow is the main source of the nonuniform pressure distribution. In addition, the convex pressure profile with a maximum at the center is theoretically predicted for diatomic gases. Finally, the existence of the Knudsen minimum in the mass flow rate is demonstrated by developing an exact analytical formula for the average temperature of the bulk flow.
Show PACS
47.45.-n Rarefied gas dynamics
47.60.Dx Flows in ducts and channels
47.40.-x Compressible flows; shock waves

Two-cell circulation in a liquid meniscus driven by a swirling gas jet

Miguel A. Herrada, Vladimir N. Shtern, and José María López-Herrera

Phys. Fluids 23, 012003 (2011); http://dx.doi.org/10.1063/1.3548858 (10 pages)

Online Publication Date: 25 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A liquid issuing from a capillary needle adopts a cone-jet structure if the liquid is further driven by a coflowing gas jet. In the present work, flow patterns appearing in this cone-jet structure are studied by the volume-of-fluid numerical method. Axisymmetric motions of the liquid and gas, both treated as viscous incompressible fluids, are simulated. As the gas/liquid mass ratio increases, the meridional circulation develops in the meniscus region of the liquid flow. As the ratio exceeds a threshold, the flow becomes time periodic and droplet generating. Swirl, added in the gas jet, affects the liquid flow in two ways. First, the threshold value increases with swirl. Second, the circulation region transforms from the bubble-like into ring-like pattern and then becomes two-cellular. As swirl further increases, the cells separate, one cell disappears, and a new cell emerges being attached to the needle wall. The predicted metamorphoses of the flow topology might be important for atomization of a liquid fuel.
Show PACS
47.32.Ef Rotating and swirling flows
47.60.Kz Flows and jets through nozzles
47.27.wg Turbulent jets
66.20.Cy Theory and modeling of viscosity and rheological properties, including computer simulation
47.55.nb Capillary and thermocapillary flows
47.55.db Drop and bubble formation

Terraced spreading of nanofilms under a nonmonotonic disjoining pressure

Kerry A. Landman and Lee R. White

Phys. Fluids 23, 012004 (2011); http://dx.doi.org/10.1063/1.3541968 (12 pages) | Cited 1 time

Online Publication Date: 26 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A thin ( ∼ nanometer) film of a viscous, essentially nonvolatile liquid spreads over a substrate controlled by the disjoining pressure Π(h) exerted by the two interfaces on one another. Such films are commonly used as hard disk lubricants in the magnetic recording industry. Macroscopic nonuniformities in the film caused by a perturbation of the uniformly spread state flow away and the film is “healed” in a time frame governed by the appropriate hydrodynamic equations. Lubrication theory may be used to derive a diffusion equation for the local film height h(x,t) as a function of position and time which shows that an effective height-dependent diffusion coefficient D(h) = −[h3/3μBξ(h)][dΠ(h)/dh] controls the spreading dynamics, where μB is the bulk liquid viscosity and ξ(h) is a function accounting for any variation of local viscosity near the substrate due to molecularity of the liquid. Such an approach is possible due to the very small ratio of the film height to the in-plane length scale of the disturbance. Provided the disjoining pressure is positive and monotonically decreasing with film thickness, the motion of the film is unexceptional, exhibiting the usual smooth profiles associated with diffusive flow with time. However, for nonmonotonic disjoining pressures, the film is experimentally observed to exhibit vertical terraces. These abrupt jumps in height do not disappear with time and they move slowly or are stationary. This phenomenon is investigated here. We demonstrate how a physically consistent “weak” solution of the diffusion equation can be constructed, where only positive values of the diffusion coefficient are sampled. The film heights at the jump discontinuity are determined by an equal area rule for the disjoining pressure. Numerical simulations for a realistic nonmonotonic disjoining pressure exhibit finite termination on the low-side and vertical terraces, thereby matching the behavior observed in experimental systems.
Show PACS
68.15.+e Liquid thin films
47.55.nd Spreading films
47.11.-j Computational methods in fluid dynamics
47.85.mf Lubrication flows
02.60.-x Numerical approximation and analysis
back to top Interfacial Flows

Numerical simulation of a liquid bridge in a coaxial gas flow

Miguel A. Herrada, José M. López-Herrera, Emilio J. Vega, and José M. Montanero

Phys. Fluids 23, 012101 (2011); http://dx.doi.org/10.1063/1.3534076 (11 pages) | Cited 7 times

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The dynamical response of an isothermal liquid bridge to a coaxial gas stream is examined from axisymmetric numerical simulations of the Navier–Stokes equations. The simulation method is previously validated by calculating the temporal evolution of the first oscillation mode in both cylindrical and axisymmetric liquid bridges. The comparison with other theoretical approaches and experiments shows good agreement in most cases, although significant discrepancies are found between the simulation and the experimental values of the damping rate for hexadecane. The simulation of a liquid bridge in a coaxial gas stream shows that a recirculation cell always appears in the liquid driven by the gas viscous stress on the free surface. The recirculation cell speed depends quasilinearly on the gas velocity for the range of gas flow rates considered. If the gas stream and gravity have the same direction, then the speed of the recirculation cell increases considerably due to the free surface deformation of the liquid bridge at equilibrium. This effect does not occur when gravity has the opposite direction because viscous dissipation in the liquid increases in this case. If the gas stream and gravity point downward, the liquid bridge shrinks at the upper part and bulges at the lower owing to the accumulation of momentum there. The same occurs for zero gravity, but noncylindrical liquid bridges deform more than cylindrical shapes with the same slenderness. If one inverts the direction of the gravity force, the interface deformation caused by the gas stream is the opposite, and its magnitude is smaller. The magnitude of the free surface deformation depends almost linearly on the gas stream velocity for both zero and normal gravity conditions.
Show PACS
47.10.ad Navier-Stokes equations
47.55.nb Capillary and thermocapillary flows
02.60.Lj Ordinary and partial differential equations; boundary value problems

Parametric stability and dynamic buckling of an encapsulated microbubble subject to acoustic disturbances

Kostas Tsiglifis and Nikos A. Pelekasis

Phys. Fluids 23, 012102 (2011); http://dx.doi.org/10.1063/1.3536646 (28 pages) | Cited 4 times

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Stability analysis of the radial pulsations of a gas microbubble that is encapsulated by a thin viscoelastic shell and surrounded by an ideal incompressible liquid is carried out. Small axisymmetric disturbances in the microbubble shape are imposed and their long and short term stability is examined depending on the initial bubble radius, the shell properties, and the parameters, i.e., frequency and amplitude, of the external acoustic excitation. Owing to the anisotropy of the membrane that is forming the encapsulating shell, two different types of elastic energy are accounted for, namely, the membrane and bending energy per unit of initial area. They are used to describe the tensions that develop on the shell due to shell stretching and bending, respectively. In addition, two different constitutive laws are used in order to relate the tensions that develop on the membrane as a result of stretching, i.e., the Mooney–Rivlin law describing materials that soften as deformation increases and the Skalak law describing materials that harden as deformation increases. The limit for static buckling is obtained when the external overpressure exerted upon the membrane surpasses a critical value that depends on the membrane bending resistance. The stability equations describing the evolution of axisymmetric disturbances, in the presence of an external acoustic field, reveal that static buckling becomes relevant when the forcing frequency is much smaller than the resonance frequency of the microbubble, corresponding to the case of slow compression. The resonance frequencies for shape oscillations of the microbubble are also obtained as a function of the shell parameters. Floquet analysis shows that parametric instability, similar to the case of an oscillating free bubble, is possible for the case of a pulsating encapsulated microbubble leading to shape oscillations as a result of subharmonic or harmonic resonance. These effects take place for acoustic amplitude values that lie above a certain threshold but below those required for static buckling to occur. They are quite useful in providing estimates for the shell elasticity and bending resistance based on a frequency/amplitude sweep that monitors the onset of shape oscillations when the forcing frequency resonates with the radial pulsation, ωf = ω0, or with a certain shape mode, ωf = 2ωn. An acceleration based instability, identified herein as dynamic buckling, is observed during the compression phase of the pulsation, evolving over a small number of periods of the forcing, when the amplitude of the acoustic excitation is further increased. It corresponds to the Rayleigh–Taylor instability observed for free bubbles, and has been observed with contrast agents as well, e.g., BR-14. Finally, phase diagrams for contrast agent BR-14 are constructed and juxtaposed with available experimental data, illustrating the relevance and range of the above instabilities.
Show PACS
47.55.dd Bubble dynamics
47.20.Ma Interfacial instabilities (e.g., Rayleigh-Taylor)
47.50.Gj Instabilities
83.60.Df Nonlinear viscoelasticity
83.50.-v Deformation and flow

Nonlinear development of oscillatory instability in a three-layer system under the joint action of buoyancy and thermocapillary effect

Ilya B. Simanovskii, Antonio Viviani, Frank Dubois, and Jean-Claude Legros

Phys. Fluids 23, 012103 (2011); http://dx.doi.org/10.1063/1.3536655 (12 pages) | Cited 3 times

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The nonlinear development of oscillatory instability under the joint action of buoyant and thermocapillary effects in multilayer system is investigated. The nonlinear convective regimes are studied by the finite difference method. The calculations have been performed for two-dimensional flows. The interfaces are assumed to be nondeforming. Rigid heat-insulated lateral walls are considered. Transitions between the flows with different spatial structures are studied. Specific types of nonlinear flows—symmetric and asymmetric oscillations—have been found. It is shown that the oscillatory flow takes place in an interval of Grashof number values bounded both from below by the quiescent mechanical equilibrium, and from above by a convecting steady state. Cavities with different lengths are considered.
Show PACS
47.20.Bp Buoyancy-driven instabilities (e.g., Rayleigh-Benard)
47.55.nb Capillary and thermocapillary flows
47.11.Bc Finite difference methods
02.70.Bf Finite-difference methods
47.55.Hd Stratified flows

Droplet charging regimes for ultrasonic atomization of a liquid electrolyte in an external electric field

Thomas P. Forbes, F. Levent Degertekin, and Andrei G. Fedorov

Phys. Fluids 23, 012104 (2011); http://dx.doi.org/10.1063/1.3541818 (10 pages)

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Distinct regimes of droplet charging, determined by the dominant charge transport process, are identified for an ultrasonic droplet ejector using electrohydrodynamic computational simulations, a fundamental scale analysis, and experimental measurements. The regimes of droplet charging are determined by the relative magnitudes of the dimensionless Strouhal and electric Reynolds numbers, which are a function of the process (pressure forcing), advection, and charge relaxation time scales for charge transport. Optimal (net maximum) droplet charging has been identified to exist for conditions in which the electric Reynolds number is of the order of the inverse Strouhal number, i.e., the charge relaxation time is on the order of the pressure forcing (droplet formation) time scale. The conditions necessary for optimal droplet charging have been identified as a function of the dimensionless Debye number (i.e., liquid conductivity), external electric field (magnitude and duration), and atomization drive signal (frequency and amplitude). The specific regime of droplet charging also determines the functional relationship between droplet charge and charging electric field strength. The commonly expected linear relationship between droplet charge and external electric field strength is only found when either the inverse of the Strouhal number is less than the electric Reynolds number, i.e., the charge relaxation is slower than both the advection and external pressure forcing, or in the electrostatic limit, i.e., when charge relaxation is much faster than all other processes. The analysis provides a basic understanding of the dominant physics of droplet charging with implications to many important applications, such as electrospray mass spectrometry, ink jet printing, and drop-on-demand manufacturing.
Show PACS
47.55.db Drop and bubble formation
47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.11.-j Computational methods in fluid dynamics
47.80.-v Instrumentation and measurement methods in fluid dynamics

Analysis of time-dependent nonlinear dynamics of the axisymmetric liquid film on a vertical circular cylinder: Energy integral model

E. Novbari and A. Oron

Phys. Fluids 23, 012105 (2011); http://dx.doi.org/10.1063/1.3541856 (12 pages) | Cited 2 times

Online Publication Date: 14 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The nonlinear dynamics of an axisymmetric liquid film falling on the outer surface of a vertical cylinder is investigated in the framework of the set of two coupled evolution equations derived recently using the energy integral method (EIM). This set of EIM evolution equations is solved numerically and its solutions are compared with the traveling wave solutions derived from it using AUTO. We find that traveling wave solutions of EIM equations can bifurcate either supercritically or subcritically from the base state. The type of bifurcation depends on the parameter set of the problem. The set of EIM equations studied here admits both traveling wave and nonstationary wave flows. We demonstrate that in the case of subcritical primary bifurcation the film dynamics is sensitive to the choice of the initial condition and coexistence of up to five different flows is possible for the same parameter set in the domain of a given periodicity. The case of supercritical primary bifurcation exhibits much lesser dependence on the initial condition, though coexistence of two different flows for the same parameter set is possible. The synergetic approach based on both direct numerical solution of the governing evolution equations and search of traveling wave solutions using AUTO facilitate a discovery of a large variety of flows and help to conclude about stability of the traveling wave flows found using AUTO.
Show PACS
68.15.+e Liquid thin films
47.35.-i Hydrodynamic waves
47.20.Ky Nonlinearity, bifurcation, and symmetry breaking
FREE

Wall energy relaxation in the Cahn–Hilliard model for moving contact lines

Pengtao Yue and James J. Feng

Phys. Fluids 23, 012106 (2011); http://dx.doi.org/10.1063/1.3541806 (8 pages) | Cited 13 times

Online Publication Date: 25 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The Cahn–Hilliard model uses diffusion between fluid components to regularize the stress singularity at a moving contact line. In addition, it represents the dynamics of the near-wall layer by the relaxation of a wall energy. The first part of the paper elucidates the role of the wall relaxation in a flowing system, with two main results. First, we show that wall energy relaxation produces a dynamic contact angle that deviates from the static one, and derive an analytical formula for the deviation. Second, we demonstrate that wall relaxation competes with Cahn–Hilliard diffusion in defining the apparent contact angle, the former tending to “rotate” the interface at the contact line while the latter to “bend” it in the bulk. Thus, varying the two in coordination may compensate each other to produce the same macroscopic solution that is insensitive to the microscopic dynamics of the contact line. The second part of the paper exploits this competition to develop a computational strategy for simulating realistic flows with microscopic slip length at a reduced cost. This consists in computing a moving contact line with a diffusion length larger than the real slip length, but using the wall relaxation to correct the solution to that corresponding to the small slip length. We derive an analytical criterion for the required amount of wall relaxation, and validate it by numerical results on dynamic wetting in capillary tubes and drop spreading.
Show PACS
47.60.Dx Flows in ducts and channels
47.32.Ef Rotating and swirling flows
47.45.Gx Slip flows and accommodation

Dynamics of nearly unstable axisymmetric liquid bridges

José M. Perales and José M. Vega

Phys. Fluids 23, 012107 (2011); http://dx.doi.org/10.1063/1.3541814 (11 pages) | Cited 1 time

Online Publication Date: 25 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The dynamics of a noncylindrical, axisymmetric, marginally unstable liquid bridge between two equal disks is analyzed in the inviscid limit. The resulting model allows for the weakly nonlinear description of both the (first stage of) breakage for unstable configurations and the (slow) dynamics for stable configurations. The analysis is made for both slender and short liquid brides. In the former range, the dynamics breaks reflection symmetry on the midplane between the supporting disks and can be described by a standard Duffing equation, while for short bridges reflection symmetry is preserved and the equation is still Duffing-like but exhibiting a quadratic nonlinearity. The asymptotic results compare well with existing experiments.
Show PACS
47.20.Cq Inviscid instability
47.60.Dx Flows in ducts and channels

The Rayleigh–Taylor instability of a surface of arbitrary cross section with pinned edges

L. E. Johns and R. Narayanan

Phys. Fluids 23, 012108 (2011); http://dx.doi.org/10.1063/1.3541819 (3 pages) | Cited 1 time

Online Publication Date: 25 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We determine the critical points of the Rayleigh–Taylor instability of a surface of arbitrary cross section having pinned edges. Often these points coincide with the diffusion eigenvalues but sometimes they do not.
Show PACS
47.20.Ma Interfacial instabilities (e.g., Rayleigh-Taylor)
02.10.Ud Linear algebra

First drop dissimilarity in drop-on-demand inkjet devices

Amin Famili, Saurabh A. Palkar, and William J. Baldy, Jr.

Phys. Fluids 23, 012109 (2011); http://dx.doi.org/10.1063/1.3543758 (6 pages) | Cited 2 times

Online Publication Date: 25 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
As inkjet printing technology is increasingly applied in a broader array of applications, careful characterization of its method of use is critical due to its inherent sensitivity. A common operational mode in inkjet technology known as drop-on-demand ejection is used as a way to deliver a controlled quantity of material to a precise location on a target. This method of operation allows ejection of individual or a sequence (burst) of drops based on a timed trigger event. This work presents an examination of sequences of drops as they are ejected, indicating a number of phenomena that must be considered when designing a drop-on-demand inkjet system. These phenomena appear to be driven by differences between the first ejected drop in a burst and those that follow it and result in a break-down of the linear relationship expected between driving amplitude and drop mass. This first drop, as quantified by high-speed videography and subsequent image analysis, can be different in morphology, trajectory, velocity, and volume from subsequent drops within a burst. These findings were confirmed orthogonally by both volume and mass measurement techniques which allowed quantitation down to single drops.
Show PACS
47.55.df Breakup and coalescence
47.80.Jk Flow visualization and imaging
06.30.Dr Mass and density
back to top Viscous and Non-Newtonian Flows

Viscoelastic drop falling through a viscous medium

Swarnajay Mukherjee and Kausik Sarkar

Phys. Fluids 23, 013101 (2011); http://dx.doi.org/10.1063/1.3533261 (8 pages) | Cited 3 times

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Deformation and sedimentation velocities of a viscoelastic drop falling through a Newtonian medium are numerically investigated using a front-tracking finite difference method. In contrast to a viscous drop, viscoelasticity deforms an initially spherical drop into an oblate shape and decreases its sedimentation velocity. Further increase of elasticity results in a dimple at the rear end, as the viscoelastic stress at the trailing end of the drop pulls the drop interface inward. The dimple becomes more prominent with increasing Deborah number, amount of polymeric viscosity, and capillary number. An approximate analysis is performed to model the stress development along the axis of symmetry, specifically its increase at the rear end that governs the dimple formation. For even higher values of Deborah number, the interfacial tension cannot balance the viscoelastic stresses leading to an unstable situation toward a toroidal shape. We numerically find the critical Deborah number for the transition. It shows an approximate inverse scaling with capillary number. For unstable cases, downward progressing dimple develops a globular end. Development of the globular end results in a sudden increase in the cross-sectional area of the drop and a sharp decrease of the settling velocity.
Show PACS
47.50.-d Non-Newtonian fluid flows
47.55.D- Drops and bubbles
47.55.nb Capillary and thermocapillary flows
47.57.Ng Polymers and polymer solutions
47.57.Qk Rheological aspects
47.11.Bc Finite difference methods

Linear instability of the horizontal throughflow in a plane porous layer saturated by a power-law fluid

A. Barletta and D. A. Nield

Phys. Fluids 23, 013102 (2011); http://dx.doi.org/10.1063/1.3532805 (7 pages) | Cited 6 times

Online Publication Date: 14 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The onset of the convective instability in the horizontal throughflow of a power-law fluid saturating a horizontal porous layer heated from below is studied. A linear stability analysis of the basic flow is carried out and the disturbance equations are solved analytically. The problem examined here is an extension of the classical Prats problem for Newtonian fluids. It is shown that the marginal stability condition, as well as the critical values of the wave number and of the Darcy–Rayleigh number, is affected by the value of the Péclet number associated with the basic flow, except for the special case of a Newtonian fluid. The limit of a vanishingly small Péclet number is considered leading to the special case of the Horton–Rogers–Lapwood (HRL) problem for a power-law fluid, i.e., the Prats problem with a vanishing basic throughflow. It is shown that the generalized HRL problem is always linearly stable for pseudoplastic fluids and always linearly unstable for dilatant fluids.
Show PACS
47.20.-k Flow instabilities
47.56.+r Flows through porous media
47.55.P- Buoyancy-driven flows; convection

A study of start-up flow of thixotropic fluids including inertia effects on an inclined plane

Wenwen Liu and Ke-Qin Zhu

Phys. Fluids 23, 013103 (2011); http://dx.doi.org/10.1063/1.3536654 (8 pages) | Cited 2 times

Online Publication Date: 14 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Despite the practical importance of thixotropic fluids, there is no reliable way at present to predict the onset of thixotropic flow. The start-up flow of thixotropic fluids including inertia effects falling down along an inclined plate is studied in this paper. The effects of the unsteady term in the NS equations on the start-up process are clarified and a criterion parameter A is presented to measure this unsteady effect. The parameter A is defined as the ratio of the Reynolds number and generalized Weissenberg number W, where W is the ratio of the characteristic time of microstructure changes and the characteristic time of flow. According to flow characteristics, we classify the motion into three cases. In case 1, avalanche happens and the initial viscosity is big. The start-up process is divided into two stages: creep and flow. Velocity profiles of both stages are discussed. In this case, if A is small enough, the inertia effects could be neglected. Otherwise, the inertial unsteady term will protract the start-up process, decrease the velocity of the free surface, and bring a thicker unyield region. In case 2, the avalanche happens and initial viscosity is small. Similar inertial unsteady effects are observed. Moreover, the unsteady term in the NS equations could delay the critical time at which the flow happens or even prevent the thixotropic material from flowing. In case 3, the avalanche could not happen. The inertial unsteady effect is only present in the start period and has no influence on the later motion.
Show PACS
47.57.Qk Rheological aspects
47.15.G- Low-Reynolds-number (creeping) flows
47.10.ad Navier-Stokes equations

Drop deformation and breakup in polystyrene/high-density polyethylene blends under oscillatory shear flow

Anuvat Sirivat, Sakchai Patako, Sumonman Niamlang, and Wanchai Lerdwijitjarud

Phys. Fluids 23, 013104 (2011); http://dx.doi.org/10.1063/1.3541967 (12 pages)

Online Publication Date: 14 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Drop deformation and breakup in polystyrene/high-density polyethylene viscoelastic melt blends were investigated under the effects of viscosity ratio, the time scale ratio, and droplet elasticity under oscillatory shear flow using an optical flow cell. The deformation was studied in terms of deformation parameters, Def∗ = a∗−c/a∗+c, where a∗ and c are the apparent drop principal axes and the minor axes of the droplets as measured from the time series of images. Amplitudes of deformation parameters are defined as the difference between the maximum and minimum values divided by two. The amplitudes increase linearly at small capillary number and nonlinearly at large capillary number, where the capillary is defined as the ratio between the matrix viscous force and the interfacial tension force. The deformation amplitude parameters decrease with increasing viscosity ratio, time scale ratio, and elasticity at any fixed capillary number. Drop breakup patterns observed are the nonsymmetric one-end tearing pattern for the system with a lower viscosity ratio and the two-end stretching and twisting for the system with a higher viscosity ratio. The critical capillary number increases with viscosity ratio but varies slightly with the time scale ratio.
Show PACS
47.55.df Breakup and coalescence
47.50.Ef Measurements
47.80.Jk Flow visualization and imaging
47.55.nb Capillary and thermocapillary flows
68.03.Cd Surface tension and related phenomena
47.54.De Experimental aspects
back to top Particulate, Multiphase, and Granular Flows
FREE

Experimental investigation into segregating granular flows down chutes

Sébastien Wiederseiner, Nicolas Andreini, Gaël Épely-Chauvin, Gaudenz Moser, Mathieu Monnereau, J. M. N. T. Gray, and Christophe Ancey

Phys. Fluids 23, 013301 (2011); http://dx.doi.org/10.1063/1.3536658 (10 pages) | Cited 8 times

Online Publication Date: 6 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We experimentally investigated how a binary granular mixture made up of spherical glass beads (size ratio of 2) behaved when flowing down a chute. Initially, the mixture was normally graded, with all the small particles on top of the coarse grains. Segregation led to a grading inversion, in which the smallest particles percolated to the bottom of the flow, while the largest rose toward the top. Because of diffusive remixing, there was no sharp separation between the small-particle and large-particle layers, but a continuous transition. Processing images taken at the sidewall, we were able to measure the evolution of the concentration and velocity profiles. These experimental profiles were used to test a recent theory developed by Gray and Chugunov [J. Fluid Mech. 569, 365 (2006)] , who derived a nonlinear advection diffusion equation that describes segregation and remixing in dense granular flows of binary mixtures. We found that this theory was able to provide a consistent description of the segregation/remixing process under steady uniform flow conditions. To obtain the correct depth-averaged concentration far downstream, it was very important to use an accurate approximation to the downstream velocity profile through the avalanche depth. The S-shaped concentration profile in the far field provided a useful way of determining what we refer to as the Péclet number for segregation, a dimensionless number that quantifies how large the segregation is compared to diffusive remixing. While the theory was able to closely match the final fully developed concentration profile far downstream, there were some discrepancies in the inversion region (i.e., the region in which the mixing occurs). The reasons for this are not clear. The difficulty to set up the experiment with both well controlled initial conditions and a steady uniform bulk flow field is one of the most plausible explanations. Another interesting lead is that the flux of segregating particles, which was assumed to be a quadratic function of the concentration in small beads, takes a more complicated form.
Show PACS
47.57.Gc Granular flow
45.70.Mg Granular flow: mixing, segregation and stratification
47.80.Jk Flow visualization and imaging
02.30.Hq Ordinary differential equations

Rheology of oscillating suspensions of noncolloidal spheres at small and large accumulated strains

Hyun-Ok Park, Jonathan M. Bricker, Michael J. Roy, and Jason E. Butler

Phys. Fluids 23, 013302 (2011); http://dx.doi.org/10.1063/1.3531745 (9 pages)

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The oscillatory rheology of a noncolloidal suspension of spheres is studied for volume fractions up to 0.50 and strain amplitudes of oscillation between 0.05 and 5.0. The stress responses to the imposed oscillatory strain can deviate from a linear response, even at the smallest strain amplitude, as quantified by both a Fourier decomposition and a least-squares analysis. The stress component that is in phase with the strain can decay by an order of magnitude over the first 15–20 oscillations, depending upon the concentration and strain amplitude. Over the same number of oscillations, the stress component in phase with the rate of strain remains nearly constant, but does change significantly over a large number of oscillations. Furthermore, the apparent values of the complex viscosity of the suspensions at steady state demonstrate a nonmonotonic dependence on strain amplitude for volume fractions larger than 0.2, confirming previously published data limited to the volume fraction of 0.4.
Show PACS
47.57.Qk Rheological aspects
82.70.Kj Emulsions and suspensions
47.32.Ef Rotating and swirling flows

Surface flows of inelastic spheres

Diego Berzi and James T. Jenkins

Phys. Fluids 23, 013303 (2011); http://dx.doi.org/10.1063/1.3532838 (7 pages) | Cited 7 times

Online Publication Date: 11 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We study flows of inelastic spheres on the surface of an erodible bed between frictional sidewalls and distinguish two regions in such flows: a dilute, diffuse region, neighboring the free surface, for which we solve a boundary-value problem based on the kinetic theory, and a dense algebraic layer, in which there is an approximate algebraic balance between production and dissipation of fluctuation energy. We take into account correlated motions between the particles at high volume fractions and employ the trapezoidal rule to solve, in an approximate way, for the flow quantities in the diffuse layer. Using boundary conditions of no-slip and yield at the bed and vanishing of the stresses and the energy flux at the free surface, we obtain analytical predictions of flow depth and mass flow rate that compare favorably with the results of experiments performed on glass spheres flowing on the surface of a heap and in half-filled rotating drums.
Show PACS
47.57.Gc Granular flow
45.70.Mg Granular flow: mixing, segregation and stratification

Dynamics of wet particles in rotating drums: Effect of liquid surface tension

P. Y. Liu, R. Y. Yang, and A. B. Yu

Phys. Fluids 23, 013304 (2011); http://dx.doi.org/10.1063/1.3543916 (9 pages) | Cited 7 times

Online Publication Date: 20 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A numerical model based on the discrete element method was developed to simulate the wet particle flow in a rotating drum. The model explicitly considered the capillary force between particles and liquid distribution within the packed bed. Physical experiments under similar conditions were carried out to validate the model, showing that the simulation and experiment results were quite comparable in terms of the flow patterns, maximum flow repose angle, and the frequency of avalanching. Flow properties in two different states were investigated with the focus on the effect of liquid surface tension. In the quasistatic state with the drum rotating at very low speeds, discrete avalanches were observed after the flow reached the maximum repose angle. However, flow properties had changed well before avalanches occurred. The microscopic analysis indicated that the strength caused by the capillary force reached a minimal when avalanches started. The maximum repose angle increased with increasing capillary force and their relationship was compared with the theoretical models based on the Mohr–Coulomb criterion and force balance. In the dynamic state, the bed showed continuous surface flow at weak surface tensions but transited into discrete avalanches characterized by the plough flow as the surface tension further increased. The flow became more dilated at high surface tensions with increased particle contacts and more uniform stress distribution. The energy and frequency of collisions between particles also decreased as the liquid surface tension increased and more collisions were observed in the region 4–5 particle diameters below the flow surface. The results would be useful to the development of a comprehensive understanding of the mechanisms of particle mixing and segregation.
Show PACS
45.70.Mg Granular flow: mixing, segregation and stratification
45.70.Ht Avalanches
47.55.nb Capillary and thermocapillary flows
47.32.Ef Rotating and swirling flows
47.20.Dr Surface-tension-driven instability
47.11.-j Computational methods in fluid dynamics

Viscous damping force during head-on collision of two spherical particles

J. S. Marshall

Phys. Fluids 23, 013305 (2011); http://dx.doi.org/10.1063/1.3546094 (9 pages)

Online Publication Date: 20 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Viscous damping is known to play a critical role in determining the restitution coefficient for the collision of two spherical particles at low and moderate Stokes numbers due to fluid motion in the squeeze-film between the particles. The classical expression for the viscous damping force of approaching spheres, valid prior to collision of the particles, has been used by several investigators to model the effect of viscous damping on the restitution coefficient. However, viscous damping also occurs during the particle collision due to the corner flow associated with a change in the radius of the contact region, within which the particle surfaces deform into flattened parallel surfaces due to high fluid pressure within the squeeze-film. The current paper derives a simple expression for the fluid damping force caused by the squeeze-film dynamics associated with a change in the contact region radius during collision. This expression is then used in conjunction with the damping force expression for spherical particles before and after collision to predict the variation of restitution coefficient with the particle Stokes number and elasticity parameter. The viscous damping force during collision exhibits sensitive dependence on the minimum approach distance separating the particle surfaces within the contact region, which, in turn, is controlled by factors such as microscopic particle surface roughness and pressure-dependent density and viscosity changes of the fluid.
Show PACS
62.10.+s Mechanical properties of liquids
66.20.Cy Theory and modeling of viscosity and rheological properties, including computer simulation
68.15.+e Liquid thin films
68.35.B- Structure of clean surfaces (and surface reconstruction)

Elastic granular flows of ellipsoidal particles

Charles S. Campbell

Phys. Fluids 23, 013306 (2011); http://dx.doi.org/10.1063/1.3546037 (10 pages) | Cited 7 times

Online Publication Date: 25 January 2011

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Granular flow rheology can be divided into two global regimes: the elastic, which is dominated by force chains, and the inertial, which is nearly free of force chains. As the propensity of a material to form force chains should be strongly influenced by particle shape, this paper is an attempt to assess the effects of shape on flow regime transitions through computer simulations of shear flow of ellipsoidal particles. On one hand, the results show that at a given concentration, ellipsoidal particles generate smaller quasistatic stress than spheres, likely a result of their ability to form denser packings. But at the same time, large aspect ratio ellipsoids more readily form force chains and demonstrate elastic behavior at smaller concentrations than spheres. This is shown to be due to a tradeoff between a shear-induced particle alignment that tends to minimize the interference of the particles and the shear flow, and the particle surface friction, which works to rotate the particles into the flow.
Show PACS
47.57.Gc Granular flow
45.70.Mg Granular flow: mixing, segregation and stratification
47.11.-j Computational methods in fluid dynamics
Page 1 of 3 Pages Return to All Sections Next Page
Close

Google Calendar

ADVERTISEMENT

Go to AIP Home   © AIP Publishing LLC
close