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

Flickr Twitter iResearch App Facebook

Search Issue | RSS Feeds RSS
Previous Issue Next Issue

Feb 2012

Volume 24, Issue 2, Articles (02xxxx)

Issue Cover Spotlight Figure

Phys. Fluids 24, 023101 (2012); http://dx.doi.org/10.1063/1.3684750 (21 pages)

R. Sattler, S. Gier, J. Eggers, and C. Wagner
Enlarge Image | Read More
Page 1 of 2 Pages Next Page | Jump to Page
back to top
RSS Feeds
FREE

Re-examining the logarithmic dependence of the mean velocity distribution in polymer drag reduced wall-bounded flow

C. M. White, Y. Dubief, and J. Klewicki

Phys. Fluids 24, 021701 (2012); http://dx.doi.org/10.1063/1.3681862 (6 pages) | Cited 1 time

Online Publication Date: 3 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A re-examination of the logarithmic dependence of the mean velocity distribution in polymer drag reduced flows shows that drag reducing polymers modify the von Kármán coefficient and, in channel flow, eradicate the log-layer at high drag reductions. It is also found that the “ultimate profile,” corresponding to the state of maximum drag reduction is not logarithmic.
Show PACS
47.27.nb Boundary layer turbulence
47.57.Ng Polymers and polymer solutions
47.27.nd Channel flow
47.50.Cd Modeling
47.27.E- Turbulence simulation and modeling
47.60.Dx Flows in ducts and channels
FREE

Oscillatory bubbles induced by geometrical constraint

M. Pailha, A. L. Hazel, P. A. Glendinning, and A. Juel

Phys. Fluids 24, 021702 (2012); http://dx.doi.org/10.1063/1.3682772 (7 pages) | Cited 1 time

Online Publication Date: 9 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We show that a simple change in pore geometry can radically alter the behavior of a fluid-displacing air finger, indicating that models based on idealized pore geometries fail to capture key features of complex practical flows. In particular, partial occlusion of a rectangular cross section can force a transition from a steadily propagating centered finger to a state that exhibits spatial oscillations formed by periodic sideways motion of the interface at a fixed distance behind the moving finger tip. We characterize the dynamics of the oscillations, which suggest that they arise from a global homoclinic connection between the stable and unstable manifolds of a steady, symmetry-broken solution.
Show PACS
47.55.dd Bubble dynamics
47.54.De Experimental aspects
FREE

Clouds of particles in a periodic shear flow

Bloen Metzger and Jason E. Butler

Phys. Fluids 24, 021703 (2012); http://dx.doi.org/10.1063/1.3685537 (6 pages) | Cited 1 time

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

multimedia

Show Abstract
We have investigated the time evolution of a cloud of non-Brownian particles subjected to a periodic shear flow in an otherwise pure liquid at low Reynolds number. This experiment illustrates the irreversible nature of particulate systems submitted to a shear. When repeating the cycles of shear, we have found that clouds of particles progressively disperse in the flow direction until reaching a threshold critical volume fraction that depends upon the strain amplitude; this critical volume fraction coincides with measurements of the threshold for reversibility found from experiments on homogeneous suspensions in periodic shear. Two distinct patterns, including a “galaxy-like” shape, are observed for the evolution of the clouds and the transition between the patterns is identified using a simple scaling analysis. Movies are available with the online version of the paper.
Show PACS
47.55.Kf Particle-laden flows
47.57.E- Suspensions
47.54.De Experimental aspects
47.60.-i Flow phenomena in quasi-one-dimensional systems
FREE

Stokes flow paths separation and recirculation cells in X-junctions of varying angle

M. Cachile, L. Talon, J. M. Gomba, J. P. Hulin, and H. Auradou

Phys. Fluids 24, 021704 (2012); http://dx.doi.org/10.1063/1.3690100 (7 pages)

Online Publication Date: 28 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Fluid and solute transfer in X-junctions between straight channels is shown to depend critically on the junction angle α in the Stokes flow regime. Experimentally, water and a water-dye solution are injected at equal flow rates in two facing channels of the junction. Planar laser induced fluorescence (PLIF) measurements show that the largest part of each injected fluid “bounces back” preferentially into the outlet channel at the lowest angle to the injection; this is opposite to the inertial case and requires a high curvature of the corresponding streamlines. The proportion of this fluid in the other channel decreases from 50% at α = 90° to 0% at a threshold angle. These counterintuitive features reflect the minimization of energy dissipation for Stokes flows. Finite elements numerical simulations of a 2D Stokes flow of equivalent geometry confirm these results and show that, below the threshold angle αc = 33.8°, recirculation cells are present in the center part of the junction and separate the two injected flows of the two solutions. Reducing further α leads to the appearance of new recirculation cells with lower flow velocities.
Show PACS
47.80.Jk Flow visualization and imaging
47.85.L- Flow control
47.60.Dx Flows in ducts and channels
02.70.Dh Finite-element and Galerkin methods
47.11.Fg Finite element methods
47.80.Cb Velocity measurements
FREE

Stokes number effects on particle slip velocity in wall-bounded turbulence and implications for dispersion models

L. H. Zhao, C. Marchioli, and H. I. Andersson

Phys. Fluids 24, 021705 (2012); http://dx.doi.org/10.1063/1.3690071 (7 pages) | Cited 1 time

Online Publication Date: 29 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The particle slip velocity is adopted as an indicator of the behavior of heavy particles in turbulent channel flow. The statistical moments of the slip velocity are evaluated considering particles with Stokes number, defined as the ratio between the particle response time and the viscous time scale of the flow, in the range 1 < St < 100. The slip velocity fluctuations exhibit a monotonic increase with increasing particle inertia, whereas the fluid-particle velocity covariance is gradually reduced for St ⩾ 5. Even if this covariance equals the particle turbulence intensity, a substantial amount of particle slip may occur. Relevant to two-fluid modeling of particle-laden flows is the finding that the standard deviation of the slip velocity fluctuations is significantly larger than the corresponding mean slip velocity.
Show PACS
47.27.nb Boundary layer turbulence
47.27.nd Channel flow
47.45.Gx Slip flows and accommodation
47.60.Dx Flows in ducts and channels
47.27.eb Statistical theories and models
47.27.ek Direct numerical simulations
back to top
RSS Feeds
back to top Micro- and Nanofluid Mechanics

Velocity slip coefficients based on the hard-sphere Boltzmann equation

Livio Gibelli

Phys. Fluids 24, 022001 (2012); http://dx.doi.org/10.1063/1.3680873 (14 pages) | Cited 3 times

Online Publication Date: 3 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We present a kinetic theory derivation of higher-order slip boundary conditions. The situation studied is that of a pressure driven isothermal gas flowing through a plane microchannel. The distribution function is expanded in terms of half-range Hermite polynomials and the system of moment equations in the expansion coefficients is analytically solved. The velocity slip coefficients, as well as their Knudsen-layer corrections, are obtained by evaluating the solution in the near continuum limit. The proposed approach is accurate and easy to implement. The results are presented for the hard-sphere Boltzmann equation and Maxwell's diffuse-specular boundary conditions, but can be extended to arbitrary intermolecular interactions and more general scattering kernels.
Show PACS
47.45.Gx Slip flows and accommodation
47.60.Dx Flows in ducts and channels
47.11.-j Computational methods in fluid dynamics
47.15.Rq Laminar flows in cavities, channels, ducts, and conduits
47.40.-x Compressible flows; shock waves
47.45.Ab Kinetic theory of gases

Microdroplet oscillations during optical pulling

Simen Å. Ellingsen

Phys. Fluids 24, 022002 (2012); http://dx.doi.org/10.1063/1.3685814 (14 pages) | Cited 2 times

Online Publication Date: 24 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
It was recently shown theoretically that it is possible to pull a spherical dielectric body towards the source of a laser beam [J. Chen, J. Ng, Z. Lin, and C. T. Chan, “Optical pulling force,” Nat. Photonics 5, 531 (2011)], a result with immediate consequences to optical manipulation of small droplets. Optical pulling can be realized, e.g., using a diffraction-free Bessel beam, and is expected to be of great importance in manipulation of microscopic droplets in micro- and nanofluidics. Compared to conventional optical pushing, however, the ratio of optical net force to stress acting on a droplet is much smaller, increasing the importance of oscillations. We describe the time-dependent surface deformations of a water microdroplet under optical pulling to linear order in the deformation. Shape oscillations have a lifetime in the order of microseconds for droplet radii of a few micrometers. The force density acting on the initially spherical droplet is strongly peaked near the poles on the beam axis, causing the deformations to take the form of jet-like protrusions.
Show PACS
47.55.D- Drops and bubbles
47.35.-i Hydrodynamic waves
back to top Interfacial Flows

Bouncing, coalescence, and separation in head-on collision of unequal-size droplets

Chenglong Tang, Peng Zhang, and Chung K. Law

Phys. Fluids 24, 022101 (2012); http://dx.doi.org/10.1063/1.3679165 (15 pages) | Cited 3 times

Online Publication Date: 1 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The dynamics of head-on collision of unequal-size droplets were experimentally and theoretically investigated, with emphasis on identifying distinct collision outcomes and interpreting the size-ratio dependence. A unified regime diagram in terms of bouncing, permanent coalescence, and separation after coalescence was identified for hydrocarbon and water droplets in the parameter space of the size ratio and a collision Weber number. Experimental results show that the transition Weber number, Web-c, that separates the bouncing and permanent coalescence regimes, weakly depends on the size ratio, while the transition Weber number, Wec-s, that separates permanent coalescence and separation regimes, significantly increases with the size ratio. A theoretical model based on energy balance and scaling analysis was developed to explain the size-ratio dependence of Wec-s. The theoretical results show good agreement with the experimental data for tetradecane and decane droplets, with a moderate discrepancy for water droplets.
Show PACS
47.55.df Breakup and coalescence
47.32.Ff Separated flows

Thermocapillary motion of a slender viscous droplet in a channel

E. Katz, M. Haj, A. M. Leshansky, and A. Nepomnyashchy

Phys. Fluids 24, 022102 (2012); http://dx.doi.org/10.1063/1.3681813 (11 pages)

Online Publication Date: 7 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We extend the previously developed low-capillary-number asymptotic theory of thermocapillary motion of a long bubble and a moderately viscous droplet in a channel [S. K. Wilson, “The effect of an axial temperature gradient on the steady motion of a large droplet in a tube,” J. Eng. Math. 29, 205 (1995)10.1007/BF00042854; A. Mazouchi and G. M. Homsy, “Thermocapillary migration of long bubbles in cylindrical capillary tubes,” Phys. Fluids 12, 542 (2000)10.1063/1.870260] toward droplets with an arbitrary viscosity. A generalized modified Landau-Levich-Bretherton equation, governing the thickness of the carrier liquid film entrained between the droplet and the channel wall in the transition region between constant thickness film and constant curvature cap, is solved numerically. The resulting droplet velocity is determined applying the mass balance and it is a function of two dimensionless parameters, the modified capillary number, Δσ*, equal to the surface tension variance over a distance of channel half-width scaled with the mean surface tension, and the inner-to-outer liquid viscosity ratio, λ. It is found that the droplet speed decreases with the increase in droplet viscosity, as expected, while this retardation becomes more operative upon the increase in Δσ*.
Show PACS
47.55.dm Thermocapillary effects
47.60.Dx Flows in ducts and channels
47.10.A- Mathematical formulations

Observation of collision and oscillation of microdroplets with extremely large shear deformation

Tatsuya Yamada and Keiji Sakai

Phys. Fluids 24, 022103 (2012); http://dx.doi.org/10.1063/1.3681810 (9 pages) | Cited 1 time

Online Publication Date: 8 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We measured the viscosity and surface tension of various liquids under large (∼106 s−1) shear deformation. Oscillation of a 10-μm size microdroplet is brought about by the head-on collision of two droplets. Since the Reynolds number is as small as 100, the motion of the liquid is stable and the dynamic image is obtained with high reproducibility by the stroboscopic method. By observing and evaluating the mechanical oscillation of the microdroplet, of which frequency ranges typically in 100 – 300 kHz, we found that the viscosity of ethylene glycol and diethylene glycol is smaller than the known literature value, which is considered to be the viscosity at zero-frequency. This phenomena can be attributed to the slow viscous relaxation of associated liquids due to the re-combination dynamics of the network of H-bonds.
Show PACS
47.55.D- Drops and bubbles
61.20.Qg Structure of associated liquids: electrolytes, molten salts, etc.
68.03.Cd Surface tension and related phenomena
66.20.Ej Studies of viscosity and rheological properties of specific liquids
62.10.+s Mechanical properties of liquids
47.80.Jk Flow visualization and imaging

The physics of aerobreakup. II. Viscous liquids

T. G. Theofanous, V. V. Mitkin, C. L. Ng, C-H. Chang, X. Deng, and S. Sushchikh

Phys. Fluids 24, 022104 (2012); http://dx.doi.org/10.1063/1.3680867 (39 pages) | Cited 4 times

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

multimedia

Show Abstract
We extend the work of Theofanous and Li [“On the physics of aerobreakup,” Phys. Fluids 20, 052103 (2008)] on aerobreakup physics of water-like, low viscosity liquid drops, to Newtonian liquids of any viscosity. The scope includes the full range of aerodynamics from near incompressible to high Mach number flows. The key physics of Rayleigh–Taylor piercing (RTP, first criticality) and of shear-induced entrainment (SIE, second and terminal criticality) are verified and quantified by new viscosity- and capillarity-based scalings for fluids of any viscosity. The relevance and predictive power of linear stability analysis of the Rayleigh–Taylor and Kelvin–Helmholtz problems (both including viscosity) is demonstrated for the RTP and the SIE regimes, respectively. The advanced stages of breakup and of the resulting particle-clouds are observed and clear definition and quantification of breakup times are offered.
Show PACS
47.55.df Breakup and coalescence
47.55.Hd Stratified flows
47.55.nb Capillary and thermocapillary flows
47.20.Ft Instability of shear flows (e.g., Kelvin-Helmholtz)
47.20.Ma Interfacial instabilities (e.g., Rayleigh-Taylor)
47.40.-x Compressible flows; shock waves

Thin films flowing down inverted substrates: Three-dimensional flow

T.-S. Lin, L. Kondic, and A. Filippov

Phys. Fluids 24, 022105 (2012); http://dx.doi.org/10.1063/1.3682001 (18 pages) | Cited 1 time

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

multimedia

Show Abstract
We study contact line induced instabilities for a thin film of fluid under destabilizing gravitational force in three-dimensional setting. In the previous work [T.-S. Lin and L. Kondic, Phys. Fluids 22, 052105 (2010)], we considered two-dimensional flow, finding formation of surface waves whose properties within the implemented long-wave model depend on a single parameter, D = (3Ca)1/3cotα, where Ca is the capillary number and α is the inclination angle. In the present work we consider fully 3D setting and discuss the influence of the additional dimension on stability properties of the flow. In particular, we concentrate on the coupling between the surface instabilities and the transverse (fingering) instabilities of the film front. We furthermore consider these instabilities in the setting where fluid viscosity varies in the transverse direction. It is found that the flow pattern strongly depends on the inclination angle and the viscosity gradient.
Show PACS
47.55.nd Spreading films
68.15.+e Liquid thin films
47.55.nb Capillary and thermocapillary flows
47.54.Bd Theoretical aspects
47.35.-i Hydrodynamic waves
47.55.np Contact lines

Measurements of liquid film thickness for a droplet at a two-fluid interface

G. Oldenziel, R. Delfos, and J. Westerweel

Phys. Fluids 24, 022106 (2012); http://dx.doi.org/10.1063/1.3684706 (18 pages) | Cited 2 times

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Coalescence of a droplet at a two-fluid interface is studied at Bond numbers larger than one and at three different values of the viscosity ratio. Both the thickness of the liquid film between the rising droplet and the two-fluid interface, and the location of film rupture are measured using laser induced fluorescence. Particle image velocimetry was applied to the flow in the film. It is found that the film thins asymetrically, and that the time interval between collision and film rupture is shorter than predicted by commonly used models. The film ruptures at an off-center location. It can be concluded that asymmetric film drainage speeds up coalescence.
Show PACS
47.55.df Breakup and coalescence
47.55.nd Spreading films
47.80.Jk Flow visualization and imaging
66.20.-d Viscosity of liquids; diffusive momentum transport
68.15.+e Liquid thin films

Dip coating with an interaction potential normal to the substrate

C. Vannozzi

Phys. Fluids 24, 022107 (2012); http://dx.doi.org/10.1063/1.3680872 (13 pages) | Cited 1 time

Online Publication Date: 16 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Dip coating in the presence of a substrate-liquid interaction potential normal to the substrate, previously theoretically investigated by R. Krechetnikov and G. M. Homsy [Phys. Fluids 17, 038101 (2005)], was revisited. Their solution procedure leads to predictions of the entrained film thickness h∞* that deviate substantially from the classical Landau-Levich law because of the impossibility to identify meniscus solutions satisfying the proper boundary conditions of zero thickness and zero apparent contact angle on the solid substrate (L-L BC’s). In contrast, in the present analysis, by choosing a different method of integration and requiring the satisfaction of the boundary condition of flat bath for large, but finite, meniscus thickness, we obtain solutions subject to L-L BC's for the same parameter range studied in Krechetnikov and Homsy's paper. Thus, the matching follows a modified Landau-Levich law, where h∞* is inversely proportional to the meniscus curvature at the substrate. Since the interaction potential changes considerably this curvature, the entrained film significantly thickens for attractive interactions or thins for repulsive ones. Similar results are also found for a potential of the Debye-Hückel form.
Show PACS
47.55.nd Spreading films
68.03.Cd Surface tension and related phenomena
47.11.-j Computational methods in fluid dynamics
47.55.Hd Stratified flows

Analytical study in the mechanism of flame movement in horizontal tubes

Kirill A. Kazakov

Phys. Fluids 24, 022108 (2012); http://dx.doi.org/10.1063/1.3684712 (34 pages)

Online Publication Date: 16 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The problem of premixed flame propagation in wide horizontal tubes is revisited. Employing the on-shell description of flames with arbitrary gas expansion, a nonlinear second-order differential equation for the front position of steady flame is derived. Solutions to this equation, obtained numerically, reveal two distinct physical regimes of laminar flame propagation controlled by the strong baroclinic effect. They differ by the front shape and flame speed, the ratio of the total consumption rates in the two regimes being 1.4 to 1.8, depending on the value of the gas expansion coefficient. Comparison with the existing experimental data on methane-air flames is made, and explanation of the main trends in the observed flame behavior is given. It is shown, in particular, that the faster (slower) regime of combustion is realized in mixtures close to (far from) the stoichiometric composition, with pronounced changeover in between.
Show PACS
47.70.Pq Flames; combustion
47.70.Fw Chemically reactive flows
47.60.Dx Flows in ducts and channels
47.40.-x Compressible flows; shock waves
47.15.Rq Laminar flows in cavities, channels, ducts, and conduits
47.11.-j Computational methods in fluid dynamics

Modeling resistance of nanofibrous superhydrophobic coatings to hydrostatic pressures: The role of microstructure

T. M. Bucher, B. Emami, H. Vahedi Tafreshi, M. Gad-el-Hak, and G. C. Tepper

Phys. Fluids 24, 022109 (2012); http://dx.doi.org/10.1063/1.3686833 (22 pages) | Cited 4 times

Online Publication Date: 23 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
In this paper, we present a numerical study devised to investigate the influence of microstructural parameters on the performance of fibrous superhydrophobic coatings manufactured via dc and ac electrospinning. In particular, our study is focused on predicting the resistance of such coatings against elevated hydrostatic pressures, which is of crucial importance for submersible applications. In our study, we generate 3D virtual geometries composed of randomly or orthogonally oriented horizontal fibers with bimodal diameter distributions resembling the microstructure of our electrospun coatings. These virtual geometries are then used as the computational domain for performing full morphology numerical simulations to establish a relationship between the coatings’ critical pressure (pressure beyond which the surface may depart from the Cassie state) and their microstructures. For coatings with ordered microstructures, we have also derived analytical expressions for the critical pressure based on the balance of forces acting on the water–air interface. Predictions of our force balance analysis are compared with those of our FM simulations as well as the equations proposed by Tuteja et al. [Proc. Natl. Acad. Sci. U.S.A. 105, 18200 (2008)]10.1073/pnas.0804872105, and discussed in detail. Our numerical simulations are aimed at providing useful information with regards to the tolerance of fibrous superhydrophobic coatings against elevated pressures, and helping with the design and optimization of the coatings’ microstructures. Our results show considerably higher pressure tolerance for the case of coatings with orthogonally oriented fibers as compared to those with randomly laid fibers when other microstructural parameters are held constant. Moreover, it is demonstrated that thickness of the coating has less influence on performance in the case of orthogonal microstructures. Coatings’ responses to other variations favor those that yield smaller-sized inter-fiber spaces. Studies are also performed investigating the effect of subtle permutations in the layer configurations of our ac-electrospun coatings, as well as the use of a hybrid coating that utilizes advantages from both dc and ac electrospinning.
Show PACS
47.50.Cd Modeling
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.08.Bc Wetting
68.03.-g Gas-liquid and vacuum-liquid interfaces
47.56.+r Flows through porous media
47.11.-j Computational methods in fluid dynamics

Cross-waves induced by the vertical oscillation of a fully immersed vertical plate

Frédéric Moisy, Guy-Jean Michon, Marc Rabaud, and Eric Sultan

Phys. Fluids 24, 022110 (2012); http://dx.doi.org/10.1063/1.3686696 (15 pages)

Online Publication Date: 24 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Capillary waves excited by the vertical oscillation of a thin elongated plate below an air-water interface are analyzed using time-resolved measurements of the surface topography. A parametric instability is observed above a well defined acceleration threshold, resulting in a so-called cross-wave, a staggered wave pattern localized near the wavemaker and oscillating at half the forcing frequency. This cross-wave, which is stationary along the wavemaker but propagative away from it, is described as the superposition of two almost anti-parallel propagating parametric waves making a small angle of the order of 20o with the wavemaker edge. This contrasts with the classical Faraday parametric waves, which are exactly stationary because of the homogeneity of the forcing. Our observations suggest that the selection of the cross-wave angle results from a resonant mechanism between the two parametric waves and a characteristic length of the surface deformation above the wavemaker.
Show PACS
47.35.Pq Capillary waves
47.20.-k Flow instabilities
47.80.Jk Flow visualization and imaging

Structure of Marangoni-driven singularities

R. Krechetnikov

Phys. Fluids 24, 022111 (2012); http://dx.doi.org/10.1063/1.3685831 (29 pages) | Cited 2 times

Online Publication Date: 28 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
This work presents an analytical study of the structure of steady Marangoni-driven singularities in the context of chemical-reaction driven tip-streaming, which identifies the conditions when such singularities are observable. As motivated by experimental observations of the conical symmetry of the problem, one can construct self-similar solutions of the Stokes equations, which are singular at the tip; these solutions, however, provide no information on the thread structure which is responsible for a resolution of the singularity via tip-streaming. The cone-tip singularity is resolved here with the help of asymptotic matching of the cone and thread solutions using slender jet approximation, which gives an explicit asymptotic formula for the thread radius and thus of the emitted droplets size as a function of physical parameters governing the problem.
Show PACS
47.55.pf Marangoni convection
02.60.Gf Algorithms for functional approximation
47.11.-j Computational methods in fluid dynamics
47.55.D- Drops and bubbles
68.03.Cd Surface tension and related phenomena
47.60.Kz Flows and jets through nozzles
back to top Viscous and Non-Newtonian Flows
FREE

The final stages of capillary break-up of polymer solutions

R. Sattler, S. Gier, J. Eggers, and C. Wagner

Phys. Fluids 24, 023101 (2012); http://dx.doi.org/10.1063/1.3684750 (21 pages) | Cited 6 times

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

multimedia

Show Abstract
The capillary break-up of a polymer solution evolves via a series of stages. After the initial instability a long-lived cylindrical filament is formed, which thins exponentially in time, while the flow is purely extensional. During the final stages of the thinning process, at which the polymers are stretched sufficiently for the filament to become unstable to a Rayleigh–Plateau-like instability, a complex flow pattern develops, which we describe here. Achieving a high spatial resolution well below the optical Rayleigh limit, we describe both the formation of individual droplets as well as that of periodic patterns. Following the periodic instability, a blistering pattern appears, with different generations of smaller droplets. At sufficiently high polymer concentrations, the filament does not break at all, but a solid polymeric fiber with a thickness well below a micron remains. The experiments were performed for various polymer and solvent systems, all of which showed the same qualitative behavior for most of the observed features.
Show PACS
47.50.Gj Instabilities
47.57.Ng Polymers and polymer solutions
47.55.df Breakup and coalescence
47.55.nb Capillary and thermocapillary flows

Viscous exchange flows

Gary P. Matson and Andrew J. Hogg

Phys. Fluids 24, 023102 (2012); http://dx.doi.org/10.1063/1.3685723 (23 pages)

Online Publication Date: 24 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Gravitationally driven exchange flows of viscous fluids with different densities are analysed theoretically and investigated experimentally within a horizontal channel. Following initiation from rest when there is a vertical boundary dividing the two fluids, the denser fluid slumps under the less dense along the underlying boundary, while the less dense fluid intrudes along the upper boundary. The motion is driven by the pressure gradients associated with the density differences between the two fluids, resisted by viscous stresses, and mathematically modelled by a similarity solution that depends on the ratio of the viscosities of the two fluids. When the viscosity of the less dense fluid is much smaller than the viscosity of the denser fluid, the shape of the interface between the fluids varies rapidly close to the upper boundary and depends weakly on the viscosity ratio within the interior of the flow. Matched asymptotic expansions are employed in this regime to determine the shape of the interface and the rates of its propagation along the boundaries. The similarity solutions are shown to be linearly stable and thus are expected to represent the intermediate asymptotics of the flow. Experiments confirm the similarity form of solutions and demonstrate close agreement with the theoretical predictions when the viscosities of the fluids are comparable, but exhibit some discrepancies when the viscosities differ more substantially. It is suggested that these discrepancies may be due to mixing between the fluids close to the boundaries, which is induced by the no-slip boundary condition. Exchange flows within porous domains are also investigated to determine the shape of the interface as a function of the ratio of the viscosities of the two fluids and using asymptotic analysis, this shape is determined when this ratio is much larger, or smaller, than unity.
Show PACS
47.60.Dx Flows in ducts and channels
47.56.+r Flows through porous media
47.11.-j Computational methods in fluid dynamics
47.51.+a Mixing
47.53.+n Fractals in fluid dynamics
back to top Laminar Flows

Strain-vorticity induced secondary motion in shallow flows

Leon P. J. Kamp

Phys. Fluids 24, 023601 (2012); http://dx.doi.org/10.1063/1.3682097 (12 pages) | Cited 1 time

Online Publication Date: 6 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
Deviations from two-dimensionality of a shallow flow that is dominated by bottom friction are quantified in terms of the spatial distribution of strain and vorticity as described by the Okubo-Weiss function. This result is based on a Poisson equation for the pressure in a quasi-horizontal (primary) flow. It is shown that the Okubo-Weiss function specifies vertical pressure gradients, which for their part drive vertical (secondary) motion. An asymptotic expansion of these gradients based on the smallness of the vertical to horizontal scale ratio demonstrates that the sign and magnitude of secondary circulation inside the fluid layer is dictated by the signs and magnitude of the Okubo-Weiss function. As a consequence of this, secondary motion as well as nonzero horizontal divergence do also depend on the strength, i.e., the Reynolds number of the primary flow. The theory is exemplified by two generic vortical structures (monopolar and dipolar structures). Most importantly, the theory can be applied to more complicated turbulent shallow flows in order to assess the degree of two-dimensionality using measurements of the free-surface flow only.
Show PACS
47.32.C- Vortex dynamics
02.30.Jr Partial differential equations
47.27.E- Turbulence simulation and modeling

Draining of a thin film on the wall of a conical container set into rapid rotation about its vertical axis

Marius Ungarish and John D. Sherwood

Phys. Fluids 24, 023602 (2012); http://dx.doi.org/10.1063/1.3682714 (21 pages) | Cited 1 time

Online Publication Date: 14 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
A theory for how rotation modifies the draining of a thin fluid film on the surface of a conical container is developed. Rapid rotation, imposed instantaneously, creates an Ekman layer that pumps fluid to the outer edge of the container. This fluid is flung out of the vessel, rather than being transferred to the interior of the container. In consequence, fluid in the vessel but outside the Ekman layer does not spin up and continues to drain towards the container base. The net result is that draining is enhanced by the outward Ekman flux and finishes in finite time (when only the thin Ekman layer remains). The governing equations can be solved by the method of characteristics, to within numerical quadrature. The amount of fluid pumped outwards (rather than draining towards the base of the container) depends upon a single parameter that characterizes the ratio of the Ekman flux to gravitational draining.
Show PACS
47.32.Ef Rotating and swirling flows
68.15.+e Liquid thin films
47.60.Dx Flows in ducts and channels
02.60.Jh Numerical differentiation and integration

Simulation of the flow around an upstream transversely oscillating cylinder and a stationary cylinder in tandem

Sheng Bao, Sheng Chen, Zhaohui Liu, Jing Li, Hanfeng Wang, and Chuguang Zheng

Phys. Fluids 24, 023603 (2012); http://dx.doi.org/10.1063/1.3683565 (20 pages)

Online Publication Date: 22 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
The flow around a transversely oscillating cylinder in tandem with a stationary cylinder was studied using the lattice Boltzmann method at Re = 100. The influences of spacing, oscillation frequency, and amplitude on the flow field were investigated in detail. It was found that, when the upstream cylinder oscillates with small amplitude, the flow pattern can be changed significantly from that of its fixed counterpart. First, the stagnation region ceases to exist. Second, the transition from the vortex suppression (VS) regime to the vortex formation (VF) regime appears earlier than when both cylinders are fixed. Moreover, the system has a wider frequency range of lock-in for both tandem cylinders in the VS regime, while the locked frequency range is slightly increased in the VF regime. The locked region of the tandem-paired cylinders is only slightly wider than that of a single oscillating cylinder. When the system is unlocked, different responses occur in the wakes of the two cylinders. Analysis of the power spectral of lift forces, lift phase portraits, and vorticity contours shows that the wake is regular under conditions of small spacing and small oscillating amplitude. However, with larger spacing, higher oscillating frequency or larger amplitude, the oscillation is powerful enough to dominate the flow field, inducing chaotic flow. The drag and lift forces of both oscillating and stationary cylinders are also discussed. The results reveal large differences between the case of one oscillating cylinder and that of two stationary tandem cylinders.
Show PACS
47.15.Rq Laminar flows in cavities, channels, ducts, and conduits
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.52.+j Chaos in fluid dynamics
47.15.Tr Laminar wakes
47.15.ki Inviscid flows with vorticity
47.11.Qr Lattice gas

Viscous pressure-driven flows and their stability in channels with vertically oscillating walls

Leonardo Espìn and Demetrios T. Papageorgiou

Phys. Fluids 24, 023604 (2012); http://dx.doi.org/10.1063/1.3690904 (12 pages) | Cited 1 time

Online Publication Date: 29 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
We study viscous flows in infinitely long two-dimensional channels which are driven by vertical periodic oscillations of their rigid walls and a prescribed horizontal pressure gradient. Exact solutions of the Navier-Stokes equations exist for these flows in the sense that a coupled system of partial differential equations (involving time and the vertical coordinate alone) determines the solutions. The flow due to the vertical periodic wall oscillations is of stagnation point form (the horizontal velocity is linear in the horizontal coordinate x) and couples with the prescribed horizontal pressure gradient to produce a horizontal velocity which is x-independent. We investigate the solutions computationally for wide ranges of Reynolds numbers as the amplitude of the wall oscillations increases. It is found that the x-independent flow loses stability (Floquet theory is used to quantify this) at values of the Reynolds number below those required to drive the purely oscillatory flow into the chaotic regime. A similar bifurcation picture emerges when the pressure gradients are time oscillatory. In addition, computations show that when the wall and pressure oscillations are synchronous, their amplitudes can be used to produce a change in the sign of the net flux across the channel over one time period. The computations identify parameter regimes that achieve a desired direction and magnitude of the time-averaged net flux across the channel.
Show PACS
47.60.Dx Flows in ducts and channels
47.10.ad Navier-Stokes equations
47.20.Ky Nonlinearity, bifurcation, and symmetry breaking
back to top Instability and Transition

Study of instabilities and transitions for a family of quasi-two-dimensional magnetohydrodynamic flows based on a parametrical model

S. Smolentsev, N. Vetcha, and R. Moreau

Phys. Fluids 24, 024101 (2012); http://dx.doi.org/10.1063/1.3680864 (21 pages) | Cited 3 times

Online Publication Date: 7 February 2012

Full Text: Read Online (HTML) | Download PDF

Show Abstract
In this study, flow phenomena associated with inflectional and boundary-layer instabilities, as well as a mixed instability mode in quasi-two-dimensional magnetohydrodynamic flows in a rectangular duct are accessed with the help of a parametrical model, where the basic velocity profile with near-wall jets and associated points of inflection are produced by imposing an external flow-opposing force. By varying this force, various instability modes and transition scenarios are reproduced. First, linear stability analysis is performed and then nonlinear effects are studied using direct numerical simulation for Hartmann numbers 100 and 200 and Reynolds numbers from 1800 to 5000. A special attention is paid to the location of the inflection point with respect to the duct wall. A more complex flow dynamics, including various vortex-wall and vortex-vortex interactions, and even negative turbulence production are observed and analyzed as the inflection point approaches the wall. The analysis of obtained results and their comparison with relevant data for magnetohydrodynamic duct flows gain an insight into what is typically called “jet instability,” including linear and nonlinear mechanisms.
Show PACS
47.27.Cn Transition to turbulence
47.27.nb Boundary layer turbulence
47.27.nf Flows in pipes and nozzles
47.27.wg Turbulent jets
47.60.Dx Flows in ducts and channels
47.20.Ib Instability of boundary layers; separation
Page 1 of 2 Pages Next Page | Jump to Page
Close

Google Calendar

ADVERTISEMENT

Go to AIP Home   © AIP Publishing LLC
close