Showing papers on "Shear flow published in 2001"

Shear-Dependent Boundary Slip in an Aqueous Newtonian Liquid

Abstract: We report direct measurements of hydrodynamic drainage forces, which show clear evidence of boundary slip in a Newtonian liquid. The degree of boundary slip is found to be a function of the liquid viscosity and the shear rate, as characterized by the slip length, and is up to $\ensuremath{\sim}20\mathrm{nm}$. This has implications for confined biological systems, the permeability of microporous media, and for the lubrication of nanomachines, and will be important in the microcontrol of liquid flow. We also show that current theories of slip do not adequately describe the experimental data.

Structure Development during Shear Flow Induced Crystallization of i-PP: In Situ Wide-Angle X-ray Diffraction Study

Abstract: In situ synchrotron wide-angle X-ray diffraction (WAXD) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s-1) using a parallel-plate shear apparatus. WAXD results were used to determine the type (α- and β-crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented α-crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented β-crystals was observed. WAXD patterns clearly showed that β-crystals grew only after the formation of oriented α-crystals in the sheared i-PP melt. The contribution of β-crystals to the total crystalline phase was as high as 65−70% at high shear rates (57 and 102 s-1) and low (20%) at low shear rates (10 s-1), which was attributed to the different amount of surface area of oriented α-crystal cylindrites generate...

The effects of interparticle interactions and particle size on reversible shear thickening: Hard-sphere colloidal dispersions

Abstract: A comparison between the effects of two colloidal stabilizing methods (electrostatic versus Brownian) on the reversible shear thickening transition in concentrated colloidal suspensions is explored Five suspensions of monodisperse silica are synthesized via the Stober synthesis and dispersed in an index matched organic solvent to minimize van der Waals interactions The residual surface charge is neutralized with nitric acid (cHNO3≈01 M) resulting in a near hard-sphere interaction that is confirmed by small angle neutron scattering measurements across a range of volume fractions Rheological measurements demonstrate the effects of neutralization on the low shear and high shear rheology, which show that the onset of shear thickening moves to lower applied shear stresses and scales inversely with particle size cubed, in agreement with theory Quantitative comparisons of both the low shear viscosity and the critical stress for shear thickening to predictions for hard spheres and literature data demonstrate

Differential constitutive equations for polymer melts: The extended Pom–Pom model

Abstract: The Pom‐Pom model, recently introduced by McLeish and Larson @J. Rheol. 42, 81‐110~1998!#, is a breakthrough in the field of viscoelastic constitutive equations. With this model, a correct nonlinear behavior in both elongation and shear is accomplished. The original differential equations, improved with local branch-point displacement, are modified to overcome three drawbacks: solutions in steady state elongation show discontinuities, the equation for orientation is unbounded for high strain rates, the model does not have a second normal stress difference in shear. The modified extended Pom‐Pom model does not show the three problems and is easy for implementation in finite element packages, because it is written as a single equation. Quantitative agreement is shown with experimental data in uniaxial, planar, equibiaxial elongation as well as shear, reversed flow and step-strain for two commercial low density polyethylene ~LDPE! melts and one high density polyethylene ~HDPE! melt. Such a good agreement over a full range of well defined rheometric experiments, i.e., shear, including reversed flow for one LDPE melt, and different elongational flows, is exceptional. © 2001 The Society of Rheology. @DOI: 10.1122/1.1380426#

Shear response of layered silicate nanocomposites

Abstract: The linear and nonlinear melt state viscoelastic properties for a series of layered silicate based intercalated polymer nanocomposites are studied to elucidate the role of highly anisotropic nanometer thick layers in altering the flow properties of such hybrids. The steady shear viscosities for the nanocomposites exhibit enhanced shear-thinning at all shear rates, with the viscosity at high shear rates being almost independent of silicate loading and comparable to that of the unfilled polymer. Further, the elasticity, as measured by the first normal stress difference, when compared at constant shear stress is surprisingly independent of the silicate loading and identical to that of the unfilled polymer. This unique combination of unfilled polymerlike viscosity and elasticity for these filled nanocomposites, is attributed to the ability of the highly-anisotropic layered silicates to be oriented in the flow direction and results in a minimal contribution by the silicate layers to both the viscosity and the ...

Selectin receptor–ligand bonds: Formation limited by shear rate and dissociation governed by the Bell model

Abstract: We have studied the principles that govern the formation and dissociation of an adhesive bond between a cell moving in shear flow and a substrate and tested different theories of how force affects bond dissociation. Viscosity relates the kinematics of fluid movement (shear rate, units of time(-1)) to shear stress (units of force/area, the product of shear rate and viscosity). At different medium viscosities, the formation of receptor-ligand bonds between a cell in the flowstream and P-selectin on the vessel wall showed a similar efficiency as a function of shear rate but not of shear stress. Therefore, bond formation was a function of shear rate and hence of the kinematics of receptor and ligand movement. By contrast, the kinetics of bond dissociation was a function of shear stress and hence of force on the bond. The different requirements for bond formation and dissociation allowed dissociation kinetics to be measured at higher forces on the bond by increasing medium viscosity. Data over an extended range of forces on the bond therefore could be collected that enabled five different proposed equations, relating force to bond dissociation, to be compared for fit to experimental data. The relationship proposed by Bell [Bell, G. I. (1978) Science 200, 618-627] fit the data significantly the best and also predicted an off-rate in the absence of force that best matched an independent measurement [Mehta, P., Cummings, R. D. & McEver, R. P. (1998) J. Biol. Chem. 273, 32506-32513].

The axisymmetric contraction-expansion: the role of extensional rheology on vortex growth dynamics and the enhanced pressure drop

Abstract: The flow of a polystyrene Boger fluid through axisymmetric contraction‐expansions having various contraction ratios (2 8) and varying degrees of re-entrant corner curvatures are studied experimentally over a large range of Deborah numbers. The ideal elastic fluid is dilute, monodisperse and well characterized in both shear and transient uniaxial extension. A large enhanced pressure drop above that of a Newtonian fluid is observed independent of contraction ratio and re-entrant corner curvature. Streak images, laser Doppler velocimetry (LDV) and digital particle image velocimetry (DPIV) are used to investigate the flow kinematics upstream of the contraction plane. LDV is used to measure velocity fluctuation in the mean flow field and to characterize a global elastic flow instability which occurs at large Deborah numbers. For a contraction ratio of D 2, a steady elastic lip vortex is observed while for contraction ratios of 4 8, no lip vortex is observed and a corner vortex is seen. Rounding the re-entrant corner leads to shifts in the onset of the flow transitions at larger Deborah numbers, but does not qualitatively change the overall structure of the flow field. We describe a simple rescaling of the deformation rate which incorporates the effects of lip curvature and allows measurements of vortex size, enhanced pressure drop and critical Deborah number for the onset of elastic instability to be collapsed onto master curves. Transient extensional rheology measurements are utilized to explain the significant differences in vortex growth pathways (i.e. elastic corner vortex versus lip vortex growth) observed between the polystyrene Boger fluids used in this research and polyisobutylene and polyacrylamide Boger fluids used in previous contraction flow experiments. We show that the role of contraction ratio on vortex growth dynamics can be rationalized by considering the dimensionless ratio of the elastic normal stress difference in steady shear flow to those in transient uniaxial extension. It appears that the differences in this normal stress ratio for different fluids at a given Deborah number arise from variations in solvent quality or excluded volume effects. © 2001 Elsevier Science B.V. All rights reserved.

Shear stress induces a time- and position-dependent increase in endothelial cell membrane fluidity

Abstract: Blood flow-associated shear stress may modulate cellular processes through its action on the plasma membrane. We quantified the spatial and temporal aspects of the effects of shear stress (τ) on th...

Partial cavity flows. Part 1. Cavities forming on models without spanwise variation

Abstract: Partial cavities that formed on the vertices of wedges and on the leading edge of stationary hydrofoils were examined experimentally. The geometry of these test objects did not vary in the spanwise direction (i.e. two-dimensional). Open partial cavities formed on a series of two-dimensional wedges and on a plano-convex hydrofoil. These cavities terminated near the point of maximum cavity thickness, and small vapour-filled vortices were shed in the turbulent cavity wake. The turbulent flow in the wake of the open cavity was similar to the turbulent shear flow downstream of a rearward-facing step. Re-entrant flow was not observed in the cavity closure of open cavities, although recirculating flow associated with a region of flow separation was detected for some cases. Predictions of a two-dimensional free-streamline model of the cavitating wedge flows were compared to the experimentally observed cavities. The model predicted the profile of the open cavity only to the point of maximum cavity thickness. Examination of the flow field near the closure of the open cavities revealed adverse pressure gradients near the cavity closure. The pressure gradients around the open cavities were sufficient to cause large-scale condensation of the cavity. Unsteady re-entrant partial cavities formed on a two-dimensional NACA0009 hydrofoil. The interface of the unsteady closed cavities smoothly curved to form a re-entrant jet at the cavity terminus, and the re-entrant flow was directed upstream. The re-entrant flow impinged on the cavity interface and led to the periodic production of cloud cavitation. These cavities exhibited a laminar flow reattachment. The flow around the closed cavity was largely irrotational, while vorticity was created when the cloud cavitation collapsed downstream of the cavity. Examination of the flow field near closure of these cavities also revealed adverse pressure gradients near the partial cavity closure, but the rise in pressure did not lead to the premature condensation of the cavity.

Turbulent shear flow over active and passive porous surfaces

Abstract: The behaviour of turbulent shear flow over a mass-neutral permeable wall is studied numerically. The transpiration is assumed to be proportional to the local pressure fluctuations. It is first shown that the friction coefficient increases by up to 40% over passively porous walls, even for relatively small porosities. This is associated with the presence of large spanwise rollers, originating from a linear instability which is related both to the Kelvin–Helmholtz instability of shear layers, and to the neutral inviscid shear waves of the mean turbulent profile. It is shown that the rollers can be forced by patterned active transpiration through the wall, also leading to a large increase in friction when the phase velocity of the forcing resonates with the linear eigenfunctions mentioned above. Phase-lock averaging of the forced solutions is used to further clarify the flow mechanism. This study is motivated by the control of separation in boundary layers.

Granular shear flow dynamics and forces: experiment and continuum theory.

Abstract: We analyze the main features of granular shear flow through experimental measurements in a Couette geometry and a comparison to a locally Newtonian, continuum model of granular flow. The model is based on earlier hydrodynamic models, adjusted to take into account the experimentally observed coupling between fluctuations in particle motion and mean-flow properties. Experimentally, the local velocity fluctuations are found to decrease more slowly with distance from the shear surface than the velocity. This can be explained by an effective viscosity that diverges more rapidly as the random-close-packing density is approached than is predicted by Enskog theory for dense hard-sphere systems. Experiment and theory are in good agreement, especially for the following key features of granular flow: The flow is confined to a small shear band, fluctuations decay approximately exponentially away from the sheared wall, and the shear stress is approximately independent of the shear velocity. The functional forms of the velocity and fluctuation profiles predicted by the model agree with the experimental results.

The molecular stress function model for polydisperse polymer melts with dissipative convective constraint release

Abstract: The molecular stress function theory for polymer melts is extended to include a new, dissipative convective constraint release process. First the Helmholtz free energy of tube segments with strain-dependent tube diameter is established neglecting constraint release, and it is demonstrated that the molecular stress is a function of the average logarithmic stretch under these conditions. Then convective constraint release is introduced as a dissipative process in the energy balance of tube deformation, which leads to a strain-dependent evolution equation for the molecular stress function. Constraint release is considered to be the consequence of different convection mechanisms for tube orientation and tube cross section. Our new, dissipative constraint release model emphasizes that tube kinematics are fundamentally different for rotational and nonrotational flows, and therefore distinguishes explicitly between simple shear and pure shear (planar extension). For the startup of simple shear and extensional flows, the predictions of our set of constitutive equations consisting of a history integral for the stress tensor and a differential evolution equation for the molecular stress function with only two nonlinear material parameters are in excellent agreement with experimental data of a polydisperse high-density polyethylene (HDPE) and a polydisperse low-density polyethylene (LDPE) melt. Also, stress relaxation after step-shear strain is described for both the HDPE and the LDPE melt.

Microrheological Modeling of Flow-Induced Crystallization

Abstract: The problem of flow-induced crystallization (FIC) of polymer melts is addressed via a microrheological approach. In particular, the Doi−Edwards model with the so-called independent alignment approximation (DE−IAA) is used to calculate the flow-induced change of free energy. Subsequently, the crystallization induction time, i.e., the nucleation characteristic time, is calculated in isothermal steady shear and uniaxial elongational flows. Asymptotic, analytical expressions for the induction time are also derived in the limit of low and high Deborah number (the product of the deformation rate and the polymer relaxation time). The DE−IAA model is found to give more realistic predictions than those of simpler, dumbbell-like models already proposed in the literature. When compared to existing FIC experimental data in shear flow, good quantitative agreement is found with the polymer relaxation time as the only adjustable parameter of the model.

Experimental Study of the Grain‐Flow, Fluid‐Mud Transition in Debris Flows

Abstract: We have performed a series of laboratory experiments that clarify the nature of the transition between fluid‐mud and grain‐flow behavior. The surface velocity structure and the speed of the nose of debris flows in channels with semicircular cross sections were measured with several cameras and visual tracers, while the mass flow rate was recorded using a load cell at the exit chamber. Other rheological tests were used to calculate independently the yield strength and matrix viscosity of the debris‐flow mixture. Shear rates were varied by nearly an order of magnitude for each mixture by changing the channel radius and slope. Shear rates were significantly higher than expected (6–55 s−1), given the modest slopes examined (10.7°–15.2°). The large values were primarily a result of the concentration of shear into narrow bands between a central nondeforming plug and the sidewall. As a result, the shear rate of interest was calculated by using the width of the shear band and the plug velocity, as oppose...

Dynamics of dilute and semidilute DNA solutions in the start-up of shear flow

Abstract: We have investigated the dynamics of dilute (10−5C*) and semidilute (⩽6C*) DNA solutions both in steady and in the start-up of shear flow by combining fluorescence microscopy, bulk rheological measurements, and Brownian dynamics simulations. First, the microscopic states, i.e., the conformational dynamics of single DNA molecules in solution during the start-up of shear flow, were examined by fluorescence microscopy. To investigate the macroscopic response resulting from the changes in the microscopic state, the bulk shear viscosity of the same DNA solutions was also measured. While the transient dynamics of individual molecules is highly variable, an overshoot in the ensemble-averaged molecular extension is observed above a critical Wi following an overshoot in shear viscosity for both dilute and semidilute DNA solutions. These two overshoots are further analyzed and explained on a physical basis from our simulation findings. Based on the physical picture, we have derived a simple scaling to predict the s...

New measurements of the flow-curves for Carbopol dispersions without slip artefacts

Abstract: The thickening properties of many commercial thickeners are difficult to measure because of wall slip artefacts. Here we report a series of experiments on a typical thickener where these artefacts have been successfully eliminated. As a result, complete, steady-state flow-curves of aqueous Carbopol 980 (the toxicologically preferred version of the older and more well-known Carbopol 940) dispersions are reported for a range of concentrations of 0.045–1.0 wt%. The vane-and-basket flow geometry was used to avoid slip problems at low shear stress, with the geometry housed in a TA AR1000-N controlled-stress rheometer, whilst a Haake RV2 viscometer with an SV2P and MV2P concentric-cylinder geometries were used at higher shear rates. The flow-curves obtained show a smooth but steep transition from a very high Newtonian viscosity at low shear stress to a much lower viscosity at high shear stress. No real yield stresses were detected, but the higher shear rate results can be fitted to the Herschel-Bulkley model, which assumes an apparent yield stress. The various model parameters are displayed as a function of Carbopol concentration.

Influence of shear flow on vesicles near a wall: A numerical study.

Abstract: We describe the dynamics of three-dimensional fluid vesicles in steady shear flow in the vicinity of a wall. This is analyzed numerically at low Reynolds numbers using a boundary element method. The area-incompressible vesicle exhibits bending elasticity. Forces due to adhesion or gravity oppose the hydrodynamic lift force driving the vesicle away from a wall. We investigate three cases. First, a neutrally buoyant vesicle is placed in the vicinity of a wall that acts only as a geometrical constraint. We find that the lift velocity is linearly proportional to shear rate and decreases with increasing distance between the vesicle and the wall. Second, with a vesicle filled with a denser fluid, we find a stationary hovering state. We present an estimate of the viscous lift force that seems to agree with recent experiments of Lorz et al. [Europhys. Lett. 51, 468 (2000)]. Third, if the wall exerts an additional adhesive force, we investigate the dynamical unbinding transition that occurs at an adhesion strength linearly proportional to the shear rate.

Microscopic theory of convective constraint release

Abstract: We develop a microscopic description of the contribution to stress relaxation in entangled polymer melts of convective constraint release, which is the release of entanglement constraints due to the effects of convective flow on chains surrounding a given chain. Our theory resolves three of the main shortcomings of the Doi–Edwards model in nonlinear rheology, in that it predicts (1) a monotonically increasing shear stress as a function of shear rate, (2) shear stress independent of molecular weight at sufficiently high shear rates, and (3) only modest anisotropies in the single chain scattering function, in agreement with experiment. In addition, our approach predicts that a stress maximum and resulting shear-banding instability would occur for living micelle solutions, as observed.

A simple constitutive equation for entangled polymers with chain stretch

Abstract: We propose a simple way of including chain stretch effects in convective constraint release theories for entangled polymers. The main idea is that the characteristic time of orientational relaxation depends in a series-parallel way on all three relevant mechanisms, i.e., reptation, constraint release (thermal and convective), and Rouse relaxation. As usual, a separate equation describes chain stretch, which however is assumed not to be affected by constraint release. The model is further simplified by writing the orientational equation in differential form. For step strains, the successful damping function of the Doi–Edwards theory is exactly preserved. Predictions in steady shear also favorably compare with typical data of nearly monodisperse polymers.

One-dimensional turbulence: vector formulation and application to free shear flows

Abstract: One-dimensional turbulence is a stochastic simulation method representing the time evolution of the velocity profile along a notional line of sight through a turbulent flow. In this paper, the velocity is treated as a three-component vector, in contrast to previous formulations involving a single velocity component. This generalization allows the incorporation of pressure-scrambling effects and provides a framework for further extensions of the model. Computed results based on two alternative physical pictures of pressure scrambling are compared to direct numerical simulations of two time-developing planar free shear flows: a mixing layer and a wake. Scrambling based on equipartition of turbulent kinetic energy on an eddy-by-eddy basis yields less accurate results than a scheme that maximizes the intercomponent energy transfer during each eddy, subject to invariance constraints. The latter formulation captures many features of free shear flow structure, energetics, and fluctuation properties, including the spatial variation of the probability density function of a passive advected scalar. These results demonstrate the efficacy of the proposed representation of vector velocity evolution on a one-dimensional domain.

Instability of Elastic Filaments in Shear Flow Yields First-Normal-Stress Differences

Abstract: Using slender-body hydrodynamics, we study the flow-induced deformation of a high-aspect-ratio elastic filament. For a filament of zero rest curvature rotating in a viscous linear shear flow, our model predicts a bifurcation to shape instabilities due to compression by the flow, in agreement with experimental observations. Further, nonlinear simulations of this shape instability show that in dilute solutions, flexibility of the fibers causes both increased shear thinning as well as significant nonzero first-normal-stress differences. These stress differences are positive for small-to-moderate deformations, but negative for large-amplitude flexing of the fibers.

Droplet deformation in dispersions with unequal viscosities and zero interfacial tension

Abstract: An analytical model is presented for the deformation of an ellipsoidal Newtonian droplet, suspended in another Newtonian fluid with different viscosity and zero interfacial tension. The theory is exact for any linear velocity field, and is not limited to small deformations. It encompasses some well-known special cases, such as Jeffery's equation for solid axisymmetric particles and Taylor's small-deformation theory for droplets. Example calculations exhibit droplet stretching, reorientation, and tumbling, and provide a reasonable match to available experimental data on transient and steady droplet shapes. The corresponding rheological theory for dilute dispersions is also derived, in a form that explicitly includes the effects of microstructure on dispersion rheology.

Droplet breakup in concentrated emulsions

Abstract: In this paper we report an experimental study on the conditions for droplet breakup in concentrated emulsions under simple shear flow. We present a set of experiments where the ratio between drop and matrix viscosity was varied from 0.1 to 22 and the volume fraction ranged from 0% to 70%. It was observed that the critical shear rate for breakup decreased by more than an order of magnitude for the most concentrated emulsions. Further, drops with viscosity ratio of 22 were seen to rupture in simple shear as soon as the emulsion concentration was raised to 40%. All these effects were conveniently explained by means of a mean field model which assumes simply that breakup of a droplet in a concentrated emulsion is determined by the average emulsion viscosity rather than the continuous phase viscosity.

Variational principles for propagation speeds in inhomogeneous media

Abstract: An important problem in reactive flows is how to estimate the speed of front propagation, especially when inhomogeneities are present. Here we prove a variational characterization of the front speed for reaction-diffusion-advectionequations in periodically varying heterogeneous media. This formulation makes it possible to calculate sharp estimates for the speed explicitly. The method can be applied to any problem obeying a maximum principle. Three examples will be analyzed in detail: a shear flow problem, a problem with rapidly oscillating coefficients, and a discretized diffusion problem. In all cases the effects of the inhomogeneous medium on the speed are discussed in comparison to the homogeneous problem. For the shear flow problem, enhancement of the speed results.

Fluid instabilities in precessing spheroidal cavities

Abstract: We study by direct numerical simulation the motion of incompressible fluid contained in an ellipsoid of revolution with ellipticity 0.1 or less which rotates about its axis of symmetry and whose rotation axis is executing precessional motion. A solution to this problem for an inviscid fluid given by Poincare (1910) predicts motion of uniform vorticity. The simulations show how the orientation of the average vorticity of a real fluid is influenced by both pressure and viscous torques exerted by the boundaries. Axisymmetric shear layers appear which agree well with those observed experimentally by Malkus (1968). Shear caused by deviations from a velocity field with uniform vorticity triggers an instability consisting of waves propagating around the average rotation axis of the fluid. The Ekman layers at the boundaries may also become unstable.

Flow analysis of an annular jet by particle image velocimetry and proper orthogonal decomposition

Abstract: This paper discusses the application of proper orthogonal decomposition (POD) to the particle image velocimetry (PIV) velocity fields of the recirculation zone of an annular jet. The annular jet is an example of complex shear flow situations. Two axisymmetrical shear layers, originating at the exit of the jet, one at the lip of the nozzle and the other in the centre of the body, eventually meet downstream or interact with each other. In this study we propose use of the POD on PIV velocity fields of the recirculation zone of this annular jet. According to POD analysis, the primary objective of this paper is to describe the mode effects on the understanding of the nature of the flow and especially the link between the first eigenfunctions of POD and the motion of the stagnation point at the extremity of the recirculation zone of the annular jet. In particular, this allows us to dissociate the oscillation and velocity fluctuations due to the turbulent flow behaviour.

Influence of grazing flow and dissipation effects on the acoustic boundary conditions at a lined wall

Abstract: The problem of sound propagation near a lined wall taking into account mean shear flow effects and viscous and thermal dissipation is investigated. The method of composite expansion is used to separate the inviscid part, in the core of the flow, from the boundary layer part, near the wall. Two diffusion equations for the shear stress and the heat flux are obtained in the boundary layer. The matching of the solutions of these equations with the inviscid part leads to a modified specific acoustic admittance in the core flow. Depending on the ratio of the acoustic and stationary boundary layer thicknesses, the kinematic wall condition changes gradually from continuity of normal acoustic displacement to continuity of normal acoustic mass velocity. This wall condition can be applied in dissipative silencers and in aircraft engine-duct systems.