Effects Of Fluid Viscoelasticity Research Articles

The effect of fluid viscoelasticity in soft lubrication

This study explores the influence of fluid viscoelasticity in soft lubrication, in which elastohydrodynamic lubrication (EHL) plays an important role. Our findings reveal that introducing polymers can significantly reduce the friction coefficient, particularly for high Deborah numbers, i.e., the ratio between the polymer relaxation time and the flow residence time, due mainly to an increased minimum film height. This augmented film thickness reduces the Newtonian pressure contributions, lowering friction. The study highlights the non-linear relationship between Deborah numbers, load, and viscoelasticity effects, as well as the complex interplay between these factors in the Pipkin space analysis. These insights provide a comprehensive understanding of the fluid viscoelasticity in soft lubricated contacts.

Formation of viscoelastic droplets in asymmetrical parallel microchannels

The dynamics of viscoelastic droplet formation in T-shaped asymmetric parallel microchannels are investigated using a high-speed camera. The effects of fluid viscoelasticity and flow rates of the two phases on the stability and uniformity of droplets, fluid flow distribution, and droplet formation period are explored. The results reveal that the viscoelasticity advances the transition from the squeezing stage to the rapid pinch-off stage, slows down the necking of rapid pinch-off stage and increases the duration of the stretching stage. It is found that the stability is significantly improved by increasing the continuous phase flow rate, and the droplet uniformity is affected by both the flow rates of the two phases and the fluid elasticity. In addition, prediction models for the fluid flow distribution, droplet size and droplet formation period are established. At last, the breakup mode of different elasticity filament and the satellite droplet size are studied. It is concluded that the increased elasticity contributes to the elimination of satellite droplets, and the satellite droplet size are affected by the fluid elasticity and the continuous phase flow rate.

The effect of fluid viscoelasticity in lubricated contacts in the presence of cavitation

In this work we study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Several studies of viscoelastic lubricants have been carried out, but none of them have considered the possibility of the presence of cavitation. To describe the effect of viscoelasticity, we use the Oldroyd-B model. By assuming that the product between ϵ, i.e. the ratio between vertical and horizontal length scales, and the Weissenberg number (Wi), i.e. the ratio between polymer relaxation time and flow time scale, is small, we can linearise the viscoelastic thin film equations, following the approach pioneered by "Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model." Consequently, the zeroth-order in ϵWi corresponds to a Reynolds equation modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We consider the flow of viscoelastic lubricants using: (i) a cosine profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. The introduction of viscoelasticity decreases the length of cavitated region in the cosine profile due to the increasing pressure distribution within the film. Consequently, the load carrying capacity increases with Wi by up to 50% in the most favorable condition, confirming the beneficial influence of the polymers in bearings. On the other hand for the pocketed profile, results show that the load can increase or decrease at higher Wi depending on the texture position in the contact. The squeeze flow problem between two plates is also modeled for viscoelastic lubricants considering an oscillating top surface. For this configuration a load reduction is observed with increasing Wi due to the additional time needed to reform the film at high Wi. Furthermore, if viscoelastic effects increase, the cavitation region widens until reaching a value of Wi for which a full-film reformation does not occur after the initial film rupture.

Dynamics of spheroids in an unbound quadratic flow of a general second-order fluid

This work employs the second-order fluid model to investigate the effect of first and second normal stresses on the motion of spheroidal particles in unbound parabolic flows, where particles migrate toward the flow center. We specifically examine the effects of fluid Weissenberg number Wi and the ratio of normal stress coefficients α = ψ2/ψ1. Previous works have considered the motion of spheroidal particles in the co-rotational limit (α = −0.5), where the effect of fluid viscoelasticity is to modify the fluid pressure but not the shear stresses. Here, we examine all ranges of α that are found for functional complex fluids such as dilute polymer solutions, emulsions, and particulate suspensions and determine how viscoelastic shear stresses alter particle migration. We use perturbation theory and the Lorentz reciprocal theorem to derive the O(Wi) corrections to the translational and rotational velocities of a freely suspended spheroid in an unbound tube or slit flow. Our results show that for both prolate and oblate particles, the viscoelasticity characterized by α significantly affects the particle cross-stream migration, but does not qualitatively change the trends seen in the co-rotational limit (α = −0.5). For a range of α (−0.9 ≤ α ≤ 0) investigated in this work, particles possess the largest mobility when α = −0.9 and smallest mobility when α = 0. Although α does not alter particle rotation at a given shear rate, we observe significant changes in particle orientation during migration toward the flow center because changes in migration speed give rise to particles experiencing different shear histories.

A note on the breathing mode of an elastic sphere in Newtonian and complex fluids

Experiments on the acoustic vibrations of elastic nanostructures in fluid media have been used to study the mechanical properties of materials, as well as for mechanical and biological sensing. The medium surrounding the nanostructure is typically modeled as a Newtonian fluid. A recent experiment however suggested that high-frequency longitudinal vibration of bipyramidal nanoparticles could trigger a viscoelastic response in water-glycerol mixtures [Pelton et al., “Viscoelastic flows in simple liquids generated by vibrating nanostructures,” Phys. Rev. Lett. 111, 244502 (2013)]. Motivated by these experimental studies, we first revisit a classical continuum mechanics problem of the purely radial vibration of an elastic sphere, also called the breathing mode, in a compressible viscous fluid and then extend our analysis to a viscoelastic medium using the Maxwell fluid model. The effects of fluid compressibility and viscoelasticity are discussed. Although in the case of longitudinal vibration of bipyramidal nanoparticles, the effects of fluid compressibility were shown to be negligible, we demonstrate that it plays a significant role in the breathing mode of an elastic sphere. On the other hand, despite the different vibration modes, the breathing mode of a sphere triggers a viscoelastic response in water-glycerol mixtures similar to that triggered by the longitudinal vibration of bipyramidal nanoparticles. We also comment on the effect of fluid viscoelasticity on the idea of destroying virus particles by acoustic resonance.

Effect of Fluid Viscoelasticity on Turbulence and Large-Scale Vortices behind Wall-Mounted Plates

Direct numerical simulations of turbulent viscoelastic fluid flows in a channel with wall-mounted plates were performed to investigate the influence of viscoelasticity on turbulent structures and the mean flow around the plate. The constitutive equation follows the Giesekus model, valid for polymer or surfactant solutions, which are generally capable of reducing the turbulent frictional drag in a smooth channel. We found that turbulent eddies just behind the plates in viscoelastic fluid decreased in number and in magnitude, but their size increased. Three pairs of organized longitudinal vortices were observed downstream of the plates in both Newtonian and viscoelastic fluids: two vortex pairs were behind the plates and the other one with the longest length was in a plate-free area. In the viscoelastic fluid, the latter vortex pair in the plate-free area was maintained and reached the downstream rib, but its swirling strength was weakened and the local skin-friction drag near the vortex was much weaker than those in the Newtonian flow. The mean flow and small spanwise eddies were influenced by the additional fluid force due to the viscoelasticity and, moreover, the spanwise component of the fluid elastic force may also play a role in the suppression of fluid vortical motions behind the plates.

Symmetry-breaking Bifurcations in T-channel Flows: Effects of Fluid Viscoelasticity

It is well known that, beyond a critical aspect-ratio-dependent Reynolds number, the flow in a three-dimensional T-channel junction, i.e. two-opposing planar channel streams joining and turning through 90 degrees, can break symmetry. For the case of two square cross-section inlets and an outlet arm of equal area (i.e. aspect ratio of two) this bifurcation is to a steady asymmetric flow. This flow bifurcation has been proposed as a method of enhancing mixing in microfluidic channels where significant increases in mixing quality are observed beyond the bifurcation. In the current work we investigate numerically the effects of viscoelasticity on this supercritical pitchfork bifurcation using a numerical finite-volume method. Results from both the upper-convected Maxwell and Oldroyd-B models show that the instability occurs at lower Reynolds numbers for viscoelastic fluids in comparison to the Newtonian base case. At higher Deborah numbers the transition leads directly to an unsteady flow.

Effects of Fluid Viscoelasticity on the Performance of an Axial Blood Pump Model

An aqueous Xanthan gum solution (XGS) was used as blood analog fluid to explore the influence of fluid viscoelasticity on the performance of an axial blood pump model. For comparison, a 39 wt% Newtonian aqueous glycerin solution (GS), the common fluid in blood pump tests, was also used as a working fluid. The experimental results showed that a higher head curve was obtained using XGS in the pump than using GS. The heads of the XGS that were computed using the viscoelastic turbulence model agreed well with the measured data. In contrast, the standard k-ε turbulence model failed to provide satisfactory predictions for the XGS. The computational results revealed that in most parts of the pump model flow fields, the Reynolds shear stress values and turbulent dissipation rates of the XGS were all lower than those of the GS. The hemolysis index of the pump model using the XGS was calculated to be only one-third of that using the GS.

NON-NEWTONIAN FLOWS IN CONTRACTING/EXPANDING BLOOD VESSELS: A THEORETICAL STUDY

The paper deals with a problem related to the flow of blood in vessels. It is known that physiological pumps produce flow by alternate contraction and expansion of the vessel. When muscles start squeezing the vessel wall, the valve at the upstream end gets closed and that at the downstream end becomes open. As a result of this, blood is pumped out in the downstream direction. During systole, when the blood in the left ventricle is forced into the aorta, the mitral valve becomes closed, while the atrioventricular valve becomes open. In a situation such as this, the left ventricle takes the form of a vessel with one end closed. In order to study such a situation, a model has been developed here that consists of a fluid-filled long cylindrical pipe with one end closed by a compliant membrane (that prevents the axial motion of the fluid, leaving radial motion completely unrestricted). An approximate solution of the Navier–Stokes equation for the flow of a second-order fluid is presented. The derived analytical expressions are computed numerically. The computed values indicate that if the Reynolds number of the flow is low, the effect of fluid viscoelasticity appears in higher-order terms.

The effects of fluid viscoelasticity on the settling behaviour of horizontally aligned spheres

A study on the interaction of particles settling in non-Newtonian fluids of shear-thinning, thixotropic and viscoelastic characteristics has been conducted. Key aspects of the rheological characteristics of the fluids that influence the interaction of the particles were examined by analysing the trajectories of two particles that are initially placed side-by-side in the fluid medium. The interaction of the particles was found to be highly dependent on the separation distance that is initially set between them. If the initial distance is smaller than a critical value, the spheres would tend to attract and converge. In cases where the initial distance is greater than this critical value, the two spheres would tend to diverge, resulting in a slight (∼20%) increase in their separation distance over their course of settling. This tendency to diverge was found to diminish as the initial distance is increased further from the critical value. The magnitude of the critical separation distance was found to be primarily dependent on the normal stresses of the fluids. A correlation was thus proposed based on this observation. In cases where the two spheres do attract and converge, it was found that the spheres tend to follow a non-symmetrical trajectory, where one of the spheres possesses a slightly lower settling velocity than the other. As a result, the spheres appear to re-arrange themselves into a vertically aligned configuration. Once aligned, the shear-thinning and thixotropic characteristics of the fluid causes the lagging sphere to accelerate and collide with the leading sphere.

Critical conditions and breakup of non-squashed microconfined droplets: effects of fluid viscoelasticity

Droplet breakup in systems with either a viscoelastic matrix or a viscoelastic droplet is studied microscopically in bulk and confined shear flow, using a parallel plate counter rotating shear flow cell. The ratio of droplet diameter to gap spacing is systematically varied between 0.1 and 0.85. In bulk shear flow, the effects of matrix and droplet viscoelasticity on the critical capillary number for breakup are very moderate under the studied conditions. However, in confined conditions a profoundly different behaviour is observed: the critical capillary numbers of a viscoelastic droplet are similar to those of a Newtonian droplet, whereas matrix viscoelasticity causes breakup at a much lower capillary number. The critical capillary numbers are compared with the predictions of a phenomenological model by Minale et al. (Langmuir 26:126–132, 2010); the model results are in qualitative disagreement with the experimental data. It is also found that the critical dimensionless droplet length, the critical capillary number, and the dimensionless droplet length at breakup show a similar dependency on confinement ratio. As a result, confined droplets in a viscoelastic matrix have a smaller dimensionless length at breakup than droplets in a Newtonian matrix, which affects the breakup mode. Whereas confined droplets in a Newtonian matrix can break up into multiple parts, only two daughter droplets are obtained after breakup in a viscoelastic matrix, up to very large confinement ratios.

Gas-assisted displacement of a viscoelastic fluid in capillary geometries

The effect of fluid viscoelasticity on the fraction of liquid deposited on the walls of capillary geometry and the pressure drop at the capillary static region are theoretically investigated using the Criminale–Ericksen–Filbey (CEF) constitutive equation to describe a non-Newtonian fluid displaced by the pressurized gas in a capillary. The singular perturbation method is used to determine the residual liquid film thickness of a viscoelastic fluid on the walls of a circular tube or a rectangular channel when displaced by another immiscible fluid. Inner and outer expansions are developed in terms of both a small parameter Ca 1 / 3 and a small parameter De / Ca 1 / 3 . The method of matched asymptotic expansion is used to match the inner and outer solutions by means of a transition region between the advancing meniscus and the entrained film where the fluid rheology has its greatest effect. A detailed analysis indicates that the residual liquid film thickness of the viscoelastic fluid tends to decrease and the pressure drop across the bubble front tends to increase as the fluid becomes more viscoelastic. The theoretical results presented in this paper are in agreement with some of the experimental data and theoretical analyses available in the literature.

Bubble dynamics in viscoelastic fluids with application to reacting and non-reacting polymer foams

The effects of fluid viscoelasticity on the expansion of gas bubbles in polymer foams for the cases of reactive and non-reactive polymers are investigated. For non-reactive polymers, bubble expansion is controlled by a combination of gas diffusion and fluid rheology. In the diffusion limited case, the initial growth rate is slow due to small surface area, whereas at high diffusivity initial growth is rapid and resisted only by background solvent viscosity. In this high Deborah number ( De) limit, we see a two stage expansion in which there is an initial rapid expansion up to the size at which the elastic stresses balance the pressure difference. Beyond this time, the bubble expansion is controlled by the relaxation of the polymer. In the model for reactive polymer systems, the polymer molecules begin as a mono-disperse distribution of a single reacting species. As the reaction progresses molecules bond to form increasingly large, branched, structures each with a spectrum of relaxation modes, which gel to form a viscoelastic solid. Throughout this process gas is produced as a by-product of the reaction. The linear spectrum for this fluid model is calculated from Rubinstein et al. [Dynamic scaling for polymer gelation, in: F. Tanaka, M. Doi, T. Ohta (Eds.), Space–Time Organisation in Macromolecular Fluids, Springer, Berlin, 1989, pp. 66–74], where the relaxation spectrum of a molecule is obtained from percolation theory and Rouse dynamics. We discretise this linear spectrum and, by treating each mode as a mode in a multimode Oldroyd-B fluid obtain a model for the non-linear rheology. Using this model, we describe how the production of gas, diffusion of gas through the liquid, and evolution of the largest molecule are coupled to bubble expansion and stress evolution. Thus, we illustrate how the rate of gas production, coupled to the rate of gas diffusion, affects the bubble size within a foam.

Effect of fluid viscoelasticity on the draw resonance dynamics of melt spinning

The effect of fluid viscoelasticity on the draw resonance dynamics of melt spinning has been examined using White-Metzner and Phan Thien-Tanner fluid models into the governing equations of the process, in a continued effort to study the effects of process conditions and material properties on draw resonance, following up the earlier study [J. Non-Newtonian Fluid Mech. 87 (1999) 165] dealing with the effect of spinline cooling on the same draw resonance. Whether or not the fluid viscoelasticity stabilizes melt spinning has turned out to coincide with whether or not the spinline tension sensitivity decreases with the increasing fluid viscoelasticity. This is because the spinning stability is always enhanced by a decrease in tension sensitivity to process disturbances and this tension sensitivity was then found in the said earlier study to be moving opposite to the level of the spinline tension: the higher spinline tension, the smaller tension sensitivity. It has been found in the present study that the effect of fluid viscoelasticity on spinning stability can be classified into two diametrically different kinds: for extension-thickening fluids an increase in viscoelasticity increases tension, decreases tension sensitivity and, thus, stabilizes the spinning, whereas it decreases tension, increases tension sensitivity and, thus, destabilizes the spinning of extension-thinning fluids.

Temperature and flow fields for the flow of a second grade fluid past a wedge

The effects of fluid viscoelasticity on the temperature and flow fields of the flow past a wedge were studied. The combined effects of the shape factor, suction/injection rates and viscoelasticity were analysed by the series expansion method, similarity transformation, fourthorder Runge-Kutta integration and the shooting method. The stream function was introduced into the momentum boundary layer equations of a second-grade fluid to eliminate the pressure terms. The energy equation is also analysed by the similarity transformation. The results indicated that the velocity profiles, temperature distributions, surface friction and heat transfer are greatly affected by the elastic parameter, the shape factor, the Prandtl number and the suction/injection rates.

Nonlinear viscoelastic effects on the harmonic generation of a finite‐amplitude acoustic wave

The effect of fluid viscoelasticity on the second‐harmonic wave generated by a planar acoustic wave is considered. It is shown that the growth, decay, wavenumber, phase, and amplitude of the second‐harmonic wave are all modified by fluid viscoelasticity. The effect on the amplitude, the most readily measured property, is discussed in detail. It is found that viscoelasticity in the form of frequency thinning causes a shift in the maximum of the second‐harmonic amplitude to distances further from the acoustic source. Finite linear and nonlinear viscoelasticity are found to cause both harmonic suppression and harmonic enhancement, depending on the properties of the fluid and the dimensionless frequency of oscillation. The harmonic suppression predicted here may be associated with changes in the noise spectrum of cavitation that have been observed for dilute polymer solutions.