Jet Of Viscoelastic Fluid Research Articles

Direct numerical simulations of turbulent viscoelastic jets

Direct numerical simulations (DNS) of spatially evolving turbulent planar jets of viscoelastic fluids described by the FENE-P model, such as those consisting of a Newtonian fluid solvent carrying long chain polymer molecules, are carried out in order to develop a theory for the far field of turbulent jets of viscoelastic fluids. New evolution relations for the jet shear-layer thickness , centreline velocity and maximum polymer stresses are derived and validated by the new DNS data, yielding , , and , respectively, where is the coordinate in the streamwise direction. It is shown that, compared with a classical (Newtonian) turbulent jet, the effect of the polymers is to reduce the spreading rate, centreline velocity decay, Reynolds stresses and viscous dissipation rate. The self-preserving character of the flow is analysed and it is shown that profiles of mean velocity, Reynolds stresses and polymer stresses are self-similar provided the proper scales are used in the normalisation of these quantities. A fundamental difference from the Newtonian jet in this regard is the necessity for two, instead of only one, different velocity and length scales to properly characterise the evolution of the turbulent flow. These extra velocity and length scales are directly related to a time scale associated with the characteristic fading memory property of viscoelastic fluids.

Nonlinear electrohydrodynamic instability through two jets of an Oldroydian viscoelastic fluids with a porous medium under the influence of electric field

In the light of the viscous potential theory, a nonlinear instability of the surface waves propagating through two jets of viscoelastic fluids is scrutinized. The flow is jetting through porous media under the action of a uniform axial electric field. The Oldroyd B model is employed to characterize the rheological behavior of the viscoelastic fluid. A weakly nonlinear theory of wave propagation is deemed. A general dispersion relation and neutral curves are addressed and plotted for the different parameters. The Routh-Hurwitz criterion is utilized to determine the stability criteria. Several special cases are reported. It found that, in absence of porous media, the stability happens for large relaxation time and small retardation time. In addition, small retardation time leads to increase the elasticity of the fluid. Therefore, the stability is determined due to the elasticity of the fluid. However, in the presence of porous medium, the elasticity of the fluid decreases with the increase of relaxation time. Thus, the elasticity has a destabilizing effects. Furthermore, fluid viscosity has showed dual role of the stability diagram. Also, the case of Maxwell fluid is illustrated graphically. It is observed that Maxwell model is less stabilizing than Oldroyd model. The method of multiple time scale with the Taylor’s expansion is employed to conduct the well-known Ginzburg-Landau equation, which governs the nonlinear stability of the system. New several regions of stability and instability are identified due to the nonlinear effects.

Numerical study on polymer nanofibers with electrically charged jet of viscoelastic fluid in electrospinning process

Electrospinning is a useful and efficient technique to produce polymeric nanofibers. Nanofibers of polymers are electrospun by creating an electrically charged jet of polymer solution. Numerical study on non-Newtonian and viscoelastic jets of polymer nanofibers in electrospinning process is presented in this work. In particular, the effect of non-Newtonian rheology on the jet profile during the electrospinning process is examined. The governing equations of the problem are solved numerically using the Keller-Box method. The effects of yield stress and power-law index on the elongation, velocity, stress and total force are presented and discussed in detail. The results show that by increasing the values of yield stress, the fluid elongation is reduced significantly.

粘弾性流体の噴流の傾斜平板着地時に形成される幾何学的パターン

粘弾性流体の噴流の傾斜平板着地時に形成される幾何学的パターン

Experimental study of atomization patterns produced by the oblique collision of two viscoelastic liquid jets

Experimental observations and analysis are presented for the formation and atomization of the fluid sheet created by obliquely colliding jets of viscoelastic fluids. Solutions of mono-disperse polystyrene (PS) in diethylphthalate and of poly-disperse polyethylene oxide (PEO) in glycerol/water mixtures were used to investigate the effects of fluid elasticity on the break-up patterns generated by the impact of two jets ejected from nozzles with an internal diameter of 0.85 mm. Various regimes of behaviour were identified which depend on the jet speed. The structures observed for these elastic fluids differ somewhat from those previously reported for Newtonian viscous fluids, and also show different behaviours depending on the degree of viscoelasticity. This study focuses on the periodic atomization, the so-called fishbone pattern, which occurs when the impinging jets form a liquid sheet which breaks up into a regular succession of ligaments and droplets. High-speed flash photography reveals that low concentrations of polymers significantly affect the evolution of the sheet and its fragmentation, the shapes of the ligaments, and the final drop sizes. The maximum fishbone angle is defined and shown to be a useful tool to describe the variation of the atomization pattern with polymer concentration. For the PS solutions the variation of maximum fishbone angle with reduced polymer concentration ( c/c * ) follows a single master curve, but although the same is true for PEO with high molecular weights, the curves remain separate for low molecular weights. Observation of the fishbone patterns formed by the oblique impact of jets may provide a useful tool to observe and characterize inter-chain interaction in high speed extensional flow of polymer solutions.

Stable Jets of Viscoelastic Fluids and Self-Assembled Cylindrical Capsules by Hydrodynamic Focusing

We demonstrate formation of long-lived cylindrical jets of a viscoelastic fluid using hydrodynamic focusing. A solution of polyacrylamide in water is driven coaxially with immiscible oil and subjected to strong extensional flow. At high flow rates, the aqueous phase forms jets that are 4-90 microm in diameter and several centimeters long. The liquid surfaces of these jets are then used as templates for assembly of microspheres into novel rigid and hollow cylinders.

On the evolution and rupture of filaments in Giesekus and FENE models

We study, through direct numerical simulation and asymptotic analysis, the evolution and break-up of jets of viscoelastic fluids of Giesekus and FENE-P type. In both cases, the jet diameter decreases linearly with time close to break-up in the inertial regime and the profiles are self-similar asymmetric. Sufficiently far from break-up, an inertialess regime develops with self-similar symmetric features. We deduce and verify numerically the scaling laws obeyed by the jet during the evolution. In particular, we describe the self-similar nature of the neck between thinning filaments and drops.

Transmission Behavior of Deformation on Viscoelastic Fluid Jet Driven by Gravity (Standing Wave Generated by Lateral Periodic Deformation).

In the previous work, we investigated the transmission behavior of deformation, caused by a lateral impulsive hit, on a viscoelastic fluid jet falling vertically in air driven by the gravity. In this work, the jet is subjected to a lateral periodic deformation at a point. A standing wave is formed on the jet at a suitable frequency and position of the deformation. The wave length of the standing wave is strongly affected by the frequency of the deformation, the deformation position and concentration of the polymer solutions. Since the flow rate does not affect the shape of the standing wave, it is found that the transmission velocity of the periodic deformation is independent of the falling velocity of the jet and the deformation transmits with a constant velocity along the jet. The normal stress acting on the jet can be calculated from the transmission velocity of the standing wave with fluid density. The elongation viscosity is estimated by the normal stress and elongation rate evaluted by the jet shape.

Transmission Behavior of Deformation on Viscoelastic Fluid Jet Driven by Gravity. Mechanism of Transmission Behavior of Pulse Deformation.

Mechanism of transmission behavior of a pulse deformation on a viscoelastic fluid jet is investigated. The jet of the viscoelastic fluid falls down in air vertically due to gravity. The pulse deformation is formed on the jet by impact of collision with a swinging rod. The deformation transmits to upward on the jet and rebounds off the nozzle exit like wave motion on a string. The motion of the deformation is recorded by a video camera to evaluate its transmission velocity. The transmission velocity decreases with increasing flow rate and it demonstrates the wave transmitts with a velocity relative to the falling jet liquid. The relative transmission velocity, which is based on a coordinate system moving with the falling jet, depends on the concentration of the polymer solutions but not affected by the flow rate and the history of deformation in reservoir and nozzle. The transition point of deformation reported in our previous paper corresponds with the position where the upward relative transmission velocity coinsides with the velocity of the falling jet.

Deformation Behavior of a Viscoelastic Fluid Jet

When a viscoelastic fluid jet falling vertically encounters a deformation from the horizontal direction, the jet shows an anomalous behavior very different from that of a Newtonian fluid jet. We observe the deformation behavior of the jet in detail and find that the jet behavior is divided into two conditions. Moreover, we derive an empirical formula of the transition point, making use of the dimensional analysis method.

A study on the deformation behavior of a viscoelastic fluid jet.

When a viscoelastic fluid jet falling down vertically inflicts with a deformation from the horizontal direction, the jet shows anomalous behavior which is very different from that of a Newtonian fluid jet. We observe the deformation behavior of the jet in detail and find that the jet behavior is divided into two types. The transition point of deformation, which is the dividing point, is investigated under various flow conditions. Moreover, we find an empirical formula of the position of the transition point making use of the dimensional analysis method.

Flexural perturbations of free jets of maxwell and Doi-Edwards liquids

Flexural perturbations of high-velocity free jets of drop liquids moving in air are reinforced by the fact that the air pressure on the concave sections of the jet surface is greater than on the convex sections. The linear and nonlinear stages of development of flexural perturbations were studied in [1–5] for viscous Newtonian fluids. The effect of elastic stresses in the fluid on the growth of flexural perturbations of jets was first examined in [6], where it was assumed in an analysis of the growth of small disturbances that surface tension was constant along the jet, i.e., the investigators actually studied a tensed string. The studies [7, 8] examined the linear stage of growth of flexural perturbations of jets of Maxwell liquids. Our goal here is to analyze the dynamics of long-wave flexural perturbations of jets of viscoelastic fluids in both the linear and nonlinear stages of development. The rheological behavior of the fluid is described by two models — the phenomenological (Maxwell) model and the physical-molecular (Doi-Edwards) model. It is shown that the disturbances are oscillatory in character in the nonlinear stage of development. Meanwhile, the results of calculations performed with the Maxwell (M) and Doi-Edwards (DE) rheological models in the given problem agree with each other quantitatively as well as qualitatively.

A one-dimensional theory for viscoelastic fluid jets, with application to extrudate swell and draw-down under gravity

A model is derived for a viscoelastic fluid jet based on a one-dimensional directed curve idealization. It incorporates the general influences on the free surface geometry of surface tension, inertia viscosity, elasticity and gravity. The model is applied to the special case of an axisymmetric torsionless jet, and the observed fluid jet phenomena of extrudate swell and draw-down under gravity are described. In particular: (a) When gravity is neglected in the model, the jet outer radius asymptotically approaches a final value which depends on the radius and velocity of the jet at the nozzle, the stress state in the fluid at the nozzle, certain material constants characterizing the fluid, and surface tension; (b) When gravity is included, the jet swells to a maximum radius, then necks down to zero radius at infinity. Some planned future applications of the model and these special solutions are indicated.

Decay of Normal Stresses Developed in Capillary Flows of Viscoelastic Fluids

The expansion of a jet of viscoelastic fluid issuing from a capillary is measured. Two methods are suggested for estimating a relaxation time of the fluid. In one the dependence of the final jet diameter on residence time in the capillary is determined, and the calculated primary normal stress differences are interpreted through a model for the response of a Bogue fluid to the sudden imposition of simple shear. In the other, the shape of the extrudate is used to give an estimate of the normal stress difference as a function of distance from the capillary exit, and the data are interpreted through the solution for the stress decay in a Bogue fluid in steady shear, subject to a sudden cessation of shear. The resulting values of relaxation times as a function of shear rate are in good agreement for these two types of experiments, and agree well with values of the relaxation time based on the equilibrium normal stress difference as a function of shear rate.

On expansion of capillary jets of viscoelastic fluids

On expansion of capillary jets of viscoelastic fluids

The Behavior of Jets of Viscoelastic Fluids

First Page

Tensile stress in jets of viscoelastic fluids. II

AbstractA viscoelastic fluid jet is treated as a moving infinite string under slowly varying tension. When the system is subjected to a sinusoidal point vibration, the resulting wave form enables the tension to be determined along the jet. The total tension is composed of a surface reaction and an internal, decaying tension. The surface reaction may be calculated from the surface tension of the jet fluid and the radius of the jet. Data for 1.4 and 1.7% carboxymethylcellulose water solutions are presented for three different jet radii and for solution temperatures of 5, 25, and 35°C. It is found that the observed tensions approach the calculated surface reaction far from the nozzle. The rate of decay is a function of concentration and temperature. The internal tension for a 1.7% solution at 25°C. was observed to decay from about 8000 to 500 dynes/cm.2 in 200 milliseconds. Relaxation times of 0.27 sec. for 1.4% and 0.32 sec. for 1.7% solutions at 25°C. describe the internal tensile properties between 40 and 200 milliseconds after ejection.

Tensile stress in jets of viscoelastic fluids. I

AbstractExperimental observations have been made on jets of a viscoelastic fluid (carboxy‐methylcellulose–water solutions). The jet was subjected to transverse vibrations by the nozzle. The resulting wave pattern of the jet was compared to the theoretical results of three mechanisms of wave propagation: tension, rigidity, and elastic stretching. The results conform to the case of tension, and thus demonstrate the existence of a tensile stress in jets of viscoelastic fluids.