Viscoelastic fluids, properties

The complete expression for tVDe is given by Darby and Pivsa-Art (1991) as a function of the viscoelastic fluid properties of the fluid (i.e., the Carreau parameters /y0, X, and p). This expression is... 

In their molten states, thermoplastic polymers behave like incompressible viscous fluids. By simply melting the matrix polymer and applying moderate forces, complex geometries can often be formed, even in the presence of fibers. Furthermore, it is interesting to note that polypropylene remains deformable over a wide temperature window while cooling from its melt temperature, before it recrystallizes. In this thermal region, it exhibits nonlinear viscoelastic fluid properties which are not... 

Fluids without any sohdlike elastic behavior do not undergo any reverse deformation when shear stress is removed, and are called purely viscous fluids. The shear stress depends only on the rate of deformation, and not on the extent of derormation (strain). Those which exhibit both viscous and elastic properties are called viscoelastic fluids. 

Actually, some fluids and solids have both elastic (solid) properties and viscous (fluid) properties. These are said to be viscoelastic and are most notably materials composed of high polymers. The complete description of the rheological properties of these materials may involve a function relating the stress and strain as well as derivatives or integrals of these with respect to time. Because the elastic properties of these materials (both fluids and solids) impart memory to the material (as described previously), which results in a tendency to recover to a preferred state upon the removal of the force (stress), they are often termed memory materials and exhibit time-dependent properties. 

Helical flow being analyzed as resultant from two independent flows (axial and circular), we may well assume that stable flow parameters (at least the flow rate) are determined primarily by viscous (flow) properties of the system, and the highelasticity effects (at superimposition of two flows) can be neglected in this case with a sufficient degree of accuracy which is reasonable from the point of view of engineering. The above assumtion was checked for correctness in 28,29) in a specific model of a viscoelastic fluid. 

Thus, the problem of flow of a viscoelastic fluid between two flat parallel plates one of which is moving in a direction transverse to the main flow is reduced to a solution of simplified system (7) at boundary conditions (1). Analysis of relationships (7) for specific boundary conditions indicates that the problem is reduced to the case of a non-Newtonian viscous fluid. In other words, the velocity profile v(y) is determined only by viscous characteristics of the media and the effect of high-elasticity properties of the melt upon velocity (flow rate) characteristics of the flow can be neglected. 

Viscoelastic fluids have elastic properties in addition to their viscous properties. When under shear, such fluids exhibit a normal stress in addition to a shear stress. For example, if a vertical rod is partly immersed and rotated in a non-viscoelastic liquid the rod s rotation will create a centrifugal force that drives liquid outwards toward the container walls, as shown in Figure 6.16(a). If, on the other hand, the liquid is viscoelastic then as the liquid is sheared about the rod s axis of rotation, a stress normal to the plane of rotation is created which tends to draw fluid in towards the centre. At some rotational speed, the normal force will exceed the centrifugal force and liquid is drawn towards and up along the rod see Figure 6.16(b). This is called the Weissenberg effect. Viscoelastic fluids flow when stress is applied, but some of their deformation is recovered when the stress is removed [381]. 

In the case of fluids without yield stress, viscous and viscoelastic fluids can be distinguished. The properties of viscoelastic fluids lie between those of elastic solids and those of Newtonian fluids. There are some viscous fluids whose viscosity does not change in relation to the stress (Newtonian fluids) and some whose shear viscosity T] depends on the shear rate y (non-Newtonian fluids). If the viscosity increases when a deformation is imposed, we define the material as a shear-thickening (dilatant) fluid. If viscosity decreases, we define it as a shear-thinning fluid. 

Boundary conditions In CFD frequently the fact is ignored that suitable boundary conditions are required at all flow boundaries. This is a major difficulty, for example, when calculating viscoelastic flow properties. We do not know the stages through which a fluid element has passed on entry into the computational domain. 

The viscoelastic fluids represent the 3rd material dass of non-Newtonian fluids. Many liquids also possess elastic properties in addition to viscous properties. This means that the distortion work resulting from a stress is not completely irreversibly converted into frictional heat, but is stored partly elastically and reversibly. In this sense, they are similar to solid bodies. The liquid strains give way to the mechanical shear stress as do elastic bonds by contracting. This is shown in shear experiments (Fig. 1.27) as a restoring force acting against the shear force which, at the sudden ending of the effect of force, moves back the plate to a certain extent. 

Finally, some fluids that undergo viscosity changes on shearing are elastic as well. These are termed viscoelastic fluids. These materials have properties of both a liquid and a solid. An excellent example is silicone putty (e.g., Silly Putty), which shows three different types of behavior depending upon the shear rate. If a piece of this material is suspended (gravity, a low shear force), it will slowly flow downward like a very viscous fluid. If it is sheared fasten it has rubbery behavior. You can observe this by rolling some of it into a ball... 

The power characteristic for viscoelastic fluids (Reher, 1969 Schiimmer, 1970) can also be described accurately by a relationship of the type Ne (Re ). Additional parameters to account for viscoelastic properties are needed only when the ratio of tangential to normal stresses is less than two (Reher, 1969). Power characteristics for the turbine stirrer in aerated non-Newtonian fluids (carboxymethylcellulose [CMC] and polyacrylamide [PAA] solutions) have been presented by Hocker and Langer (1962). 

The major characteristic of a polymeric reactor that is different from most other types of reactors discussed earlier is the viscous and often non-Newtonian behavior of the fluid. Shear-dependent rheological properties cause difficulties in the estimation of the design parameters, particularly when the viscosity is also time-dependent. While significant literature on the design parameters for a mechanically agitated vessel containing power-law fluid is available, similar information for viscoelastic fluid is lacking. 

As a matter of fact we do think that a better understanding of the mathematical properties of the models for viscoelastic fluid flows is fundamental in order to select a good constitutive equation, and to develop and implement numerical codes of practical use. 

Humans and other organisms are filled with complex fluids. Blood is a suspension containing around 40% by volume blood cells, which are flexible puckered disks roughly 10 / m in diameter, suspended in clear plasma, which is itself a viscoelastic fluid containing interacting macromolecules. The viscoelastic properties of blood determine the pumping load... 

These materials exhibit both viscous and elastic properties. In a purely Hookean elastic solid, the stress corresponding to a given strain is independent of time, whereas for viscoelastic substances the stress will gradually dissipate. In contrast to purely viscous liquids, on the other hand, viscoelastic fluids flow when subjected to stress, but part of their deformation is gradually recovered upon removal of the stress. 

HA is the basis of the lubricant and "shock absorber" properties of synovial fluid. Osteoarthritis is the most common disease of joints, and correlates with a deterioration of synovial HA. Intra-articular administration of HA is a widely used therapy for OA, providing relief of pain, and other symptoms. The first arthroscopic viscosurgical application of HA was in 1989 [150]. There are several preparations of partially cross-linked HA that are now used in this context. However, only one preparation will be discussed here. Synvisc , also known as hylan G-F 20, is a viscoelastic fluid containing modified HA produced from rooster combs. Hylans are cross-linked derivatives of HA. Synvisc contains hylan A (average molecular size 6x10 Da) and hylan B, a hydrated gel in a buffered salt solution. 

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