Elastic turbulence

Figure 8.10. Effect of melt temperature on onset of elastic turbulence in polyethylene. (After Howells...

Figure 8.11. Effect of molecular weight on critical shear stress at onset of elastic turbulence in poly(methyl methacrylate). (After Howells and Benbow )...

In addition to elastic turbulence (characterised by helical deformation) another phenomenon known as sharkskin may be observed. This consists of a number of ridges transverse to the extrusion direction which are often just barely discernible to the naked eye. These often appear at lower shear rates than the critical shear rate for elastic turbulence and seem more related to the linear extrudate output rate, suggesting that the phenomenon may be due to some form of slip-stick at the die exit. It appears to be temperature dependent (in a complex manner) and is worse with polymers of narrow molecular weight distribution. 

At fixed filling ratio, using a hybrid filler may decrease extruded material swelling, raise the critical deformation parameters at which elastic turbulence of the melt may begin [366]. 

Elastic turbulence here means the distortion of the surface evenness and shape of the extruded article. A review of the wealth of experimental data and reasons for this phenomenon is outside the scope of this paper. Detailed information concerning this problem will be found in [359, 360]. 

Li, F.-C., Oishi, M., Oshima, N., Kawaguchi, Y., and Oshima, M., Statistical characteristics of elastic turbulence in a free-surface swirling flow, New Trends in Fluid Mechanics Research, Proceedings of the 5th International Conference on Fluid Mechanics, Shanghai, China, August 15-19, pp. 91-94, Tsinghua University Press and Springer (2007). 

FIG. 15.40 Effect of melt temperature on onset of elastic turbulence in polyethylene. From Brydson (1981, Gen Ref, his Fig. 5.6 as reproduced from Howels and Benbow, 1962). Courtesy The Plastics 8t Rubber Institute. 

A. Groisman and V. Steinberg. Elastic turbulence in a polymer solution flow. Nature, 405 53-55, 2000. 

G.V. Vinogradov and L.I. Ivanova. Wall slippage and. elastic turbulence of polymer in the rubbery state. RheoL Acta, 1968, 7(3), 243-255. 

The same effect is observed in melts of macromolecular substances. Since, in this case, the liquids are elastic, however, additional elastic oscillations of small liquid particles occur. The uneveness of the oscillations causes an elastic turbulence. This occurs at much lower flow velocities than the normal turbulence, i.e., at low Reynolds numbers. Elastic turbulence can also be recognized by the fact that the flow rate increases much more sharply with increasing pressure in the elastic turbulence region than in the laminar flow region in normal liquids the increase in the flow rate is less in the turbulence region than in the laminar flow region. Elastic turbulence becomes apparent in the processing of plastics at what is called the melt break. 

Nonlaminar Flow. Ideally, a melt flows in a steady, streamlined pattern in and out of a die. Actually, the extrudate is distorted, causing defects called melt fracture or elastic turbulence. To reduce or eliminate this problem, the entry to the die is tapered or streamlined. 

The length of time for which the liquid remains in the spinneret orifices is 0.1-100 ms. The relaxation times for this process, on the other hand, lie between 100 and 1000 ms. Relaxation processes are therefore important in spinning. They are particularly evident in the Bams effect and in elastic turbulence. 

It is known that a viscoelastic fluid, e.g., a solution with a trace amount of highly deformable polymers, can lead to elastic flow instability at Reynolds number well below the transition number (Re 2,000) for turbulence flow. Such chaotic flow behavior has been referred to as elastic turbulence by Tordella [2]. Indeed, the proper characterization of viscoelastic flows requires an additional nondimensional parameter, namely, the Deborah number, De, which is the ratio of elastic to viscous forces. Viscoelastic fluids, which are non-Newtonian fluids, have a complex internal microstructure which can lead to counterintuitive flow and stress responses. The properties of these complex fluids can be varied through the length scales and timescales of the associated flows [3]. Typically the elastic stress, by shear and/or elongational strains, experienced by these fluids will not immediately become zero with the cessation of fluid motion and driving forces, but will decay with a characteristic time due to its elasticity. 

The elastic dissipation is responsible for some effects such as the great pressure losses, the swelling of the extruded polymer, melt fracture, elastic turbulence. 

Elastic turbulence One final topic we will mention here before closing the chapter is the elastic turbulence. The term describes the turbulence caused by the elastic forces arising in non-Newtonian fluids. In the situations where mixing is needed, the elastic forces in a non-Newtonian fluid can be used to create turbulence. The analysis by Joo and Shaqfeh (1992) is particularly useful for the flow of non-Newtonian fluids in microchannels but is beyond the scope of the text here. 

Sometimes confused with melt fracture (also known as elastic turbulence, bambooing, and distortion) is sharkskin. In thermoplastics this occurs as tiny transverse ridges on the surface of an extrudate. It is found to occur above a critical linear output rate. The writer has suggested that this must be due to tearing of weak elastic melts as the surface of the extrudate accelerates in velocity, relative to the centre, as it leaves the die. Most formal studies have been made on thermoplastics but the phenomenon, or something very much like it, is observed with rubbers. 

Experiments on torsional flow and an overview of the onset and properties of elastic turbulence, as these low Reynolds number instabilities are sometimes known, is in... 

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