摘要:

Viscoelastic flows are studied experimentally using two‐dimensional L‐shaped channels. Velocity and pressure distributions have been measured for various flow rates and for various channel widths on the downstream side in order to clarify the causes and the conditions under which a reverse flow is generated upstream of the re‐entrant corner. The present experiment shows that the reverse flow is a maximum in extent when the mean velocity and the channel width downstream of the re‐entrant corner are varied. Pressure distributions on the channel wall rise in the flow direction in the upstream vicinity of the re‐entrant corner. This pressure rise is a maximum in magnitude and in extent under the same conditions as the reverse flow. The Weissenberg effect, peculiar to viscoelastic fluids, may be the cause of the reverse flow and the pressure rise upstream of the re‐entrant corner.

ISSN:0148-6055    

DOI:10.1122/1.550130

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

A new finite element technique has been developed for using integral‐type constitutive equations in viscoelastic flow simulations with recirculating regions. The present method is relatively inexpensive and in the meantime can reach higher elasticity levels without numerical instability, compared with the best available similar calculations in the literature. The new method has been used to simulate entry flows of polymer melts in circular contractions using the K‐BKZ integral model. The numerical results show that LDPE melts exhibit a rapid corner vortex growth as the flow rate [or a dimensionless recoverable shear (SR) value] is increased, while HDPE melts do not produce a significant corner vortex growth even at very high flow rates, confirming existing numerical and experimental data. A comprehensive parametric study of the effects of elongational viscosity on vortex growth has also been carried out. It has been found that a combination of LDPE shear properties including the normal stresses with an HDPE‐like (strain‐thinning) elongational viscosity gives performance similar to an HDPE melt (in terms of the corner vortex growth), while a combination of HDPE shear properties with an LDPE‐like (strain‐thickening) elongational viscosity gives performance similar to an LDPE melt. This finding convincingly shows that the strain‐thickening elongational viscosity is primarily responsible for the rapid vortex growth exhibited by LDPE melts, while the strain‐thinning elongational viscosity of HDPE melts is responsible for their slow vortex growth. In other words, a strain‐thickening elongational viscosity enhances vortex growth withSR, while a strain‐thinning elongational viscosity stunts vortex growth withSR.

ISSN:0148-6055    

DOI:10.1122/1.550131

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

The orientation of anisotropic solid particles suspended in thermoplastics is examined by wide angle x‐ray diffraction (WAXD). The state of orientation is assessed as a function of the deformation or processing history of a filled material. The technique is based onaprioriassessing the relationship of the position of crystallographic planes in the anisotropic particles. Our study uses mica, talc, and DuPont Kevlar fibers in which the orientations of crystallographic planes are well established. WAXD pole figures have been obtained for systems with varying processing histories. The results are represented in terms of orientation factors of the type which have been developed to represent polymer chain and crystallographic orientation in fibers and films. Specific application is made to: (i) filament extrusion, (ii) melt spinning, (iii) sheet extrusion, (iv) compression molding, (v) cold rolling, and (vi) injection molding. The differences in orientation development of suspended fibers and flakes in these various processing operations are described. Generally fibers orient in the direction of flow and flakes orient with their normals perpendicular to surfaces in extrudates and molded products.

ISSN:0148-6055    

DOI:10.1122/1.550132

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

A tensor description of the orientational structure in a fiber suspension provides an efficient way to compute flow‐induced fiber orientation, but the scheme requires an accurate closure approximation for the higher‐order moments of the orientation distribution function. This paper evaluates a number of different closure approximations, comparing transient orientation calculations using the tensor equations to a full calculation of the distribution function. We propose a new hybrid closure approximation for three‐dimensional orientation, formed by modifying the scalar measure of fiber alignment. The new scheme is tested in a variety of flow fields against the commonly employed quadratic L1 and H&L2). None of these closure approximations provide accurate solutions for all the flow and orientation fields. The quadratic closure exhibits stable dynamic behavior, but predicts neither the correct transient behavior nor accurate steady‐state values, especially for nearly random to intermediately aligned orientations and rotational flow fields. Hinch and Leal’s closures work quite well for low to intermediate alignment, but one form (H&L2) displays artificial oscillations in simple shear flow for strong alignment. The other composite, H&L1, makes up for this deficiency in simple shear flow, but it is consistently less accurate than H&L2 in other flows and gives physically impossible values in biaxial elongation. Our new hybrid closure is always well behaved. In fact, it is the only approximation other than the quadratic closure that never exhibits artificial oscillations or pathological behavior. Its steady‐state predictions are slightly better than the quadratic form in shearing flows and performs best for combined shearing/stretching flow over a wide range of orientations.

ISSN:0148-6055    

DOI:10.1122/1.550133

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

Flow into or out of two opposed nozzles immersed in a liquid offers a means of measuring an extension‐dominated response of a liquid up to extension rates as high as 1000 s−1and more. We evaluate the concept as it applies to Newtonian liquids, by solving the Navier–Stokes system for complete fields of stress and extension rate. We examine possibilities of inferring nearly extensional stress over some internal area from a measured force (torque, actually) on one nozzle assembly, of resolving that stress into its pressure and viscous parts, and of having nearly uniform extension rate over the same area so that an extensional viscosity can be reliably deduced; the choice of the area and the control volume of which it is part proves pivotal. This examination leads to an instrument design optimization problem which we solve approximately for the near‐best nozzle bore, separation, and exterior shape, and for the near‐best choice of control volume. Results are compared with experimental findings and implications for measurements of non‐Newtonian liquids are touched on.

ISSN:0148-6055    

DOI:10.1122/1.550134

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

It is known that evaluating oscillatory rheological shear experiments in the usual way may lead to complete misinterpretation of the experimental data, especially for low viscosity fluids in the high frequency range. We used a modern type of oscillatory rheometer with a stiff transducer. When measuring Newtonian liquids artificial, second‐order fluid‐like data for the complex shear modulus were found. In order to explain those findings, it is essential to take into account the influence of fluid inertia. Accordingly a method, based on theoretical considerations, is suggested for the correction of experimental data for slightly elastic fluids in order to obtain the true viscoelastic material functions.

ISSN:0148-6055    

DOI:10.1122/1.550135

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

Three solutions of the Leslie–Ericksen equations for low‐molar mass rod‐like nematic liquid crystals are obtained for pressure‐driven radial outflow between concentric parallel disks. There are two competing hydrodynamic effects on the orientation: extensional flow (strongest in the centerline region) and shear (strongest near the bounding surfaces). At low pressure drops the in‐plane orientation is stable; the centerline orientation is normal to the flow direction. For each radial distance there is a critical pressure drop at which the elongational torques twist the material out‐of‐plane of flow; left and right rotations are possible. The transition from in‐plane mode to out‐of‐plane modes is analogous to the second order Frederiks transition. The two out‐of‐plane modes are characterized by a secondary azimuthal flow. At certain pressure drops all the stationary modes can coexist in the flow cell. The radius of the boundary between in‐ and out‐of‐plane modes increases with increasing pressure drops. There is a minimum pressure drop to observe the transition at the edge of the entrance hole, and there is also another minimum pressure drop at which the out‐of‐plane modes fill the whole flow cell.

ISSN:0148-6055    

DOI:10.1122/1.550136

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550137

出版商:The Society of Rheology    

年代:1990

数据来源: AIP