The flow visualization experiments of shear-thinning aqueous polyacrylamide solutions through a rectangular channel with an abrupt contraction were carried out, using not only the tracer method but also dye-solution injection technique in order to study the transition from a steady flow regime to an unstable flow regime and the structure of the unstable flows.
It is found that two basic transition patterns can be identified accoding to the flow properties of polymer solutions and channel geometry. The one follows a rapid enhancement of the salient corner vortex in flows of rather concentrated solutions through a channel with a large contraction ratio, and the unstable flow is characterized by the fact that two salient corner vortices grow and decay alternately. The other is observed in rather dilute solution flows through a channel with a small contraction ratio. In this case the salient corner vortices are almost completely diminished. In the tracer-visualization experiments, it is clear that diverging flow occurs first and the radius of curvature of the main flow around the salient corner becomes reduced with an increase in flow rate. The so-called bundle-like streams are then generated near the salient corners in unstable flows as the flow rate is further increased.
The dye-solution injection technique reveals that a stream bundle consists of two counterrotatingvortex tubes, i.e. Goertler-like vortices, whose cross-section is approximately rectangular.

View full abstract

Flow patterns and fiber orientation in the two-dimensional merging flow of fiber suspensions are calculated using Dinh-Armstrong model. The assumption that all fibers lie tangent to streamline is excluded from the present calculation. Therefore fiber orientation is obtained by integration of the Jeffery's equation along a streamline.
The flow pattern of fiber suspension is different from that of Newtonian fluid, and a vortex occurs near the corner. The dependence of the vortex height on Reynolds number and a parameter a is discussed. Fibers lie tangent to a streamline in the main flow, but not in the vortex region. Furthermore, the aplication of Stress-relief-mechanism is examined for onset of the vortex.

View full abstract

In a previous paper, the helix numbers in crepe fabric was analysed with the aid of a mechanical model. In that model, the crepe was assumed to be formed due to weft loading by twisting moment, application of linear elastic restraint force on the weft by warps, and the increase of apparent moment of inertia of filling area by weft restraints. In the present paper, the validity of such a model has been verified experimentally with different silk and cotton crepe fabrics. Helix numbers in the fabrics have been found decrease with the decreasing number of twist, with the decreasing temperature of refining water, and with the use of thicher yarns in the fabric. The use of hard twisted yarns in succession (twisted in the same direction) also reduces the number of helixes. The effect of warp and weft densities on helix number has been found very small.

View full abstract