Effect of the second and the third normal stress difference on occurrence of instability in viscoelastic fluid flow

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The aim of this research is investigating the relation of the normal stress differences to the occurrence condition of the flow instability in viscoelastic fluid flow. Specially, the effect of the normal stress differences on the flow instability in the two-dimensional Poiseuille flow cell and the concentric cylinders flow cell was measured by the double-beams optical measurement technique. The aqueous solution of a surfactant CTAB (Cetyltrimethylammonium bromide) was used as a test fluid. NaSal (Sodium salicylate) is added as a counter ion. CTAB and NaSal were dissolved 0.23mol/l and 0.03mol/l in the distilled water. This sample forms wormlike micelles at a room temperature. The flow property of this fluid was measured by a strain-controlled rheometer and the result showed that this fluid is a Maxwell type linear viscoelastic fluid at low shear rate region as same as a mono-disperse polymer solution. It was clarified by using the concentric cylinder flow cell that at start-up flow thi … More s fluid showed the flow-induced structure change at certain strain and started the flow instability when shear rate was higher. In addition, a parallel plates flow cell made by two glass plates was designed and built up to generate two-dimensional Poiseuille flow. This flow cell is designed to measure the flow birefringence from two directions simultaneously and the space between the glass plates can be changed for various experimental conditions. The flow birefringence measurement device was finished to build and the data acquisition software was also completed. The fluid becomes opaque when shear rate and strain exceed 3 l/s and 25, respectively. In this region, the stress-optic coefficient is changed by shear rate and it means that the flow-induced structure change occurred. The second and the third normal stress differences are related with the stress-optic coefficient. So, the occurrence of the flow instability might be affected by these normal stress differences. The details are investigating by this experimental result continuously. Less