突然平面收缩中的粘弹性流动转变
Viscoelastic flow transitions in abrupt planar contractions
摘要：
我们目前的实验证据的全球粘弹性流动转变在2：1，8：1和32：1平面收缩下的惯性条件。光片可视化和激光多普勒测速技术被用来探测与这些不稳定性的发病空间结构和时间尺度。结果报告的临界Weissenberg数表征流体流速。对于一个给定的收缩比和聚合物流体，一个二维的，稳定的流动与收敛流线过渡到一个二维的模式与发散流线超过临界Weissenberg数。在更高的Weissenberg数，空间过渡到三维流观察。上游Weissenberg数的发病空间不稳定性和收缩比之间的关系。对于收缩比基本上大于1，我们观察到，进一步增加的流速的结果在第二个时间过渡到一个三维的，随时间变化的流。这种复杂的流动是由于应力-曲率相互作用和中性方向边界壁引起的三维扰动的综合作用而产生的。与其他几何形状的研究比较表明，边界形状深刻地影响流动转变的顺序和性质。
英文摘要：
We present experimental evidence of global viscoelastic flow transitions in 2:1, 8:1 and 32:1 planar contractions under inertia-less conditions. Light sheet visualization and laser Doppler velocimetry techniques are used to probe spatial structure and time scales associated with the onset of these instabilities. The results are reported in terms of critical Weissenberg numbers characterizing the fluid flow rates. For a given contraction ratio and polymer fluid, a two-dimensional, steady flow with converging streamlines transitions to a two-dimensional pattern with diverging streamlines beyond a critical Weissenberg number. At even higher Weissenberg numbers, spatial transition to three-dimensional flow is observed. A relationship between the upstream Weissenberg number for the onset of this spatial instability and the contraction ratio is derived. For contraction ratios substantially greater than unity, we observe that further increase in the flow rate results in a second temporal transition to a three-dimensional, time-dependent flow. This complex flow arises due to a combination of the effects of stress-curvature interaction and three dimensional perturbations induced by the walls bounding the neutral direction. Comparison with studies on other geometries indicates that boundary shapes deeply influence the sequence and nature of flow transitions.
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