We present an experimental study of the flow of yield stress materials through annular abrupt expansions–contractions, to evaluate the flow invasion into the cavity formed in the larger cross section region. Steady inertialess flows of Carbopol® aqueous dispersions were investigated. The flow pattern reveals yielded and unyielded regions, which were visualized using tracer particles, laser sheets, and a digital camera. The yield surfaces were identified in the experiments by choosing large enough exposure times that allow sufficient particle displacement in the yielded region. To estimate the amount of fluid that remains stagnant in the cavity, we defined the invasion ratio, a quantity that was determined through image processing for different combinations of the governing parameters. The influence of the cavity diameter and axial length, eccentricity, and inlet velocity on the invasion ratio was investigated. Fore-aft asymmetric yield surfaces were observed for all tests, probably due to elastic effects.

中文翻译：

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屈服应力材料通过环形突然膨胀-收缩的流动

我们提出了屈服应力材料通过环形突然膨胀-收缩流动的实验研究，以评估流动侵入形成在较大横截面区域中的空腔。研究了 Carbopol® 水性分散体的稳定无惯性流动。流动模式揭示了屈服和未屈服区域，这些区域使用示踪粒子、激光片和数码相机进行了可视化。在实验中通过选择足够大的曝光时间来确定屈服面，从而在屈服区域中允许足够的粒子位移。为了估计腔内滞留的流体量，我们定义了侵入率，这是通过图像处理针对不同控制参数组合确定的量。型腔直径和轴向长度的影响，研究了偏心率和入口速度对侵入比的影响。所有测试都观察到前后不对称屈服面，可能是由于弹性效应。