Flow instabilities in intersecting geometries

今年的重点是探索交叉缝隙几何形状内的高雷诺数流动。我们的实验观察表明，超过一定的临界流量(雷诺数，Re)，流动变得不稳定，出现稳定的涡流。我们发现，通过改变几何形状和Re的横截面的长宽比，我们可以控制流场中出现的涡旋的数量及其结构。在前人研究的基础上，我们提出了一个模型来描述中心涡的速度和涡度分布。当Re增大到某一临界值时，我们观察到流动变得非定常和周期性，其特征频率取决于几何形状的长径比和外加的Re。我们已经刻画并解释了支配在这种流动类型中观察到的周期波动的不同机制，我们的结果得到了我们合作者所做的数值模拟的支持。这项工作的手稿目前正在准备提交。此外，在ESPCI的PMMH实验室，我们研究了球形颗粒(直径80微米的聚苯乙烯颗粒，8%的通道宽度)与交叉缝隙几何图形中的涡流之间的相互作用。我们的观察表明，对于牛顿流体，颗粒将均匀分布在流场中，颗粒在涡旋结构中旋转。当加入少量聚合物时，颗粒将绕着涡流流动，而不能在涡流结构中旋转。

This year, the focus was on exploring high Reynolds number flow within the cross-slot geometry. Our experimental observations reveal that above a certain critical flow rate (Reynolds number, Re) the flow becomes unstable and a steady vortex flow appears. We discover that by changing the aspect ratio of the cross-section of the geometry and the Re, we can control the number of vortices that appear in the flow field and their structure. We suggested a model based on previous studied to describe the velocity and vorticity profiles of the central vortex. When increasing the Re to a critical value we observed that the flow becomes unsteady and periodic with characteristic frequencies that depends on the aspect ratio of the geometry and on the imposed Re. We have characterized and explained the different mechanisms that govern the periodic fluctuations that are observed in this flow type and our results are supported with numerical simulations done by our collaborators. The manuscript for this work is currently being prepared for submission.Additionally, at the PMMH lab at ESPCI, we studied the interaction between spherical particles (80 micrometers diameter polystyrene particles, 8% of channels width) and the vortex flow in the cross-slot geometry. Our observations reveal that for a Newtonian fluid the particles will be evenly distributed in the flow field, the particles swirl inside the vortical structure. When adding small amount of polymers, the particles will flow around the vortex and will not be able to swirl through the vortical structure.