Flow instabilities in intersecting geometries

今年的重点是探索交叉槽几何结构中的高雷诺数流动。实验结果表明，在一定的临界流量（雷诺数，Re）以上，流动变得不稳定，出现稳定的涡旋流动。我们发现，通过改变几何截面的展弦比和Re，可以控制流场中出现的涡的数量及其结构。我们在前人研究的基础上提出了一个描述中心涡速度和涡度分布的模型。当Re增加到一个临界值时，我们观察到流动变得不稳定和周期性，其特征频率取决于几何形状的长径比和施加的Re。我们已经描述并解释了在这种流动类型中观察到的控制周期性波动的不同机制，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，通道宽度的8%）与交叉槽几何形状的涡流之间的相互作用。我们的观察表明，对于牛顿流体，颗粒在流场中均匀分布，颗粒在涡旋结构内旋转。当加入少量聚合物时，颗粒会绕着漩涡流动，而不能通过漩涡结构旋转。

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.