Shift In Shedding Frequency In Multiphase Flow Past A Cylinder

涡流是从热交换器的管子和无数其他常见的具有实际意义的流动中脱落的。梭子涡会在设备中激发振动，进而导致低效运行或损坏，这是一种已知的现象，通常会在设计中考虑在内。然而，如果流过像管子这样的物体的液体含有气体或蒸汽气泡，则脱落频率可能会发生变化，从而激发潜在的意外频率。对于脱落频率发生变化的流动，人们仍然只有部分了解。本研究的目的是为了更好地理解多相流与钝体(即非流线型物体)相遇的物理机制。这项研究将为K-12学生提供解释气泡-流体物理现象的记录可视化。这种互动将向不同的学生群体介绍这些流动的相关物理知识，目标是增加对STEM领域的兴趣。为了更深入地了解多相流中的旋涡脱落，对名义上二维流动中气泡流过圆柱体的相关物理机制进行了精心策划的基础研究。这项研究将利用最先进的实验技术、理论和数值模拟来彻底了解各种参数范围内的流动。数值模拟将使用自适应网格的高精度流动解算器和大涡模拟模型。气泡大小对气泡夹带到边界层和涡核的概率和速率有可预测的影响。小气泡充当被动示踪剂，而大气泡则以浮力为主。然而，对于中等大小的气泡，气泡在旋涡中捕获的概率与气泡大小之间存在显著的非线性依赖关系。对于气泡在旋涡中以不成比例的速度聚集并增加局部含气率的情况，脱落频率有增强的移动。因此，要研究的问题包括：控制涡核中气体积累的主要机制是什么，分散相对底层流动的修改是否影响了除尾迹平均密度修改之外的其他机制对涡旋脱落的影响，以及涡旋脱落频率是否如先前研究人员的数据所暗示的那样出现分叉？这一裁决反映了美国国家科学基金会的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Vortices are shed from tubes in heat exchangers and in countless other commonly encountered flows of practical importance. The shed vortices can excite vibration in the equipment, which in turn can lead to inefficient operation or damage, which is a known phenomenon and usually accounted for in the design. However, if the liquid flow past an object like a tube contains gas or vapor bubbles, the shedding frequency can shift and hence excite potentially unexpected frequencies. The flows with a shift in the shedding frequency remain only partially understood. The goal of this research is to better understand the physical mechanisms of multiphase flows encountering a bluff body (i.e. an object that is not streamlined). This research will provide recorded visualizations for K-12 students explaining the bubbly-fluid physical phenomena. This interaction will introduce a diverse student population to the relevant physics of these flows with the goal of increasing interest in the STEM field. To gain a deeper understanding of vortex shedding in multiphase flows, a carefully planned fundamental study is undertaken on the physical mechanisms relevant to a bubbly flow past a cylinder in a nominally two-dimensional flow. The research will utilize state-of-art experimental techniques, theory, and numerical modelling to gain a thorough understanding of the flow over a wide range of parameters. The numerical simulations will be carried out using high-order accurate flow solvers with adaptive meshes and large eddy simulation models. The bubble size has a predictable effect on the probability and rate of bubble entrainment to boundary layers and vortex cores in a flow past a cylinder. Small bubbles act as passive tracers, whereas for large bubbles buoyancy dominates. However, for intermediate size bubbles there is a notable nonlinear dependence on probability of bubble capture in the vortices on bubble size. For conditions where bubbles accumulate in the vortices at disproportionate rate and increasing local void fraction, there is an enhanced shift in shedding frequency. Hence, the questions to be studied include: what are the dominant mechanisms controlling gas accumulation in the vortex cores, does the modification of the underlying flow by the dispersed phase affect vortex shedding by mechanisms other than modification of average density in wake, and does the vortex shedding frequency bifurcate as suggested by previous investigators’ data?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.