Shift In Shedding Frequency In Multiphase Flow Past A Cylinder

涡流从热交换器中的管中脱落，并且在无数其他具有实际重要性的常见流动中脱落。脱落的涡流会激发设备中的振动，这又会导致低效的操作或损坏，这是一种已知的现象，通常在设计中考虑。然而，如果流经物体（如管道）的液体包含气体或蒸汽气泡，则脱落频率可能发生偏移，从而激发潜在的意外频率。在脱落频率的变化的流动仍然只是部分了解。本研究的目的是更好地理解多相流遇到海崖体（即非流线型物体）的物理机制。这项研究将为K-12学生解释气泡流体物理现象提供记录的可视化。这种互动将向不同的学生群体介绍这些流的相关物理学，以提高他们对STEM领域的兴趣。为了更深入地了解多相流中的旋涡脱落现象，本文对二维圆柱体泡状流的物理机制进行了细致的基础研究。该研究将利用最先进的实验技术，理论和数值建模，以全面了解各种参数的流动。数值模拟将使用具有自适应网格和大涡模拟模型的高阶精确流动求解器进行。气泡尺寸对气泡卷吸到圆柱绕流边界层和涡核的概率和速率有可预测的影响。小气泡作为被动示踪剂，而大气泡浮力占主导地位。然而，对于中等大小的气泡，有一个显着的非线性依赖于气泡尺寸的旋涡中的气泡捕获的概率。对于气泡以不成比例的速率在涡流中积累并增加局部空隙率的条件，脱落频率有增强的偏移。因此，要研究的问题包括：什么是占主导地位的机制控制气体积聚在涡核中，修改的底层流动的分散相是否影响涡脱落的机制以外的修改的平均密度在尾流，和涡脱落频率分叉的建议，由以前的研究者的数据？该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估的支持。

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.