Shift In Shedding Frequency In Multiphase Flow Past A Cylinder

在热交换器和无数其他常见的具有实际重要性的流动中，漩涡是从管道中脱落的。棚子涡会激发设备的振动，从而导致设备的低效运行或损坏，这是一种已知的现象，通常在设计中考虑到。然而，如果液体流过含有气体或蒸汽气泡的物体，则脱落频率会发生变化，从而激发潜在的意想不到的频率。随着脱落频率的变化，流动仍然只是部分地被理解。本研究的目的是为了更好地理解多相流遇到钝体（即非流线型物体）的物理机制。这项研究将为K-12学生提供可视化记录，解释气泡流体物理现象。这种互动将向不同的学生群体介绍这些流动的相关物理，目的是增加对STEM领域的兴趣。为了更深入地了解多相流中的旋涡脱落，在名义上的二维流动中，对气泡流过圆柱体的物理机制进行了精心策划的基础研究。该研究将利用最先进的实验技术、理论和数值模拟来全面了解大范围参数下的流动。数值模拟将使用高阶精确的自适应网格流动求解器和大涡模拟模型进行。气泡的大小对气泡夹带到边界层和涡核的概率和速率具有可预测的影响。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，在漩涡中气泡捕获的概率与气泡大小有显著的非线性关系。当气泡以不成比例的速度积聚在涡流中，并且局部空隙率增加时，脱落频率的变化会增强。因此，需要研究的问题包括：控制涡核中气体聚集的主要机制是什么？分散相对下伏流动的改变是否通过其他机制而不是通过尾迹平均密度的改变来影响涡的脱落？涡的脱落频率是否像以往研究者的数据所表明的那样出现分叉？该奖项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知识价值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.