Characterizing Transition to Turbulence in Pulsatile Pipe Flow

过渡和湍流脉动流已经在包括生物医学流动在内的各种应用中被观察到，例如流过心脏、主动脉和脑的流动。过渡流会引起压力和壁面剪应力等流动参数的显著波动，这可能会导致重大的实际后果。例如，这些波动会加速包括心脏病、狭窄等在内的血管疾病的进展；此外，它们还会削弱材料，使其失效，即加速动脉瘤的破裂。因此，为了准确地评估这些生物医学条件，重要的是要考虑和量化过渡流效应的作用。不幸的是，脉动流向湍流的转变仍然是一个鲜为人知的流型。因此，目前的研究大多忽略了过渡性流动的影响。因此，该项目的主要目的是提供对脉动过渡流态的基本了解，并建立能够估计其功能影响的通用工具。该项目还将侧重于扩大与实验流体动力学方法相关的培训和教育资源。此外，通过该项目收集的大量实验数据将被公之于众，并用于建立一个全球可访问的实验过渡和湍流数据存储库。该项目的目标是建立关于脉动管流中驱动过渡的机理的基本知识，并开发能够估计间歇和过渡进程的首个同类工具。到目前为止，探索这种流动机制的研究仅限于一次性贡献，并产生了相互矛盾的发现。由于对这一流动形式缺乏共识和理解，生物医学研究越来越多地认为平均雷诺数低至250的流动是过渡性流动。这突出了解决这一领域的关键知识差距的紧迫性。该项目使用一系列严格的实验粒子跟踪测速(PTV)研究来满足这一需求。该项目将侧重于三个具体目标：(I)调查轴流因素，包括脉动频率和输入脉动波形形状对过渡到湍流的作用；(Ii)评估过渡到湍流的开始和发展对径向流动不稳定性的敏感程度，例如管道曲率或横截面闭塞或膨胀；(Iii)开发分析工具，以估计任意流动的间歇性、过渡进程和关键流动参数(例如，压力)的预期波动水平。这项工作将是该领域第一个全面的实验研究，也是第一个联合和参数评估影响脉动过渡的轴向和径向流动因素的实验研究。该项目预计将极大地促进目前对脉动过渡性流动的理解，并建立可翻译的工具，使未来的研究能够量化这种流动制度在任何应用领域的存在和影响。该奖项反映了NSF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。

Transitional and turbulent pulsatile flows have been observed in a variety of applications including biomedical flows such as flow through the heart, aorta, and brain. Transitional flow causes significant fluctuations in flow parameters such as pressure and wall shear stress, which can precipitate major practical consequences. For example, these fluctuations can accelerate the progression of vascular diseases including heart disease, stenoses, etc.; additionally, they can weaken materials to failure, i.e., expediting the rupture of aneurysms. Thus, to accurately evaluate these biomedical conditions, it is important to consider and quantify the role of transitional flow effects. Unfortunately, transition to turbulence in pulsatile flows remains a poorly understood flow regime. As a result, current studies most often ignore the effects of transitional flow. Therefore, the principal aim of this project is to provide a fundamental understanding of the pulsatile transitional flow regime as well as build universal tools capable of estimating its functional effects. This project will also focus on expanding training and educational resources related to experimental fluid dynamic methodologies. Additionally, the vast amount of experimental data collected through this project will be made publicly available and used to establish a globally accessible repository of difficult-to-obtain experimental transitional and turbulent flow data.The goal of this project is to establish fundamental knowledge regarding the mechanisms that drive transition in pulsatile pipe flow as well as develop first-of-their-kind tools capable of estimating intermittency and transition progression. To date, studies exploring this flow regime have been limited to one-off contributions and produced contradictory findings. The lack of consensus and understanding of this flow regime has increasingly led to biomedical studies controversially asserting flows with mean Reynolds numbers as low as 250 as being transitional in nature. This punctuates the urgency of addressing the critical knowledge gap in this domain. This project delivers on this need using a rigorous series of experimental particle tracking velocimetry (PTV) studies. The project will focus on three specific aims: (i) Investigate the role of axial flow factors including frequency of pulsation and input pulsatile waveform shape on transition to turbulence; (ii) Assess how the onset and development of transition to turbulence is sensitive to radial-flow instabilities such as pipe curvature or cross-sectional area occlusions or expansions; (iii) Develop analysis tools to estimate intermittency, transition progression, and expected fluctuation levels of key flow parameters (e.g., pressure) for any arbitrary flow. This effort will represent the first comprehensive suite of experimental studies in this domain as well as the first experimental studies to jointly and parametrically evaluate axial and radial flow factors that affect pulsatile transition. This project is expected to dramatically advance the current understanding of pulsatile transitional flow as well as establish translatable tools to enable future studies to quantify the presence and effects of this flow regime in any application area.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.