Influence of geometry, rheology and compliance on the transition to turbulence in pulsatile pipe flow

心血管系统中紊乱的血流模式被认为会引发疾病。因此，近年来人们对了解心血管系统在什么条件下发生不稳定性和湍流转变越来越感兴趣。除了在规范系统中研究转变的固有困难外，心血管流动还包括一些额外的复杂性。大动脉中的血流是搏动的，动脉是灵活的，具有复杂的几何形状，而血液本身是一种非牛顿流体。这些特征中的一些已经在文献中进行了研究，通常是单独的和简单的设置。脉动效应的研究主要集中在直、光滑刚性管道中。即使在这个正则系统中，跃迁也受到脉动的很大影响。根据流动参数、脉动频率和形状的不同，在脉动的某些阶段，流动非常容易受到突然出现湍流的影响。对于由生理波形驱动的流动来说更是如此，这表明湍流在心血管系统中的分布可能比之前认为的要广泛。柔性管壁和非牛顿流体对脉动管道流动的影响在文献中很少得到关注。虽然在稳定的管道流动中，柔性和剪切变薄流变学通常会延迟湍流转变，但在脉动情况下，它们的影响在很大程度上仍然未知。本项目的目标是扩展在刚性管道中脉动管流的湍流转捩方面所获得的知识，并系统地研究心血管流的另一个特征。这里考虑了三个不同的问题。首先，柔性管壁直管中的脉动流动。第二，复杂流体在刚性管道中的脉动流动。第三，简单几何形状的脉动流，类似于人类主动脉的某些部分。重点将是确定每个新特征是否会影响为简单情况确定的关键机制，或者它是否甚至引入了新的湍流关键路径。为了做到这一点，我们将开发数值工具来执行瞬态增长分析，并调整已有的软件来执行直接的数值模拟。将与研究单位内的实验室实验进行直接比较。最终的长期目标是定义心血管系统中湍流的完整路线图，并使用实验和数值结果以简单的方式对其进行建模。

The presence of disturbed flow patterns in the cardiovascular system is thought to trigger diseases. Therefore, in recent years there has been an increasing interest on understanding under what conditions instabilities and turbulence transition occur in the cardiovascular system. Apart from the inherent difficulties of studying transition even in canonical systems, cardiovascular flow includes several additional complexities. Blood flow in the large arteries is pulsatile, the arteries are flexible and exhibit complex geometries, and blood itself is a non-Newtonian fluid. Some of these features have been investigated in the literature, usually separately and in simple setups. The effect of the pulsation has been mostly studied in straight and smooth rigid pipes. Even in this canonical system, transition is greatly affected by the pulsation. Depending on the flow parameters, frequency and shape of the pulsation, the flow can be highly susceptible to the sudden appearance of turbulence during certain phases of the pulsation. This is even more so for flows driven with physiological waveforms, suggesting that turbulence could be more widespread in the cardiovascular system than what was previously thought. The influence of flexible walls and non-Newtonian fluids on pulsatile pipe flow has received little attention in the literature. While both compliance and e.g., shear-thinning rheology usually delay turbulence transition in steady pipe flow, in the pulsatile case their effects remain largely unknown. The goal of this project is to extend the knowledge gained on turbulence transition in pulsatile pipe flows in rigid pipes, and systematically study one additional feature of cardiovascular flow. Three different problems are considered. Firstly, pulsatile flow in straight pipes with flexible walls. Secondly, pulsatile flow of complex fluids in rigid pipes. Thirdly, pulsatile flow in simple geometries which resemble certain sections of the human aorta. The focus will be on determining if each new feature affects the critical mechanism identified for the simple case, or if it even introduces new critical paths to turbulence. In order to do so we will develop numerical tools to perform transient growth analyses, and adapt pre-existing software to perform direct numerical simulations. Direct comparisons to laboratory experiments within the research unit will be performed. The ultimate, long-term goal is to define a complete roadmap to turbulence in the cardiovascular system and model it in a simple way using the experimental and numerical results.