Characterizing Transition to Turbulence in Pulsatile Pipe Flow

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.