Author: Ai, Chunhui
Title: Fluid-structure interaction of compliant vessels with pulsatile flows
Advisors: Tang, Hui (ME)
Degree: Ph.D.
Year: 2022
Subject: Blood -- Circulation
Fluid mechanics
Cardiovascular system -- Physiology
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xviii, 136 pages : color illustrations
Language: English
Abstract: Cardiovascular diseases have been one of the leading causes of death in the world, usually involving pathological alterations in blood vessels. Some vascular diseases are related to abnormalities in vessel geometry, such as arterial aneurysm and stenosis. Unfortunately, at the initial stage, these diseases generally have no symptoms. With recent technologies, like magnetic resonance imaging (MRI), X-ray, and ultrasound, we can get information in tissues and vessels non-invasively, such as flow velocities, deformations or even wall shear stress (WSS) . However, these techniques may not have enough temporal and spatial resolution, hence are not competent to capture enough details of flow and its evolution especially when fluid-structure interaction has important effect. From the perspective of fluid dynamics, the blood flow inside vessels could provide rich information, such as vortices, velocity profiles, for medical use. Many simulation works have been conducted on this type of problems. Compared to simulations, experimental investigations on vascular models could be more challenging: the lower density of tracer particles and strong reflection near the wall in conventional particle image velocimetry (PIV), let alone the measurement of pulsatile flow inside elastic vessels with tangible deformations. WSS is a significant indicator on the progression of diseases such as aneurysms and atherosclerosis. But very little was reported on WSS from existing experiment studies mainly due to the difficulty in measurement. Moreover, most of these studies only focused on rigid vessel models to avoid the complication caused by passive deformation of the vessel walls, such as the change of flow field and resulting WSS. To bridge these gaps, in the present work we experimentally studied the interaction of various rigid/compliant vascular models with the inside pulsatile flows, with the focus on the temporal and spatial distributions of WSS, and thus achieved a better understanding from the perspective of fluid dynamics.
Four sets of geometrically simplified, transparent vascular models or phantoms were built using silicone elastomer: one purely straight, one with a bulge of 50% expansion in diameter, one with symmetric stenosis of 50% reduction in diameter, and one with asymmetric stenosis of 50% reduction in diameter. Both rigid and compliant models were constructed. The flexibility of these vascular models was adjusted by tuning the thickness of vessel walls. To facilitate PIV measurements, a mixture of water/glycerol/NaI was adopted as the Newtonian working fluid to match the refractive index of the silicone phantom. The pulsatile flow was produced using a programmable pump, in a physiological waveform similar to that in human body. The Reynolds number was chosen to be about 500, and the Womersley number was chosen to be about 8, both in the range of real blood flows. Two pressure sensors were applied in the flow circuit to record the pressure at the exit of pump and at the entrance of phantom. With time-resolved PIV measurements, the unsteady flow field inside the model was obtained and analyzed, along with the deformation of the vessel wall. The WSS was estimated directly from the recorded images using the interfacial PIV technique.
First, measurements on rigid models were conducted to explore the effect of vessel geometry on the flow dynamics. It was found that in the straight phantom, WSS varies with time in a similar waveform as the flow rate, and hardly depends on physical locations. In the bulge phantom, WSS changes in both space and time, and the peak value occurs after the center of bulge. As for the two stenosed phantoms, the narrowed flow passage causes WSS to increase right before the throat. Time-averaged WSS increases generally from the bulge phantom, the straight phantom, to the concentric stenosis phantom, and reaches the highest value in the eccentric stenosis phantom. These WSS features were found to be closely related to the variation of flow rate and the formation of vortex structures.
Second, the effect of vessel compliance on the flow dynamics and WSS was studied. The results showed that compliant phantoms experienced much gentler pressure variation with much lower peak pressure. The peak WSS was reduced remarkably and the WSS distribution became more spread compared with the rigid vessel cases. Apart from that, compliant models tended to have larger deformation, and the vortices in compliant models were much weaker, compared with the rigid cases.
Last, the flow and WSS around two eccentric stenoses arranged in tandem were studied. Two representative settings were considered: i.e., the two stenoses were either on the same side or on the opposite side. The effect of the distance between these two stenoses were studied. Results showed that the flow and WSS around the downstream stenosis were significantly affected by the upstream stenosis if they were too close. The arrangement of the two stenoses also led to prominent difference until their distance became large enough.
This research enriches our understanding of the fluid-structure interaction between pulsatile flows and vessel walls, which could shed some light on the medical diagnosis and treatment of cardiovascular diseases.
Rights: All rights reserved
Access: open access

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