Date

2021-9

Author

Türk, Semih

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Abdominal Aortic Aneurysm (AAA) is defined as the enlargement of the largest artery in the abdominal cavity, the abdominal aorta. The vascular rupture begins to pose a risk after a certain enlargement, and the rupture is described as one of the most critical emergencies in medicine. The disease is labeled as the 14th disease with the highest mortality rate. Thus, predicting the progression of the disease is vitally important. Hence, researchers are trying to identify and standardize all biological and mechanical factors on growth and rupture. Wall shear stress and the wall shear stress metrics have a significant impact on vascular dilation and rupture. Recently, researches focused on understanding the relationship between the wall shear stress and the rupture correlated occurrences such as calcification and fat deposits with numerical and experimental studies. Due to the difficulties encountered with the near-wall measurements in experimental methods, most studies are conducted by numerical methods. The present study aims to characterize the flow structure in abdominal aortic aneurysm and analyze the distribution of the wall shear stress of the AAA by using Particle Image Velocimetry (PIV.) The study compares the accuracy of PIV data in terms of proximity to the wall and velocity profile fits in terms of WSS prediction. For that purpose, simple and axisymmetric aneurysm geometry and Newtonian blood mimicking fluid are used. The experiments are conducted in two steady (Re =300 and Re=900) and one physiological (Remean ≈ 300 and α=7.17) case. Close-up studies are conducted in order to see the effect of spatial resolution increase for the steady cases. The results indicate that the first PIV data closest to the wall overestimates the velocities for the whole field experiment approach. On the other hand, close-up studies overestimate multiple data point closest to the wall. However, results of close-up studies are obtained by using closer data points to the wall compared to the whole field since the spatial resolution of the close-up approach is more superior than the whole field. For obtaining accurate WSS estimation, different near-wall profiles are investigated, such as second-order polynomial, third order polynomial, cubic spline, etc. 3rd order polynomial and cubic spline fits are the two methods that give expected results. Using cubic spline fits for the wall shear stress calculation is convenient since the spline fit method gives robust results in the case of complex flows. Regions with low wall shear stress and high oscillatory shear index are considered hazardous for growth and rupture. For the current aneurysm model, this combination and high risk are observed at distal/proximal edges and regions with secondary vortices in case of physiological flow.

Subject Keywords

Abdominal aortic aneurysm, Wall shear stress, Blood flow, Hemodynamics, Particle image velocimetry

URI

https://hdl.handle.net/11511/92148

Collections

Graduate School of Natural and Applied Sciences, Thesis