PSI - Issue 45

Xiaochen Wang et al. / Procedia Structural Integrity 45 (2023) 88–95 Xiaochen Wang/ Structural Integrity Procedia 00 (2023) 000 – 000

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3.3. Time-averaged wall shear stress Time-averaged wall shear stress (TAWSS) is one of the key parameters to be assessed in computational fluid dynamics, particularly in studies that focus on arterial analysis. The formulation of TAWSS is usually defined as (6) where w  is the instantaneous vector of wall shear stress (Carpenter et al., 2023). Figure 4(a) displays TAWSS contour maps of different AAA models. Despite slight variations, all four models exhibit very similar wall shear stress and TAWSS distributions and values. This consistency is due to the predominant influence of the inflow boundary conditions on wall shear stress. The aortic sac region is observed to have a lower TAWSS, which is often attributed to the presence of flow abnormalities such as flow recirculation and helical flow. Prolonged exposure to low TAWSS can result in the loss of endothelial cells on the intraluminal wall, making the AAA wall stiffer (Doyle and Norman, 2016). It should be noted that TAWSS cannot be solely relied on as an indicator for high-risk rupture locations. However, the pattern of TAWSS can indicate critical regions that may have a lower dissection energy and tissue strength threshold, as low TAWSS is related to material changes and the potential accumulation of ILT due to phenotypic regulation. Ruptures in AAA are commonly found at the proximal neck region and are correlated with low TAWSS zones. Low wall shear stress, which is often caused by flow abnormalities such as recirculation, can result in an increase in AAA growth rate and thrombosis (Wang et al., 2022). Additionally, low TAWSS can cause aortic wall hypoxia and lead to disruption (Mutlu et al., 2023). In conclusion, TAWSS contour maps of AAA models show similar wall shear stress and TAWSS distributions and values due to the influence of inflow boundary conditions. However, low TAWSS in the aortic sac region can indicate critical regions that may have a higher risk of rupture, growth rate increases, thrombosis, and disruption due to flow abnormalities and material changes in the intraluminal wall. (a) (b) 0 1 TAWSS= , T w dt T  

Fig. 4. (a) Time-averaged wall shear stress contours for models without and with ILT; (b) Blood flow streamline for models without and with ILT at diastole phase.

3.4. Haemodynamic patterns In Fig. 4(b), the visualisation of blood velocity streamlines within the lumen of AAA models, both with and without the presence of thrombus, is presented. The results show that, across all models, a recirculation zone is present during diastole phase, regardless of the presence of ILT. The turbulence intensity in the lumen of the models without ILT is 12% higher compared to that with ILT. These complex flow patterns, along with the stress distributions discussed earlier, suggest that high levels of recirculation can have negative impact on wall shear stress. An interesting finding in this study is the presence of helical flows in models with ILT at the beginning of the aneurysmal region. Unlike recirculation, helical flow operates differently by promoting smoother flow along the aortic wall (Morbiducci et al., 2015). However, the exact mechanics and potential risks associated with helical flow in AAA are still unknown and require further research. This highlights the importance of continuing to investigate the complex haemodynamics within the lumen of AAA and its potential impact on the stability of the aneurysm. Additionally, according to the review by Qiu et al. (Qiu et al., 2019), rupture of AAA is more likely to occur at the interface between different flow patterns. Thus, in the case of models with ILT, the area where the recirculation zone meets the helical flow is at a higher risk of rupture. Therefore, it is important to consider these flow patterns and their interactions when evaluating