Model studies of the flow in abdominal aortic aneurysms during resting and exercise conditions.

Pulsatile flow in abdominal aortic aneurysm (AAA) models has been examined in order to understand the hemodynamics that may contribute to growth of an AAA. The model studies were conducted by experiments (flow visualization and laser Doppler velocimetry) and by numerical simulation using physiologically realistic resting and exercise flow conditions. We characterize the flow for two AAA model shapes and sizes emulating early AAA development through moderate AAA growth (mean and peak Reynolds numbers of 362 < Re(mean) < 1053 and 3308 < Re(peak) < 5696 with Womersley parameter 16.4 < alpha < 21.2). The results of our investigation indicate that AAA flow can be divided into three flow regimes: (i) Attached flow over the entire cycle in small AAAs at resting conditions, (ii) vortex formation and translation in moderate size AAAs at resting conditions, and (iii) vortex formation, translation and turbulence in moderate size AAAs under exercise conditions. The second two regimes are classified in the medical literature as disturbed flow conditions that have been correlated with atherogenesis as well as thrombogenesis. Thus, AAA disturbed hemodynamics may be a contributing factor to AAA growth by accelerating the degeneration of the arterial wall. Our investigation also concluded that vortex development is considerably weaker in an asymmetric AAA. Furthermore, turbulence was not observed in the asymmetric model. Finally, our investigation suggests a new mode of transition to turbulence: vortex ring instability and bursting to turbulence. The transition process depends on a combination of the pulsatile flow conditions and the tube cross-sectional area change.