Cyclic flow characteristics in an idealized asymmetric abdominal aortic aneurysm model

Abstract

The flow characteristics and the corresponding hydrodynamic stability in an idealized asymmetric abdominal aortic aneurysm (AAA) model have been investigated using a laser Doppler anemometer. A rectified sine waveform was used to simulate aortic flow conditions (Re(delta) = 806 and alpha = 12.2). The flow around the distal neck of the AAA model undergoes transition and becomes turbulent for a fraction of time shortly after the commencement of the deceleration phases at every flow cycle while the rest of the flow inside the model stayed laminar throughout the cycle. As a result of non-symmetric vortical structure development inside the model, the distribution of turbulent shear stresses was found to be highly uneven along the radial direction of the model; this is in contrast to results found by the present authors in the symmetrical AAA model. The maximum turbulent shear stress found at the straight side of the distal neck are four times more than the maximum turbulent shear stress measured at the most dilated side of the distal neck. One of the interesting biological implications of the results is that the outward dilation of the arterial wall may be a physiological response to avoid the high turbulent shear load from the momentarily turbulent blood flow.

Abdominal aortic aneurysm detection by common femoral artery Doppler ultrasound waveform analysis

“…The CFA waveform in the presence of an AAA shows high levels of spectral broadening, typical of disturbed or turbulent flow in the deceleration phase of the systolic peak. These changes seem to be consistent with the intermittent turbulent bursts observed in the flow within an AAA model during the deceleration phase of the flow cycle in the study byYip and Yu (2003) shown inFigure 1.15. This suggests that AAA may alter the distal flow pattern.…”

“…Yip and Yu (Yip and Yu 2003) also extended their work to investigate sinusoidal flows within idealised, rigid, asymmetric AAA models. They observed that as flow entered the asymmetric AAA model, recirculating vortices were formed either side of the proximal neck, with the vortex on the bulge side larger than on the straight side.…”

“…During deceleration, turbulent flows subject the walls to high turbulent shear stresses. For sinusoidal flows Yip and Yu (Yip and Yu 2002;Yip and Yu 2003) estimated the wall shear stress from the instantaneous velocity profiles at different phases in the flow cycles. The peak stress was observed at the distal end of the aneurysm models during the deceleration phase.…”

“…Therefore, biomechanics plays a key role in the development, enlargement and rupture of aortic aneurysms (Steinman, Vorp et al 2003). Turbulent shear stresses are much more damaging to endothelial cells than laminar shear stresses of the same magnitude (Yip and Yu 2003). Turbulent fluid stress is known to induce and promote vascular endothelial cell turnover and may cause blood cell and platelet damage.…”

“…Phase variation o f stream wise velocity measured where turbulent bursts appeared during deceleration phase in AAA model(Yip and Yu 2003) …”