Steady flow in models of abdominal aortic aneurysms. Part I: Investigation of the velocity patterns.

As an investigation into the mechanical factors that lead to rupture of abdominal aortic aneurysms, both color Doppler flow imaging and laser Doppler velocimetry measurements of steady flow through a series of aneurysm models are presented. The flow pattern in each model consisted of a core of relatively fast-moving fluid traveling through the center of the dilation, surrounded by an outer annulus of slowly recirculating fluid. At flow rates below a Reynolds number of 1750 +/- 150, the flow was smooth, steady, and laminar. At higher flow rates (Reynolds number above 2250 +/- 250), the flow was always irregular and turbulent. Between these fully laminar and fully turbulent regimes the flow was intermittently turbulent. Larger models showed a tendency to become turbulent at lower Reynolds numbers than smaller models. In addition, turbulence was amplified in the distal half of the model dilation, with the largest models producing velocity fluctuations as great as 35% of the time-average centerline velocities. These data suggest that larger aneurysms in vivo may be subject to more frequent and intense turbulence than smaller aneurysms. Concomitantly, increased turbulence may contribute significantly to risk of rupture, as is discussed in Part II.