Cyclic transition to turbulence in rigid abdominal aortic aneurysm models

The hydrodynamic stability of cyclic flows inside rigid abdominal aortic an eurysm (AAA) models was investigated. Rectified sinc waveforms were used to simulate aortic flow conditions (Re-mean = 1600-2100 and alpha = 7.2-12.2) . Depending on the bulge geometry (D/d and L/d ratios), AAA flows can be br oadly classified into three regimes, namely types A, B and C, respectively. While type A has no vortex formation, type B and C have distinctive lamina r vertical structures that are very different from one another. The type of flow regimes would also determine where and when the transition to turbule nce would occur and the portion of the cycle at which the flow remains turb ulent in the bulge. The stability characteristics of types B and C are obta ined from the linear stability analysis performed on the unsteady velocity profiles measured at different phases of a cycle. Based on the linear stabi lity analyses, instability is found to initiate in the bulge for types B an d C through the formation of vortical structures. Instability grows progres sively during the acceleration phase and transition to turbulence in the bu lge occurs shortly after the commencement of the deceleration phase in all cases investigated. The mechanisms of transition to turbulence for types B and C are discussed. Although transition to turbulence appears in all the c ases investigated here, fully laminar flows in types B and C are predicted to exist by the linear stability theory under extreme flow conditions. Fina lly, the in vivo biological implications of the in vitro results were discu ssed, (C) 2001 Published by The Japan Society of Fluid Mechanics and Elsevi er Science B.V. All rights reserved.