The hydrodynamic stability of cyclic ows inside rigid abdominal aortic aneurysm (AAA) models was investigated. Rectiÿed sine waveforms were used to simulate aortic ow conditions (Re mean = 1600-2100 and = 7:2-12.2). Depending on the bulge geometry (D=d and L=d ratios), AAA ows can be broadly classiÿed into three regimes, namely types A, B and C, respectively. While type A has no vortex formation, type B and C have distinctive laminar vortical structures that are very di erent from one another. The type of ow regimes would also determine where and when the transition to turbulence would occur and the portion of the cycle at which the ow remains turbulent in the bulge. The stability characteristics of types B and C are obtained from the linear stability analysis performed on the unsteady velocity proÿles measured at di erent phases of a cycle. Based on the linear stability analyses, instability is found to initiate in the bulge for types B and C through the formation of vortical structures. Instability grows progressively during the acceleration phase and transition to turbulence in the bulge 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 cases investigated here, fully laminar ows in types B and C are predicted to exist by the linear stability theory under extreme ow conditions. Finally, the in vivo biological implications of the in vitro results were discussed.
The airflow of a 10,000 rpm hard disk drive is simulated using the LES turbulent model. The critical parameters for using LES model to get the correct/reliable result are discussed. The flow induced vibration of the head gimbal assembly is calculated with the obtained fluid force acting on the head gimbal assembly (HGA) and compared with measured results. Good agreement is observed between the measured and simulated results. The optimization of the airflow is further carried out to reduce the flow induced vibration of the HGA. It is found that almost 50% reduction of flow induced vibration of the HGA can be achieved with the proposed modifications for airflow optimization.
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