Understanding the fl uid mechanics behind transverse wall shear stress of Biomechanics

The patchy distribution of atherosclerosis within arteries is widely attributed to local variation in hae- modynamic wall shear stress (WSS). A recently-introduced metric, the transverse wall shear stress (transWSS), which is the average over the cardiac cycle of WSS components perpendicular to the tem- poral mean WSS vector, correlates particularly well with the pattern of lesions around aortic branch ostia. Here we use numerical methods to investigate the nature of the arterial fl ows captured by transWSS and the sensitivity of transWSS to in fl ow waveform and aortic geometry. TransWSS developed chie fl y in the acceleration, peak systolic and deceleration phases of the cardiac cycle; the reverse fl ow phase was too short, and WSS in diastole was too low, for these periods to have a signi fi cant in fl uence. Most of the spatial variation in transWSS arose from variation in the angle by which instantaneous WSS vectors deviated from the mean WSS vector rather than from variation in the magnitude of the vectors. The pattern of transWSS was insensitive to in fl ow waveform; only unphysiologically high Womersley numbers produced substantial changes. However, transWSS was sensitive to changes in geometry. The curvature of the arch and proximal descending aorta were responsible for the principal features, the non- planar nature of the aorta produced asymmetries in the location and position of streaks of high transWSS, and taper determined the persistence of the streaks down the aorta. These results re fl ect the importance of the fl uctuating strength of Dean vortices in generating transWSS. per-sistent fl ow disturbances accelerates atherogenesis and promotes thin cap