Analysis of extreme wall-shear stress and vortical structure through direct numerical simulation of transitional flow in a stenosed carotid artery

Blood flow in human arteries is mostly laminar, but recent advances in measurement techniques and numerical simulations have given evidence of turbulence in physiological blood flow, especially in stenosed carotid arteries. Although research has observed this onset of turbulence by focusing on several hemodynamic factors, questions remain regarding extreme wall-shear stress (WSS) events and the associated vortical structures when the laminar-turbulent transition occurs. Here, by conducting the direct numerical simulation of transitional flow in a stenosed carotid artery, we demonstrate the frequent occurrence of an extreme retrograde WSS event and its relationship to the surrounding vortical structures. The laminar-to-turbulent transition is initiated by the breakdown of intense shear layers developed from the stenosis region of the internal carotid artery (ICA). The extreme retrograde WSS frequently occurs during the peak systole and deceleration phases. At peak systole, large transverse vortices generated by the oscillation and roll-up of shear layers induce the extreme WSS. In the deceleration phase, specifically, we observe a group of hairpin-like vortices that move in a similar convection velocity in the recirculation zone, reminiscent of hairpin packets in wall turbulence. The hairpin-like vortices evolve from a quasi-streamwise vortex near the wall, and new vortices are generated in the upstream that ultimately form a hairpin packet consistent with the auto-generation mechanism. During this process, we observe the regions of extreme retrograde WSS and intense Reynolds shear stress caused by the coherent induction of the vortices, which may contribute to platelet activation and red blood cell damage.