Direct numerical simulation of stenotic flows, Part 1: Steady flow

Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) stenosed tubes have been performed, with the motivation of understanding the biofluid dynamics of actual stenosed arteries. The spectral-element method, providing geometric flexibility and high-order spectral accuracy, was employed for the simulations. The steady flow results are examined here while the pulsatile flow analysis is dealt with in Part 2 of this study. At inlet Reynolds numbers of 500 and 1000, DNS predicted a laminar flowfield downstream of an axisymmetric stenosis and comparison to previous experiments showed good agreement in the immediate post-stenotic region. The introduction of a geometric perturbation within the current model, in the form of a stenosis eccentricity that was 5% of the main vessel diameter at the throat, resulted in breaking the symmetry of the post-stenotic flowfield by causing the jet to deflect towards the side of the eccentricity and at a high enough Reynolds number of 1000, jet breakdown occurred in the downstream region. The flow transitioned into turbulence about five diameters away from the stenosis, with velocity spectra taking on a broadband nature, acquiring a 5/3 slope that is typical of turbulent flows. Transition was accomplished by the breaking up of streamwise, hairpin vortices into a localized turbulent spot, reminiscent of the turbulent pu observed in pipe flow transition, within which r.m.s. velocity and turbulent energy levels were highest. Turbulent fluctuations and energy levels rapidly decayed beyond this region and flow relaminarized. The acceleration of the fluid through the stenosis resulted in wall shear stress (WSS) magnitudes that exceeded upstream levels by more than a factor of thirty but low WSS levels accompanied the flow separation zones that formed immediately downstream of the stenosis. Transition to turbulence in the case of the eccentric stenosis was found to manifest itself in large temporal and spatial gradients of WSS, with significant axial and circumferential variations in the turbulent section.