Experimental investigation of transition to turbulence in arteriovenous grafts

In patients with end-stage renal disease, dialysis is used to filter their blood, a function normally performed by the kidneys. In order to provide an access site for dialysis and to generate the required flow rates, an arteriovenous graft is surgically attached to both the artery and vein. Transition to turbulence in arteriovenous grafts occurs in vivo at Reynolds numbers approximately three times lower than in experiments or numerical simulations performed in vitro using similar geometries and flow conditions. Previous in vivo experiments have shown correlations to exist between vein wall vibration, which is probably caused by turbulence induced pressure fluctuations, and intimal thickening, which is the cause of graft failure. Of interest is a model that can predict levels and existence of turbulence in order to determine the role of biomechanical forces in the localization of intimal thickening.;Many experiments and simulations have been performed to predict the levels and existence of turbulence in vivo, however the results do not match the in vivo values. The six possible reasons that have been cited for the discrepancy are complex geometry, pulsatility, the non-Newtonian nature of blood, wall compliance, flow ratio, and inlet flow conditions. The goal of this thesis was to determine the importance of complex geometry, flow ratio, and the non-Newtonian nature of blood on transition to turbulence in order to help in the identification of the reason for the discrepancy. Although complex geometry, flow ratio, and the shear thinning aspect of the non-Newtonian nature of blood were found to affect transitional flow to a certain degree, none of these alone could account for the discrepancy that is observed in vivo. The fluid solid interaction within a non-Newtonian fluid was found to have a substantial effect on transition to turbulence and requires further examination to determine if it is the cause of the discrepancy.;Additionally, the experiments performed alluded to various techniques to lower vein wall vibration. In vivo experiments at the University of Chicago incorporating these into the graft design showed a statistically significant decrease in the amount of intimal thickening 8 weeks after implementation compared to standard grafts. Thus, the work done in this thesis has led to an improvement in graft design and has given a better understanding of the reason for early transition in vivo.