Experimental and Numerical Studies of Flows Related to the Processes of Atherosclerosis

Abstract: This thesis concerns the prediction of the fluid mechanical conditions in some specific large vessels in the human body. Large vessels are special in the way that the Reynolds number is relatively high and in the case of arteries the flow is pulsatile. The aim is to contribute to the knowledge of flow patterns in large vessels, and ways to predict and measure fluid mechanical parameters such as shear stresses acting on the vessel wall, turbulent velocity fluctuations and mass transport of different molecules in the blood. There are strong indications that wall shear stress and mass transport are linked to the selective development and progress of atherosclerosis in the vascular tree. Two specific flow situations are treated in this thesis, the flow conditions in constricted large arteries and those in an arterialized vein punctured by a cannula. The fluid mechanical conditions in constricted pipes approximating the conditions in stenotic large arteries where transition to turbulence occur have been studied numerically by LES (large eddy simulations) and experimentally by DPIV (digital particle image velocimetry). It is demonstrated that the numerical methodology employed can predict transition to turbulence in the post-stenotic jet and give good estimates of the turbulent velocity fluctuations present behind moderate and severe constrictions. Both steady and pulsatile flow cases are considered. A method to improve the accuracy of whole plane velocity measurements in the presence of strong shear is proposed by a combination of multi-pass DPIV and DPTV. The methodology is tested on laminar and turbulent pipe flow and it is demonstrated that the method offers significant improvements in the estimation of near wall turbulent statistics and instantaneous wall shear stress compared to cross-correlation DPIV. To predict concentration of various substances PLIF (planar laser induce fluorescence) is introduced, and the combination of DPIV and PLIF is used to simultaneously measure the instantaneous velocity- and concentration field in liquid flows. The method was tested on mixing in the near field of a free turbulent jet and the results agree well with numerical simulations of the flow. An attractive feature of the method is its ability to provide estimates of the turbulent mass transport terms in the plane. Measurements by DPTV and PLIF were used to quantify the velocity field including the wall shear stress, and the mixing patterns around a cannula puncturing a vein during hemodialysis treatment. The measurements were compared with and complemented by numerical simulations. The results from these studies in terms of wall shear stress variation and mixing patterns are analyzed for different flow rates in the vein and the cannula. The results are interpreted in terms of risk factors for the development of atherosclerosis.