The aorta is the largest artery leaving the heart. Under normal conditions the flow is claimed to be laminar, but it may also become turbulent. Turbulence and the associated increased shear stress are believed to be associated mostly with pathological blood flow phenomena. Although in the last decades there have been many scientific investigations on the flow characteristics in a human aorta, it is still unknown whether and under what circumstances aortic flow becomes turbulent. Despite numerous studies investigating individual aspects for laminar/turbulent flow transition, to date their interplay at realistic flow conditions is still poorly understood. Therefore in this thesis work relevant flow parameters will systematically be varied and the resulting flow patterns and flow states will be studied. In particular it will be asked: What are the local causes for transition? What regions become turbulent? What are the properties of turbulence in these regions? Are transition and geometry related, due to flow separation, or is free shear flow in the middle of the vessel mainly responsible? In special regions of interest the field of velocity derivatives will be investigated, including the fields of vorticity and strain that are responsible for dissipating the kinetic energy of the flow.

Up to recently, there was no non-intrusive whole field experimental access to particle trajectories, velocities, velocity fluctuations and velocity derivatives in turbulent flows. With the recent development of PTV, it is possible to identify and analyze turbulent flow regions accurately and on the necessary level of detail. The flow will be first adjusted to closely mimic in-vivo flow conditions as obtained from MRI measurements. The accuracy of the MRI velocities will be assessed by comparing with PTV measurements of the same flow in the same setup. Parameters of a ‘healthy patient reference flow’ and an ‘unhealthy patient reference flow’ will be varied systematically to investigate their influence on laminar turbulent transition. Milestones of our project are: (i) Setup of in-vitro PTV experiment in an aorta replica to match in-vivo flow conditions. (ii) Understand the conditions that lead to transition in aortic flow. (iii) Quantify flow properties in regions before, during and after transition. The expected outcome has technical, engineering and fundamental aspects: The quality of MRI velocity measurements will be assessed. Detailed velocity information will produce benchmark data for computational fluid dynamics codes that are used in industry. The understanding of the flow phenomenon transition will be advanced.

To summarize, in this project, the main aim is to advance the hydrodynamic understanding of aortic flows. To achieve this, transition from laminar to turbulent flow in an elastic and transparent anatomically accurate replica of a human aorta is analyzed. The inflow conditions are taken from in-vivo magnetic resonance imaging (MRI) measurements, in order for them to be as realistic as possible. Particle Imaging Velocimetry (PIV) and 3D Particle Tracking Velocimetry (3D-PTV), two image-based non intrusive measurement techniques are applied to a pathological and healthy human aorta replicas.

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