No abstract

Atherosclerosis is the most fatal cardiovascular disease. As disease progresses, stenoses grow inside the arteries blocking their lumen and altering blood flow. Analysing flow dynamics can provide a deeper insight on the stenosis evolution. In this work we combined Eulerian and Lagrangian descriptors to analyze blood flow dynamics and fluid transport in stenotic aortic models with morphology, mechanical and optical properties close to those of real arteries. To this end, vorticity, particle residence time (PRT), particle's final position (FP) and finite time Lyapunov's exponents (FTLE) were computed from the experimental fluid velocity fields acquired using ultrasonic particle imaging velocimetry (Echo-PIV). For the experiments, CT-images were used to create morphological realistic models of the descending aorta with 0%, 35% and 50% occlusion degree with same mechanical properties as real arteries. Each model was connected to a circuit with a pulsatile programmable pump which mimics physiological flow and pressure conditions. The pulsatile frequency was set to ≈0.9 Hz (55 bpm) and the upstream peak Reynolds number (Re) was changed from 1100 to 2000. Flow in the post-stenotic region was composed of two main structures: a high velocity jet over the stenosis throat and a recirculation region behind the stenosis where vortex form and shed. We characterized vortex kinematics showing that vortex propagation velocity increases with Re. Moreover, from the FTLE field we identified Lagrangian coherent structures (i.e. material barriers) that dictate transport behind the stenosis. The size and strength of those barriers increased with Re and the occlusion degree. Finally, from the PRT and FP maps, we showed that independently of Re, the same amount of fluid remains on the stenosis over more than a pulsatile period.

Several Lagrangian methods were used to analyze the mixing processes in an experimental model of a constricted artery under pulsatile flow. Upstream Reynolds number Re was changed between 1187 and 1999, while the pulsatile period $T$ was fixed at 0.96s. Velocity fields were acquired using Digital Particle Image Velocimetry (DPIV) for a region of interest (ROI) located downstream of the constriction. The flow is composed of a central jet and a recirculation region near the wall where the vortex forms and sheds. To study the mixing processes, finite-time Lyapunov exponents (FTLE) fields and concentration maps were computed. Two Lagrangian coherent structures(LCS) responsible for mixing fluid were found from FTLE ridges. A first LCS delimits the trailing edge of the vortex, separating the flow that enters the ROI between successive periods. A second LCSdelimits the leading edge of the vortex. This LCS concentrates the highest particle agglomeration, as verified by the concentration maps. Moreover, from particle residence time maps (RT) the probability of a fluid particle of leaving the ROI before one cycle was measured. As Re increases, the probability of leaving the ROI increases from 0.6 to 0.95. Final position maps$r{_f}$ were introduced to evaluate the flow mixing between different subregions of the ROI. These maps allowed us to compute an exchange index between subregions, $\bar{\mathrm{EI}}$, which shows the main region responsible for the mixing increase with Re. Finally, by integrating the results of different Lagrangian methods, a comprehensive description of the mixing and transport of the flow was provided.

Turbulent fountains are widespread natural phenomena with numerous industrial applications. Extensive research has focused on the temporal evolution and maximum height of these fountains, as well as their dependence on Reynolds and Froude numbers. However, the minimum height of the surrounding ambient fluid, which is removed by the fountain due to the entrainment effect, has received little attention. In this study, we investigate the dependence of this minimum height on the characteristics of the fountain and demonstrate how to control it. Our findings present important implications for technological applications of turbulent fountains, particularly in contaminant withdrawal. We discuss the potential of our results to improve the efficiency of such applications.