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Experimental study of flow pulse interacting with walls

Li, Q. (2020). Experimental study of flow pulse interacting with walls. (Unpublished Doctoral thesis, City, University of London)


Experimental studies are reported which investigate flow pulses interacting with walls. Such unsteady time-dependent impacts occur in many engineering applications as well as in nature, e.g. in fluid jets where the periodic generation of vortices around the jet core impact on solid or flexible walls. The thesis starts with the fundamental investigation of the impact of an elementary vortex structure, i.e. the vortex ring, on a wall. Comparing a flat wall with a wall containing a fakir like surface composed of slender circular posts shows a large influence of the roughness on the impact history. It is observed that a tailored lattice of the posts in hexagonal distribution causes the rapid growth of secondary vortex structures in the azimuthal instability mode of number N=6 arrangement at the outer edge of the primary ring in the form of six lobes which are aligned with the orientations of preferential “pathways” in the lattice. Rotating the layer with the hexagonal lattice results in the same rotation of the secondary flow pattern with the jets’ orientation lock-in with the orientation of the lattice. The layer with a random distribution of the posts at the same number density is not able to repeat this observation and no regular secondary flow pattern is seen. To further study the effect of mixing within the canopy, a thin homogeneous bed of particles is placed within the canopy and their resuspension by vortex impact is studied. When comparing a hexagonal arrangement to a random arrangement, preferential “pathways” is observed to promote re-suspension, as the random path of the particles around the filaments is affected in a non-linear manner by the local resistance. Furthermore, if the filaments are flexible, the efficiency of resuspension is increased by the amount it allows the effective canopy-height to reduce due to the reconfiguration of the flexible structures.

Extended to biomechanical field, the wall shear stress (WSS) induced by the natural flow pulse during the systolic beat of the heart is investigated in a transparent model of the human aorta. A traditional mechanical valve prosthesis, the SJM Regent bileaflet mechanical heart valve (BMHV), is compared with a newer version of a trileaflet mechanical heart valve (TMHV), the Lapeyre-Triflo FURTIVA. Elastic micro-pillar structures, calibrated as WSS sensors by micro-Particle-Image Velocimetry measurements, are applied to the wall along the ascending aorta (AAo). The peak amplitudes in WSS oscillations in the BMHV are ob-served to be almost twice that of the values seen in the TMHV. Flow field analysis illuminates that these peaks are linked to the jet-like flows generated in the valves interacting with the aortic wall. The side-orifice jets generated by the BMHV travel along the aortic wall in the AAo, impacting the wall throughout the AAo. However, the jets generated by TMHV impact further downstream in the AAo and results in a reduced WSS. The three-dimensional (3D) structure and evolution of the jet-like flow when interacting with the aortic wall are also obtained by additional 3D scanning PIV measurements.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments: Doctoral Theses
School of Science & Technology > School of Science & Technology Doctoral Theses
School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
[thumbnail of Li, Qianhui_PhD Thesis_Final_New.pdf] Text - Accepted Version
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