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Subject: PIV

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    Dynamic Stall Characteristics of Pitching Finite-Aspect-Ratio Wings
    (North Dakota State University, 2021) Ullah, Al Habib
    In this study, an experimental investigation was performed to characterize the dynamic stall of pitching wings and provide confirmation of the existence of the arch-shaped vortex for moderate sweep wing. Dynamic stall is a complex flow, which happens because of a sudden change of incident angle during the pitching motion. The pitching motion of a wing can cause instability in the shear layer and generate the separation burst at certain angles. For a pitching wing, the dynamic stall vortex is characterized by the formation of an arch-shaped vortex to the evolution of a ring-shaped vortex. The leg of the arch-shaped vortex causes a negative pressure region on the airfoil surface and can, in fact, generate greater lift. However, in certain conditions, the detachment of the arch-shaped vortex from the airfoil surface can cause high pressure and vibration in the structures. The formation of the arch-shaped vortex and its evolution were systematically investigated using cutting-edge flow diagnostic techniques, and the physics of the dynamic stall is explained in addition to providing the confirmation of the theory developed based on Computational Fluid Dynamics. The study was done using Particle Image Velocimetry (PIV) and Pressure-Sensitive Paint for three sweep angle wings. The wings, with an aspect ratio of AR=4 and a NACA 0012 section assembled with round-tip, are considered for the current experimental study. The sweep angles = 0, 15, and 30 degrees were considered to compare the flow phenomena. The PIV results show the formation of a laminar separation bubble and its evolution to a dynamic stall vortex. The increase of sweep angle causes the formation of such vortices towards the wing tip. In the process of finding the footprint of the vortices and pressure distribution on the surface of the wings, a single-shot lifetime method using fast porous paint was used. The results show the existence of suction pressure and later grows towards the trailing edge of the wing due to the formation of a dynamic stall vortex. In addition, at Re=2x10^5 and reduced frequency k=0.2, a moderate sweep airfoil shows the apparent footprint of the arch-shaped vortex, which confirms the current theory.
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    Experimental Studies of Pulsatile Flow Characteristics of Aortic Models under Normal and Diseased Conditions
    (North Dakota State University, 2021) Zhang, Ruihang
    Heart disease is the leading cause of death globally. Aorta is extremely important because of its critical function in blood circulation. Abnormal hemodynamics of aortic valve and arch is related to many severe diseases and has intrigued a growing of fluid dynamic researches over decades. However, due to the complexity of transient flow and fluid-structure interaction, many aspects of aortic hemodynamics have not been fully understood. The goal of this dissertation is to design and construct an in-vitro cardiovascular flow simulator for PIV hemodynamics research and understand the pulsatile flow characteristics of human aortic valve and arch under normal and diseased conditions. First, we investigated the fluid dynamics of a complaint aortic root model under varied cardiac outputs. High turbulence kinetic energy was observed after peak systole. A reduction in cardiac outputs resulted in a lower post-systole turbulence, smaller circumferential deformation, smaller geometric orifice area, and a shortened valve-opening period. Second, we investigated the pulsatile flow through stenotic aortic valve models. Results indicated that a severe prosthetic stenosis causes significant changes in the flow fields downstream. The hemodynamic changes, e.g., increased jet velocity and viscous shear stress, were associated with the stiffened leaflet materials, rather than the stent base structure. Third, we presented a combined experimental and numerical study of the pulsatile flow characteristics within Gothic and Romanesque aortic arch models. The results revealed significantly different primary and secondary flow characteristics between two models. Low and oscillatory wall shear stress and the abnormal secondary flow in the Gothic arch are correlated to vascular endothelial cell remodeling and might provide hints to the increased risks of atherosclerosis, late systemic hypertension, and other cardiovascular complications. Overall, this dissertation provides physical insights into pulsatile flow characteristics through aortic valve and arch models under varied normal and diseased conditions. In-vitro experiments using PIV can capture prominent flow characteristics within prosthetic aortic models, providing better controllability and spatial resolution that complements clinical diagnosis and a source of validation for computational simulations. Future improvements of artificial models’ designs and the advanced flow diagnostic techniques can further enhance the accuracy and credibility of in-vitro flow researches.