Browsing by Author "Ullah, Al Habib"

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    Advanced Measurements and Analyses of Flow Past Three-Cylinder Rotating System
    (North Dakota State University, 2020) Ullah, Al Habib
    Interaction of flow structures from a three-cylinder system is complex and important for fundamental and engineering applications. In this study, experiments using hotwire, 2D PIV, and Tomography are to be conducted to characterize the fluid flow at various Re number and rotation speeds. The Reynolds number considered based on the diameter of the single-cylinder ranges from 37 to 1700. The peaks in the frequency spectrum obtained from the hotwire study show a unique relation of Strouhal number as a function of static incident angle, RPM, and Reynolds number. From the 2D PIV and 3D tomography experiment, vorticity and velocity results characterize the interaction of wake flow from individual cylinders and as a function of the rotational speeds. Besides, the Standard deviation map shows the turbulence intensity variation at the various static and rotating conditions. The obtained results at static conditions are found to be consistent with the previous computational study.
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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.