Experimental investigation of three-dimensional separated flows at low reynolds numbers on finite aspect ratio wings

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Hayostek, Shelby Kateri-Rose
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Electronic thesis
Aeronautical engineering
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Flow separation is a complex, three-dimensional phenomenon that leads to a loss in aerodynamic performance such as a decrease in lift and an increase in drag. It occurs when the flow detaches from a surface and creates a deficit leading to this degradation. Flow separation has been studied for awhile now with some of the earliest work on the phenomenon dating back to the early 1900s with Prandtl's studies, however due to the complexity behind separation many assumptions needed to be made in order to simplify the problem. One major assumption was that a given flow is two dimensional. From this assumption, many studies have been done, paving the ground work in understanding how flow separates, however recently it has been highlighted that flows, especially over wings, cannot be treated as two-dimensional but rather must be studied in full to be able to understand this phenomenon completely. Once separation is understood, flow control technologies can then be implemented in order to mitigate the effects of separation with the eventual goal of improving the performance of a given air vehicle. The current work is the first step in understanding three-dimensional separation by experimentally exploring how various parameters affect a separated flowfield on a simple geometry. The wing used had a symmetric NACA 0015 airfoil profile and was finite, meaning that end effects cannot be ignored. The parameters of boundary conditions, sweep angle, aspect ratio, Reynolds number, and angle of attack were varied to understand how each parameter affected flow separation over the wing and which one had the strongest influence. The experiments were conducted in two water tunnels using two experimental methods: dye flow visualizations to qualitative visualize the flowfield and stereo particle image velocimetry to obtain a quantitative understanding of the flow around the wings. This work includes a wide parametric work with many cases, however a small subset was studied in great detail as some of the features were the same among the cases or had limited differences. As stated, this work is the first step in understanding this phenomenon, but it is part of a much larger project. The work presented focused on low chord-based Reynolds numbers (600 and 1,000) to cross-validate with DNS and TriGlobal Stability analysis in order to fill in the gaps in understanding three-dimensional flow separation. In the following phases on this project, the complexity of the wing geometry will slowly be increased to higher Reynolds numbers, taper ratio, wing twist, etc., until a complete air vehicle can be studied, where what was learned about separation in simpler geometry can be applied to more complex flowfields. This will lead to better implementation of different flow control techniques to increase the vehicle's aerodynamic performance. The parameters that were found to have the largest influence on the flowfield were the aspect ratio, the sweep angle, and the different boundary conditions. On any given finite span, low aspect ratio, cantilevered wing, there are three regions along the span: a wall region, a wake region, and a tip region. It was found that at smaller aspect ratios, the end effects played a significant role in the development of the wake by mitigating the shedding and resulting in a smaller separated region as the end regions dominate the flowfield. At the higher aspect ratios, the end effects had less of an influence on the overall global flowfield allowing shedding to develop in the wake. Along with that, in the unswept case, secondary structures of alternating rotation developed in the wake as a result of the interaction between the separated flow and the end regions. In the swept cases, a large spanwise component developed, pushing the flow outboard towards the tip. This spanwise flow interacted with the separated region forming secondary structures as well with a large dominant swirl vortex along the span. Due to the outboard flow, the most outboard structure merged with the tip vortex. When comparing boundary conditions, the influence of the boundary layer led to the formation and mitigation of the vortical structures. This in turn affected the wake as when a boundary layer was absent, the number of structures present would decrease. To compare the various cases, both time-averaged and instantaneous images were used to understand the formation of the structures seen. The present study has shown the complexity of three-dimensional flow separation even on relatively simple geometries by exploring in detail how various structures are formed, advect, and interact with each other. With this understanding, further and more complex studies can be conducted to further aide in filling in the gaps of this complex phenomenon.
August 2021
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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