Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
2 results
Search Results
Item Measurements and Modeling of the Unsteady Flow around a Thin Wing(2018) Manar, Field; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unsteady separated flows are encountered in many applications (e.g. dynamic stall in helicopters and wind turbines). Recent efforts to better understand the problem of unsteady separated aerodynamics have been prompted by growing interest in creating small-scale flight vehicles, termed micro air vehicles (MAVs). Because of their small size, all MAVs operate at low Reynolds numbers. In that regime, flow separation is a common occurrence either due to Reynolds number effects or aggressive motion. The dominant and most studied feature of these flows is the leading edge vortex (LEV). The LEV receives its circulation from a shear layer emanating from the leading edge of the wing, where the production of circulation occurs. In spite of its importance to the flow and the resulting forces, the production of circulation has received relatively little experimental attention. To fill this gap, water tank experiments on a surging flat plate wing at a high angle of attack have been performed at varying Reynolds number, acceleration, angle of attack, and aspect ratio. These experiments measured time resolved forces, LEV location, LEV circulation, and leading edge circulation production. These data were then used to explore how the LEV and the circulation production reacts to changes in kinematic parameters. This resulted in the proposal of a new relationship between the wake state and the leading edge circulation production, termed the boundary layer analogy (BLA). Additionally, existing potential flow modeling techniques were implemented and evaluated against the present experimental data. This analysis focused on evaluating the suitability of applying the Kutta condition at the leading edge. The Kutta condition was found to be a valid leading edge condition capable of predicting the LEV circulation seen in experiments. Representing the shed wake with multiple vortices was found to be necessary to capture the dynamics of vortex roll up and shedding. Other models struggle to account for these events, though simpler models may offer a better route to intuitive understanding of the fluid dynamic origin of the forces. The experimental data collected here, coupled with the novel analysis of the modeling techniques in the light of the leading edge circulation measurements, constitutes a significant step forward in the modeling and understanding of unsteady separated flows.Item Unsteady Low Reynolds Number Aerodynamics of a Rotating Wing(2012) Kolluru Venkata, Siddarth; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Micro air vehicles (MAVs) are small, unmanned aircraft useful for reconnaissance. The small size of MAVs presents unique challenges as they operate at low Reynolds numbers O(10^4), and they share a flight regime with insects rather than conventional aircraft. The low Reynolds number regime is dominated by poor aerodynamic characteristics such as low lift-to-drag ratios. To overcome this, birds and insects utilize unsteady high lift mechanisms to generate sufficient lift. A leading edge vortex (LEV), one of these unsteady lift mechanism, is thought to be responsible for the high lift generated by these natural fliers, but the factors which contribute to the formation, stability, and persistence of LEVs are still unclear. The objectives of this study are to: 1) qualitatively understand the formation of the LEV by evaluating the effect of wing acceleration profiles, wing root geometry, Reynolds number, and unsteady variations of pitch, 2) quantify whether high lift can be sustained at low Reynolds numbers on a rotary wing in continuous revolution, and 3) determine the effect of wing flexibility on the unsteady lift coefficient. Experiments were performed on a rotating wing setup designed to model the translational phase of the insect wing stroke during hover. Experiments were performed in a water tank at Reynolds numbers between 5,000 and 25,000, and the flow was investigated using dye flow visualization, as well as lift and drag force measurements. A rigid wing and a simple one degree-of-freedom flexible wing were tested. Dye flow visualization on a rotating wing showed the formation of a coherent LEV near the wing root. The LEV became less coherent further outboard, and eventually burst. As the wing continued to rotate, the location where the LEV burst moved inboard. Dye injection within the burst vortex showed the formation of multiple small scale shedding vortices that traveled downstream and formed a region of recirculating flow (i.e., a burst vortex). Parameter variations in this experiment included velocity profiles, acceleration profiles, and Reynolds numbers. High lift and drag coefficient peaks were measured during the acceleration phase of the wing stroke at Reynolds numbers of 15,000 and 25,000. After the initial peak, the coefficients dropped, increased, and eventually attained a ``steady-state" intermediate value after 5 chord-lengths of travel, which they maintained for the remainder of the first revolution. When the wing began the second revolution, both the lift and drag coefficients decreased, and leveled out at a second intermediate value. Force measurements on a chordwise flexible wing revealed lower lift coefficients. For all of the cases tested, high lift was achieved during the acceleration phase and first revolution of the wing stroke, though values dropped during the second revolution.