Theses and Dissertations - Department of Aerospace Engineering and Mechanics

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Browsing Theses and Dissertations - Department of Aerospace Engineering and Mechanics by Author "Agrawal, Ajay K."

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    An experimental study of flow separation over a flat plate with 2D transverse grooves
    (University of Alabama Libraries, 2013) Jones, Emily; Lang, Amy W.; University of Alabama Tuscaloosa
    Nature has long been an inspiration for research in engineering. In particular, the biological surfaces of aquatic swimmers have been studied for their potential as drag reducing surfaces. The hydrodynamic benefit of riblets, or grooves embedded parallel to the flow, which appear on many aquatic biological surfaces, have been well documented and implemented in practical engineering applications. However the skin of dolphins is embedded with grooves that run perpendicular to the flow of water over their bodies. It is theorized that the transverse grooves present on dolphin skin trap vortices between them, creating a partial slip condition over the surface and inducing turbulence augmentation in the boundary layer, thus controlling boundary layer separation over the dolphin's skin. Similarly, sharks are covered with scales that are flexible at the base and capable of bristling, forming grooves running transverse to the flow. It is theorized that the scales bristle when encountering a reversing flow, thereby trapping vortices between the scales and, similarly, delaying boundary layer separation. In an attempt to test this hypothesis and study these affects, a spinning cylinder was used in a water tunnel to induce separation over a flat plate with 2 mm, rectangular transverse grooves and sinusoidal grooves of similar scaling. The results were compared to tripped, turbulent boundary layer separation occurring over a flat plate without grooves using time-resolved particle image velocimetry. The strength of the adverse pressure gradient was varied, and the observed delay in flow separation and other affects upon the boundary layer are discussed.
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    An investigation of the performance potential of a liquid oxygen expander cycle rocket engine
    (University of Alabama Libraries, 2016) Stapp, Dylan Thomas; Branam, Richard D.; University of Alabama Tuscaloosa
    This research effort sought to examine the performance potential of a dual-expander cycle liquid oxygen-hydrogen engine with a conventional bell nozzle geometry. The analysis was performed using the NASA Numerical Propulsion System Simulation (NPSS) software to develop a full steady-state model of the engine concept. Validation for the theoretical engine model was completed using the same methodology to build a steady-state model of an RL10A-3-3A single expander cycle rocket engine with corroborating data from a similar modeling project performed at the NASA Glenn Research Center. Previous research performed at NASA and the Air Force Institute of Technology (AFIT) has identified the potential of dual-expander cycle technology to specifically improve the efficiency and capability of upper-stage liquid rocket engines. Dual-expander cycles also eliminate critical failure modes and design limitations present for single-expander cycle engines. This research seeks to identify potential LOX Expander Cycle (LEC) engine designs that exceed the performance of the current state of the art RL10B-2 engine flown on Centaur upper-stages. Results of this research found that the LEC engine concept achieved a 21.2% increase in engine thrust with a decrease in engine length and diameter of 52.0% and 15.8% respectively compared to the RL10B-2 engine. A 5.89% increase in vacuum specific impulse was also observed. The implications of these results could lead to significant launch cost savings and replacement of aging expander cycle technology in the rocket propulsion industry. In order to fully validate the results of this research, more knowledge is required regarding the heat transfer characteristics of supercritical oxygen for rocket thrust chamber cooling. Future work in this topic will focus on experimental LOX heat transfer research and model optimization to improve heat transfer estimations in the baseline model developed in this research and further explore the optimal performance potential and limitations of the LEC engine.
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    Turbulent separation control effects of mako shark skin samples on a naca 4412 hydrofoil
    (University of Alabama Libraries, 2011) Smith, Jonathon Andrew; Lang, Amy W.; University of Alabama Tuscaloosa
    Shark skin is investigated as a means of passive flow separation control due to its preferential flow direction and the potential for its scales to obstruct low-momentum backflow resulting from an adverse pressure gradient. In this study, the effect of the scales on flow reversal is observed in a tripped turbulent boundary layer by comparing the flow over a NACA 4412 hydrofoil with a smooth, painted surface to that over the same hydrofoil with samples of mako shark skin affixed to its upper surface. These samples were taken from the shark's flank region because the scales at that location have been shown to have the greatest angle of bristling, and thus the best potential for separation control. All sets of flow data in this study were obtained using Time-Resolved Digital Particle Image Velocimetry and recorded at multiple angles of attack (between 8 and 16 degrees) and two Reynolds numbers. The flow was primarily analyzed by means of the backflow coefficient (a value based on the percentage of time that flow in a region over the hydrofoil is reversed), average backflow magnitude, and the time history of instantaneous flow velocity values at specific points in the boundary layer over the hydrofoil models. Results showed that at angles of attack of 12° and below, the shark skin generated a slightly larger, higher magnitude region of reversed flow than was seen over the painted surface. At an angle of attack of 16°, the backflow region of the shark skin surface was significantly reduced in size and magnitude compared to that of the painted surface. These results support the hypothesis that in order for the scales to be an effective means of flow control, sufficient shear must be present in the backflow to cause them to bristle fully.