UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
4 results
Search Results
Item Urban Air Mobility: Effects of increasing three-dimensionality on fixed and rotary wings in unsteady aerodynamic environments(2024) Wild, Oliver Dominik; Jones, Anya; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The rapidly growing field of electric vertical takeoff and landing aircraft, air taxis, and urban air mobility vehicles promises transformative solutions to alleviate urban congestion, accelerate deliveries, and revolutionize transportation systems. Central to the successful integration of these futuristic modes of transportation is a comprehensive understanding of their aerodynamics, particularly in the context of unsteady airflow encountered in urban environments. This work explores the foundational aspects essential for achieving efficient and safe urban air mobility operation. The focus lies on the integration of rotary and translatory wings in gusty and unsteady flow environments since – unlike conventional fixed-wing aircraft – many urban air vehicles utilize rotor systems for both vertical takeoff and forward flight. The research framework is structured around three interconnected pillars: advancing rotary wings, fixed-wing-gust encounters, and the synthesis of rotary wings in gusty conditions. The combined results from these three pillars are fundamental in reaching the future goal of efficient and safe urban air mobility. The first pillar investigates the aerodynamic characteristics of advancing rotary wings, particularly concerning flow structures, blade loading, and the influence of the trailing edge geometry using experimental, numerical, and modeling techniques. A comparison between a standard NACA0012 airfoil profile and an elliptical profile is conducted at advance ratios ranging from 0.00 to 1.00 at pitch angles from 7 deg to 25 deg. Four main vortex structures were detected in reverse flow. At the aerodynamic leading edge, a strong interference of the tip vortex with the reverse flow dynamic stall vortex was identified when blade flapping was restricted. Dynamic stall vortices advect closer to the blade surface for the blunt elliptical airfoil, thus reducing the wake area in reverse flow. Overall, the vortex structures that form on the ellipse are more coherent than those on the NACA0012. A 29% pitching moment increase was measured in the reverse flow region with sharp trailing-edged blades compared to blunt blades. The blunt trailing-edged blade delayed flow separation and thus prevented the formation of a reverse flow dynamic stall vortex, reducing the pitching moment. The second pillar delves into the three-dimensional dynamics of fixed-wing-gust encounters, aiming to understand the formation of leading-edge vortices and their impact on lift generation. Emphasis is placed on exploring strong transverse gust encounters and the effects of sideslip angle on leading edge vortex formation, with the objective of devising predictive models for lift generation under varied gust scenarios. Experimental investigations in a towing tank and the employment of a strip theory Küssner model show a peak lift coefficient decrease with decreasing gust ratios and increasing sideslip angles. The model accurately predicts the experimental results at gust entry as well as within the gust. Flow reattachment is delayed due to the formation of a leading-edge vortex inducing reverse flow on the wing suction side, resulting in a non-zero wing forcing at gust exit. The third pillar examines the effects of gusts on both hovering and advancing rotors. It synthesizes the findings from the previous two pillars, mirroring real-world conditions occurring on urban air mobility vehicles. Gusts cause an increase in blade flapping and lagging moments, and a nose-down pitching moment in both hovering and advancing rotors. In forward flight, the moment response mirrors a wing-gust encounter. A lower advance ratio broadens the moment peaks. Reverse flow shows a smaller moment response but a wider azimuth angle impact. Increased gust and advance ratios amplify moment disturbances, with gust encounters on the retreating blade more sensitive to gust ratio changes. By integrating insights from rotary wings and gust encounters, this research provides a comprehensive understanding of aerodynamic phenomena crucial for the development of efficient and safe urban flight vehicles. Through this multidisciplinary approach, this thesis contributes to advancing the fundamental understanding of aerodynamic challenges in urban air mobility, paving the way for the development of innovative solutions to propel the future of urban air mobility.Item PERFORMANCE OF ELECTRIC MEDIUM-SIZED VARIABLE-RPM ROTOR AND SHROUDED ROTOR(2022) Ryseck, Peter Christian; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electric variable RPM rotors are increasingly being used for propulsion and control of unmanned air vehicles. As these vehicles scale to carry heavier payloads of 50 to 400 lbs (20 to 180 kgs) in the group 2 and 3 UAS category, there are concerns about their aerodynamic performance and handling quality degradation. Therefore, there is a need to develop a systematic experimental testing procedure to measure loads on these systems to evaluate performance and augment Computational Fluid Dynamic (CFD) validation tools. In this work, a universal electric powered test rig is designed and fabricated for hover and wind tunnel tests of open and shrouded rotors. Steady hover results are validated using blade element momentum theory. These predictions incorporate an empirical correction approach in conjunction with an interpolation scheme to capture Reynolds number variation along the span of the blade and variation with RPM. Results show good agreement with the interpolation method for the low Reynolds number rotor tested (Re_tip<500,000). For the variable RPM rotor, transient step and chirp inputs are also presented. System identification showed linear frequency responses between thrust and torque with RPM and RPM-square in hover. Therefore, when modeling this rotor, steady inflow appears adequate in the frequency range of interest (0.4 to 60 rad/sec). In addition to an open rotor, the electric motor-rotor test stand was used to test a shrouded rotor in hover and forward flight to systematically compare performance results. Test data showed the shrouded rotor gained 15% thrust for the same power in hover with the best configuration. For low speed forward flight, lift-to-drag ratio was found to increase by 8 to 10% for the shrouded rotor system over the isolated rotor.Item Aerodynamic Analysis of an MAV-Scale Cycloidal Rotor System Using a Stuctured Overset RANS Solver(2010) Yang, Kan; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A compressible Reynolds-Averaged Navier-Stokes solver was used to investigate the performance and flow physics of the cycloidal rotor (cyclocopter). This work employed a computational methodology to understand the complex aerodynamics of the cyclocopter and its relatively unexplored application for MAVs. The numerical code was compared against performance measurements obtained from experiment and was seen to exhibit reasonable accuracy. With validation of the flow solver, CFD predictions were used to gain qualitative insight into the flowfield. Time histories revealed large periodic variations in thrust and power. In particular, the virtual camber effect was found to significantly influence the vertical force time history. Spanwise thrust and flow visualizations showed a highly three-dimensional flowfield with large amounts of blade shedding and blade-vortex interaction. Overall, the current work seeks to provide unprecedented insight into the cyclocopter flowfield with the goal of developing an accurate predictive tool to refine the design of future cyclocopter configurations.Item Development of Mach Scale Rotors with Composite Tailored Couplings for Vibration Reduction(2004-11-29) Bao, Jinsong; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of composite tailored couplings in rotor blades to reduce vibratory hub loads was studied through design, structural and aeroelastic analysis, fabrication, and wind tunnel test of Mach scale articulated composite rotors with tailored flap-bending/torsion couplings. The rotor design was nominally based on the UH-60 BLACK HAWK rotor. The 6-foot diameter blades have a SC1095 profile and feature a linear twist of -12 deg. The analysis of composite rotor was carried out using a mixed cross-section structural model, and UMARC. Five sets of composite rotor were fabricated, including a baseline rotor without coupling, rotors with spanwise uniform positive coupling and negative coupling, and rotors with spanwise dual-segmented coupling (FBT-P/N) and triple-segmented coupling. The blade composite D-spar is the primary structural element supporting the blade loads and providing the desired elastic couplings. Non-rotating tests were performed to examine blade structural properties. The measurements showed good correlation with predictions, and good repeatability for the four blades of each rotor set. All rotors were tested at a rotor speed of 2300 rpm (tip Mach number 0.65) at different advance ratios and thrust levels, in the Glenn L. Martin Wind Tunnel at the University of Maryland. The test results showed that flap-bending/torsion couplings have a significant effect on the rotor vibratory hub loads. All coupled rotors reduced the 4/rev vertical force for advance ratios up to 0.3, with reductions ranging from 1 to 34%. The mixed coupling rotor FBT-P/N reduced overall 4/rev hub loads at advance ratios of 0.1, 0.2 and 0.3. At a rotor speed of 2300 rpm and an advance ratio of 0.3, the FBT-P/N rotor achieved 15% reduction for 4/rev vertical force, 3% for 4/rev in-plane force and 14% for 4/rev head moment. The reductions in the 4/rev hub loads are related to the experimentally observed reductions in 3/rev and 5/rev blade flap bending moments. Through the present research, it has been experimentally demonstrated that structural couplings can significantly impact rotor vibration characteristics, and with suitable design optimization (coupling strength and spanwise distribution) they can be used to reduce vibratory hub loads without penalties.