Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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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 NUMERICAL SIMULATION AND VALIDATION OF HELICOPTER BLADE-VORTEX INTERACTION USING COUPLED CFD/CSD AND THREE LEVELS OF AERODYNAMIC MODELING(2014) Amiraux, Mathieu; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rotorcraft Blade-Vortex Interaction (BVI) remains one of the most challenging flow phenomenon to simulate numerically. Over the past decade, the HART-II rotor test and its extensive experimental dataset has been a major database for validation of CFD codes. Its strong BVI signature, with high levels of intrusive noise and vibrations, makes it a difficult test for computational methods. The main challenge is to accurately capture and preserve the vortices which interact with the rotor, while predicting correct blade deformations and loading. This doctoral dissertation presents the application of a coupled CFD/CSD methodology to the problem of helicopter BVI and compares three levels of fidelity for aerodynamic modeling: a hybrid lifting-line/free-wake (wake coupling) method, with modified compressible unsteady model; a hybrid URANS/free-wake method; and a URANS-based wake capturing method, using multiple overset meshes to capture the entire flow field. To further increase numerical correlation, three helicopter fuselage models are implemented in the framework. The first is a high resolution 3D GPU panel code; the second is an immersed boundary based method, with 3D elliptic grid adaption; the last one uses a body-fitted, curvilinear fuselage mesh. The main contribution of this work is the implementation and systematic comparison of multiple numerical methods to perform BVI modeling. The trade-offs between solution accuracy and computational cost are highlighted for the different approaches. Various improvements have been made to each code to enhance physical fidelity, while advanced technologies, such as GPU computing, have been employed to increase efficiency. The resulting numerical setup covers all aspects of the simulation creating a truly multi-fidelity and multi-physics framework. Overall, the wake capturing approach showed the best BVI phasing correlation and good blade deflection predictions, with slightly under-predicted aerodynamic loading magnitudes. However, it proved to be much more expensive than the other two methods. Wake coupling with RANS solver had very good loading magnitude predictions, and therefore good acoustic intensities, with acceptable computational cost. The lifting-line based technique often had over-predicted aerodynamic levels, due to the degree of empiricism of the model, but its very short run-times, thanks to GPU technology, makes it a very attractive approach.Item Analysis of Factors Affecting the Aerodynamics of Low Reynolds Number Rotating Wings(2013) Schlueter, Kristy Lynn; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A computational analysis was performed to address the effects of walls on wings rotating at a Reynolds number of 120. For rotation angles less than one revolution, a tip clearance of 0.5 chord-lengths is sufficient to approximate a wing rotating in an infinitely large volume of fluid. However, for a maximum rotation of two revolutions, a tip clearance of 5.0 chords is necessary. At the start of the second revolution, the wing encounters its wake, and the wake structure is significantly affected by low tip clearances. Lift and drag forces were measured experimentally for wings rotating at a Reynolds number of 10,000 while parameters including root cutout were varied. Root cutout significantly alters the lift and drag coefficients, including the location of a second local maximum in both lift and drag. The root-relative method of force non-dimensionalization provided the best comparison of force coefficients between cases with different root cutouts.Item SCHLIEREN SEQUENCE ANALYSIS USING COMPUTER VISION(2013) Smith, Nathanial Timothy; Lewis, Mark J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Computer vision-based methods are proposed for extraction and measurement of flow structures of interest in schlieren video. As schlieren data has increased with faster frame rates, we are faced with thousands of images to analyze. This presents an opportunity to study global flow structures over time that may not be evident from surface measurements. A degree of automation is desirable to extract flow structures and features to give information on their behavior through the sequence. Using an interdisciplinary approach, the analysis of large schlieren data is recast as a computer vision problem. The double-cone schlieren sequence is used as a testbed for the methodology; it is unique in that it contains 5,000 images, complex phenomena, and is feature rich. Oblique structures such as shock waves and shear layers are common in schlieren images. A vision-based methodology is used to provide an estimate of oblique structure angles through the unsteady sequence. The methodology has been applied to a complex flowfield with multiple shocks. A converged detection success rate between 94% and 97% for these structures is obtained. The modified curvature scale space is used to define features at salient points on shock contours. A challenge in developing methods for feature extraction in schlieren images is the reconciliation of existing techniques with features of interest to an aerodynamicist. Domain-specific knowledge of physics must therefore be incorporated into the definition and detec- tion phases. Known location and physically possible structure representations form a knowledge base that provides a unique feature definition and extraction. Model tip location and the motion of a shock intersection across several thousand frames are identified, localized, and tracked. Images are parsed into physically meaningful labels using segmentation. Using this representation, it is shown that in the double-cone flowfield, the dominant unsteady motion is associated with large scale random events within the aft-cone bow shock. Small scale organized motion is associated with the shock-separated flow on the fore-cone surface. We show that computer vision is a natural and useful extension to the evaluation of schlieren data, and that segmentation has the potential to permit new large scale measurements of flow motion.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 EXPERIMENTAL INVESTIGATION OF WING-FUSELAGE INTEGRATION GEOMETRIES INCLUDING CFD ANALYSES(2008-04-24) Supamusdisukul, Jirapat; Barlow, Jewel; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A wind tunnel experiment exploring the influence of interface geometry on wing-body aerodynamics is described and the results are presented. The investigation focuses on the interference effects that occur for several wing-body geometries that are considered candidates for a design of an airplane intended to operate at low subsonic speeds at high altitude. The geometries of the test models were developed by Aurora Flight Sciences as in the process of evolving a preliminary design for a potential future unmanned aerial vehicle. With the support of the Glenn L. Martin Wind Tunnel at the University of Maryland, an experimental program has been carried out in which force data were obtained to identify the most promising wing-fuselage geometries for future detailed development. The research also included computational fluid dynamics simulations to explore flow characteristics around these wing-fuselage systems in greater detail than was possible in the experiments. The experimental data and simulation results are discussed in this thesis.Item Rotor Hover Performance and System Design of an Efficient Coaxial Rotary Wing Micro Air Vehicle(2007-03-02) Bohorquez, Felipe; Pines, Darryll J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Size restrictions force MAVs to operate in a low Reynolds number aerodynamic regime that results in poor aerodynamic performance of conventional airfoils and rotor configurations. A computerized hover test stand was used for the systematic testing of single and coaxial small-scale rotors. Thin circular arcs were chosen for blade manufacturing because of their good aerodynamic characteristics and simplified parameterization. Influence of airfoil geometry on single rotor hover performance was studied on untwisted rectangular blades. Non rectangular blades were used to study coupled airfoil and blade parameters. Performance gains were obtained by introducing large negative twist angles over short radial distances at the blade tips. A parametric study of the blade geometries resulted in maximum figures of merit of 0.65. Coaxial rotor performance at torque equilibrium was explored for different trims and operating conditions. It was found that the upper rotor was marginally affected by the lower one at spacings larger than 35% of the rotor radius, and that it produced about 60% of the total thrust. Experiments showed that power loading was maximized when higher collectives were used at the lower rotor, resulting in sizable differences in rotational speed between rotors. The CFD solver INS2d was used for a two-dimensional parametric aerodynamic study of circular arc airfoils. Lift, drag, and moment coefficients were explored over a range of Reynolds numbers. Lift predictions were satisfactory; however, drag was under-predicted at low angles of attack. The CFD database was integrated to a BEMT rotor model through a parameterization that coupled blade planform with twist distribution and airfoil shape. Thrust and maximum FM predictions were satisfactory for rectangular and non-rectangular blades with maximum cambers of 6% and below. The BEMT model was extended to the coaxial rotor case, producing good thrust and power predictions with errors within 5% of the experimental measurements. The approach validated the use of analytical and numerical tools commonly used in full-scale analysis, and proved to be a powerful system design tool. A fully functional coaxial MAV was developed based on the aerodynamic studies performed. It has been used as a testing platform for control system and algorithms.Item Low Reynolds Number Validation Using Computational Fluid Dynamics with Application to Micro Air Vehicles(2005-12-05) Schroeder, Eric Joseph; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The flow physics involved in low Reynolds number flow is investigated computationally to examine the fundamental flow properties involved with Micro Air Vehicles (MAV). Computational Fluid Dynamics (CFD) is used to validate 2-D, 3-D static and hover experimental data at Reynolds numbers around 60,000, with particular attention paid to the prediction of laminar separation bubble (LSB) on the upper surface of the airfoil. The TURNS and OVERFLOW flow solvers are used with a low Mach preconditioner to accelerate convergence. CFD results show good agreement with experimental data for lift, moment, and drag for 2-D and static 3-D validations. However, 3-D hover thrust and Figure of Merit results show less agreement and are slightly overpredicted for all measured collectives. Areas of improvement in the hover model include better vortex resolution and wake capturing to ensure that all the flow physics are accurately resolved.Item Characterization of Transient Pressure Loads in the Reservoir of a Hypersonic Blowdown Tunnel(2005-05-03) Smith, Kerrie Anne; Lewis, Mark J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)When flow through a hypersonic blowdown tunnel is initiated by the bursting of a diaphragm, expansion of the process gas into the downstream vacuum of the facility creates a strong rarefaction wave. This rarefaction propagates upstream, generating significant pressure drops in upstream components, such as a heater. These pressure drops can be attenuated with the use of a metering orifice, which requires an accurate prediction of the pressure drop for proper sizing. So as to be generally applicable and to provide physical insight, a closed-form or simple numerical solution for determining this pressure drop is preferred over computational fluid dynamics. Three methods are investigated: acoustic reflection, flow pattern assumption, and the Method of Characteristics. By examining the three methods in conjunction, tradeoffs between complexity and physical accuracy can be analyzed. Ultimately, this study shall lead to the design of an experiment to verify the accuracy of the three methods.