Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item Experimental Evaluation of Circulation Control Aerodynamics on a Cylindrical Body(1987) Ngo, Hieu Thien; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)In this study, an experimental investigation is conducted on a two-dimensional circulation control cylinder with blowing taking place from a single spanwise slot to determine its aerodynamic characteristics. The results include detailed pressure distributions (both chordwise and spanwise) for a range of momentum coefficients and slot locations. The measured results showed that the lift coefficients up to 4.8 were produced at momentum coefficients of 0.14 in a turbulent flow condition. The experimental results of lift coeffficients Were correlated satisfactorily with analytical results. The surface flow patterns were observed using the oil and smoke techniques. Also flow field surveys of the model Were obtained using total pressure, yaw and pitch probes. A color video display technique was used to present the results of the flow field surveys. Based on this evidence, a flow field model of the circulation control cylinder is presented.Item An Expert System for Helicopter Conceptual Design(1987) Babuska, Vit; Fabunmi, James A.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The objective of this thesis is to demonstrate the applicability of expert systems in helicopter conceptual design by developing an expert assistant which aids the engineer in defining a feasible design configuration. The expert assistant combines some experiential knowledge of the design engineer with a typical conceptual design algorithm to guide the engineer to a reasonable baseline design. The expert assistant was developed on a personal computer using the expert system shell INSIGHT2+®. The design algorithm employed is SSPl, a helicopter weight and sizing program developed at the US Army Applied Technologies Laboratory. A set of heuristic rules was developed which attempts to simulate the thinking of an expert design engineer using SSP1 for helicopter conceptual design. The result, a Prototype expert assistant which aids an engineer in the conceptual design phase, demonstrates the feasibility of expert systems in helicopter design.Item Navier Stokes Solutions for Chemical Laser Flows: Steady and Unsteady Flows(1979) Kothari, Ajay Prasannajit; Anderson, John D. Jr; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)This work consists of an overall effort to apply a detailed and accurate computational fluid dynamic technique to the solution of practical high energy laser flows. In particular, a third generation of super sonic diffusion chemical laser analysis is introduced, namely, the complete solution of the Navier-Stokes equations for the laminar, super sonic mixing flow fields fully coupled with chemical kinetics for both the hot and cold reactions for HF. Multicomponent diffusion is treated in a detailed fashion. Solutions are obtained, firstly, for "cold flows", where the effects of chemical reactions and vibrational relaxation are not included. Although such a situation is purely artificial, the results do isolate some of the fluid dynamic aspects of chemical laser flows, and provide a set of data to be compared later with hot flow calculations. A set of numerical experiments using four different time dependent finite difference schemes show that relatively minor changes in the differencing procedure can lead to major variations in the results. A modification of the well-known Maccormack approach appears to be the best suited for mixing flows associated with chemical lasers. A comparison is next made between cold flows (with fully coupled chemical kinetics). the results show that temperature distributions are affected the most and velocity distributions the least by chemical energy heat release. The results have an impact on the interpretation of cold flow aerodynamic experiments in the laboratory, and their proper extrapolation to the real chemical laser flows. also, comparisons between the present Navier Stokes results and other, more approximate, existing calculations are made. Gradients are calculated as a natural part of the Navier Stokes solutions. Results are given for steady flows with large pressure gradients where advantages of the Navier Stokes solutions are delineated. In addition, the effect of unsteady fluctuations intentionally introduced at the cavity inlet are studied. Specifically, sinusoidal fluctuations in one stream and then both streams (primary and secondary) in various quantities e.g. pressure, density, u velocity and v velocity were simulated. Of these, the oscillations in v velocity with approximate frequency and amplitude produced a remarkable improvement in mixing. Such unsteady fluctuations also yielded peak laser gain which were larger by almost a factor of two compared to the steady case. the flow at which the upstream boundary has so far, in the above mentioned cases been assumed to be uniform with real effects like Boundary Layer and Base Flow having been neglected. For comparison purposes these effects are next included. the boundary layer profile and velocity at the inlet is shown to feed production of gain substantially. Base flow calculations were attempted but were not successfulItem The Influence of Variable Flow Velocity on Unsteady Airfoil Behavior(1991) van der Wall, Berend G.; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The importance of unsteady aerodynamics for prediction of rotor dynamics is unquestioned today. The purpose of unsteady aerodynamic models is to represent the effect of unsteady airfoil motion on the lift, moment and drag characteristics of a blade section. This includes unsteady motion (arbitrary motion) of the airfoil in angle of attack (pitch) and vertical movement (plunge), as well as the effects of an airfoil traveling through a vertical gust field. However, the additional degrees of freedom, namely the fore-aft motion and the unsteady freestream variations commonly are acknowledged, but neglected in virtually all analyses. Since the effect of unsteady freestream results in a stretching and compressing of the shed wake vorticity distribution behind an airfoil, it will have an effect on the airfoil characteristics. The subject of this thesis is to provide a review of the analytic and experimental work done in the area of unsteady freestream and unsteady fore-aft motion, to clarify the limits of the various theories, and to show the differences between them. This will be limited to the attached flow regime since all theories are based on the small disturbance assumption in incompressible flow. As far as possible the theories are compared with experimental data, however most of the available experimental data are confined to stalled flow conditions and are not useful here. In addition to the theories, a semiempirical mathematical model will be used based on the aerodynamics of indicial functions. The purpose is to show the differences of using the theories of unsteady airfoil motion in a constant flow, and those accounting for unsteady freestream flow. This will help to justify whether it is necessary to include the unsteady freestream effect in comprehensive rotor codes. Finally, a generalisation of Isaacs unsteady aerodynamic theory for an airfoil undergoing a frequency spectra in pitch and plunge in a freestream oscillating with the fundamental frequency is presented here for the first time. Therein the axis of rotation of the airfoil is a free parameter.Item An Experimental Investigation of the Effects of Leading Edge Modification on the Post-Stall Characteristics of an NACA 0015 Wing(1979) Saini, Jugal Kishore; Jones, Everett; Winkelmann, Allen E.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)The effects of leading edge modifications on the stalling characteristics of an NACA 0015 panel wing model were investigated in a series of low speed wind tunnel tests. The modification typically consisted of adding a 14% Clark Y glove onto a portion of the leading edge. Six-component balance data, pressure distribution measurements and oil flow visualization tests were completed at a Reynolds number based on chord of 2.0 x 10^6 for increasing and decreasing angles of attack from 0° to 50°. The leading edge modifications produce stabilizing vortices at stall and beyond. These vortices have the effect of fixing the stall pattern of the wing such that various portions of the wing upper surface stall nearly symmetrically. This results in a higher lift on the modified wing as compared to the lift on the unmodified wing in the post-stall region. The lift curve slope of the modified and unmodified wings remained essentially constant at 0.071 per degree. Two lift-coefficient peaks were obtained for the baseline NACA 0015 wing at angles of attack of 17° and 30°. The twin-peak behavior of the lift curve was also observed on the modified wings. The drag coefficient obtained with several modified configurations was smaller than the drag coefficient of the baseline NACA 0015 wing in the pre-stall region. Also a smaller center of pressure shift with angle of attack was observed with several modified configurations. Considering a smoother variation of lift, pitching moment, rolling moment at stall and a smaller drag and center of pressure movement to be desired criteria, the best configuration tested consisted of placing the glove on the entire leading edge except for a gap at 25% to 50% of the semispan.Item NUMERICAL SIMULATION OF THE BLUE WHIRL: A REACTING VORTEX BREAKDOWN PHENOMENON(2019) Chung, Joseph Dong il; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The blue whirl is a small, stable, spinning blue flame that evolved spontaneously in recent laboratory experiments while studying turbulent, sooty fire whirls. It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a new potential way for low-emission combustion. This thesis uses numerical simulations to present, for the first time, the flame and flow structure of the blue whirl. These simulations show that the blue whirl is composed of three different flames - a diffusion flame and a premixed rich and lean flame - all of which meet in a fourth structure, a triple flame which appears as a whirling blue ring. The results also show that the flow structure emerges as the result of vortex breakdown, a fluid instability which occurs in swirling flows. This thesis also presents the development and testing of the numerical algorithms used in the simulation of the blue whirl. This work is a critical step forward in understanding how to use this new form of clean combustion.Item The Effects of Model Scaling on Sediment Transport in Brownout(2013) Glucksman-Glaser, Mark Samuel; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The phenomenon of “brownout” is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.Item A Magnetorheological Energy Absorber for Enhanced Crashworthiness in Drop-Induced Impacts(2019) Pierce, Rebecca; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis uses a multidisciplinary approach to investigate the enhanced crashworthiness of a magnetorheological energy absorber (MREA). Magnetorheological (MR) fluids have been considered for use in crashworthiness applications because they can be modified to adjust for parameters such as an occupant’s weight or the impact velocity of a crash. This study first reviews an existing soft landing control algorithm for an MREA vertically stroking crew seat and applies it to several crash scenarios. The combined addition of a bumper and optimized yield force is found to successfully reduce the jerk at the end of the MREA stroke without introducing new discontinuities in the acceleration profile. Secondly, this study explores the use of mesocarbon microbeads (MCMBs) in MR fluids. The MCMBs are found to increase the yield force produced in an MR damper. An endurance study further reveals the durability of the yield force-enhancing effect up to 100,000 cycles.Item Guided Lamb Wave Structural Health Monitoring Techniques for Aircraft Applications(2019) McCullum, Jacob Ryan; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Growing aerospace industry interest in structural health monitoring (SHM) has led to the development of many damage detection and localization techniques which make use of guided Lamb Waves (GLW). To continue this growth, further development of these techniques is necessary with an industry-focused mindset through studies with realistic, complex aircraft structures. The present study applies GLW techniques to two aircraft structures and examines the feasibility of their use for practical SHM applications. Particularly this work focuses on evaluating the effects of complex structural features found in aircraft, examining the human interaction with GLW techniques, and enhancing GLW techniques using nontraditional dual PZT transducers. Several damage case studies are performed showing that damage can be detected and located, and limitations to the techniques are characterized. Moreover, the use of dual PZT transducers shows improvements to damage localization techniques which potentially enable greater flexibility for aircraft applications.Item An Experimental Investigation of Hypersonic Boundary-Layer Transition on Sharp and Blunt Slender Cones(2019) Kennedy, Richard Edward; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the instabilities leading to the laminar-to-turbulent transition of a hypersonic boundary layer is a key challenge remaining for the design of efficient hypersonic vehicles. In the present study, experiments are performed in three different facilities at freestream Mach numbers between 6 and 14 to characterize instability mechanisms leading to transition on a 7-degree half-angle slender cone. Second-mode instability waves are visualized using a high-speed schlieren setup with the camera frame rate and spatial resolution optimized to allow individual disturbances to be tracked. In order to facilitate quantitative time-resolved measurements, a method of calibrating the schlieren system and novel image-processing algorithms have been developed. Good agreement is observed between the schlieren measurements, surface pressure measurements, and parabolized stability equation computations of the second-mode most-amplified frequencies and N factors. The high-frequency-resolution schlieren signals enable a bispectral analysis that reveals phase locking of higher harmonic content leading to nonlinear wave development. Individual disturbances are characterized using the schlieren wall-normal information not available from surface measurements. Experiments are also performed to investigate the effect of nose-tip bluntness. For moderate to large bluntness nose tips, second-mode instability waves are no longer visible, and elongated structures associated with nonmodal growth appear in the visualizations. The nonmodal features exhibit strong content between the boundary-layer and entropy-layer edges and are steeply inclined downstream. Simultaneously acquired surface pressure measurements reveal high-frequency pressure oscillations typical of second-mode instability waves associated with the trailing edge of the nonmodal features.