Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item Transient Dynamics of Helicopter Rotor Wakes Using a Time-Accurate Free-Vortex Method(2001) Bhagwat, Mahendra J.; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)A second-order accurate predictor-corrector type algorithm has been developed to obtain a time-accurate solution of the vortical wake generated by a helicopter rotor. The rotor blade flapping solution was fully integrated with the wake geometry solution using the same time-marching algorithm. The analysis was used to predict the locations of wake vortex filaments under transient flight conditions, where the rotor wake may not be periodic at the rotational frequency. Applications of this analysis include prediction of the rotor induced velocity field and blade airloads during transient flight and maneuvers. The stability of the rotor wake structure is important from the perspective of free-vortex wake models. The wake stability was examined using a linearized stability analysis, and the rotor wake was shown to be physically unstable. Therefore, the stability of the numerical algorithm is an important consideration in developing robust wake methodologies. Both the stability and accuracy of the numerical wake solutions algorithms was rigorously examined. The straight-line vortex segmentation used in the present analysis was shown to be second-order accurate. The overall numerical solution was also demonstrated to converge with a second-order accuracy. A technique for increasing the order of accuracy for high resolution solutions is also described. Along with a formal (mathematical) verification of solution accuracy, the numerical solution for the rotor wake problem was compared with experimental results for both steady-state and transient operating conditions. The steady-state wake model was shown to give good predictions of rotor wake geometry, induced inflow distribution as well as performance trends. Under transient conditions, such as those following a pitch input during a maneuver, the time-accurate wake model was shown to correctly model the dynamic response of rotor wake. In axial descent passing through the vortex ring state, the present analysis was shown to properly model the associated power losses as shown by experimental results. The present analysis was also shown to give improved predictions of wake distortions during simulated maneuvering flight with various imposed angular rates of the rotor.Item RBCC Engine-Airframe Integration on an Osculating Cone Waverider Vehicle(2001) O'Brien, Timothy F.; Lewis, Mark J.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)An analytical vehicle study is performed that integrates a rocket-based combined cycle engine with an osculating cones waverider-based fuselage. The integration of the two concepts brings about an interesting design challenge: predicting the aerodynamic performance of a high-speed fuselage design across the full range of Mach numbers from take-off to orbit that a rocket-based combined-cycle engine will operate. The aerodynamic performance of this class of vehicles is analyzed for on- and off-design Mach numbers and angles of attack. Analytical aerodynamic models are developed for the off-design behavior of both the fuselage of the vehicle and the engine. These models arc combined to predict the powered performance of this class of vehicle along a trajectory. The models developed arc rapid enough that they may be applied to initial design studies, optimization algorithms, or trajectory analyses. The aerodynamic model for the fuselage is based on the tangent-wedge, tangent-cone, and shock-expansion theories for hypersonic flow, and the linearized, small perturbation, velocity potential equations for supersonic and transonic flow. Each model is validated with numerical solutions for an example Mach 12 vehicle design. The results show an accurate prediction of the trends in lift and drag of the vehicle fuselage across a range of Mach numbers between 0.4 and 15. The aerodynamic engine model is based on Prandtl-Meyer flow and the oblique shock relations for the internal compression system, and quasi-one dimensional flow (including finite-rate chemistry) for the combustor flowfield. The strut-based compression model is validated with numerical solutions for a range of Mach numbers between 2.5 and 6. The combustor flowfield model is validated by comparison to two experimental hydrogen-fueled scramjet engines. The results showed that this class of geometry generates very little lift at low speeds (below Mach 3) and will require lift augmentation. The transonic drag rise is modeled analytically and numerically, with maximum inviscid drag coefficient occurring at Mach 1.2. Engine integration has a large effect on off-design behavior, including maximum lift-to-drag ratio and zero lift angle of attack.Item FLOW INDUCED CAVITY RESONANCE FOR TURBULENT COMPRESSIBLE MIXING ENHANCEMENT IN SCRAMJETS(2002) Nenmeni, Vijay Anand Raghavendran; Yu, Kenneth H.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In a Scramjet combustor, flow residence time is very short and fuel-air mixing can be adversely affected by compressibility effect. Thus, it is important to study mixing enhancement techniques for reducing the characteristic mixing time. It is also important to examine the feasibility of using them in practical settings. One of the promising mixing enhancement techniques is based on flow-induced cavity resonance, which generates large-scale coherent structures in the shear layer for faster mixing. Of particular interest is whether this technique, which is passive in nature, can be used over a wide range of flow conditions, expected in Scramjet operation. In this thesis, physical mechanisms governing the use of flow-induced cavity resonance were examined experimentally using Schlieren visualization of the flowfield and spectral analysis of resulting pressure oscillations. Various cavities with the length between 0.125 and 1.25 inch and the depth between 0.125 and 0.25 inch were placed inside a Mach 2 flow tunnel, which simulated the Scramjet internal flowfield. The properties of supersonic flow were further modified in the inlet, upstream of the cavity section, by changing the upstream stagnation pressure between 35 psig and 120 psig, which resulted in inlet shock trains of different strength. The objective was to characterize and compare the enhancement mechanism under various off-design conditions. In all, nine different cavity cases were tested under six different stagnation pressure settings. For each case, spark Schlieren images were taken and pressure oscillations inside the cavity were measured. The Schlieren images provided qualitative understanding of the physics while the pressure measurements were used to quantify the amplitude and frequency of dominant oscillations. Also from the images, inlet Mach number was deduced by measuring the Mach wave angles. The data were summarized to shed more light on reliability of the mixing enhancement mechanism under off-design inlet conditions. The results indicated that flow-induced cavity resonance mechanism was robust over a wide range of flow conditions. Also, mode-switching behavior of the cavities was observed, which could modify the mixing enhancement rate. Further, helium injection studies were conducted to gain qualitative assessment of the effect of cavity resonance on mixing.Item Application of Compound Compressible Flow to Hypersonic Three-Dimensional Inlets(2009) Bussey, Gillian Mary Harding; Lewis, Mark J.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A method for correcting flow non-uniformities and incorporating multiple oblique shocks waves into compound compressible flow is presented. This method has several applications and is specifically presented for the problem of creating a streamline-traced hypersonic three-dimensional inlet. This method uses compound compressible flow theory to solve for the freestream flow entering a pre-defined duct with a desired downstream profile. This method allows for multiple iterations of the design space and is computational inexpensive. A method is also presented for modeling a laminar or turbulent boundary layer to compare inlet designs and to determine the viscous correction to the inlet. Two different Mach 6 designs were evaluated, with a rectangular capture area and circular combustor with a uniform temperature, pressure, and Mach number profile. Comparison with other three-dimensional inlets indicates those designed with this method demonstrate good inviscid performance. These inlets also have the ability to correct incoming flow non-uniformities.Item Pressure-Constrained, Reduced-DOF, Interconnected Parallel Manipulators with Applications to Space Suit Design(2009) Jacobs, Shane Earl; Akin, David L; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the concept of a Morphing Upper Torso, an innovative pressure suit design that incorporates robotic elements to enable a resizable, highly mobile and easy to don/doff spacesuit. The torso is modeled as a system of interconnected, pressure-constrained, reduced-DOF, wire-actuated parallel manipulators, that enable the dimensions of the suit to be reconfigured to match the wearer. The kinematics, dynamics and control of wire-actuated manipulators are derived and simulated, along with the Jacobian transforms, which relate the total twist vector of the system to the vector of actuator velocities. Tools are developed that allow calculation of the workspace for both single and interconnected reduced-DOF robots of this type, using knowledge of the link lengths. The forward kinematics and statics equations are combined and solved to produce the pose of the platforms along with the link tensions. These tools allow analysis of the full Morphing Upper Torso design, in which the back hatch of a rear-entry torso is interconnected with the waist ring, helmet ring and two scye bearings. Half-scale and full-scale experimental models are used along with analytical models to examine the feasibility of this novel space suit concept. The analytical and experimental results demonstrate that the torso could be expanded to facilitate donning and doffing, and then contracted to match different wearer's body dimensions. Using the system of interconnected parallel manipulators, suit components can be accurately repositioned to different desired configurations. The demonstrated feasibility of the Morphing Upper Torso concept makes it an exciting candidate for inclusion in a future planetary suit architecture.Item Comparison of Optic Flow in the Visible Light and Infrared Specturms(2008) Chinn, Michael William; Humbert, James S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Insects use a method of Wide Field Integration (WFI) to navigate efficiently through unknown environments. Using these natural paradigms, various WFI based forms of navigation can be implemented based on electro-mechanical vision devices on robotic vehicles. However, under low light and/or suspended particles in the environment, these methods become less useful. One solution to this problem is to use infrared vision sensors rather than visible light sensors. This would allow insect-like navigation for autonomous vehicles under a variety of lighting conditions, including a total lack of visible light. The results show that, using infrared sensors, it is possible to navigate under a variety of lighting conditions, even where visible light sensors become ineffective.Item A Study of Selected Aspects of Electromagnetic Formation Flight(2008) Gardner, Peter Nathaniel; Sedwick, Raymond; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electromagnetic Formation Flight (EMFF) is a technique for electromagnetically controlling the relative position and velocity of satellites in close proximity, without using propellant.\nAn optimal design for an EMFF system for clusters of small satellites was calculated. Trends in parameters were identiï¬ ed, taking into account thermal issues.\nA power transfer system, using strongly coupled magnetic resonance, was simulated, using the same coils as the EMFF system. The eï¬ ciencies were calculated for the same parameters.\nA scheme for EMFF control was tested, in which two satellites at a time were active, with their dipoles aligned with each other on-axis. This system was shown to keep clusters of four satellites within speciï¬ ed boundaries.Item Three-Dimensional Motion Coordination in a Time-Varying Flowfield(2009) Hernandez-Doran, Sonia; Paley, Derek A; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Decentralized algorithms that stabilize three-dimensional formations of unmanned vehicles in a time-varying flowfield have applications in environmental monitoring in the atmosphere and ocean. This thesis provides Lyapunov-based algorithms to control a system of self-propelled particles traveling in three dimensions at a constant speed relative to a spatiotemporal flowfield. A particle's inertial velocity is the sum of its velocity relative to the flowfield plus the velocity of the flowfield. Multiple particles can be steered to form parallel, helical, and circular formations. A special case of the three-dimensional model is also studied, in which the particles travel on the surface of a sphere. In this case, we provide Lyapunov-based algorithms that stabilize circular formations in a time-varying flowfield on a rotating sphere. Because we are interested in using unmanned-vehicle formations for environmental monitoring, we simulate our results using numerical simulations of time-varying flowfields that resemble tornadoes and hurricanes.Item UNDERSTANDING THE ROLE OF HEAT RECIRCULATION IN ENHANCING THE SPEED OF PREMIXED LAMINAR FLAMES IN A PARALLEL PLATE MICRO-COMBUSTOR(2009) Veeraragavan, Ananthanarayanan; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the role of heat recirculation in enhancing the flame speeds of laminar flames stabilized in a parallel plate reactor by: 1) developing analytical models that account for conjugate heat transfer with the wall and 2) making measurements of temperature profiles in a simulated microcombustor using non-intrusive FTIR spectroscopy from which heat recirculation is inferred. The analytical models have varying degrees of complexity. A simple heat transfer model simulates the flame by incorporating a concentrated heat release function along with constant temperature wall model. The next level model accommodates conjugate heat transfer with the wall along with a built in heat loss model to the environment. The heat transfer models identify the thermal design parameters influencing the temperature profiles and the Nusselt number. The conjugate heat transfer model is coupled with a species transport equation to develop a 2-D model that predicts the flame speed as an eigenvalue of the problem. The flame speed model shows that there are three design parameters (wall thermal conductivity ratio ( &kappa ), wall thickness ratio ( &tau ) and external heat loss parameter ( NuE )) that influence the flame speed. Finally, it is shown that all these three parameters really control the total heat recirculation which is a single valued function of the flame speed and independent of the velocity profile (Plug or Poiseuille flow). On the experimental side, a previously developed non-intrusive diagnostic technique based on FTIR spectroscopy of CO2 absorbance is improved by identifying the various limitations (interferences from other species, temperature profile fitting, ... etc) and suggesting improvements to each limitation to make measurements in a silicon walled, simulated microcombustor. Methane/Air and Propane/Air flames were studied for different equivalence ratios and burning velocities. From the temperature profiles it can be seen that increasing the flame speed pushes the flames further up the channel and increases the combustors inner gas and outer wall temperatures (measured using IR thermography). The temperature profiles measured are used to make a 2-D heat recirculation map for the burner as a function of the equivalence ratio and burning velocity. The experimental results are compared to the analytical models predictions which show a linear trend between flame speed and heat recirculation.Item Mechanisms of Sediment Entrainment and Transport in Rotorcraft Brownout(2009) Johnson, Bradley Stephen Curtis; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To advance the understanding of the phenomenon of rotorcraft brownout, a dual-phase flow environment induced by a small-scale rotor hovering above a sediment bed was studied using high-speed flow visualization and particle image velocimetry (PIV). The high frame rate of the camera, combined with advanced particle recognition and tracking software, permitted an understanding of the temporal evolution of the rotor wake in ground effect simultaneously with the processes of sediment entrainment and transport by the rotor flow. High-resolution near-wall PIV measurements showed that large excursions in the surface boundary layer were produced by the convecting rotor wake vortices. These excursions acted to suppress an equilibrium state in the boundary layer within the zone of vortex impingement on the ground. The highest sediment entrainment levels were observed to occur within this impingement zone, which can be attributed to the increase in groundwash and wall shear produced beneath the vortices. Once entrained, significant quantities of sediment were then trapped and locally suspended by the vortex-induced upwash field. This effect resulted in a noticeable level of intermittency in the initial vertical transport of sediment from the ground. The ground and upwash flow velocities were shown to strengthen significantly during the viscous merging of adjacent wake vortices. This mechanism proved fundamental in defining the concentration of suspended sediment, as well as the maximum height to which sediment could be transported. Sediment particles reaching sufficient heights were observed to recirculate into the rotor wake, and convect back towards the ground at a high speed. This process caused sediment ejection by means of bombardment or "splash." The classical process of saltation bombardment was also visualized for larger particles whose inertia prevented them from being suspended in the vortical flow. While providing new insight into the time- and length-scales associated with sediment transport by a rotor wake, the observations made here also bring into question the validity of equilibrium particle flux models currently being used for brownout simulations.