A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item FUNDAMENTAL UNDERSTANDING OF HELICOPTER AEROMECHANICS ON MARS THROUGH CHAMBER TESTING AND HIGH-FIDELITY ANALYSIS(2020) Escobar, Daniel; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The fundamental aeromechanics of rotary-wing flight on Mars is explored. The exploration is based on chamber testing of Mars-like low Reynolds number rotors and the development of comprehensive analysis and comprehensive analysis coupled with computational fluid dynamics for systematic investigation of aeromechanical phenomena--critical for weights and packaging for Mars. The investigation includes rotor airloads, structural loads, and control loads, comparison of hingeless and articulated hubs, hover and forward flight, and the impact of fuselage aerodynamics. The coaxial configuration is the baseline platform for this work. The use of a helicopter on Mars would dramatically increase the speed, range, and coverage of exploration by providing access to caves, craters, over polar ice, along icy scarps and recurring slope lineae that are just plain inaccessible or too dangerous for rovers. Many factors go into the design of a Mars helicopter from launch/entry loads to power to controls to packaging. Aeromechanics is only one factor, but the principal factor for efficient and effective flight that impacts everything else. This work is focused on this principal factor. Current knowledge extrapolated from Earth would allow for short hops into the Mars atmosphere. Deeper understanding of Martian aeromechanics is needed to design larger more capable aircraft. Accurate predictions are needed for performance, blade loads, control loads, and blade strike behavior. True high-fidelity is needed for unlike on Earth decades of data sets do not exist on Mars. In fact there is not even a single data set. Thus clever and innovative means of verification and validation must be found. The objective of this thesis is to carry out all of these tasks. The key conclusions are: (1) the design of aircraft, hub, blades, and controls are substantially different on Mars because of its unique aeromechanics, (2) an articulated hub can in fact have lesser danger of blade strike, (2) a hingeless hub can experience lower or only marginally higher (6-7%) flap bending moments, (3) control / pitch link loads are dramatically impacted more by choice of Mars airfoils than rotor hubs, (4) lifting-line analysis does not even begin to capture the precise magnitudes of blade passage impulsive loads, and (5) fuselage aerodynamics is irrelevant in preliminary design. These, and other interesting phenomena will be the topics of this dissertation.Item Identification of State-Space Rotor Wake Models with Application to Coaxial Rotorcraft Flight Dynamics and Control(2019) Hersey, Sean Patrick; Celi, Roberto; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modern aerodynamic analysis tools, such as free-vortex wake models and CFD-based techniques, include fewer theoretical limitations and approximations than classical simplified schemes, and represent the state-of-the-art in rotorcraft aerodynamic modeling, including for coaxial and other advanced configurations. However, they are impractical or impossible to apply to many flight dynamics problems because they are not formulated in ordinary differential equation (ODE) form, and they are often computationally intensive. Inflow models, for any configuration type, that couple the accuracy of high-fidelity aerodynamic models with the simplicity and ODE form of dynamic inflow-type theories would be an important contribution to the field of flight dynamics and control. This dissertation presents the methodology for the extraction of linearized ODE models from computed inflow data acquired from detailed aerodynamic free-vortex wake models, using frequency domain system identification. These methods are very general and applicable to any aerodynamic model, and are first demonstrated with a free wake model in hover and forward flight, for a single main rotor, and subsequently for the prediction of induced flow off the rotor as well, at locations such as the tail or fuselage. The methods are then applied to the extraction of first order linearized ODE inflow models for a coaxial rotor in hover. Subsequent analysis concluded that free-vortex wake models show that the behavior of the inflow of a coaxial configuration may be higher-order. Also, tip-path plane motion of a coaxial rotor causes wake distortion which has an impact on the inflow behavior. Therefore, the methodology is expanded to the identification of a second order inflow representation which is shown to better capture from all of the relevant dynamics from free-vortex wake models, including wake distortion. With ODE models of inflow defined for an advanced coaxial configuration, this dissertation then presents a comparison of the fully-coupled aircraft flight dynamics, and the design of an explicit modeling-following feedback controller, with both a free-vortex wake identified model and a momentum theory based approach, concluding that accurate inflow modeling of coaxial rotor inflow is essential for investigation into the flight dynamics and control design of advanced rotor configurations.Item FLOW AND ATOMIZATION CHARACTERISTICS OF CRYOGENIC FLUID FROM A COAXIAL ROCKET INJECTOR(2007-11-28) Gautam, Vivek; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High thrust-to-weight ratio and consistent performance over a range of operating conditions make cryogenic rocket engines one of the best options for space propulsion. However, future space explorations and missions to moon, mars and beyond require improvements in our present knowledge of the rocket engine combustion technology. In order to help improve the performance and reliability of current rocket engine combustors, several key issues need to be considered. Injector performance is one such issue related to the development of a new generation of rocket engine combustors. Previous research has suggested that coaxial injectors are most preferable for injection of cryogenic propellants inside the combustion chamber because of their simple design, low losses and high combustion stability. An experimental facility was designed and fabricated to simulate a single element shear coaxial injector. Gases of different densities were injected through the annulus between the two injector tubes over a large range of velocities, while liquid nitrogen flows through the inner tube. In this research, liquid nitrogen was used to simulate liquid oxygen because it is very similar to liquid oxygen, chemically inert, easy and safe to install in laboratory testing. High speed cinematography and Schlieren imaging have been used to examine the evolutionary flow behavior and global features of the liquid nitrogen jet, while PIV imaging was used to characterize the gaseous flow. This research has analyzed the transient behavior and unfolds the detailed evolutionary characteristics of both the cryogenic liquid and gaseous phase evolving from the shear coaxial injector for the first time. The effect of density ratio, velocity ratio and momentum ratio on the behavior of steady-state liquid nitrogen jet from a coaxial injector at atmospheric pressure has also been examined in detail. The impact of these parameters on primary instability of liquid core, the shear/spreading angle and its potential core length have been examined. Furthermore, the impact of some of the important non-dimensional numbers such as, Reynolds number, Weber number and Prandtl number, have been examined to develop scaling laws for the prediction of cryogenic potential core lengths. New correlations have been provided that describes the cryogenic jet behavior under simulated rocket injector operating conditions.