The system under study is that of a two-dimensional, two-degree-of-freedom airfoil (NACA 0012) in a steady subsonic airstream with external forcing. This airfoil is flexibly mounted in both degrees-of-freedom, and thus, describes an aeroelastic system. Non-linearities arising from the aerodynamics are responsible for the phenomenon of dynamic stall when the airfoil oscillates past the static-stall angle of attack. These non-linearities also cause the system to produce non-linear classes of motion, the most important of which is chaotic motion. Aeroelastic instabilities are also present in the system. This thesis explores the instabilities present in this system as well as its non-linear behaviour. / A semi-empirical numerical model revolving around the concept of an indicial response is used to model the non-linear aerodynamics in both degrees-of-freedom. The structural components of the system are modeled using simple linear elements, such as translational and torsional springs. Structural damping is ignored. Simple force and moment balancing equations allow for the derivation of the pertinent aeroelastic equations, which are then solved using numerical techniques. / Self-excited oscillations, examples of aeroelastic instability, were found in the one-degree-of-freedom system for oscillations about the static-stall angle. Binary flutter, another form of aeroelastic instability, was found in the two-degree-of-freedom system. Every class of non-linear motion (equilibrium, periodic, quasi-periodic and chaotic) was discovered in the non-linear analysis, and several routes to chaos were discovered. These routes included the quasi-periodic route, period-doubling route and intermittency route. Some of the routes discovered compared well with classical examples.

Space manipulators mounted on a free-floating base are structurally flexible mechanical systems. For some applications, it is necessary to control the attitude of the base by the use of on-off thrusters. However, thruster operation produces a rather broad frequency spectrum that can excite sensitive modes of the flexible system. This situation is likely to occur especially when the manipulator is moving a big payload. The excitation of these modes can introduce further disturbances to the attitude control system; therefore, undesirable fuel replenishing limit cycles may develop. To investigate these dynamic interactions, the dynamics model of an N-flexible-joint space manipulator is developed and used to describe a three-flexible-joint manipulator mounted on a six-degree-of-freedom spacecraft dubbed in this thesis "spatial system". Since the attitude controller assumes the use of on-off thrusters, which are nonlinear devices, the describing-function technique, an approximate method for the analysis of nonlinear systems, is used to analyze four different control systems using an approximate two-mass system. This technique is adapted to be used in conjunction with the root-locus concept, thus providing a different picture of the problem and helping in the control system design. A parametric study is performed to compare the control schemes using those analytical tools, which is validated with simulations using the spatial system. It is shown that the three proposed variations of a classical control scheme are very effective to minimize such undesirable dynamic interactions. Finally, Mecano, a commercial mechanical-system modelling and analysis software tool, is adapted for the study of control systems and used to study the effect of link elasticity on the robustness of the proposed control schemes.

The separation bubble at the leading edge of a thin sharp-edged aerofoil in steady, incompressible and two-dimensional flow was studied. A simple method, using irrotational flow and source singularities, has been developed for predicting bubble reattachment length, drag and lift. / For a flat-plate aerofoil predictions compare favourably with new experiments. The non-dimensional reattachment length $x sb R over rm c$ is proportional to the square of the incidence $( alpha)$ and the slope $x sb R over rm c alpha sp2$ depends on the growth of the outer part of the separated shear layer. The value of the term $x sb R over rm c alpha sp2$ was determined experimentally as $ pi over 0.08$. At incidences above 2$ sp circ$, the bubble drag becomes increasingly dominant when compared with the skin friction drag. Although the details of the bubble geometry are not simulated, the lift and stall are predicted fairly well. / The theory is extended to a circular-arc aerofoil. This part of the study is much less satisfactory. New experimental measurements do not appear to be sufficiently accurate to provide the empiricism to support the extended theory which must now account for regions of separated flow near the trailing edge.

This paper examines the dynamics of a tether connected multi-spacecraft system, arranged in a wheel-spoke configuration, in the vicinity of the L 2 Lagrangian point of the Sun-Earth system. First, the equations of motion of a N-body system are obtained and equilibrium configurations of the system are determined and small motions about one of these configurations are analyzed. Then, a numerical analysis of the free tether libration is carried out for a three-mass case when the system is near L2 and the parent mass is assumed to be in a halo orbit of different sizes. Finally, a set of control goals are defined and a time domain state feedback control system is integrated into the numerical model. The performance of the control system is tested under different conditions.

The thermal oxidation of fine aluminum (Al) powders in carbon dioxide (CO2) gas was investigated for the purpose of metal-based propulsion fuel development. The thermal behavior and reaction energy was studied using simultaneous thermogravimetric (TG) analysis and differential scanning calorimetry (DSC). The reactivities of Al powders with nanometer and micrometer-scale average particle sizes were compared. The particle morphology was examined at different stages of the process using field emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM). The corresponding chemical changes were analyzed by X-ray diffraction spectrometry (XRD) and energy dispersion X-ray spectrometry (EDS). / Nano- and micrometer-scale Al powders exhibited different calorimetric behaviour. The primary oxidation occurred at around 533°C for the nanopowders and at 1045°C for the micropowders. The mechanism of the oxide growth, particle deformations, and hollow oxide shells are discussed. A low-temperature transformation of the amorphous Al2O3 coating to crystalline gamma-Al 2O3 is identified as a trigger to the exothermic reaction in the case of the Al nanopowders. Carbon was also shown to be involved in the reaction.

Space manipulators mounted on a free-floating base are structurally flexible mechanical systems. For some applications, it is necessary to control the attitude of the base by the use of on-off thrusters. However, thruster operation produces a rather broad frequency spectrum that can excite sensitive modes of the flexible system. This situation is likely to occur especially when the manipulator is moving a big payload. The excitation of these modes can introduce further disturbances to the attitude control system, and therefore, undesirable fuel replenishing limit cycles may develop. To investigate these dynamic interactions, an approximate two-mass system, where the manipulator is replaced with an equivalent spring-and-dashpot system, is used to reproduce the relative motion of the payload with respect to the spacecraft. A dynamic model of two-flexible-joint planar manipulator was derived to obtain its natural frequencies and then, to determine the corresponding spring stiffness and damping coefficient of the approximate system. SInce the attitude controller assumes the use of on-off thrusters, which are nonlinear devices, the describing function technique, an approximate method for the analysis of nonlinear systems, is used to perform a parametric study investigating the significant parameters of three models studied. This study provides some guidelines for the design of attitude control systems when flexibility is a major concern. As well, this study shows that one of the three models studied is a very good alternative to the actual attitude controllers. Finally, simulations are executed to confirm these results and to study the addition of noise and model uncertainties in the three selected models.

In the aeronautical applications, many problems involving boundaries of unspecified geometry are of interest, such as the indirect problems of determining the shape of an airfoil to generate a specified pressure distribution, supersonic nozzle design based on reflection-suppression condition, flexible-membrane airfoils or wings, jet-flapped airfoils, and others. In these cases, the utilization of the Eulerian formulations of the Euler equations of motion, may lead to long iterative computations for successive shapes of the boundaries, until the final geometry is reached. For this type of problems, the Lagrangian formulations using the streamline coordinates are more suitable, since the geometrically-unspecified boundaries are always represented by streamlines. / Computational methods based on Lagrangian formulations have been recently developed for supersonic flows for the solution of the system of Euler equations. These Lagrangian formulations use the stream-function and the Lagrangian-time or -distance coordinates. In our present study, the numerical method applied to solve aerodynamic problems of unspecified geometry, is based on a finite volume discretization in the Lagrangian coordinates (stream-function and Lagrangian-distance), in which the flux values are determined by using the Riemann problem solution. Improvements leading to a better resolution of the shock waves are included. / The method has been first validated for nozzles and airfoils of specified geometry, by comparison with analytical results and previous solutions obtained using Eulerian formulations. Then, the method has been applied for the solution of the above mentioned problems of unspecified geometry. In the cases of the flexible-membrane airfoils and the jet-flapped airfoils, an analytical solution has been developed in addition to the numerical solution.

The formulation and implementation of a far field boundary condition (FFBC) model for compressible flows is reported in this thesis. This FFBC model, developed for quasi-one-dimensional and two-dimensional flows, aims to permit a substantial reduction of the computational domain, leading to a considerable improvement in the computational efficiency. The present FFBC approach uses asymptotic expansions of the Riemann variables, which are truncated up to the required degree of accuracy. Then, the far field perturbation equations are integrated in time and applied in conjunction with the solution calculated within the computational domain. The propagating information from the computational domain is determined along the outgoing characteristic fronts, based on the estimation of the wave front orientation. / The proposed FFBC model is implemented in conjunction with an implicit finite-difference flow field solver using an alternating direction implicit (ADI) scheme for solving the Euler equations. The discretized form of the governing equations are solved using a time-marching technique until the steady-state solution is reached. An accurate procedure for the solid boundary treatment was also used. / The proposed FFBC model was used for solving typical problems of confined and external compressible flows in subsonic and transonic regimes. For the transonic regime, the proposed FFBC model has been extended for the case of non-isentrophic outgoing flows, which appear behind the shock waves. The solutions obtained are compared with previous theoretical and numerical results. This comparison shows that the proposed FFBC model can generate accurate solutions using a substantially reduced computational domain, which reduces by an order of magnitude the size of the block tridiagonal matrices to be inverted. This leads to a corresponding improvement in the overall computational efficiency.

<p> Ground teleoperation of a satellite servicing spacecraft is a challenging task for a human operator, especially when there is significant communications delay between the control station and spacecraft. On-orbit operations are further complicated by a communications time delay between the ground and spacecraft. Operator performance can be improved with the use of a graphical simulation of the robot. By displaying the robot's commanded position, graphical simulation can also mitigate some effects of time delay. This work implemented a visualization tool and commanded display to assist operation of a remote dexterous manipulator. A Fitts' Law experiment was designed to determine the effectiveness of the commanded display in reducing the impact of time delay. The experiment was conducted with a six degree of freedom manipulator over a range of time delays, from 0.0 to 6.0 seconds. The experimental results were analyzed to assess the reduction of task completion time and operator workload.</p>

<p> West Virginia University (WVU) has created the Small Microgravity Research Facility (SMiRF) drop tower through a WVU Research Corporation Program to Stimulate Competitive Research (PSCoR) grant on its campus to increase direct access to inexpensive and repeatable reduced gravity research. In short, a drop tower is a tall structure from which experimental payloads are dropped, in a controlled environment, and experience reduced gravity or microgravity (i.e. "weightlessness") during free fall. Currently, there are several methods for conducting scientific research in microgravity including drop towers, parabolic flights, sounding rockets, suborbital flights, NanoSats, CubeSats, full-sized satellites, manned orbital flight, and the International Space Station (ISS). However, none of the aforementioned techniques is more inexpensive or has the capability of frequent experimentation repeatability as drop tower research. These advantages are conducive to a wide variety of experiments that can be inexpensively validated, and potentially accredited, through repeated, reliable research that permits frequent experiment modification and re-testing. </p><p> Development of the WVU SMiRF, or any drop tower, must take a systems engineering approach that may include the detailed design of several main components, namely: the payload release system, the payload deceleration system, the payload lifting and transfer system, the drop tower structure, and the instrumentation and controls system, as well as a standardized drop tower payload frame for use by those researchers who cannot afford to spend money on a data acquisition system or frame. In addition to detailed technical development, a budgetary model by which development took place is also presented throughout, summarized, and detailed in an appendix. After design and construction of the WVU SMiRF was complete, initial calibration provided performance characteristics at various payload weights, and full-scale checkout via experimentation provided repeatability characteristics of the facility. Based on checkout instrumentation, Initial repeatability results indicated a drop time of 1.26 seconds at an average of 0.06g, with a standard deviation of 0.085g over the period of the drop, and a peak impact load of 28.72g, with a standard deviation of 10.73g, for a payload weight of 113.8 lbs. </p><p> In order to thoroughly check out the facility, a full-scale, fully operational experiment was developed to create an experience that provides a comprehensive perspective of the end-user experience to the developer, so as to incorporate the details that may have been overlooked to the designer and/or developer, in this case, Kyle Phillips. The experiment that was chosen was to determine the effects of die swell, or extrudate swell, in reduced gravity. Die swell is a viscoelastic phenomenon that occurs when a dilatant, or shear-thickening substance is forced through a sufficient constriction, or "die," such that the substance expands, or "swells," downstream of the constriction, even while forming and maintaining a free jet at ambient sea level conditions. A wide range of dilatants exhibit die swell when subjected to the correct conditions, ranging from simple substances such as ketchup, oobleck, and shampoo to complex specially-formulated substances to be used for next generation body armor and high performance braking systems. To date, very few, if any, have researched the stabilizing effect that gravity may have on the phenomenon of die swell. By studying a fluid phenomenon in a reduced gravity environment, both the effect of gravity can be studied and the predominant forces acting on the fluid can be concluded. Furthermore, a hypothesis describing the behavior of a viscoelastic fluid particle employing the viscous Navier-Stokes Equations was derived to attempt to push the fluid mechanics community toward further integrating more fluid behavior into a unified mathematical model of fluid mechanics. While inconclusive in this experiment, several suggestions for future research were made in order to further the science behind die swell, and a comprehensive checkout of the facility and its operations were characterized. As a result of this checkout experience, several details were modified or added to the facility in order for the drop tower to be properly operated and provide the optimal user experience, such that open operation of the WVU SMiRF may begin in the Fall of 2014.</p>