# Aerospace Engineering

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- ItemNumerical Solutions for Two- and Three-Dimensional Non-Reacting Flowfields in an Internal Combustion Engine(1977) Griffin, Michael Douglas; Anderson, John D. Jr; Jones, Everett; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The numerical solution for the flowfield established in a spark- ignition internal combustion engine during the four-stroke (intake, compression, power, exhaust) cycle is considered. Only fluid-dynamic effects are treated with combustion simulated by constant- volume heat addition near top-dead-center on the compression stroke. The working fluid is assumed to be air of constant specific heat, with both viscous and inviscid models considered. Two- and three-dimensional engine models are examined, with the three-dimensional models including both rectangular and cylindrical geometries. The difficulties associated with obtaining numerical solutions in cylindrical coordinates for three-dimensional non-axisymmetric problems when the centerline is included in the region of interest are discussed. A new method which avoids the coordinate- singularity problems associated with such cases is presented and used to obtain the first known four-stroke inviscid-flow solution for a three- dimensional cylindrical engine model. Similar results are presented for a three-dimensional rectangular model, and for the first known two-dimensional four-stroke calculation for a viscous fluid. The inviscid three-dimensional results are compared with each other and with previously obtained two-dimensional inviscid-flow calculations. The use of two-dimensional models is found to be justified for the non- reacting flowfields considered, since the results obtained from a two-dimensional calculation in the valve plane are apparently not strongly dependent on the flowfield perpendicular to the valve plane. It is found that significant flowfields do exist in all I.C. engine models considered. It is shown that the unit-cell-Reynolds-number criterion limits viscous flow calculations to Reynolds numbers of approximately one ten-thousandth the realistic value, and that this produces flowfields which are strongly piston-dominated. In contrast, inviscid results show marked circulatory patterns, which are more realistic. The velocity patterns which develop in the three-dimensional cylindrical engine model are shown to exhibit a marked swirl in planes parallel and perpendicular to the cylinder axis.
- ItemNavier 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 successful
- ItemAn 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 0Â° to 50Â°. 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 17Â° and 30Â°. 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.
- ItemDynamics of a Helicopter-Slung Load System(1980) Sampath, Prasad; Barlow, Jewel B.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Stability of a tandem rotor helicopter (347/HLH) carrying a slung cargo container has been investigated. Lagrange equations were used to write the equations of motion. The cables of the sling were modeled as massless rigid extensible rods, which collapse under compressive loads. Extensibility was provided by considering the rods as linear spring with viscous damping. Aerodynamics of the cable were neglected. Tabulated static aerodynamic data were considered for the helicopter as well as the load. The equations were divided into two sets, one representing the towing vehicle (referred to as Subsystem 1) and the other representing the slung load (referred to as Subsystem 2). Subsystem 2 corresponds to a wind tunnel model of a slung load.
- ItemAn 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+Â®. 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.
- ItemThe Phase Space Portrait of an Integrable Dynamical System(Science, 1990-02-16) Coffey, Shannon; Deprit, AndrÃ©; Deprit, Etienne; Healy, LiamFor an integrable dynamical system with one degree of freedom, "painting" the integral over the phase space proves to be very effective for uncovering the global flow down to minute details. Applied to the main problem in artificial satellite theory, for instance, the technique reveals an intricate configuration of equilibria and bifurcations when the polar component of the angular momentum approaches zero.
- ItemA Toolbox for Nonlinear Dynamics(Springer-Verlag, 1991) Coffey, Shannon; Deprit, AndrÃ©; Deprit, Eitenne; Healy, Liam; Miller, BruceUsing the main problem of artificial satellite theory as an illustration, we review several developments which have had a significant impact on research in nonlinear dynamics. On the mathematical front, we point to the theory of Lie transformations; in the area of computational software, we explain how massively data parallel machines open the way for symbolic solution of large problems. Finally, we show how color graphics assist in the qualitative analysis of dynamical systems.
- ItemThe 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.
- ItemPaint by number: Uncovering phase flows of an integrable dynamical system(American Institute of Physics, 1991-09) Healy, Liam; Deprit, EtienneGiven an integrable dynamical system with one degree of freedom, "painting" the integral over phase space proves to be a powerful technique for uncovering both global and local behavior. This graphical technique avoids numerical integration, employing instead a nonlinear method of assigning contrasting colors to the energy values to distinguish subtle details of the flow.
- ItemParallel Computing for Space Surveillance(MIT Lincoln Laboratory, 1992) Healy, Liam; Coffey, ShannonThis paper reports on an application of massively parallel processors to multiple satellite propagation and the calculation of miss distances between objects (COMBO). Unlike serial computations, we do not pre-filter the data but rather sort the data set in a way that dramatically cuts the number of comparisons required in order to be assured of a complete catalog-to-catalog comparison. The same general algorithm allows two logical sets to be compare to each other. Run time for this demonstration code on an 8K Connection Machine is about one second per time step, including propagation, complete catalog-to-catalog calculation of miss distances, plotting satellite positions, and recording of the miss distances to a file. Propagation of the objects is performed with an analytic propagator, using J2 only at present, though the code may easily be extended to other propagators. We demonstrate a second application of parallel computing to the problem of debris propagation resulting from a satellite breakup. The spread of such debris into n pieces is simulated by replicating the element set for the original satellite n times, then altering each to represent a distribution of velocities to the center of mass.
- ItemComputation of error effects in nonlinear Hamiltonian systems using Lie algebraic methods(American Institute of Physics, 1992-06) Healy, Liam; Dragt, Alex; Gjaja, IvanThere exist Lie algebraic methods for obtaining transfer maps around any given trajectory of a Hamiltonian system. This paper describes an iterative procedure for finding transfer maps around the same trajectory when the Hamiltonian is perturbed by small linear terms. Such terms often result when an actual system deviates from an ideal one due to errors. Two examples from accelerator physics are worked out. Comparisons with numerical computations, and in simple cases exact analytical calculations, demonstrate the validity of the procedure.
- ItemDeterministic Studies of Debris Hazards with Parallel Processors(European Space Agency, 1993-04-05) Healy, Liam; Coffey, ShannonA new generation of parallel processing computers makes possible the ability to propagate all objects in the space surveillance catalog with simulated objects, and detect close approaches. With this capability, it is possible to test deterministically debris scenarios, without resorting to statistical models. To compare the positions of objects we have developed two methods, an all-to-all comparison and a one-to-all comparison. For the former, a seive significantly reduces computation time; for the latter, direct comparison is possible in parallel. We show results from several simulations, including simulated multiple sources of debris, hazard to the space station, and close contacts amongst the catalog itself, to show potential for debris studies. The techniques described here have potential application the general problem of catalog maintenance.
- ItemClose Conjunction Detection on Parallel Computer(American Institute of Aeronautics and Astronautics, 1995-07) Healy, LiamClose conjunction detection is the task of finding which satellites will come within a given distance of other satellites. The algorithms described here are implemented on the Connection Machine (CM) in a program called CM-COMBO. It will find close conjunctions of satellites over a time range for one, a few, or all satellites against the original or another catalog and works with an arbitrary propagator. The problem of comparing an entire catalog against itself is beyond the computing power of current serial machines. This program does not prefilter any orbits and does not make assumptions about the type of orbit (that it be nearly circular, for instance). This paper describes the algorithm for this computation, the implementation on the CM, and resuls of several studies using this program.
- ItemSymbolic and Parallel Computation in Celestial Mechanics(Society for Industrial and Applied Mathematics, 1996) Healy, LiamOne aspect of celestial mechanics is the computation of the long-term orbits of celestial bodies. This type of computation is complicated by the interaction of the many bodies that need to be considered to derive accurate long-term behavior. For reasons explained in this chapter, it is necessary to do this symbolically rather than numerically. Symbolic computations performed on a Lisp machine are described. The visualization of the solution is accomplished in a massively parallel SIMD machine.
- ItemHigh-Frequency Nonlinear Vibrational Control(IEEE, 1997-01) Shapiro, Benjamin; Zinn, B. T.This paper discusses the feasibility of high-frequency nonlinear vibrational control. Such control has the advantage that it does not require state measurement and processing capabilities that are required in conventional feedback control. Bellman et al. [1] investigated nonlinear systems controlled by linear vibrational controllers and proved that vibrational control is not feasible if the Jacobian matrix has a positive trace. This paper extends previous work to include nonlinear vibrational controllers. A stability criteria is derived for nonlinear systems with nonlinear controllers, and it is shown that a nonlinear vibrational controller can stabilize a system even if the Jacobian matrix has a positive trace.
- ItemAutomatic Rendering of Astrodynamics Expressions for Efficient Evaluation(American Astronautical Society, 1998) Healy, Liam M.; Travisano, Jeffrey J.In this paper, we describe the automatic rendering of expressions computed using symbolic manipulation. Computations from astrodynamics frequently can be put in a fixed hierarchy of polynomials and Fourier series. Once in this form, FORTRAN subprograms can be generated automatically in a form that lends itself to numerical evaluation. The goal of the current work is to present an approach for using symbolic manipulation techniques to produce a Fortran representation of the normalized Hamiltonian and other supporting equations representing as many of the actual physical effects on satellites as possible.
- ItemSymmetry Approach to Extension of Flutter Boundaries via Mistuning(American Institute of Aeronautics and Astronautics, 1998-05) Shapiro, BenjaminA general framework is presented for analyzing and optimizing stability increases resulting from mistuning. The framework given is model independent and is based primarily on symmetry arguments. Difficult practical issues are transformed to tractable mathematical questions. It is shown that mistuning analysis reduces to a block circular matrix eigenvalue/vector problem that can be solved efficiently even for large problems. Similarly, the optimization becomes a standard linear constraint quadratic programming problem and can be solved numerically. Because the methods given are model-independent, they can be applied to various models and allow the researcher to easily conclude which models accurately capture mistuning and which do not. A simple quasisteady model for flutter in a cascade is used to illustrate and validate results in this paper.
- ItemMATHEMATICAL MODEL OF ADAPTIVE MOTOR CONTROL(1999) Kosha, Makiko; Sanner, Robert M.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)An adaptive control law incorporating a biologically inspired neural networks for robot control is used as a mathematical model of human motor control and the motor control adaptation. Modeling human motor control strategy is made difficult due to the redundancies in the human motor control system. This control model is able to overcome the difficulties of the human motor control modelling, and include the learning capability of the motor control strategy which was omitted in human motor control studies until now. By adaptively piecing together a collection of elementary computational elements, the proposed model develops complex internal models which are used to compensate for the effects of externally imposed forces or changes in the physical properties of the system. In order to examine the form of human motor control adaptation in detail, a computer simulation was developed with a two dimensional model of the human arm which utilized the proposed adaptive motor control model. The simulation result show that the model is able to capture the characteristics of the motor control adaptation seen in human experiments reported by [14], [46]. For cont inuation of this research, an experimental apparatus was designed and built for the human motor control study. This apparatus is a cable driven, two-dimensional manipulator which is used to apply specified disturbance forces to the human arm. The preliminary experiment conducted with this test apparatus show a strong correlation to the simulation data and other experimental data reported on human reaching motions.
- ItemSolving for Mistuned Forced Response by Symmetry(American Institute of Aeronautics and Astronautics, 1999-03) Shapiro, BenjaminThe introduction of mistuning in jet-engine bladed disks can lead to large changes in stability and forced response. Even small random mistuning (within the bounds of manufacturing tolerance) can lead to unacceptable response and high-cycle fatigue. Meanwhile, intentional mistuning may improve stability and forced response under manufacturing uncertainty. This paper presents a general framework for predicting forced response as a function of mistuning. Because the forced response problem is an almost singular linear problem, its solution is highly nonlinear in the mistuning parameters. Our methods exploit symmetry arguments and eigenstructure perturbation to provide a method valid for any model. It is shown that, by perturbing eigenvectors in the numerator and the inverse of eigenvalues in the denominator (exploiting symmetry in both computations), we can accurately approximate the forced response as a function of mistuning. Results are demonstrated for a simple lightly damped model, and the consequent sharp nonlinear behavior is captured almost perfectly. We also show that intentional mistuning may guarantee improved stability and forced response under fixed manufacturing tolerances. Thus, intentional mistuning should be considered as a practical means of increasing safety and enhancing engine performance.
- ItemThe Main Problem in Satellite Theory Revisited(Springer Science+Business Media B.V., 2000) Healy, Liam M.Abstract. Using the elimination of the parallax followed by the Delaunay normalization, we present a procedure for calculating a normal form of the main problem (J2 perturbation only) in satellite theory. This procedure is outlined in such a way that an object-oriented automatic symbolic manipulator based on a hierarchy of algebras can perform this computation. The Hamiltonian after the Delaunay normalization is presented to order six explicitly in closed form, that is, in which there is no expansion in the eccentricity. The corresponding generating function and transformation of coordinates, too lengthy to present here to the same order; the generator is given through order four.