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This paper investigates trajectories for spaceborne observatories with long times of flight high above the ecliptic plane. A large normal displacement is necessary for mitigating the effect of noise generated by the interplanetary (zodiacal) dust cloud, thus increasing resolution and reducing the sizes of the telescopes. Flyby paths using multiple-gravity- assist maneuvers near Earth and Venus are used to reach Jupiter, where the inclination is increased and the final orbit is produced. The trajectory design is performed using a memetic algorithm, which is a combination of global and local optimizers. The global search for optimal trajectories with minimal energy requirements and short transfer times to the highly inclined destination orbit is performed using a niching genetic algorithm improved by a gradient-based local optimization. The optimization yielded three candidate paths, each accompanied by, at most, one deep-space maneuver. The resulting energy-efficient orbits are characterized by large displacements normal to the ecliptic plane, thereby providing prolonged observation times in the interplanetary-dust-noise-free celestial sphere. [ABSTRACT FROM AUTHOR]

The increasing satellite congestion at the geostationary altitude requires positioning a few satellites in the same geostationary slot, a technique known as collocation. In this paper, we develop a geostationary orbit satellite collocation algorithm using relative orbital element corrections, which represent the differences between the impulsive orbital element corrections of any two spacecraft in a geosynchronous slot. The main idea is that formulating the problem of collocation in terms of relative orbital element corrections leaves some of the final values of the orbital elements unconstrained. The freedom rendered by this modeling can be used to find impulsive maneuvers minimizing a given performance index. The minimum distance between satellites that guarantees collision-free motion is incorporated into the design process to find necessary and sufficient conditions for the relative eccentricity and inclination vectors, guaranteeing safe collocation. The proposed collocation algorithm is illustrated in a simulation. [ABSTRACT FROM AUTHOR]

Hypersonic vehicle development, particularly for hypersonic airbreathers, will require robust-design optimization to achieve performance targets. Vehicle shape optimization will play an important role. This paper presents the first application to hypersonic vehicle shape optimization of a reduced parametric section shape representation coupled with a surrogate-assisted evolutionary-algorithm optimization approach. The particular case considered is the minimization of total drag of the nose cone of a hypersonic flight experiment. The computational fluid dynamics solver used here is ANSYS CFX. Single-point optimization at Mach 3 and Mach 8 is performed at altitudes relevant to those Mach numbers for a typical hypersonic flight experiment ascent trajectory. Without surrogate assistance, the optimized shapes for each Mach number were both found to result in significant drag-force reductions (1.39% at Mach 3 and 1.96% at Mach 8) when compared with the baseline blunted standard ogive nose-cone shape. When the surrogate assistance (which is a radial-basis-function network approximation to the drag dependence on the shape parameters) was introduced, the optimized shapes yielded nearly the same drag reduction as without surrogate assistance, but also resulted in significant savings in the computational cost. Finally, the performance of the optimum shape derived at Mach 3 is evaluated at Mach 8 and vice versa to illustrate the robustness of the nose-cone shapes derived using such an approach. [ABSTRACT FROM AUTHOR]

This paper presents the modeling, algorithm development, and laboratory testing of a two-axis digital sun sensor. The basis for our modeling is the Sinclair Interplanetary SS-330, a sensor that employs a single linear array to provide two-axis measurements of the sun vector. The detector illumination simulation includes consideration of Fresnel diffraction, finite pixel size, the extended nature of the sun, and a detector-noise model to synthesize artificial images. Simulated images agree with actual sensor images to within about 3.2% (root mean square). Algorithm studies contrast the performance of existing peak-detection routines (e.g., peak detections and centroiding) with more sophisticated parametric algorithms. Laboratory studies confirm that these modified algorithms can provide a threefold performance improvement over the baseline sensor processing. These results also evaluate the simulation's ability to provide accurate estimates of sensor performance. Although the simulations qualitatively replicate many behaviors, they are not yet accurate enough to be used in place of hardware validation. [ABSTRACT FROM AUTHOR]

Supersonic inlet buzz and, in particular, its onset were studied through the analysis of pressure records. These records are issued from an experimental study that was presented in a previous paper. The onset of buzz is a nonstationary phenomenon, and so the pressure signals could not be analyzed using classical signal-processing tools such as the Fourier transform, which does not take into account the possible evolutions in time. That is why time-frequency methods (spectrogram, wavelet transform, and Wigner-Ville distribution) were used. The evolution in time of the energy levels corresponding to buzz frequencies were studied. This analysis shows the existence of precursor phenomena that can appear several tenths of seconds before the onset of buzz. The detection of these precursors could, with the appropriate activation of a control device, allow the avoidance of buzz. Two change-detection algorithms (cumulative sum and generalized likelihood ratio) were tested on experimental pressure signals and proved their ability to successfully detect these precursors. [ABSTRACT FROM AUTHOR]

Autopilot systems are capable of reliably following flight plans under normal circumstances, but even the most advanced flight-management systems cannot provide robust response to most anomalous events including in-flight failures. This paper describes an emergency flight-management architecture that can be applied to piloted or autonomous aircraft, with focus on the design and implementation of an adaptive flight planner (AFP) that dynamically adjusts its model to compute feasible flight plans in response to events that degrade aircraft performance. A two-step landing-site selection/trajectory generation process defines safe emergency plans in real time for situations that require landing at an alternate airport. A constraint-based search algorithm selects and prioritizes feasible emergency landing sites, then the AFP synthesizes a segmented trajectory to the best site based on post-failure flight dynamics. The AFP architecture is general for any failure situation; however, operational success is guaranteed only with accurate post-failure performance characterization and a trajectory generation strategy that respects degraded flight envelope boundaries. A real-time segmented trajectory planning algorithm and case study results are presented for total loss of thrust failure scenarios. This emergency is surprisingly common and necessitates an immediate approach and landing without a go-around option. [ABSTRACT FROM AUTHOR]

Computations of complex flow fields around three-dimensional high-lift configurations using an adaptive Cartesian grid method and a cell-center finite-volume method are carried out and compared to wind-tunnel experiments. Because of complexities in both configuration and flowfield, there are great difficulties in Numerical simulation of flowfields around transport aircraft high-lift configurations with leading slats and trailing flaps. In this paper, a modified Cartesian grid method is introduced to simulate the complex flow using the adaptive grid technique. The main emphasis is given to a multizone technique and a face-to-face algorithm and its applications. The multizone technique is employed to simplify the great difficulties in grid generation. The face-to-face algorithm is constructed to precisely establish the information exchange of flowfields on interfaces between zones. We solve Euler equations with a conventional algorithm, which includes a cell-center finite-volume method and an explicit four-stage Runge-Kutta time-stepping scheme in combination with a dual-time stepping approach. For three three-dimensional high-lift configurations, the numerical results are presented and analyzed. The computational results are in good agreement with the available experimental data show that the present method is feasible and effective for solving the flow around complex three-dimensional high-lift systems. [ABSTRACT FROM AUTHOR]

The issue of monitoring aircraft performance during takeoff has been repeatedly considered in the last 50 years. This has been in response to concern over the low safety margins associated with this phase of flight and the lack of adequate objective performance-related information available to the crew during the takeoff maneuver. Two major difficulties, however, have to date inhibited the successful introduction of an instrument providing such information on the flight deck, namely, the provision of a reliable quantification of actual performance during the takeoff maneuver and the provision of an adequate display. This paper focuses on the former issue and evaluates the accuracy of a predictive performance monitoring algorithm developed at Cranfield University. The major hurdles that are associated with takeoff performance monitoring are briefly discussed, and the results of flight-based tests using data from large jet-transport operations presented. These results, which confirm a robustness that warrants the application of the algorithm in real-time performance monitoring, are also compared with those of an earlier flight test program using a twin turboprop aircraft. [ABSTRACT FROM AUTHOR]

This paper presents an algorithm to estimate unknown parameters of parachute models from flight-test data. The algorithm is based on the simultaneous-perturbation-stochastic-approximation method to minimize the prediction error (difference between model output and test data). The algorithm is simple to code and requires only the model output. Analytical gradients are not necessary. The algorithm is used to estimate aerodynamic and apparent mass coefficients for an existing parachute model. [ABSTRACT FROM AUTHOR]

The paper summarizes a retrofit reconfiguration methodology that augments the production flight control system of an F/A-18 to compensate for changes in aircraft stability and control derivatives caused by aerodynamic control surface failures. The retrofit architecture relies on an indirect-adaptive model-following control approach that exploits onboard models of the closed-loop aircraft dynamics in the control computations. An output-error parameter identification process is used off-line to model the nominal behavior of the F/A-18 inner closedloop (airframe and baseline controller). The identified models are employed to describe the desired behavior for the controller and to provide regularization for an online parameter identification process. The online system identification algorithm updates the parameters of the models used for control decision, and a regularized recursive least-squares method is chosen to enable rapid adaptation to unforeseen failures and unmodeled dynamics while being robust to periods of low excitation. The retrofit reconfigurable controller was evaluated in high-fidelity F/A-18 simulations across a wide envelope of flight conditions with single and multiple control surface failures. In 80 to 90% of the failure cases, the system provided a significant improvement in stability and handling qualities compared with the unaugmented production control system. Piloted simulations were carried out with Navy and Boeing pilots in fall 2002, and flight tests are planed for 2004. [ABSTRACT FROM AUTHOR]

This paper presents a comparison between three continuum-based topology optimization methods: the hybrid cellular automaton method, the optimality criteria method, and the method of moving asymptotes. The purpose of the study is to highlight the differences between the three. The optimality criteria method and the method of moving asymptotes are well established in topology optimization. The hybrid cellular automaton method is a recently developed gradient-free technique that combines both local design rules based on the cellular automaton paradigm and the finite element analysis. The closed-loop controllers used in the hybrid cellular automaton method are used to modify the mass distribution in the design domain to find an optimum material layout. The hybrid cellular automaton and optimality criteria methods and the method of moving asymptotes are described and applied in a comparative study to three sample problems. The influence of different algorithm control parameters is shown in this work. The paper demonstrates that, for the sample problems presented, the hybrid cellular automaton method generally required the fewest number of iterations to converge to a solution compared with the optimality criteria method and the method of moving asymptotes. The final topologies generated using the hybrid cellular automaton method typically had the lowest compliance and exhibited the fewest number of intermediate densities at the solution. [ABSTRACT FROM AUTHOR]

This paper proposes a methodology for the analysis and design of systems subject to parametric uncertainty in which design requirements are specified via hard inequality constraints. Hard constraints are those that must be satisfied for all parameter realizations within a given uncertainty model. Uncertainty models given by norm-bounded perturbations from a nominal parameter value (i.e., hyperspheres) and by sets of independently bounded uncertain variables (i.e., hyperrectangles) are the focus of this paper. These models, which are also quite practical, allow for a rigorous mathematical treatment within the proposed framework. Hard-constraint feasibility is determined by sizing the largest uncertainty set for which the design requirements are satisfied. Assessments of robustness are attained by comparing this set with the actual uncertainty model. These assessments do not suffer from the numerical deficiencies of sampling-based methods. Strategies that enable the comparison of the robustness characteristics of competing design alternatives, the approximation of the robust design space, and the systematic search for designs with improved robustness are also proposed. Because the problem formulation is generic and the tools derived only require standard optimization algorithms for their implementation, this methodology is applicable to a broad range of engineering problems. [ABSTRACT FROM AUTHOR]

An estimation of aerodynamic models of impaired aircraft using an innovative differential vortex-lattice method tightly coupled with extended Kalman filters is discussed. The proposed approach significantly reduces the order of the estimation problem by exploiting the prior knowledge about the undamaged aircraft and the detected information on the approximate location and extent of damage. Three different extended Kalman filter formulations are developed and their comparative analyses are performed through numerical simulations. Algorithms given in this paper can be used as the basis for online derivation of an aircraft performance model, which can then form the basis for designing safe landing guidance laws for damaged aircraft. [ABSTRACT FROM AUTHOR]

This paper presents a strategy called patch assembly for assembling overlapping grid systems. This strategy has been designed to deal efficiently with specific grid assembly cases. These are cases where the chimera technique is used to add a new feature, described in a patch grid, to a geometry defined in a background grid. The patch assembly algorithm automatically performs the blanking of the background grid using simple and robust algorithms. In particular, the algorithm is not based on any geometric mask, which can be difficult to build. The blanking performed ensures a minimum overlap between grids. This grid assembly strategy is applied to a simple two-dimensional case and to a big three-dimensional wing-body configuration for modeling control surfaces. On both configurations, results are compared to results obtained on grid setups manually set with masks. A favorable influence of the minimum overlap produced has been observed on these configurations. [ABSTRACT FROM AUTHOR]

This paper presents the development of a multigrid parallel algorithm for a nonlinear aeroelastic analysis. The aeroelastic model consists of 1) a nonlinear structural model that captures in-plane, out-of-plane, and torsional couplings; 2) an unsteady viscous aerodynamic model that captures compressible flow effects for transonic flows with shock/boundary-layer interaction; and 3) a solution methodology that assures a tightly coupled solution of the nonlinear structure and the fluid flow, including a consistent geometric interface between the highly deforming structure and the flowfield. A domain-decomposition parallel computation algorithm based on a message-passing interface was developed for the flow solver. A three-level multigrid algorithm was implemented in the flow solver to further reduce the computational time. A grid generation and deformation algorithm was developed concurrently with the flowsolver in order to improve the efficiency of the computation. The grid deformation methodology kept the mesh topology unchanged as the structure deformed. Consequently, it was not necessary for either the parallel computation or the multigrid algorithm to update their communication pointers while the structure deformed. The validation of the numerical solver was done using experimental results of the F-5 wing. The aeroelastic solver was then used to assess the effect of structural nonlinearities on the aeroelastic response of the heavy Goland wing. [ABSTRACT FROM AUTHOR]

The objective of this paper is to examine nonlinear normal modes and their bifurcations in cyclic periodic structures. The nonlinear normal modes are computed using a numerical technique that combines shooting and pseudoarclength continuation. Unlike perturbation techniques, the resulting algorithm can investigate strongly nonlinear regimes of motion. This study reveals that modal interactions may occur without necessarily having commensurate natural frequencies in the underlying linear system. In addition, a countable infinity of such modal interactions are shown to exist in the system. [ABSTRACT FROM AUTHOR]

Many industrial applications involve flows periodic in time. Such flows are not simulated with enough efficiency when using classical unsteady techniques, as a transient regime must be bypassed. New techniques, dedicated to time- periodic flows and based on Fourier analysis, have been developed recently. Among these, the time spectral method casts a time-periodic flow computation in several coupled steady computations, corresponding to a uniform sampling of the period. Up until now, the steady states were reached using an explicit pseudotime algorithm. Thus, very small time steps were needed and the convergence rate slowed down when increasing the number of harmonics. In this paper, a block-Jacobi approach is presented to solve the stationary problems with an implicit algorithm. Numerical simulations show, on one hand, the good quality of the results and, on the other hand, the interest of the proposed method to reduce the sensitivity of computations to a large number of harmonics. [ABSTRACT FROM AUTHOR]

In this paper we present the application of a hyper radial basis function network as a topology description function. A hyper radial basis function network is used to parameterize the material distribution (density) for topology optimization. The density of each finite element in the continuum topology optimization domain is governed by a single network of hyper radial function bases. Thus, the topology optimization problem is to determine the parameters governing the hyper radial basis function network to satisfy certain design criteria. Here we present the solution to a minimum compliance topology design. A hyper radial basis function network is used to parameterize the material density during the finite element analysis stage. An efficient optimization algorithm that makes use of perturbation and sequential linear programming is developed to obtain the hyper radial basis function network parameters. Examples are presented to demonstrate the proposed approach and to compare it with the traditional solid isotropic microstructures with penalization topology optimization. Some of the advantages of the proposed approach are that it can yield checkerboard-free, manufacturable topologies using coarse-mesh finite element analysis models as opposed to the traditional approach, which requires fine meshes. This results in a reduction in the solution time for finite element analysis while conserving the ability to yield a smooth topology. [ABSTRACT FROM AUTHOR]

In this paper, the precise time-step integration method by step-response and impulsive-response matrices is further developed by expanding the dimension of the matrices so as to avoid computing the particular solutions separately. Two new precise time-step integration algorithms with excitations described by second-order and first-order differential equations are proposed. The first method is a direct extension of the existing algorithms. However, the extended system matrices are not symmetrical. In the second method, the Duhamel integrals are used as the particular solutions. As a result, the responses can be expressed in terms of the given initial conditions and the step-response matrix, the impulsive-response matrix and a newly derived Duhamel-response matrix. The symmetry property of the system matrices can be used in the computation. However, it will first require a calculation of the Duhamel-response matrix and its derivative. To reduce the computational effort, the relation between the Duhamel response matrix and its derivative is established. A special computational procedure for periodic excitation is also discussed. Numerical examples are given to illustrate the present highly efficient algorithms. [ABSTRACT FROM AUTHOR]

Engineering codes are powerful tools for the conceptual design stage. Use of these codes in early design stages enables higher fidelity computational fluid dynamics calculations or wind-tunnel tests to be performed on more mature configurations. Two widely used engineering tools for conceptual level supersonic/hypersonic aerodynamic prediction are tangent-wedge and tangent-cone methods. Unfortunately, these analysis tools lack the ability to systematically account for effects of real-gas flow across the oblique shock wave. This paper derives an extension to the classical oblique shock wave equations where the effects of real-gas flow changes are modeled using theoretical models borrowed from statistical mechanics. Using an enthalpy balance across the shock wave, the Einstein model for a diatomic gas is analytically integrated to give a relationship of temperature, ratio of specific heats, and velocity on the downstream side of the shock wave. This relationship replaces the energy equation used in the ideal-gas oblique shock analysis. This method is adapted to conical flowfield assuming an initial shock angle, and using the new oblique shock algorithm to compute properties immediately behind the conical shock. The resulting model is compared to equilibrium real-gas calculations and shown to give accurate results well into the hypersonic flow regime, approximately Mach 20. [ABSTRACT FROM AUTHOR]

The scope of takeoff monitoring is to provide a measure of aircraft performance during the takeoff run and to identify whether this lies within the predetermined limits of scheduled performance or otherwise. Used in conjunction with a good display, such a system, provided it is adequately reliable, will provide critical situational awareness to the crew and will thus contribute towards improving safety during takeoff. Cranfield University has developed a takeoff performance monitor based on an algorithm that determines estimates of relevant parameters from the history of the particular takeoff run, rather than relying on scheduled estimates. This, however, resulted in a delay between the start of the run and the generation of the first estimate of performance. This paper describes the improvements in performance achieved by the introduction of a modification that uses measured thrust-to-weight ratio to significantly shorten the delay and improve prediction accuracy, particularly at the start of the run. [ABSTRACT FROM AUTHOR]

This paper presents a method for computing the thermoacoustic modes in combustors. In the case of a nonisothermal reacting medium, the wave equation for the pressure fluctuations contains a forcing term related to the unsteady heat release. Depending on the phase relationship between the acoustics and the flame, certain linear modes may become unstable, leading to thermoacoustic instabilities. The relevant Helmholtz equation is derived and two approaches for solving the corresponding nonlinear eigenvalue problem are proposed. The first one is based on an asymptotic expansion of the solution, the baseline being the acoustic modes and frequencies for a steady (or passive) flame and appropriate boundary conditions. This method allows a quick assessment of any acoustic mode stability but is valid only for cases where the coupling between the flame and the acoustic waves is small in amplitude. The second approach is based on an iterative algorithm where a quadratic eigenvalue problem is solved at each subiteration. It is more central processing unit demanding but remains valid even in cases where the response of the flame to acoustic perturbations is large. Frequency-dependent boundary impedances are accounted for in both cases. A parallel implementation of the Arnoldi iterative method is used to solve the large eigenvalue problem that arises from the space discretization of the Helmholtz equation. Several academic and industrial test cases are considered to illustrate the potential of the method. [ABSTRACT FROM AUTHOR]

Analytical target cascading is a hierarchical multilevel multidisciplinary design methodology. In analytical target cascading, top-level design targets (i.e., specifications) are propagated to lower-level design problems in a consistent and efficient manner. In this paper, a modified Lagrangian dual formulation and coordination for analytical target cascading are developed to enhance a formulation and coordination proposed earlier in the literature. The proposed approach guarantees all the properties established earlier and additionally offers new significant advantages. As established previously for the convex case, the proposed analytical target cascading coordination converges to a global optimal solution with corresponding optimal Lagrange multipliers in the dual space. The Lagrange multipliers can be viewed as the weights for deviations in analytical target cascading formulations. Thus the proposed coordination algorithm finds the optimal solution and the optimal weights for the deviation terms simultaneously. The enhancement allows for target cascading between levels, for the use of augmented Lagrangian to improve convergence of the coordination algorithm, and for prevention of unboundedness. A guideline to set the step size for subgradient optimization when solving the Lagrangian dual problem is also proposed. [ABSTRACT FROM AUTHOR]

Direct aeroacoustic computations require nonreflective boundary conditions that allow disturbances to leave the domain freely without anomalous reflections. In the present paper we analyze a series of nonreflective boundary conditions already published in the literature and propose an improved outflow nonreflective boundary condition with reflection characteristic that is reduced greatly. For the solution of the linearized Euler equations, a sixth-order compact finite-difference algorithm is used together with a sixth-order explicit digital filter that suppresses the highfrequency spurious oscillations in the solution. A number of representative test cases are presented. The new outflow boundary condition is recommended for the simulation of sound produced by turbulence. [ABSTRACT FROM AUTHOR]