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There are still some challenges, such as grid quality, numerical stability, and boundary schemes, in the practical application of high-order finite difference schemes for complex configurations. This study presents some improved strategies that indicate potential engineering applications of high-order schemes. The formally fifth-order weighted compact nonlinear scheme developed by the authors is implemented on point-matched multiblock structured grids, which are generated over complex configurations to ensure the grid quality of each component block. The information transmission between neighboring blocks is carried out by new characteristic-based interface conditions that directly exchange the spatial derivatives on each side of an interface by means of a characteristic-based projection to keep the high-order accuracy and high resolution of a spatial difference scheme. The high-order scheme combined with the interface conditions is shown to be asymptotically stable. The engineering-oriented applications of the high-order strategy are demonstrated by solving several two- and three-dimensional problems with complex grid systems. DOI: 10.2514/1.J050285

A semi-analytical model for the prediction of the broadband noise due to the interaction between turbulent rotor wakes and a rotor in contra-rotating open rotors is presented. The unsteady loading of the rear rotor is modeled using classical isolated flat-plate theory. Strip theory is used to treat the spanwise variations of aerodynamic quantities and blade geometry. The turbulent wake is assumed to be homogeneous and isotropic turbulence that is modulated by a train of wake profiles. The model is presented in detail and insight into its modal behavior is provided. A parameter study is conducted to investigate the effects of blade number, rotor-rotor gap and rotor speeds on broadband noise emissions due to rotor-wake/rotor interaction in contra-rotating open rotors. Scaling laws for sound power levels have been established analytically and show good agreement with the results of the parameter study. DOI: 10.2514/1.J050566

A hybrid mesh generation method is described to discretize complex geometries. The idea behind this hybrid method is to combine the orthogonality and directionality of a structured grid, the efficiency and simplicity of a Cartesian grid, and the flexibility and ease of an unstructured grid in an attempt to develop an automatic, robust, and fast hybrid mesh generation method for configurations of engineering interest. A semistructured quadrilateral grid is first generated on the wetted surfaces. A background Cartesian grid, covering the domain of interest, is then constructed using a Quadtree-based Cartesian Method. Those Cartesian cells overlapping with the semistructured grids or locating outside of computational domain are then removed using an Alternating Digital Tree method. Finally, an unstructured grid generation method is used to generate unstructured triangular cells to fill all empty regions in the domain as a result of the trimming process. The automatic placement of sources at the geometrical irregularities is developed to render these regions isotropic, thus effectively overcoming the difficulty of generating highly stretched good-quality elements in these regions. The self-dividing of the exposed semistructured elements with high aspect ratio and the adaptation of the background mesh using the cell size information from the exposed semistructured elements for generating Cartesian cells are introduced to improve the quality of unstructured triangular elements and guarantee the success of the unstructured grid generation in the void regions. The developed hybrid grid generation method is used to generate a hybrid grid for a number of test cases, demonstrating its ability and robustness to mesh complex configurations. DOI: 10.2514/1.J050491

There are various condensation methods for substructure techniques in structural dynamics. A generalized condensation method that comprises most of the classical condensation techniques and allows for arbitrary mode shapes within a standardized approach is described here. Within the framework of this method, a flexible boundary method is introduced that allows for elastic and mass-loaded boundaries for eigenmode determination, as well as for mode shapes that reflect the influence of damping. The relationship to other approaches taken from existing literature is examined. For damped structures, the flexible boundary method provides a condensation process that takes into account the influence of the complex eigenmodes of structures with nonproportional and high damping. To couple substructures with different component damping, the equivalent structural damping approach is provided. The problems associated with diagonal system damping of substructures and the full triple matrix product are overcome. DOI: 10.2514/1.J050484

Reliability-based design often uses the Monte--Cario method as a sampling procedure for predicting failure. The combination of designing for very small failure probabilities (~[10.sup.-8] - [10.sup.-6]) and using computationally expensive finite element models, makes Monte--Carlo simulations very expensive. This paper uses an improved sampling procedure for calculating the probability of failure, called separable Monte--Carlo method. The separable Monte--Carlo method can improve the accuracy of the traditional crude Monte--Carlo when response and capacity are independent. In previous research, accuracy of separable Monte--Carlo for a simple limit state was estimated via expectation calculus for a simple form of the limit state. In this paper, error estimates for a general limit state are developed through bootstrapping, and it is demonstrated that the estimates are reasonably accurate. Separable Monte--Carlo allows us to choose different sample sizes of the response and capacity in the limit state, and the paper demonstrates that bootstrapping may be used to estimate the contribution of the response and capacity to the total error. When the accuracy of the probability of failure is not good enough, the paper proposes reformulation of the limit state as another way to reduce uncertainty associated with the expensive random variable (usually the response). The accuracy of the bootstrapping estimates and the effectiveness of regrouping is demonstrated with an example of prediction of failure in a composite laminate with the Tsai--Wu failure criterion. DOI: 10.2514/1.J050439

The present work has the objective of demonstrating the capabilities of a spectral finite volume scheme implemented in a cell-centered finite volume context for unstructured meshes. The two-dimensional Euler equations are considered to represent the flows of interest. The spatial discretization scheme is developed to achieve high resolution for flow problems governed by hyperbolic conservation laws. Roe's flux difference splitting method is used as the numerical approximate Riemann solver. Several applications are performed in order to assess the method capability compared to data available in the literature and also compared to an weighted essentially nonoscillatory scheme. There is good agreement with the comparison data, and efficiency improvements over the weighted essentially nonoscillatory method are observed. The features of the present methodology include an implicit time-marching algorithm; second-, third-, and fourth-order spatial resolution; exact high-order domain boundary representation; and a hierarchical moment limiter to treat flow solution discontinuities. DOI: 10.2514/1.J050395

The use of surrogates (also known as metamodels) for facilitating optimization and statistical analysis of computationally expensive simulations has become commonplace. Surrogate models are usually fit to be unbiased (i.e., the error expectation is zero). However, in certain applications, it might be important to safely estimate the response (e.g., in structural analysis, the maximum stress must not be underestimated in order to avoid failure). In this work we use safety margins to conservatively compensate for fitting errors associated with surrogates. We propose the use of cross validation for estimating the required safety margin for a desired level of conservativeness (percentage of safe predictions). The approach was tested on three algebraic examples for two basic surrogates: namely, kriging and polynomial response surface. For these examples we found that cross validation is effective for selecting the safety margin. We also applied the approach to the probabilistic design optimization of a composite laminate. This design under uncertainty example showed that the approach can be successfully used in engineering applications. DOI: 10.2514/1.J050327

Aerospace, aviation particularly, need, in any era, the strongest and most thermostable materials available, often at nearly any price. The space elevator, space ships (especially during atmospheric reentry), rocket combustion chambers, thermally challenged engine surfaces, hypersonic aircraft materials are better now than any available, with undreamed performance as the reward if obtained. As shown in this research, the offered new material allows to greatly improve all characteristics of space ships, rockets, engines, and aircraft, and design new types space, propulsion, and aviation systems. At present the term 'nanotechnology' is well known--in its ideal form, the flawless and completely controlled design of conventional molecular matter from molecules or atoms. Such power over nature would offer routine achievement of remarkable properties in conventional matter, and creation of metamaterials where the structure not the composition brings forth new powers of matter. But even this yet unachieved goal is not the end in material science possibilities. The writer herein offers the idea of design of new forms of nuclear matter from nucleons (neutrons, protons), electrons, and other nuclear particles. He shows this new 'AB Matter' has extraordinary properties (for example, tensile strength, stiffness, hardness, critical temperature, superconductivity, supertransparency, zero friction, etc.), which are up to millions of times better than corresponding properties of conventional molecular matter. He shows concepts of design for space ships, rockets, aircraft, sea ships, transportation, thermonuclear reactors, constructions, and so on from nuclear matter. These vehicles will have unbelievable possibilities (e.g., invisibility, ghostlike penetration through any walls and armor, protection from nuclear bomb explosions and any radiation flux, etc.). People may think this as fantasy. But 15 years ago most people and many scientists thought nanotechnology was fantasy. Now many groups and industrial laboratories, even start-ups, spend hundreds of millions of dollars for development of nanotechnological-range products (precise chemistry, patterned atoms, catalysts, metamaterials, etc.) and we have nanotubes (a new material which does not exist in nature) and other achievements are beginning to come out of the pipeline in prospect. Nanotubes are stronger than steel by a hundred times--surely an amazement to a 19th century observer if he could behold them. Nanotechnology, in near term prospect, operates with objects (molecules and atoms) having the size in nanometer ([10.sup.-9] m). The writer here outlines perhaps more distant operations with objects (nuclei) having the size in the femtometer range ([10.sup.-15] m, millions of times smaller than the nanometer scale). The name of this new technology is femtotechnology. DOI: 10.1061/(ASCE)AS.1943-5525.0000042 CE Database subject headings: Explosion; Radiation; Aerospace engineering; Material properties; Nanotechnology. Author keywords: Femtotechnology; Nuclear matter; Artificial AB Matter; Superstrength matter; Superthermal resistance; Invisible matter; Superprotection from nuclear explosion and radiation.

The clear-air turbulence is difficult to detect and predict and yet affects the flight safety the most. A transport aircraft in encountering a severe turbulence frequently develops a sudden plunging motion with the abrupt change in altitude. The aerodynamic forces and moments involved are dynamic and nonlinear and are not well known. The main objective in the present paper is to present the dynamic and nonlinear aerodynamics estimated with the flight data recorder data of a transport aircraft through a fuzzy-logic algorithm. The algorithm is utilized to establish a thrust model and aerodynamic models for all aerodynamic coefficients including the effects of turbulence. The static and dynamic stability derivatives for the subject transport aircraft are demonstrated to be highly nonlinear and unsteady in response to a severe clear-air turbulence in transonic flight. DOI: 10.1061/(ASCE)AS.1943-5525.0000025 CE Database subject headings: Fuzzy sets; Algorithms; Turbulence; Air flow; Aerodynamics. Author keywords: Fuzzy-logic algorithm; Clear-air turbulence; Flight data recorder (FDR); Plunging motion.

The feasibility of using the transfer matrix method (TMM) to analyze open-variable thickness circular cylindrical shells exposed to a high-temperature field is explored theoretically. In the approach to the problem, the thermal degradation (TG) of thermoelastic characteristics of the material is considered. Natural frequencies and mode shapes for the cylindrical shells are investigated in detail by combining the vibration theory with the TMM. The governing equations of vibration for this system are expressed by the matrix differential equations, and the coefficient matrices are derived. After the relationship between the transfer matrix and the coefficient matrix is established, the fourth-order Runge-Kutta method is used numerically to solve the matrix equation. Once the transfer matrix of single component has been obtained, the product of each component matrix can compose the matrix of the entire structure. The frequency equations and mode shape are formulated in terms of the elements of the structural matrices. Finite-element numerical simulation has validated the present formulas of natural frequencies. Numerical illustrations, supplying pertinent information on the implications of the TG, are presented for various curvatures, aspect ratios, boundary conditions, and thickness ratios, and the pertinent conclusions are outlined. DOI: 10.1061/(ASCE)AS.1943-5525.0000035 CE Database subject headings: Shells; Vibration; Thermal factors; Matrix methods. Author keywords: Transfer matrix method (TMM); Shells; Vibration; Thermal degradation.

This paper describes a spacecraft trajectory planning algorithm based on the calculus of variations which can solve 6-degree-of-freedom spacecraft docking and proximity operations problems. The design of a cost functional which trades off fuel use, obstacle clearance distance, and arrival time is discussed. The nonlinear orbital dynamic equations are treated as dynamic constraints. The Euler-Lagrange equations for this functional are derived, as is the Pontryagin criteria for the optimal control input given realistic saturating on-off thrusters. The indirect collocation method is chosen to solve the attendant boundary-value problem for its lack of sensitivity to initial conditions; continuation is used to further improve the algorithm's robustness. The manipulation of the Euler-Lagrange equations and the transversality condition into a form suitable for use with existing collocation codes is discussed. Results are shown for an end-to-end docking maneuver with a tumbling satellite. DOI: 10.1061/(ASCE)AS.1943-5525.0000019 CE Database subject headings: Spacecraft; Orbits; Space exploration; Algorithms. Author keywords: Spacecraft mission planning.

Nonlinear filtering theory, based upon the Fokker-Planck equation and Bayes' rule, has been around since the 1970s. This method does not require linearized dynamics and/or measurement models, and also the assumption of the Gaussian process is not necessary. However, it is extremely difficult to obtain an analytical solution of the Fokker-Planck equation with only a few exceptions, and normally the solution is obtained through numerical approaches. In this paper, this type of nonlinear filtering method is reinvestigated and applied to a relative orbit estimation problem. The time evolution of the state conditional probability density function between measurements is obtained by solving the associated Fokker-Planck equation numerically using the numerical scheme called the alternating direction implicit method. To further mitigate the computational load, an enhanced moving domain technique is employed and the probability distribution function is partitioned. DOI: 10.1061/(ASCE)AS.1943-5525.0000024 CE Database subject headings: Filters; Aerospace engineering; Orbits; Measurement; Distribution functions. Author keywords: Nonlinear filtering; Fokker-Planck equation; Bayes estimation; Aerospace control; Alternative direction implicit method.

Fire risk in high-rise buildings is of special concern to the fire community, since it is crucial but still technically extremely challenging to significantly improve the current fire fighting capabilities when a calamity happens above the 'seven story limit.' In fact, at the moment, there is a lack of effective means for fire fighting in this rare but potentially deadly and costly accidents. To address this problem, in this work we provide a preliminary exploration of a new concept for fire suppression: the use of rotorcraft vehicles for aerial fire fighting in high-rise buildings. If available, such system could effectively support fire fighting efforts conducted with conventional means; moreover, it could provide for one of the very few alternatives to classic systems in certain critical cases such as postearthquake fires. At first, we analyze the operative context with the help of a few reference realistic scenarios which allow for the estimation of some key physical parameters. Next, we evaluate two alternative fire extinguishing technologies, and we identify in the water impulse cannon the solution which seems to be the most effective and compatible with the use on-board a rotorcraft for the scopes considered in this work. The analysis leads us to the identification of a target vehicle as a possible candidate platform for the development of a fire fighting helicopter. Finally, we propose a preliminary design of the fire fighting kit, and we evaluate the handling qualities of the vehicle during operations with reference to the ADS-33 normative. DOI: 10.1061/(ASCE)AS.1943-5525.0000021 CE Database subject headings: Fires; Aircraft; Buildings, high-rise. Author keywords: Fire fighting; Aircraft design; Rotorcraft vehicles.

Shock-wave interaction with an elastic structure is investigated using a coupled numerical analysis approach, which considers solid-fluid interaction within an arbitrary Lagrangian-Eulerian framework. The analysis is performed considering a compressive shock wave, where the shock front is followed by constant pressure. An analysis procedure, which considers the change in the fluid domain due to the deformation of the solid and changes in the overpressure due to the movement of the elastic structure, is developed. Approximate numerical procedures for solving the Riemann problem associated with the shock are implemented within the Godunov finite volume scheme for the fluid domain. The influences of parameters such as structural stiffness and mass of the system on the displacement, velocity, and energy of the elastic structure following the shock-wave incidence are investigated. Immediately after the contact of the shock wave with the solid surface the pressure at the face of the elastic solid rises to a value which is equal to that obtained off of a fixed rigid wall. Subsequently, the motion of the piston produces changes in the applied pressure. The overpressure applied to the elastic system does not have a fixed profile but it depends on its elastic stiffness and structure mass. It is shown that there is a continuous exchange of energy between the air and the moving elastic structure, which produces a damped motion of the solid. The effect of damping is considerable for the cases of low elastic stiffness and low structural mass, where the resulting motion of the solid is nonoscillatory. The conventional analysis procedure, which ignores the energy exchange between the air and the moving solid, predicts an undamped oscillatory response of the structure for all cases considered. It is shown that neglecting the interaction between the air and solid can produce significant error in the total energy of the structure and the dynamic load factor when the resulting motion is nonoscillatory. DOI: 10.1061/(ASCE)AS.1943-5525.0000022 CE Database subject headings: Blasting; Elastic structures; Structural response; Fluid-structure interactions. Author keywords: Shock; Blast; ALE; Riemann; Overpressure; Transient; Dynamic; Structure.

Compressible large-eddy simulations of turbulent flow over a wall-mounted hump with active flow control are performed and compared with previous experiments. The flow is characterized by the unsteady separation before the steep trailing edge, which naturally reattaches downstream of the hump to form an unsteady turbulent separation bubble. The low Mach number large-eddy simulation demonstrated a good prediction of surface pressure coefficient, separation-bubble length, and velocity profiles compared with experiments. The effect of compressibility on the baseline flow is documented and analyzed and is found to increase the separation-bubble size, due to a reduced growth rate. Control is applied just before the natural separation point via steady suction and zero-net-mass-flux oscillatory forcing, and steady suction is shown to be more effective in decreasing the size of the separation bubble and pressure drag for the control parameters investigated. Controlled flow at a compressible subsonic Mach number is applied, and found to be slightly less effective than the same control parameters at low Mach numbers. DOI: 10.2514/1.44756

The replacement of the analysis portion of an optimization problem by its equivalent metamodel usually results in a lower computational cost. In this paper, a conventional nonapproximative approach is compared against three different metamodels: quadratic-interpolation-based response surfaces, Kriging, and artificial neural networks. The results obtained from the solution of four different case studies based on aircraft design problems reinforces the idea that quadratic interpolation is only well-suited to very simple problems. At higher dimensionality, the usage of the more complex Kriging and artificial neural networks models may result in considerable performance benefits. DOI: 10.2514/1.45790

This paper deals with aerodynamic shape optimization using a high-fidelity solver. Because of the computational cost needed to solve the Reynolds-averaged Navier--Stokes equations, the performance of the shape must be improved using very few objective function evaluations, despite the high number of design variables. In our framework, the reference algorithm is a quasi-Newton gradient optimizer. An adjoint method inexpensively computes the sensitivities of the functions, with respect to design variables, to build the gradient of the objective function. As usual, aerodynamic functions show numerous local optima when the shape varies, and a more global optimizer is expected to be beneficial. Consequently, a kriging-based optimizer is set up and described. It uses an original sampling refinement process that adds up to three points per iteration by using a balancing between function minimization and error minimization. To efficiently apply this algorithm to high-dimensional problems, the same sampling process is reused to form a cokriging (gradient-enhanced model) based optimizer. A comparative study is then described on two drag-minimization problems depending on 6 and 45 design variables. This study was conducted using an original set of performance criteria, characterizing the strength and weakness of each optimizer in terms of improvement, cost, exploration, and exploitation. DOI: 10.2514/1.45331

This paper deals with the numerical study of installation effects, a subject which comes under the more general purpose of aircraft noise prediction and reduction. As an example of installation effects, we study here the potential acoustic shield that an empennage airfoil could constitute against the aft fan noise emitted by a coaxial engine. To that end, we make use of a structured computational aeroacoustics solver (sAbrinA code), taking benefits from its coupling functionality in order to handle more easily the solid obstacles that are to be accounted for. Several full-3D simulations are then conducted, which involve an exhaust installed over an empennage airfoil and allotted with two thermodynamics conditions (quiescent medium and takeoff flight). Being validated against boundary element method results, such calculations provide a validation of the numerical strategy adopted here as well as a good illustration of its abilities to solve installed engine noise problems. Coming in addition to such methodological outcomes, these calculations deliver important conceptual insights, demonstrating the potential high efficiency of empennage airfoils in terms of aft fan noise reduction. DOI: 10.2514/1.42153

Formulas for calculating the effect of centerbody scattering on the sound radiated from an advanced open rotor are presented. The effects of blade sweep and distributed blade loading are considered. Mach number effects are also implicitly included in the model. The work extends a previously published method and applies it to a practical situation in which scattering by the centerbody has a significant effect on the radiated sound field. DOI: 10.2514/1.44659

This paper deals with the experimental validation of an analytical trailing-edge noise model dedicated to low-speed fans operating in free field. The model is intrinsically related to the aerodynamics of the blades and should lead to a useful fast-running tool to be included in a blade-design process in an industrial context. The investigations are made on a two-bladed low-speed axial fan without shroud, installed inside an anechoic room. The blades are instrumented with two sets of embedded small-size microphones (2.5 mm diam), and the wall-pressure signals are acquired via a slip ring mounted on the fan axis. The chord-based Reynolds number is about 200,000, and the tip Mach number about 0.07. The data base is completed by far-field measurements made with a single microphone on a moving support. The analytical model is based on a previously published extension of Amiet's trailing-edge noise theory. A blade is split into several strips in the spanwise direction, and the model is applied to each strip. For this the input data are interpolated from the measurements performed with the aforementioned sets of microphones. The trailing-edge noise model is more reliable for observer positions within [+ or -]30[degrees] from the fan-rotation plane. DOI: 10.2514/1.43840

Measurements and predictions are made of a short-cowl coflowing jet with a bypass ratio of 8:1. The Reynolds number is 300,000, and the inlet Mach numbers are representative of aeroengine conditions. The low Reynolds number of the measurements makes the case well suited to the assessment of large-eddy-simulation-related strategies. The nozzle concentricity is carefully controlled to deal with the emerging metastability issues of jets with coflow. Measurements of mean quantities and turbulence statistics are made using both laser Doppler anemometry and particle image velocimetry. The simulations are completed on 6 x [10.sup.6], 12 x [10.sup.6], and 50 x [10.sup.6] cell meshes. To overcome near-wall modeling problems, a hybrid large-eddy-simulation--Reynolds-averaged-Navier--Stokes-related method is used. The near-wall Reynolds-averaged-Navier--Stokes layer is helpful in preventing nonphysical separation from the nozzle wall. DOI: 10.2514/1.44534

When carrying out design searches, traditional variable screening techniques can find it extremely difficult to distinguish between important and unimportant variables. This is particularly true when only a small number of simulations are combined with a parameterization that results in a large number of variables of seemingly equal importance. Here, the authors present a variable reduction technique that employs proper orthogonal decomposition to filter out undesirable or badly performing geometries from an optimization process. Unlike traditional screening techniques, the presented method operates at the geometric level instead of the variable level. The filtering process uses the designs that result from a geometry parameterization instead of the variables that control the parameterization. The method is shown to perform well in the optimization of a two-dimensional airfoil for the minimization of drag-to-lift ratio, producing designs better than those resulting from traditional kriging- based surrogate model optimization and with a significant reduction in surrogate tuning cost. DOI: 10.2514/1.41420

A new approach for the distribution of grid points on the surface and in the volume has been developed. In addition to the point and line sources of previous works, the new approach uses surface and volume sources for automatic curvature-based grid sizing and convenient point distribution in the volume. A new exponential growth function produces smoother and more efficient grids and provides superior control over the distribution of grid points in the field. All types of sources support anisotropic grid stretching, which not only improves the grid economy but also provides more accurate solutions for certain aerodynamic applications. The new approach does not require a threedimensional background grid as in the previous methods. Instead, it makes use of an efficient bounding-box auxiliary medium for storing grid parameters defined by surface sources. The new approach is less memory intensive and more efficient computationally. The grids generated with the new method either eliminate the need for adaptive grid refinement for certain class of problems or provide high-quality initial grids that would enhance the performance of many adaptation methods. DOI: 10.2514/1.41355

In this study, a deterministic compressible solver is coupled to a nonintrusive stochastic spectral projection method to propagate several aerodynamic uncertainties through a transonic steady flow around a NACA0012 airfoil. The stochastic model is solved in a generalized polynomial chaos framework. This approach combines the advantage of not modifying the existing deterministic solver while maintaining accurate representations of the stochastic solution and its statistics. The major difficulty of this work is to deal with deterministic transonic flows for which aerodynamics nonlinearities are reported in the uncertain probabilistic space. The efficiency of the present methodology are evaluated for the propagation of random disturbances associated with the angle of attack and the freestream Math number. An error analysis is carried out in order to determine appropriate physical and stochastic discretization levels. Different stochastic flow regimes are analyzed in details by means of various postprocessing procedures, including error bars, probabilistic density function of the aerodynamic field, and Sobol's coefficients. DOI: 10.2514/1.42637

The basic formulation of the least-squares method, based on the L2 norm of the residuals, is still widely used today for identifying elastic constants of aerospace materials from experimental data. While this method often works well, methods that can benefit from statistical information, such as the Bayesian method, may sometimes be more accurate. We seek situations with significant difference between the material properties identified by the two methods. For a simple three-bar truss example we illustrate three situations in which the Bayesian approach systematically leads to more accurate results: different sensitivity of the measured response to the parameters to be identified, different uncertainty in the measurements, and correlation among response components. When all three effects add up, the Bayesian approach can be much more accurate. Furthermore, the Bayesian approach has the additional advantage of providing the uncertainty in the identified parameters. We also compare the two methods for a more realistic problem of identification of elastic constants from natural frequencies of a composite plate. DOI: 10.2514/1.40922

An investigation of inlet injection in a scram jet having a three-dimensional inlet and an elliptical combustion was undertaken. Experiments were conducted using a test flow simulating a Mach 8.1 flight condition with an altitude of 32 km. The objective was to determine the feasibility of reducing the combustion-chamber length by injection of hydrogen fuel in the inlet. The study involved a self-starting three-dimensional inlet with an overall geometric contraction ratio of 5.80, of which 2.08 was internal. The fuel was injected through multiple portholes in the forward portion of the inlet to allow significant mixing upstream of the combustion chamber. The divergent elliptical combustion chamber had a length sized for fuel ignition and combustion only and used a fluid-dynamic ignition scheme requiring no physical obstructions to the towpath. Results indicated that inlet injection produced robust combustion at good combustion efficiency over a large fueling range up to an equivalence ratio of 0.92. A further key result was that no evidence of combustion was observed in the inlet. These experiments suggest that fuel injection in the inlet is a promising concept for higher-speed scramjet applications. DOI: 10.2514/1.J050052

Two techniques are proposed in a new regularization method for the inverse identification of local damages in a structure. One technique is the introduction of a new side condition, and the other technique restricts the range of variation of the regularization parameter by consistently choosing the optimal point on the L-curve. Both techniques fully make use of the information from results obtained in previous iteration steps. A plane frame structure is studied with two damaged elements and different levels of noise and model errors. Numerical results show that the proposed consistent regularization method is very effective at improving the results of the ill-conditioned inverse-problem phenomenon, compared with the Tikhonov regularization. DOI: 10.2514/1.43322

For many engineering and aerospace applications, detection and quantification of multiscale damage in fiber-reinforced composite structures is increasing in importance. Consequently, the development of an efficient and cost-effective diagnosis scheme that can accurately sense, characterize, and evaluate the existence of any form of damage will offer significant potential for improving the performance, reliability, and extending the operational life of these complex systems. We present an approach to characterize and classify different damage states in composite laminates by measuring the change in the signature of the resultant wave that propagates through the anisotropic media under forced excitation. The wave propagation is measured using surface-mounted piezoelectric transducers. Sensor signals collected from test specimens with various forms of induced damage are analyzed using a pattern-recognition algorithm known as the one-class support vector machines. The one-class support-vector-machine algorithm performs automatic anomaly detection and classification of damage signatures using various features from the sensor readings. The results obtained suggest that the one-class support-vector-machine algorithm, along with appropriate preprocessing techniques, can often achieve better accuracy than the popular k-nearest-neighbor method in detecting and classifying anomalies caused by structural defects, even when the perturbations caused in the sensor signals due to different damage states are minimal. DOI: 10.2514/1.37282

A virtual reality simulation system for a manned maneuvering unit (MMU) and a training system for an astronaut space walk is introduced. The system can simulate an astronaut's outer space walk by means of manipulating the MMU, and it also can be used as a training environment by taking advantage of virtual reality properties of imagination, interaction, and immersion. As in microgravity space, an astronaut's moving limbs cause changes in the MMU posture; these changes result in great offsets to correct direction and have a great effect on the MMU's manipulation performance. A multibody astronaut model is developed by using relative coordinates according to the motion characteristics of an astronaut equipped with MMU. Motion information on the user's limbs are captured by means of a motion capture subsystem and are mapped to an astronaut in virtual environment to simulate the astronaut's limb motions while the astronaut is navigating in outer space. It is an open-chain system for an astronaut multibody model. As an astronaut can control his limbs, the whole system dynamics is a hybrid forward and inverse dynamic problem. Recursive dynamic formulations of a previous method are introduced to solve the forward dynamic problem. For the inverse dynamic part, because the system can get motion characteristics of parts of the relative coordinates, unknown generalized driving forces are applied on these known relative coordinates. All the unknown generalized driving forces and unknown relative coordinates can be combined in a set of hybrid dynamic formulations based on a forward dynamic formulation. The motion characteristics of an astronaut multibody model can be obtained by solving the hybrid formulations. Finally, some simulation results for astronaut forward-flying mode with moving limbs are presented; these results show that the movement of an astronaut's limbs has great effects on astronaut navigation and MMU manipulation performance. The multibody astronaut model has the capability of simulating real space walk with an MMU equipped astronaut. DOI: 10.1061/(ASCE)AS.1943-5525.0000018 CE Database subject headings: Dynamic models; Simulation; Aerospace engineering; Hybrid methods. Author keywords: Manned maneuvering unit; Virtual reality; Multibody; Hybrid dynamic formulation.

Based on momentum conservation principle of space manipulator, this paper presents precapture configuration of space manipulator capturing object. Two unique precapture configurations, 'straight-arm capture' (SAC) and 'generalized straight-arm capture' (GSAC), were proposed based on the single-arm system; then GSAC was extended to the dual-arm system and the corresponding angular relation was obtained. This configuration satisfies GSAC and can reduce the coupling angular momentum during capture operation and the burden of postimpact control. This avoids the limitation of joint velocity when controlling the compound (manipulator and object) and guarantees the stability of system. The simulation results show the effectiveness of this method. DOI: 10.1061/(ASCE)AS.1943-5525.0000016 CE Database subject headings: Spacecraft; Space exploration; Control systems; Velocity. Author keywords: Space manipulator; Generalized straight-arm capture; Angular momentum; Base attitude.

We consider the question of how to reconcile integrated forces and moments from a pressure-sensitive paint measurement with measured forces and moments from a balance. We show that it is possible to compute the smallest change in pressure distribution that would be required to bring the two sets of data into agreement. We refer to this as the gap distribution and show that it can be expressed in terms of a set of basis functions that are determined by the geometry of the test article. The use of these gap basis functions allows discrepancies in forces and moments to be expressed in terms of a common unit of measure, namely the magnitude of the implied gap distribution. We apply this gap analysis to data from a wind-tunnel test of the NASA Orion command module, for which both pressure-sensitive paint data and balance data are available. Results of the analysis confirm earlier suspicions that there was a problem with the normal component of force of the balance. Still, it is shown that application of the gap correction leads to improved pressure-sensitive paint data, as determined by the level of agreement with pressure tap data. The analysis procedure involving a pressure-gap distribution should be applicable to the comparison of balance data to integrated forces and moments from computational fluid dynamics calculations and other techniques. DOI: 10.2514/1.38376

Three-dimensional simulations are employed to examine the effect of inlet distortion and model fidelity with Reynolds averaged Navier--Stokes or large eddy simulation approaches for a generic circular cross-sectional supersonic combustor at a flight condition of Mach 6 and an altitude of about 24.2 km. To examine inlet distortion effects on combustion, frozen and finite rate chemistry simulations are performed on combustors connected to two different types of streamline-traced inlets (denoted 'Scoop' and 'Jaws') with the Wilcox k-[omega] turbulence model. For comparison, uniform inflow boundary condition to the combustor is also simulated. The metrics employed include qualitative assessments related to flow structure as well as quantitative values of fuel combustion efficiency and thrust ratios. The results indicate a complex overall effect of distortion due to inlet design. For Jaws, the increased pressure loss associated with distortion is mitigated slightly by improved combustion efficiency and better thrust performance. The Scoop inlet has lower distortion and better recovery, but the combustion coefficient is lower than Jaws. In the second part of this study, finite rate chemistry results with unsteady Reynolds averaged Navier--Stokes are compared with those from large eddy simulation with an uniform inflow profile. Differences in transient processes include the manner in which large-and small-scale structures originate and evolve in the cavity recirculation and downstream regions. The finer details observed with the large eddy simulation model have significant consequences on the overall field, including the unsteady position of the shock structure arising from the interaction of the incoming flow with the cavity shear layer, combustion processes, and injection jet interactions. DOI: 10.2514/1.43646

Reynolds-averaged and hybrid Reynolds-averaged/large-eddy simulations have been applied to a supersonic coaxial jet flow experiment. The experiment was designed to study compressible mixing flow phenomenon under conditions that are representative of those encountered in scram jet combustors. The experiment used either helium or argon as the inner jet nozzle fluid, and the outer jet nozzle fluid consisted of laboratory air. The inner and outer nozzles were designed and operated to produce nearly pressure-matched Math 1.8 flow conditions at the jet exit. The purpose of the computational effort was to assess the state of the art for each modeling approach and to use the hybrid Reynolds-averaged/large-eddy simulations to gather insight into the deficiencies of the Reynolds-averaged closure models. The Reynolds-averaged simulations displayed a strong sensitivity to choice of turbulent Schmidt number. The initial value chosen for this parameter resulted in an overpredictiou of the mixing layer spreading rate for the helium case, but the opposite trend was observed when argon was used as the injectant. A larger turbulent Schmidt number greatly improved the comparison of the results with measurements for the helium simulations, but variations in the Schmidt number did not improve the argon comparisons. The hybrid Reynolds-averaged/large-eddy simulations also overpredicted the mixing layer spreading rate for the helium ease, while underpredicting the rate of mixing when argon was used as the injectant. The primary reason conjectured for the discrepancy between the hybrid simulation results and the measurements centered around issues related to the transition from a Reynolds- averaged state to one with resolved turbulent content. Improvements to the inflow conditions were suggested as a remedy to this dilemma. Second-order turbulence statistics were also compared with their modeled Reynolds-averaged counterparts to evaluate the effectiveness of common turbulence closure assumptions. DOI: 10.2514/1.43771

Understanding the physics of supersonic combustion is the key to design a performing engine for scramjet-powered vehicles. Despite studies on supersonic combustion dating back to the 1950s, there are still numerous uncertainties and misunderstandings on this topic. The following questions need to be answered: How does compressibility affect mixing, flame anchoring, and combustion efficiency? How long must a combustor be to ensure complete mixing and combustion while avoiding prohibitive performance losses? How can reacting turbulent and compressible flows be modeled? Experimental results in the past have shown that supersonic combustion of hydrogen and air is feasible and takes place in a reasonable distance, which is a necessary requirement in actual hypersonic vehicles powered by supersonic combustion ramjets. These results are explained based on a theoretical analysis of the physical mechanisms driving mixing and combustion in supersonic airstreams, where they are found to be different from those in the incompressible regime. In particular, the classic Kolmogorov scaling is shown to be no longer strictly valid, and the flame regime is predicted to be significantly affected by compressibility and different from that of subsonic flames. This analysis is also supported by the results of the numerical simulations presented, showing that by generating sufficiently intense turbulence, a supersonic combustion flame is short and can indeed anchor within a small distance from fuel injectors, with the flame typically burning in the so-called flamelets-in- eddies regime. DOI: 10.2514/1.43652

The flowfield downstream of a strut-based injection system in a supersonic combustion ramjet is investigated using large-eddy simulation with a new localized dynamic subgrid closure for compressible turbulent mixing. Recirculations are formed at the base of the strut in the nonreacting flow and trap some of the injected fluid. The high levels of turbulence along the underexpanded hydrogen jets and in the shear layer lead to a high level of mixing of fuel and freestream fluids. Furthermore, the shear layer unsteadiness permits efficient large-scale mixing of freestream and injected fluids. In the reacting flowfield, the flame anchoring mechanism is, however, found to depend more on a recirculation region located downstream of the injectors than on their sides. A region of reverse flow is formed that traps hot products and radicals. Intermittent convection of hot fluid toward the injector occurs and preheats the reactants. DOI: 10.2514/1 .43647

A theoretical and experimental aeroelastic study of a typical airfoil section with control surface free play for nonzero angle of attack in low subsonic flow is presented. The study includes the flutter and limit cycle oscillation behavior and also the linear and nonlinear aeroelastic responses excited by periodic gust loads. The theoretical approach uses Peters's finite state airloads model. The experimental investigation has been carried out in the Duke University wind tunnel using a rotating slotted cylinder gust generator. The theoretical and experimental results show that the serf-excited aeroelastic limit cycle oscillation is sensitive to the effect of initial pitch angle. For the gust response, the effect of initial pitch angle is smaller for the plunge and pitch responses and is larger for the flap response. The fair to good quantitative agreement between theory and experiment verifies that the present analytical approach has reasonable accuracy and good computational efficiency for nonlinear aeroelastic response analysis of such systems. DOI: 10.2514/1.44538

We present and discuss results on the development and application of extremizing feedback control to high-speed and high Reynolds number axisymmetric jets. In particular, we demonstrate control authority on the near-field pressure of a Mach 0.9 jet with a Reynolds number based on jet diameter of 6.4 c [10.sup.5]. Open-loop forcing experimental results are presented, wherein localized are filament plasma actuators are shown to have two distinct effects on the irrotational near-field pressure, similar to their effects on the far-field acoustics reported earlier. At low forcing Stroubal numbers near the jet preferred mode, a large amplification in the pressure fluctuations is observed. At much higher forcing Strouhal numbers, an attenuation is observed in the pressure fluctuations over a broad range of excitation frequencies, especially in the axisymmetric mode of the pressure fluctuations. Previous experiments have shown that forcing the jet with these low and high Strouhal numbers results in jet mixing enhancement and farfield noise reduction, respectively. Two different gradient-free extremizing feedback control algorithms have been developed and implemented, each of which can perform online minimum seeking, as well as maximum seeking. Both methods demonstrate fast convergence to the optimum followed by steady operation. We also show that the far-field acoustic spectrum in steady-state operation of the closed-loop control is quite similar to that observed with optimal open-loop forcing. DOI: 10.2514/1.44012

A turbulence model that calculates the turbulent Prandtl and Schmidt numbers as part of the solution is presented. This model also accounts for compressibility effects, and addresses turbulence/chemistry interaction. Its predictions are compared with two experiments: an axisymmetric case involving mixing, hydrogen combustion, and ethylene combustion; and a three-dimensional ethylene mixing experiment. Fair to good agreement is indicated in the cases where data is available. Chemical mechanisms are found to have an influence on autoignition for hydrogen combustion cases, and on ignition location and flame size for hydrogen/ethylene combustion cases using reduced mechanisms. DOI: 10.2514/1.43213