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This paper handles the problem of accuracy loss for flow solutions on computational domains with a heterogeneous discretization on the subdomains. The problem occurs, e.g., when a composite multiblock mesh comprises blocks with different grid resolutions on the common boundary face. The problem is especially troublesome for high-accuracy solutions, in which tailoring of heterogeneous meshes frequently brings the loss of accuracy on a global scale. This paper focuses on the treatment of non-point-to-point structured multiblock grids, in which face-matched block boundaries are not necessarily point-matched. In the context of high-accuracy characteristic finite volume schemes for three-dimensional Navier--Stokes equations, a new approach is proposed that handles composite grids without the need in geometrical adjustment of boundary cells or any other grid-related preprocessing. In the proposed approach, interblock cell clusters are formed that contain boundary cells from the neighboring blocks of different resolutions. For each such cluster, the residuals of the Navier--Stokes equations are computed and then distributed among the cells of the host cluster by directly minimizing the flux imbalance in an appropriate norm. Outside these clusters, the residuals are computed in the regular way. The method allows automatic preservation of the conservative property of fluxes by placing constraints upon the optimum, it does not change the stability properties of the basic high-accuracy scheme and ensures a sufficiently high level of approximation on grids with reduced resolution. The results that include a number of numerical tests for two- and three-dimensional wings indicate good accuracy and robustness of the method and its applicability to full-scale Navier--Stokes computations. DOI: 10.2514/1.J05000

A dynamic hysteresis model developed by modifying the relay operator in the classical Preisach model and adding a dynamic relaxation operator to simulate the change in electrical resistance with strain in conductive polymer nanoeomposites is proposed. Data from the conductivity test on a conductive polymer nanocomposite sample are presented to show the hysteretic variation of electrical resistance with strain. A hysteresis model developed by modifications on the relay operators and addition of a dynamic relaxation operator is described. Effectiveness of the model in simulating the behavior of conductive polymer nanocomposites is explained. A modal function based identification procedure to determine the parameters associated with the model is described. Efficiency of the model is discussed by comparison of the result obtained from the model with the experimental result. DOI: 10.2514/1.J050189

Simulations of plasma flow through a magnetic nozzle were conducted using the time-dependent, three-dimensional magnetohydrodynamies code MACH3. Modeling of magnetic nozzle flow using constant, classical isotropic, and classical tensor resistivity provides preliminary quantitative depiction of the core-plasma flow, evolution of the magnetic field, and conversion of stagnation enthalpy to directed exhaust thrust energy, and serves as verification for the numerical model. For stagnation conditions of 100 eV and 0.355 MPa, steady-state modeling using helium propellant demonstrates a nearly isentropic expansion through the nozzle to exhaust speeds near 160 km/s. The extent of the contribution from the magnetic diffusion and the mass-flux penetration to the thickness of the current layer strongly depends on plasma resistivity. Plasma-field interaction results in a reduction of approximately 50 % of the directed axial thrust when compared with a solid-wall nozzle of equivalent Mach number design. At fixed plasma pressure and applied field the exhaust velocity scales appropriately as the square root of the stagnation temperature. DOI: 10.2514/1.J050123

The effect of the interaction strength on the unsteady behavior of a planar shock wave impinging on a low Reynolds turbulent boundary layer is investigated. This is achieved by means of a variation in incident shock angle under otherwise constant flow conditions. In addition, the effect of an order-of-magnitude variation in the Reynolds number is considered. This has been done for equivalent interaction strength, based on a similar probability of occurrence of instantaneous flow separations. The measurement technique employed is two-component planar particle image velocimetry. Common mechanisms for the large-scale reflected-shock unsteadiness are deduced by means of conditional statistics based on the separation bubble height. The results indicate that both upstream and downstream mechanisms are at work, the dominant mechanism depending on the interaction strength. No significant dependence on the Reynolds number was observed for interactions with a similar probability of instantaneous flow separations. DOI: 10.2514/1.J050093

To assess their application to rotorcraft, the two-dimensional aerodynamics of deployable Gurney flaps, referred to as miniature trailing-edge effectors, are explored using computational fluid dynamics. These deployable devices have a height of only a few percent of the airfoil chord and deploy normal to the airfoil surface near the trailing edge. Their small size, low inertia, and small added mass make them well suited for deployment in high-frequency applications such as those needed for rotorcraft. A combination of wind-tunnel measurements using airfoils fitted with Gurney flaps and unsteady circulatory theory are used to validate the computational fluid dynamics, and grid and time-resolution studies are used for code verification. Qualitative and quantitative agreement of the effects of a Gurney flap with experiments and theory suggest that the computational fluid dynamics results are valid. These investigations examine the effects of the chordwise positioning and deployment frequency of the miniature trailingedge effectors. In doing so, their operation on both static and dynamically pitching airfoils is considered. Through these studies, a number of physical insights into miniature trailing-edge effectors and Gurney flap aerodynamics have been obtained. These insights have led to the introduction of a scaling parameter that easily accounts for compressibility effects, an understanding of the aerodynamic consequences due to positioning miniature trailingedge effectors upstream of the trailing edge, and an assessment on the benefits of using of miniature trailing-edge effectors for active stall alleviation on an airfoil oscillating in pitch. DOI: 10.2514/1.J050070

Based on previous studies with microphones and electronic equipment in a cryogenic environment, a test array for cryogenic application has been developed. Electret microphones, electronic equipment, and fairing material have been successfully tested by the authors under cryogenic conditions and have demonstrated long-term stability. Subsequently, acoustic array measurements performed in a cryogenic wind tunnel for various temperatures in the range of 100 to 300 K, using a single-rod configuration, are described in this paper. The Mach number was in the range 0.1 to 0.3, and the Reynolds number (based on the cylinder diameter of 2.5 mm) was in the range of 5 x [10.sup.3] to 8 x [10.sup.4]. The experiments demonstrated the technical feasibility of performing aeroacoustic array measurements at these low temperatures. A systematic study of Reynolds- and Mach-number effects on the radiated noise of a single-rod configuration has been conducted. The results showed good agreement between theory and measured sound radiation. A Reynolds number dependency of the measured and predicted sound power can be shown. DOI: 10.2514/1.J050083

This paper introduces a nonparametric fitting method for the interpolation of aerodynamic observations over a large range of multiple angles of attack. The method is based on the employment of smoothing thin-plate spline class functions, a well-renewed mathematical tool for multivariate data mining based on the generalization of the univariate natural cubic splines, in which a roughness penalty criterion is used to produce very smooth predictive hypersurfaces. Compared with other methods, such as parametric or even conventional nonparametric methods, the use of a smoothing thin-plate spline is more effective, in that the predictive surface comes directly from the observed points, thus minimizing any intervention of the analyst aimed at introducing model parameters. This forms the basis for a very reliable fitting technique, in which model construction can be relatively easy to implement. An application of the method is carried out on a case study representative of some experimental data coming from a wind-tunnel campaign on a typical three-dimensional fuselage-shaped body, aimed at the acquisition of its aerodynamic coefficients over a rather extensive attitude range. Specifically, the application is focused on the body lift coefficient as a function of both angle of attack and sideslip angle. The data set is also interpolated using concurrent response-surface methods' namely, a linear model, a bivariate spline, a radial basis function network, a support vector regression technique, a regression kriging, and a moving-least-squares approach, alternatively known as local polynomial regression. Results of data fitting are assessed using a cross-validation approach and reveal a clear superiority of smoothing thin-plate spline over the other methods, leading to a more regular fitted surface and a more reliable prediction tool, even when some observations are omitted. This is important per se, but acquires even more significance when an aerodynamic test campaign is to be planned with the minimum number of experimental observations. DOI: 10.2514/1.J050028

The derivation and validation of a broadband combustion noise model is presented. The random partlcle-mesh approach for combustion noise is a hybrid computational fluid dynamics/computational aeroacoustics method and reties on the stochastic reconstruction of combustion noise sources in the time domain. The stochastic reconstruction of unsteady sound sources based on statistical turbulence quantities from a reacting Reynolds-averaged Navier-Stokes simulation is realized with the random particle-mesh method. In the present paper, the modeled combustion noise sources are derived for the use in conjunction with the finearized Euler equations for the computation of the acoustic propagation. Two open, nonpremixed, turbulent jet flames (DLR-A and DLR-B), which differ in their fuel outlet velocity and their respective Reynolds number, are used for the validation of the particle-mesh for combustion noise approach. Results of the reacting flow computations and the subsequent acoustic simulations are compared with measurements. Excellent agreement is found between the computed narrow band sound spectra and the experimental data. DOI: 10.2514/1.45535

This paper presents a generalized Galerkin method based on the dynamic flexibility matrix expansion technique. The main objective is to develop an analysis to predict the modes and frequencies of a real structure when the design verification methods used to conduct on-the-ground tests can only be performed on a pseudomaterial structure. It is critical to develop analytical methods that correctly predict the dynamic behavior of real structure in order to guarantee a successful design of complex structures. Oftentimes, a full-scale spacecraft modal test cannot be performed on the real model due to hazardous payloads it carries. Stable and relatively inexpensive materials are often used to replace these hazardous materials during the test. The basic idea for analyzing these test modes of pseudomaterial structures is based on eigenvalue reanalysis techniques. This method is relatively easy to understand and can be implemented into the existing method. It is numerically accurate and efficient and can be applied to assist in large, complex space structure designs. DOI: 10.2514/1.45282

The paper introduces a new approach for the numerical solution of full-potential unsteady flows based on an independent approximation of the density and velocity potential fields. The solution procedure relies on an unstructured, node-based, finite volume approximation, with linear shape functions and nonreflecting farfield boundary conditions. An improved upwind density biasing allows us to stabilize the solution in supersonic regions. In view of linearized aeroelastic stability and response analyses, unsteady boundary conditions are accounted for by means of a density flow transpiration. Time marching solutions are dealt using first/second-order implicit schemes, whose unconditional linearized stability properties are demonstrated. A few applications are presented to validate the method. DOI: 10.2514/1.J050013

Jet engines are tested in so-called test cells, in which the desired environmental conditions are controlled. The jet plume is exhausted out of the test cell, usually through a diffuser. Under certain testing conditions, high-intensity pressure fluctuations (170 dB) can arise in such facilities. Their frequency is close to that of an acoustic normal mode of the exhaust-diffuser, but the mechanisms driving the resonance are unclear. In the present work the resonance phenomenon is studied using numerical simulations of a model configuration, which shares the key features of actual facilities: an underexpanded supersonic jet, a finite-length solid-wall shroud surrounding the jet, and the receptivity of acoustic disturbances to excite jet instabilities at the nozzle. For this configuration a high-amplitude resonance, qualitatively similar to that of experiments in actual facilities, is observed for a symmetric overexpanded [M.sub.jet] = 1.2 jet. This strongly resonant case is contrasted with a nonresonant ducted [M.sub.jet] = 1.5 jet in the same geometry, as well as with a free jet at [M.sub.jet] = 1.2. The hydrodynamic mechanism of this resonance is studied using linear stability analysis. The presence of excited acoustic modes of the duct is revealed by the Fourier analysis of the data. A numerical experiment shows that slight artificial damping of just the most excited acoustic mode suppresses the resonance. DOI: 10.2514/1.45191

The present paper modifies and extends the recently developed equivalent static load method for the optimization of dynamically loaded linear elastic finite element systems with a huge number of degrees of freedom. In the equivalent static load method, dynamic loads have been transformed into equivalent static loads. This leads to an equivalent static response optimization with multiple loading conditions instead of a dynamic optimization problem. In the present paper, the equivalent static load algorithm is modified and extended by introducing a fatigue analysis in the iterative optimization procedure, where damage is used as suitable termination criterion of the iteration, as well as for the determination of a single and meaningful equivalent static load that leads to maximal damage in the structure. During the evolution process the structure is systematically stiffened by using the solid isotropic microstructure with penalization approach until a user-defined damage level is reached. Three standard examples from literature and an industrial application with a large number of degrees of freedom (600,000) demonstrate the computational efficiency of the proposed method. DOI: 10.2514/1.45196

Fluidic oscillators are actuators that are essentially constituted of a flow vane with no moving parts. They are very effective in generating an oscillating velocity field, and because of their robustness and potential to meet most application requirements they have been thoroughly investigated in previous years. In this work fluidic oscillators have been embedded in an airfoil representative of the outboard sections of wind turbine blades, and subsequently tested at full-scale Reynolds numbers 2.0.[10.sup.6][less than or equal to] Re [less than or equal to] 4.8.106 in the laminar wind tunnel at the University of Stuttgart. The effects of the unsteady actuation on the lift and drag strongly depend upon Re, the level of actuation, and the state of the airfoil surface. However, strong improvements have been obtained throughout the whole testing envelope, with relative lift increase spanning from a minimum of 10 to over 60% and substantial stall margin extension. In addition, employing fluidic oscillators strongly reduces the suction surface boundary-layer thickness and the unsteadiness of the mean flow velocity. DOI: 10.2514/1.42836

This paper discusses the effects of a thin flexible rectangular polymer film (fin) attached to the upper surface of a NACA0012 airfoil model on the natural low-frequency oscillation in poststall flows. The flexible fin passively manipulates flow structures, which leads to suppression of the low-frequency oscillation and reduction of drag. Detailed measurements of the velocity fields and kinematics of fin deformation show the significant effects of a flexible fin on the development of the flow structures in the fully-separated-flow region. The generating and controlling mechanisms of the low-frequency oscillation are explored. DOI: 10.2514/1.J050205

We report our findings from a computational study on wing-tip vortices. The main emphasis of the simulations is to compute the formation of a tip vortex around a blunt wing tip and its interaction with the wing and tip side-edge surfaces. Comparisons of simulation results with available experimental data are done to assess the prediction capability of the simulations. The simulations are performed on a computational grid containing 110 million points total. The blunt tip geometry actually gives rise to the formation of two vortices. The primary vortex forms over the upper surface of the wing, and the secondary vortex forms off of the side edge. These two vortices merge together around the tip trailing edge to form the tip vortex. Simulations at the experimental Reynolds number of 1.8 x [10.sup.6] are performed using several subgrid-scale models. It is shown that the simulation performed without an explicit subgridscale model, also known as implicit large eddy simulation, produces the best agreement with the experimental measurements. Although some differences between the simulation results and experimental data still exist, the overall agreement between the experiment and simulation is found to be satisfactory. DOI: 10.2514/1.J050147

The design of a gas turbine, or one of its constituent modules, is generally approached with some specific operating condition in mind (its design point). Unfortunately, engine components seldom exactly meet their specifications and do not operate at just one condition, but over a range of power settings. This simplification can then lead to a product that exhibits performance worse than nominal in real-world conditions. The integration of some consideration of robustness as an active part of the design process can allow products less sensitive to the presence of the noise factors commonly found in real-world environments to be obtained. To become routinely used as a design tool, minimization of the time required for robustness analysis is paramount. In this study, a nonintrusive polynomial chaos formulation is used to evaluate the variability in the performance of a generic modular-core compression system for a three-spool modern gas turbine engine subject to uncertain operating conditions with a defined probability density function. The standard orthogonal polynomials from the Askey scheme are replaced by a set of orthonormal polynomials calculated relative to the specific probability density function, improving the convergence of the method. DOI: 10.2514/1.J050012

An efficient high-fidelity plate model is developed for heterogeneous multilayer laminates made of functionally graded material. Taking advantage of the smallness of the ratio of the thickness to the characteristic wavelength of the deformation of the reference surface, we apply the variational-asymptotic method to rigorously decouple the three-dimensional, anisotropic elasticity problem into a one-dimensional through-the-thickness analysis and a two-dimensional plate analysis. The through-the-thickness analysis provides constitutive relations for the plate analysis as well as the recovery information for the three-dimensional fields, reducing the complex three-dimensional elasticity model to a simple two-dimensional plate model with an excellent tradeoff between efficiency and accuracy. The present model is valid for large displacements and global rotations and can capture all the geometric nonlinearity of a plate when the strains are small. A few examples are used to validate this model. DOI: 10.2514/1.J050016

The near-surface flow structure and topology on a delta wing of moderate sweep angle are investigated using a technique of high-image-density digital particle image velocimetry. The focus is on the time evolution of the surface topology during relaxation of the flow after termination of a pitching maneuver for a wide range of pitch rates. The near-surface patterns at the instant corresponding to termination of the maneuver are a strong function of the magnitude of pitch rate and show substantial differences. On the other hand, irrespective of the severity of the flow distortion at the end of the pitching maneuver, the relaxation of the flow involves the same sequence of universal topological states. Furthermore, it is demonstrated that there exists a critical universal state that marks an abrupt transformation between two distinctly different states of the near-surface topology. Moreover, it is demonstrated that the changes of the topological features observed during the early stages of the relaxation process are analogous to the alterations of the surface patterns obtained for the stationary wing at smaller angles of attack. DOI: 10.2514/1.J050097

An adjoint-based methodology for design optimization of unsteady turbulent flows on dynamic unstructured grids is described. The implementation relies on an existing unsteady three-dimensional unstructured grid solver capable of dynamic mesh simulations and discrete adjoint capabilities previously developed for steady flows. The discrete equations for the primal and adjoint systems are presented for the backward-difference family of time-integration schemes on both static and dynamic grids. The consistency of sensitivity derivatives is established via comparisons with complex-variable computations. The current work is believed to be the first verified implementation of an adjoint-based optimization methodology for the true time-dependent formulation of the Navier--Stokes equations in a practical computational code. Large-scale shape optimizations are demonstrated for turbulent flows over a tiltrotor geometry and a simulated aeroelastic motion of a fighter jet. DOI: 10.2514/1.J050035

A novel method for solving static boundary-value problems named the integrated local Petrov--Galerkin sine method is introduced. The method uses the process of numerical indefinite integration based on the double-exponential transformation to develop basis functions for a local Petrov--Galerkin type numerical method. Because the developed basis functions do not satisfy the Kronecker delta property, essential boundary conditions are imposed using the traditional penalty method and the Lagrange multiplier method. Three basis functions are introduced, and the accuracy and efficiency of the method is examined for two problems: a one-dimensional tapered bar with vanishing tip area and a two-dimensional plane-stress elasticity problem. The numerical results indicate that the integrated local Petrav--Galerkin sinc method can provide greater accuracy than the sine method based on Interpolation of highest derivative. For the two example problems studied, the method's high rate of convergence can provide greater accuracy of stresses for the same computational cost as a displacement-based C0-continuous and a mixed finite element. However, the still in development integrated local Petrov--Galerkin sinc method suffers from requiring a more fully populated stiffness matrix and relatively high computational cost of the matrix factorization. DOI: 10.2514/1.45892

This paper presents stability analyses for the boundary-layer flow over broad cones (half-angle [psi] > 40[degrees]) rotating in imposed axial flows. Preliminary convective instability analyses are presented that are based on the Orr--Sommerfeld equation for a variety of axial-flow speeds. The results are discussed in terms of the limited existing experimental data and previous stability analyses on related bodies. The results of an absolute-instability analysis are also presented which are intended to further those by Garrett and Peake [Garrett, S. J., and Peake, N., 'The Absolute Instability of the Boundary Layer on a Rotating Cone,' European Journal of Mechanics, B/Fluids, Vol. 263, 2007, pp. 344-353.] through the use of a more rigorous steady-flow formulation. Axial flow is seen to delay the onset of both convective and absolute instabilities. DOI: 10.2514/1.J050021

Based on the availability of measured natural frequencies of structures, the interval analysis technique was proposed for structural damage identification. Influences of uncertainties in the measurements and modeling errors on the identification were investigated. Because of the lack of information on measurement uncertainties, the interval description was adopted for the measured natural frequencies in this paper. Via the first-order Taylor series expansion, the interval bounds of the elemental stiffness parameters of both undamaged and damaged structures were derived by using the model updating based on the initial analytical finite element model. The damage can be identified by comparing the differences between the two models, where the quantitative measure of the possibility of damage existence in the elements is introduced. A larger value of possibility of damage existence implied a higher possibility of damage occurrence. The damage identifications for a steel cantilever beam and a steel cantilever plate were performed by the presented method, which is validated by Monte Carlo simulation. Moreover, the case of the multidamage identification, the number of the used natural frequencies, and the effects of damage level and uncertainty level on the damage detection were studied as well. The numerical results proved the validity and applicability of the presented interval analysis method. DOI: 10.2514/1.45325

In aerospace engineering, various problems such as restoring impaired experimental flow data can be handled by gappy proper orthogonal decomposition. Similar to gappy proper orthogonal decomposition, probabilistic principal component analysis can approximate missing data with the help of an expectation-maximization algorithm, yielding an expectation-maximization algorithm for probabilistic principal component analysis (expectation-maximization principal component analysis). Although both gappy proper orthogonal decomposition and expectation-maximization principal component analysis address the same missing-data-estimation problem, their antithetical formulation perspectives hinder their direct comparison; the development of the former is deterministic, whereas that of the latter is probabilistic. To effectively differentiate both methods, this research provides a unifying least-squares perspective to qualitatively dissect them within a unified least-squares framework. By virtue of the unifying least-squares perspective, gappy proper orthogonal decomposition and the expectation-maximization principal component analysis turn out to be similar in that they are twofold: basis and least-squares coefficient evaluations. On the other hand, they are dissimilar because the expectation-maximization principal component analysis, unlike gappy proper orthogonal decomposition, dispenses with either a gappy norm or a proper orthogonal decomposition basis. To illustrate the theoretical analysis of both methods, numerical experiments using simple and complex data sets quantitatively examine their performance in terms of convergence rates and computational cost. Finally, comprehensive comparisons, including theoretical and numerical aspects, establish that the expectation-maximization principal component analysis is simpler and thereby more efficient than gappy proper orthogonal decomposition. DOI: 10.2514/1.45750

This paper presents two experimental investigations of uncertainty in natural frequencies of linear structures estimated from measured frequency response function under dynamic loading. Experiments were conducted on 100 nominally identical realizations of two structures: a fixed--fixed beam and a thin plate with one edge fixed and the remaining three edges free. In the first set of experiments, on a fixed-fixed beam, 12 identical masses were placed at random spatial locations (generated by a computer) along the length of the beam. Each random arrangement of the masses constitutes one realization, and 100 such realizations were individually subjected to impulse loading to obtain frequency response functions. The total random mass is about 2% of the total mass of the beam. In the second set of experiments, 10 spring-mass oscillator units were attached to the cantilevered plate at random spatial locations determined from a computer-generated random matrix. Although the beam experiments represent parametric uncertainty in the mass matrix, the plate experiment pertain to unmodeled dynamics, which in turn results in randomness in both the mass and stiffness matrices. The results obtained from these experiments may be useful for the validation of many random eigenvalue analysis and prediction methods currently available to structural dynamicist. This paper is limited to comparisons with Monte Carlo simulation of deterministic finite elements models of the two structures. It is concluded that the method of estimation of natural frequencies from frequency response functions and the spatial location of the measurements has significant influence upon the first two moments (mean and standard deviation) of the natural frequency ensemble. Furthermore, although the Monte Carlo simulation estimates of the mean and standard deviation are in reasonable agreement with experiments at higher frequencies, the probability density function differ appreciably, within the limits of the sample size investigated in this study. DOI: 10.2514/1.44668

A novel, Cartesian-grid-based flow solver is developed for predicting complex high-Reynolds-number turbulent flowfields. The Cartesian grid generator is based on the cut-cell approach using cell merge and Cartesian layer techniques. Cartesian layers imitate the structured grid approach in which the mesh is stretched gradually. Local refinement is added based on local surface curvature. As a turbulence closure model, the two equation k-[omega]-TNT turbulence model is successfully implemented using an unconditionally positive-convergent implicit time integration scheme. The overall flow solver's robustness and accuracy are verified using three challenging test cases. The numerical results convincingly demonstrate the robustness and accuracy of the flow solver, especially in predicting aerodynamic forces. DOI: 10.2514/1.45817

The Taylor--Culick flow corresponds to the flow injection through the lateral wall of a semi-infinite channel. This flow is intrinsically unstable downstream of a critical axial position. The flow disturbances develop in vortices that could couple with the acoustics of the channel, generating pressure oscillations. This is what occurs in large solid rocket motors in which the Taylor flow is induced by the propellant combustion. Depending mainly on the Reynolds number, three different flow regimes can be observed: laminar, transitional, or turbulent. The current paper presents an experimental and numerical investigation of that flow instability. The setup consists of a rectangular shape channel in which air is introduced through the bottom wall due to a porous plate. Quantitative measurements are performed with particle image velocimetry while unsteady numerical simulations are carried out using large eddy simulation. A special injection system for the seeding particles and a special arrangement for the laser sheet provide good quality particle image velocimetry data. The numerical simulations are validated against the particle image velocimetry results. They complete the particle image velocimetry investigation by following chronologically the instantaneous phenomenon. The experiments and the numerical simulations are performed at different injection velocities and channel heights. DOI: 10.2514/1.44616

Computational aeroelasticity has become an active area of research in the past decade. Effort has been put into coupling between computational fluid dynamic and finite element solvers and into model reduction to make the resulting simulations more useful for practical analysis. This paper is the latest in a series that describe research toward making eigenvalue-based stability analysis routine for large-scale computational-fluid-dynamic-based semidiscrete systems. The particular contribution of this paper is to formulate the problem in a framework that exploits the Schur complement. This effectively allows the different parts of the system Jacobian to be treated in a decoupled way, with the final result being a small nonlinear eigenvalue problem for the stability analysis. The calculation of this small system can be done robustly in parallel. Results to illustrate the performance of the method are presented for model wings and full aircraft test cases. DOI: 10.2514/1.40961

A three-dimensional unsteady reacting flowfield that is generated by transverse hydrogen injection into a supersonic mainstream is numerically investigated using detached-eddy simulation and a finite-rate chemistry model. Grid refinement with the grid-convergence-index concept is applied to the instantaneous flowfield for assessing the grid resolution and solution convergence. Validation is performed for the jet penetration height, and the predicted result is in good agreement with experimental trends. The results indicate that jet vortical structures are generated as the interacting counter-rotating vortices become alternately detached in the upstream recirculation region. Although the numerical OH distribution reproduces the experimental OH--planar-laser-induced fluorescence well, there are some disparities in the ignition delay times due to the restricted availability of experimental and numerical data. The effects of the turbulence model on combustion are identified by a comparative analysis of the Reynolds-averaged Navier--Stokes and detached-eddy simulation approaches. Their effects are quantified by the production of [H.sub.2]O, which is the primary species of hydrogen combustion. DOI: 10.2514/1.41165

The Schrodinger equation of a charged particle in a uniform electric field can be specified in either a time-independent or a time-dependent gauge. The wave functions in these two gauges are related by a phase factor reflecting the gauge symmetry of the problem. We show that the effect of such a gauge transformation connecting the two wave functions can be mimicked by the effect of two successive extended Galilean transformations. An extended Galilean transformation connects two reference frames such that one is accelerating with respect to the other. [c] 2010 American Association of Physics Teachers. [DOI: 10.1119/1.3290866]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2009.03.005 Byline: Chen-Chi Chu (a), Chieh-Li Chen (b) Keywords: Photovoltaic cells; Sliding mode control; Robustness; Maximum power point tracking Abstract: Due to nonlinear I-V characteristics of photovoltaic cells, an maximum power point tracking algorithm is adopted to maximize the output power. In this paper, an approach for peak power tracking using the sliding mode control is proposed. The proposed controller is robust to environment changes and load variations. The stability and robustness of the controller are addressed. The performance of the controller is verified through simulations and experiments. It demonstrated that the proposed approach can be implemented effectively and economically. Author Affiliation: (a) Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, USA (b) Department of Aeronautics and Astronautics, National Cheng Kung University, 1 University Rd., Tainan, Taiwan Article History: Received 30 April 2007; Revised 26 January 2009; Accepted 2 March 2009 Article Note: (miscellaneous) Communicated by: Associate Editor Elias Stefanakos

This paper introduces a new technique for robust aeroelastic analysis that extends standard linear flutter analysis to take deterministic uncertainty and variation into account. The basic principle of the proposed [mu]-p method is to exploit structured-singular-value (or [mu]) analysis to investigate if the system uncertainties can make the flutter determinant zero for a given flutter eigenvalue p. This makes it possible to compute regions of feasible eigenvalues in the complex plane as well as extreme eigenvalues that can he used to predict damping bounds and perform robust flutter analysis. The capability to predict damping bounds at subcritical flight conditions is a very attractive feature of the new method, as flight testing is rarely taken to the flutter point. The [mu]-p formulation also opens up new possibilities to bound the magnitude of the system uncertainties based on frequency and/or damping estimates from flight testing. In the final part of the paper, the [mu]-p framework is successfully applied to perform robust aeroelastic analysis of a low-speed wind-tunnel model. DOI: 10.2514/1.35859

The primary goal of this research is to develop a framework for dealing with multi-objective, multidisciplinary optimization problems with a large number of variables. The proposed method is expected to provide a relatively uniformly spaced, widely distributed Pareto front. To achieve this end, a novel integration of the adaptive weighted sum method within a concurrent subspace optimization framework is presented. In the bilevel framework of concurrent subspace optimization, the adaptive weighted sum is used to make tradeoffs among multiple, conflicting objectives. To obtain better distributed solutions, two modifications are made. First, an additional equality constraint in suboptimization for each expected solution is relaxed because it causes slow convergence within the bilevel optimization framework. The probability of entrapment in local minima can also be reduced. Second, the mesh of the Pareto front patches is modified due to the low efficiency of the original scheme. The proposed method is demonstrated with three multidisciplinary design optimization problems: 1) a numerical multidisciplinary design optimization test problem with a convex Pareto front, available within the NASA multidisciplinary design optimization Test Suite; 2) a test problem with a nonconvex Pareto front, which is not easily solved; and 3) a conceptual design of a subsonic passenger aircraft, which consists of two objectives, four design variables, five coupling behavior variables, seven constraints in aerodynamics, and weight discipline. The primary results show that the proposed method is promising with regard to obtaining a uniformly spaced, widely distributed, and smooth Pareto front and is applicable in the design of large-scale, complex engineering systems such as aircraft.

First-principle-based models of the dynamics of flow systems are often of limited use for model-based control design, not only because of their nonlinear and infinite-dimensional nature, but also because the control input is generally specified as a boundary condition. Proper orthogonal decomposition and Galerkin projection are among the most effective and commonly used methods to obtain reduced-order models of flow dynamics. However, the final form of these models may not account for the presence of a forcing or control input. From a control design perspective, it is desirable to obtain a reduced-order model in which the control input appears explicitly in the dynamic equations. In this paper, two methods for control input separation are introduced and comparatively evaluated in experimentally based reduced-order modeling of cavity flow, both in their ability to reconstruct the forced flowfield and to provide models suitable for feedback control design. The proposed methods, namely, 1) actuated proper orthogonal decomposition expansion and 2) [L.sub.2] optimization, extend the baseline flow model through the use of innovation vectors, which capture the deviation of the actuated flow from the baseline space. The new methods address some of the issues associated with the subdomain separation technique employed in our previous works. Linear-quadratic regulator controllers, built using models obtained from the new methods, have been tested on a cavity flow experiment. Although the new models perform satisfactorily and comparably to our previous models in terms of suppression of cavity tones, they offer a substantial advantage in terms of the required input power to achieve a similar or better performance.

In this work, both fully implicit time-domain and nonlinear frequency-domain methods are used to compute aeroelastic solutions in transonic flows. Specifically, flutter boundaries for a two-dimensional NACA64A010 airfoil are calculated and compared with preexisting numerical results. The second-order backward-difference time-accurate scheme will serve as our numerical benchmark when determining the computational efficiency of the nonlinear frequency-domain method. Comparable results between the time-domain and nonlinear frequency-domain approaches will further justify the nonlinear frequency-domain method as an efficient alternative to the study of unsteady periodic problems. A temporal resolution study will establish the required number of modes or time steps per period required for unsteady transonic flows.

A shape optimization methodology for reducing the initial shock pressure rise on the ground of a supersonic aircraft is presented. This methodology combines elements from the linearized aerodynamic theory, such as the Whitham F function, with elements from the nonlinear aerodynamic theory, such as the prediction of lift distribution by an Euler or a Navier-Stokes flow solver. It is applied to the optimization of two different airplane concepts developed by Reno Aeronautical and Lockheed Martin, respectively, for the Defense Advanced Research Projects Agency's Quiet Supersonic Platform program. For Reno Aeronautical's laminar-flow supersonic aircraft, the initial shock pressure rise on the ground is reduced by a factor close to 2, from 1.224 psf (58.605 N/[m.sup.2]) at a freestream Mach number of 1.5 to 0.671 psf (32.127 N/[m.sup.2]), while maintaining constant lift. For Lockheed Martin's point of departure aircraft, a tenfold reduction of the initial shock pressure rise on the ground is demonstrated, from 1.623 psf (77.71 N/[m.sup.2]) at a freestream Mach number of 1.5 to 0.152 psf (7.278 N/[m.sup.2]), also while maintaining constant lift. DOI: 10.2514/1.27607

Localized arc filament plasma actuators were used to control an axisymmetric Mach 0.9 jet with a Reynolds number based on the nozzle exit diameter of 7.6 x [10.sup.5]. Eight actuators, distributed azimuthally inside the nozzle, near the nozzle exit, were used to excite various instabilities and azimuthal modes of the jet over a large Strouhal number range ([St.sub.DF] of 0.1 to 5.0). Time-resolved pressure measurements were used to investigate the development of actuation perturbations in the jet, particle image velocimetry measurements were used to evaluate the control effects on the turbulence field, and far-field sound was measured to evaluate the control effects on the radiated acoustic field. The jet responded to the forcing over a large range of excitation Strouhal numbers with varying degrees. As expected, the low Strouhal number seeded disturbances grew slowly, saturated farther downstream, and stayed saturated for a longer time before decaying gradually. The saturation and decay of the seeded perturbations moved farther upstream as their Strouhal number was increased. Seeded perturbations with higher azimuthal modes exhibited faster decay. Particle image velocimetry results showed that when exciting the jet's preferred-mode instability at lower azimuthal modes, the jet potential core was shortened and the turbulent kinetic energy was increased significantly. At higher Strouhal numbers and higher azimuthal modes, forcing had less of an impact on the mean velocity and turbulent kinetic energy. Far-field acoustic results showed a significant noise increase (2 to 4 dB) when the jet is excited around the jet's preferred-mode instability Strouhal number ([St.sub.DF] = 0.2-0.5), in agreement with the results in the literature. Noise reduction of 0.5 to over 1.0 dB is observed over a large excitation Strouhal number range; this reduction seems to peak around [St.sub.DF] = 1.5 to 2.0 at a 30-deg angle, but around [St.sub.DF] = 3.0 to 3.5 at a 90deg angle. Although forcing the jet with higher azimuthal modes is advantageous for noise mitigation at a 30-deg angle and lower Strouhal numbers, the effect is not as clear at higher forcing Strouhal numbers and at a 90-deg angle. DOI: 10.2514/1.27499

A method of actively stabilizing a hybrid vibration absorber driven by a force proportional to the velocity detected at the attachment position of the absorber is investigated. This method can be applied to improve model-independence of robust performance while allowing for multimode controllability for the case where a hybrid vibration absorber is desired as an actuator. Robustness not only against variations in a primary structure to be controlled but also against parameter identification errors in a vibration absorber system is discussed. It is illustrated that although this method transforms the hybrid absorber equation with a spring term into the full active absorber equation, the important advantage of a hybrid absorber over a full active absorber, that is, the power-saving effect near the first resonant frequency of a primary structure can be retained. Application examples are presented also for the vibration isolation and active noise control problems. DOI: 10.2514/1.17273

A comprehensive strategy for developing and implementing discrete adjoint methods for aerodynamic shape optimization problems is presented. By linearizing each procedure in the entire optimization problem, transposing each linearization, and reversing the sequential order of operations, the adjoint of the complete optimization problem, including flow equations and mesh motion equations is constructed in a modular and verifiable fashion. This construction is also shown to produce minimal memory overheads and retain the same convergence characteristics of the original analysis problem in the sensitivity analysis. These techniques are implemented in a three-dimensional unstructured multigrid Navier--Stokes solver and demonstrated on a transonic drag reduction problem for a wing body configuration. DOI: 10.2514/1.22743

Among the time-integration methods used in solid mechanics, Newmark family of algorithms (along with Galerkin finite element) has been most popular and has gathered much attention over the years. Recently, the time discontinuous Galerkin method (TDG) and discontinuous Galerkin method (DGM) are being investigated by the researchers due to their advantages in wave propagation problems. In this study, TDG and DGM are combined to have space-time discontinuous Galerkin formulation for the first time for solid mechanics problems. The basic formulations are presented and some examples are studied. DOI: 10.1061/(ASCE)0893-1321(2007)20:2(128) CE Database subject headings: Discontinuities; Elasticity; Dynamics; Numerical models; Mechanics.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.compeleceng.2006.02.002 Byline: S.S. Li (a), R.S. Chen (b), X.F. Sun (b), M.W. Jie (b), X.D. Ye (b) Keywords: Block adaptive filtering; Data reusing; Toeplitz preconditioned Abstract: We present a new block adaptive algorithm as a variant of the Toeplitz-preconditioned block conjugate gradient (TBCG) algorithm. The proposed algorithm is formulated by combining TBCG algorithm with a data-reusing scheme that is realized by processing blocks of data in an overlapping manner, as in the optimum block adaptive shifting (OBAS) algorithm. Simulation results show that the proposed algorithm is superior to the block conjugate gradient shifting (BCGS), TBCG and Toeplitz-OBAS (TOBAS) algorithms in both convergence rate and tracking property of input signal conditioning. Author Affiliation: (a) Department of Electronic Engineering, Naval Aeronautical Engineering Institute, China (b) Department of Communication Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China Article History: Received 8 January 2004; Revised 11 January 2006; Accepted 1 February 2006

Theory, methodology, and example applications are developed and shown for the systematic analysis of overall vehicle forces in terms of irreversibility and heat. The methodology presented involves analyzing and deconstructing vehicle forces using individual stream tubes as components within the overall fluid control volume in which the vehicle is embedded. This provides the capability for the complete fluid/thermodynamic 'audit' of vehicle performance in terms of irreversibility, combustion (heating), and fluid dynamic flow turning and area change. Sample results are shown for a simplified hypersonic vehicle configuration modeled with constant specific heats and Rayleigh heating. The role of overall entropy generation and wake mixing processes in the production of vehicle forces in atmospheric flight is next discussed and clarified. Specifically, the direct analytical relationship between entropy, wake mixing processes, and overall force production for the vehicle is developed from fundamental considerations of the global control volume with inclusion of the wake in the analysis. This analysis is demonstrated using the same simplified high-speed configuration and is finally developed for the completely general problem of an aerospace vehicle with variable specific heats, thermal loading, variable composition, fuel injection, and chemical reaction.

The double pendulum, a simple system that exhibits complex dynamics, is used to demonstrate a method based on Hamilton's weak principle (HWP) for assembling and solving the maximum coordinate set equations for a multibody system. Results calculated using HWP are compared to a reference solution obtained by integrating the ordinary differential equations (ODE) in minimum coordinate set form. The HWP solution is shown to be comparable to the ODE solution in accuracy and computational efficiency. These results suggest an alternative architecture for multibody system analysis, which implements the maximum coordinate set form of the system equations, and a simple, efficient, and robust solution algorithm.

A numerical analysis was carried out to study the interaction of the reflected shock with incident boundary layer and contact surface in reflected shock tunnels and the driver gas contamination of the hot flow, in tailored conditions. Navier-Stokes equations were solved with a multiblock finite volume method in a shock tube and a nozzle. Different suction devices, such as a corner slit, a corner slit prolongated by a sleeve, and a conical ring, were tested for their capability of delaying this contamination effect. The results show that the driver gas passes through a bifurcated shock foot as a jet and causes premature contamination of the driven gas. They also show why the suction induced by a corner slit is inefficient for delaying the contamination. Located just behind the bifurcated shock foot after the shock/interface interaction, the sleeve device captures driver gas upstream and leads to an improvement in the driver gas arrival time by a factor of 1.8. Finally, based on the present numerical investigation, a new conical ring device is proposed, with a horizontal and vertical suction, which further increases this factor to 2.5.

Response surface methods have been used for a variety of applications in aerospace engineering, particularly in multidisciplinary design optimization. We investigate the use of kriging models as alternatives to traditional second-order polynomial response surfaces for constructing global approximations for use in a real aerospace engineering application, namely, the design of an aerospike nozzle. Our objective is to examine the difficulties in building and using kriging models to create accurate global approximations to facilitate multidisciplinary design optimization. Error analysis of the response surface and kriging models is performed, along with a graphical comparison of the approximations. Four optimization problems are also formulated and solved using both sets of approximation models to gain insight into their use for multidisciplinary design optimization. We find that the kriging models, which use only a constant 'global' model and a Gaussian correlation function, yield global approximations that are slightly more accurate than the response surface models.

The article describes the effect of axial load on conical shell vibration. Free vibration characteristics and shell sensitivity to its semivertex angle are analyzed.