Showing papers in "AIAA Journal in 2003"

Evolutionary Optimization of Computationally Expensive Problems via Surrogate Modeling

Abstract: We present a parallel evolutionary optimization algorithm that leverages surrogate models for solving computationally expensive design problems with general constraints, on a limited computational budget. The essential backbone of our framework is an evolutionary algorithm coupled with a feasible sequential quadratic programming solver in the spirit of Lamarckian learning. We employ a trust-region approach for interleaving use of exact modelsfortheobjectiveandconstraintfunctionswithcomputationallycheapsurrogatemodelsduringlocalsearch. In contrastto earlier work, we construct local surrogatemodels using radial basis functionsmotivated by theprinciple of transductive inference. Further, the present approach retains the intrinsic parallelism of evolutionary algorithms and can hence be readily implemented on grid computing infrastructures. Experimental results are presented for some benchmark test functions and an aerodynamic wing design problem to demonstrate that our algorithm converges to good designs on a limited computational budget.

Recent Developments in the Research on Pulse Detonation Engines

Abstract: Introduction I Nprinciple,detonationsare an extremelyefŽ cientmeans of combustinga fuel-oxidizermixture and releasing its chemical energy content. During the past 60 years or so, there have been numerous researcheffortsat harnessingthepotentialof detonationsfor propulsion applications.1 There is a renewed interest lately on intermittent or pulsed detonations engines. Eidelman et al. and Eidelman and Grossmann3 have reviewed some of the initial research as well as work done in the late 1980s on pulse detonation engines (PDEs). The basic theory, design concepts, and the work in the early 1990s related to pulse detonationengines have been discussedby Bussing and Pappas.4 The focus of a more recent review5 is on performance estimates fromvarious experimental, theoreticaland computational studies. More recently, work related to nozzles for PDEs has been discussed. Other reviews7i9 discussing the objectives and accomplishments of various programs are also available.The objective of this paper is to update the previousreviews, focusingon themore recent developmentsin the researchon PDEs. The review is restricted toworkopenlyavailablein the literaturebut includesongoingefforts around the world. Currently, there are several programs sponsored by OfŽ ce of Naval Research (ONR), U.S. Air Force, NASA, Defense Advanced Research Projects Agency, and other agencies in the United States as well as several parallel efforts in Belarus, Canada, France, Japan, Russia, Sweden, and other countries.The results from some of these programs are just beginning to be published.A summary of recent progress and the various organizationsand people involved in PDE research in Japan has been presented.9 Reports of the basic PDE research sponsoredby ONR are available in the proceedingsof a recurringannualmeeting(forexample, seeRef. 10).Recentwork conducted outside the United States has been reported at international meetings on detonations such as those held in Seattle11 (for more information, see http://www.engr.washington.edu/epp/icders/) and Moscow.12 Although an attempt is made to cover a broad range of the reported research, the shear volume of papers presented with PDEs in the title make it impractical to be exhaustive. Rather than providing a chronologicalreport, an attempt is made here to discuss the recent progress in terms of broad topic areas. The key issues that need to be resolved have been addressed in a number of papers (e.g., Refs. 13 and 14). The speciŽ c order in which to discuss the various topics was determined by considering the schematic of an idealized, laboratory pulse detonation engine shown in Fig. 1. This idealizedengine is representativeof the device

Particle swarm optimization

Abstract: Gerhard Venter (gventer_vrand.conl) *Vanderpla(ds Research and Development, bit.1767 S 8th St'reef. Suite 100, Colorado Springs. CO 80906Jaroslaw Sobieszczanski-Sobieski (j.sobieski:_larc.nasa.gov) *A_4SA Lcmgley Research Ce,_terMS 240, Hampton, I:4 23681-2199The purpose of this paper is to show how the search algorithm, known as par-ticle swarm optimization performs. Here, particle swarm optimization ks appliedto structural design problems, but the method.has a much wider range of possi-ble applications. The paper's new contributions are improvements to the particleswarm optimization algorithm and conclusions and recommendations as to theutility of the algorithm. Results of numerical experiments for both continuousand discrete applications are presented in the paper. The results indicate that theparticle swarm optimization algorithm does locate the constrained minimum de-sign in continuous applications with very good precision, albeit at a much highercomputational cost than that of a typical gradient based optimizer. However, thetrue potential of particle swarm optimization is primarily in applications withdiscrete and/or discontinuous functions and variables. Additionally, particleswarm optimization has the potential of e3_icient computation with very largenumbers of concurrently operating processors.

Comparison of Heat Transfer Augmentation Techniques

Abstract: Dr. Phil Ligrani is currently Professor of Mechanical Engineering and Director of the Convective Heat Transfer Laboratory at the University of Utah and a Fellow of the American Society of Mechanical Engineers. He has beenworking on convection heat transfer and fluid mechanics research problems since he received his Ph.D. degree from the Department of Mechanical Engineering at Stanford University in 1980. From 1979 to 1982, he was an Assistant Professor in the Turbomachinery Department of the von Karman Institute for Fluid Dynamics, Rhode-Saint-Genese, Belgium. From 1982 to 1984, he worked in the Department of Aeronautics of the Imperial College of Science and Technology, University of London. From 1984 to 1992, he was an Associate Professor in the Department of Mechanical Engineering of the U.S. Naval Postgraduate School. In his research, he has investigated the ultra-small-scale motions that exist near walls in turbulent boundary layers, the effects of surface roughness on turbulent boundary layers, transitional phenomena in curved channels including the development and structure of Dean vortex pairs, and innovative schemes for internal cooling and surface heat transfer augmentation, such as dimpled surfaces and swirl chambers, as well as a variety of gas turbine heat transfer and blade cooling problems. He served as Guest Editor for the journal Measurement Science and Technology from 1998 to 2000, and he will serve as Associate Technical Editor for the Journal of Heat Transfer from 2003 to 2006. He has published approximately 150 journal papers, conference papers, and book chapters. In 1995, he was presented with the "Professor of the Year" award at the University of Utah for outstanding classroom teaching. Some of his other activities and recognitions include a Guest Professorship in 2000 at the Institut fur Thermische Stroemungs-maschinen-Universitaet Karlsruhe, a Visiting Senior Research Fellowship from 1982 to 1983 at the Imperial College of Science and Technology-University of London, a NASA Space Act Tech Brief Award in 1991 for "Development of Subminiature Multi-Sensor Hot-Wire Probes," and the Carl E. and Jessie W. Menneken Faculty Award in 1990 for Excellence in Scientific Research. E-mail: ligrani@mech.utah.edu.

Algorithm Developments for Discrete Adjoint Methods

Abstract: This paper presents a number of algorithm developments for adjoint methods using the 'discrete' approach in which the discretisation of the non-linear equations is linearised and the resulting matrix is then transposed. With a new iterative procedure for solving the adjoint equations, exact numerical equivalence is maintained between the linear and adjoint discretisations. The incorporation of strong boundary conditions within the discrete approach is discussed, as well as a new application of adjoint methods to linear unsteady flow in turbomachinery.

Lumped Element Modeling of Piezoelectric-Driven Synthetic Jet Actuators

Abstract: : This paper presents a lumped element model of a piezoelectric-driven synthetic jet actuator. A synthetic jet, also known as a zero net mass-flux device, uses a vibrating diaphragm to generate an oscillatory flow through a small orifice or slot. In lumped element modeling (LEM), the individual components of a synthetic jet are modeled as elements of an equivalent electrical circuit using conjugate power variables. The frequency response function of the circuit is derived to obtain an expression for Q(sub out)/V(sub AC), the volume flow rate per applied voltage. The circuit is analyzed to provide physical insight into the dependence of the device behavior on geometry and material properties. Methods to estimate the model parameters are discussed, and experimental verification is presented. In addition, the model is used to estimate the performance of two prototypical synthetic jets, and the results are compared with experiment.

Large-Eddy Simulation of Supersonic Cavity Flowfields Including Flow Control

Abstract: Large-eddy simulations of supersonic cavity flowfields are performed using a high-order numerical method. Spatial derivatives are represented by a fourth-order compact approximation that is used in conjunction with a sixth-order nondispersive filter. The scheme employs a time-implicit approximately factored finite difference algorithm, and applies Newton-like subiterations to achieve second-order temporal and fourth-order spatial accuracy. The Smagorinsky dynamic subgrid-scale model is incorporated in the simulations to account for the spatially underresolved stresses. Computations at a freestream Mach number of 1.19 are carried out for a rectangular cavity having a length-to-depth ratio of 5:1. The computational domain is described by 2.06×10 7 grid points and has been partitioned into 254 zones, which were distributed on individual processors of a massively parallel computing platform. Active flow control is applied through pulsed mass injection at a very high frequency, thereby suppressing resonant acoustic oscillatory modes

Flexible Flapping Airfoil Propulsion at Zero Freestream Velocity

Abstract: Thrust generation for an airfoil plunging at zero freestream velocity, the case relevant to hovering birds and insects, has been studied. The objective was to investigate the effect of airfoil stiffness. Particle image velocimetry and force measurements were taken for three airfoils of relative bending stiffnesses 1:8:512 in a water tank. The deformation of the flexible airfoils produces an angle of attack that varies periodically with a phase angle with respect to the plunging motion. Amplitude and phase of this combined plunging/pitching motion play a major role in the flowfield and thrust generation. Vortex pairs or alternating vortex streets were observed depending on the amplitude and phase lag of the trailing edge. The strength of the vortices, their lateral spacing, and the time-averaged velocity of the induced jet were found to depend on the airfoil flexibility, plunge frequency, and amplitude. Direct force measurements confirmed that at high plunge frequencies the thrust coefficient of the airfoil with intermediate stiffness was greatest, although the least stiff airfoil can generate larger thrust at low frequencies. It is suggested that there is an optimum airfoil stiffness for a given plunge frequency and amplitude. The thrust/input-power ratio was found to be greater for the flexible airfoils than for the rigid airfoil.

Aeroelastic Dynamic Analysis of a Full F-16 Configuration for Various Flight Conditions

Abstract: An overview is given of recent advances in a three-field methodology for modeling and solving nonlinear fluid-structure interaction problems, and its application to the prediction of the aeroelastic frequencies and damping coefficients of a full F-16 configuration in various subsonic, transonic, and supersonic airstreams is reported. In this three-field methodology the flow is described by the arbitrary Lagrangian-Eulerian form of the Euler equations, the structure is represented by a detailed finite element model, and the fluid mesh is unstructured, dynamic, and updated by a robust torsional spring analogy method. Simulation results are presented for stabilized, accelerated, low-g, and high-g flight conditions, and correlated with flight-test data. Consequently, the practical feasibility and potential of the described computational-fluid-dynamics-based computational method for the flutter analysis of high-performance aircraft, particularly in the transonic regime, are discussed.

Effect of Vibrational Nonequilibrium on Hypersonic Double-Cone Experiments

Abstract: Recent numerical simulations of hypersonic double-cone experiments overpredict the heat-transfer rate to the model by about 20%. We present a systematic analysis of the experimental facility and the physical modeling to explainthisdiscrepancy.Nozzlee owe eldsimulationsareusedtoinvestigatetheeffectofvibrationalnonequilibrium in the test section. These simulations show that the vibrational modes of the nitrogen gas freeze near the nozzle throatconditions, resulting inanelevated vibrationaltemperatureinthetestsection. Thislowersthekineticenergy e ux, reducing the heat transfer to the model. The effect of slip boundary conditions is also studied, and it is shown that weak accommodation of vibrational energy at the surface further reduces the heat-transfer rate to the model. The combination of these two effects brings the predicted heat-transfer rate into agreement with the experiments. In addition, weak e ow nonuniformity in the test section is shown to slightly modify the predicted separation zone, further improving the agreement.

Hybrid Simulation Approach for Cavity Flows: Blending, Algorithm, and Boundary Treatment Issues

Abstract: The maturation of high-performance computer architectures and computational algorithms has prompted the development of a new generation of models that attempt to combine the robustness and efficiency offered by the Reynolds averaged Navier-Stokes equations with the higher level of modeling offered by the equations developed for large eddy simulation. The application of a new hybrid approach is discussed, where the transition between these equation sets is controlled by a blending function that depends on local turbulent flow properties, as well as the local mesh spacing. The utilization of local turbulence properties provides added control in specifying the regions of the flow intended for each equation set, removing much of the burden from the grid-generation process. Moreover, the model framework allows for the combination of existing closure model equations, avoiding the difficulty of formulating a single set of closure coefficients that perform well in both Reynolds averaged and large eddy simulation modes. Simple modifications to common second-order accurate Reynolds averaged Navier-Stokes algorithms are proposed to enhance the capturing of large eddy motions

Concept Selection Using s-Pareto Frontiers

Abstract: We introduce the notion of s-Pareto optimality and show how it can be used to improve concept selection in engineering design. Specie c design alternatives are classie ed as s-Pareto optimal when there are no other alternatives from the same or any other general design concept that exhibit improvement in all design objectives. Further, we say that the set of s-Pareto design alternatives comprises the s-Pareto frontier. Under the proposed approach the s-Pareto frontier plays a paramount role in the concept selection process, as it is used to dee ne and classify concept dominance. Thes-Pareto frontier-based concept selection method can becharacterized as onethat capitalizes on the benee ts of computational optimization during the conceptual phase of design, before a general design concept has been chosen. An introduction to s-Pareto optimality and a method for generating s-Pareto frontiersaredeveloped.Anapproachforusing s-Paretofrontierstoperformconceptselectionisalsopresented.The methods proposed can effectively aid in the elimination of dominated design concepts, keep competitive concepts, and ultimately choose a specie cdesign alternative from theselected design concept. A truss design problem is used to illustrate the usefulness of the method. Nomenclature g = vector of inequality constraints h = vector of equality constraints J = aggregate objective function mi = number of points along Ni Ni = ith vector dee ning the utopia plane n = number of design objectives nx = number of design variables T k = relaxation/slack variable for concept k X P = generic point on the utopia plane x = vector of design variables ± = increment by which feasible space is reduced π = vector of design objectives (or design metrics) π i¤ = ith anchor point π ¤k = optimum design objective value for concept k π si¤ = s-anchor point for the ith objective

PEGASUS 5: An Automated Preprocessor for Overset-Grid Computational Fluid Dynamics

Abstract: An all new, automated version of the PEGASUS software has been developed and tested. PEGASUS provides the hole-cutting and connectivity information between overlapping grids and is used as the final part of the grid-generation process for overset-grid computational fluid dynamics approaches. The new PEGASUS code (Version 5) has many new features: automated hole cutting, a projection scheme for fixing small discretization errors in overset surfaces, more efficient interpolation search methods using an alternating digital tree and a stencil-jumping scheme, hole-size optimization based on adding additional layers of fringe points, and an automatic restart capability. The new code has also been parallelized using the message-passing interface standard. The parallelization performance provides efficient speedup of the execution time by an order of magnitude, and up to a factor of 30 for very large problems. The results of two example cases are presented: a three-element high-lift airfoil and a complete Boeing 777-200 aircraft in a high-lift landing configuration

Effective Coefficients of Piezoelectric Fiber-Reinforced Composites

Abstract: The effective coefe cients of piezoelectric e ber-reinforced composites (PFRC) have been determined through micromechanical analyzes. The method of cells (MOC) and the strength of materials (SM) approach have been employed to predict the coefe cients. A constant electric e eld is considered in the direction transverse to the e ber direction and is assumed to be the same both in the e ber and the matrix. MOC and SM predictions for the effective piezoelectric coefe cient of the PFRC assessing the actuating capability in the e ber direction are in excellent agreement. It has been found for the piezoelectric e bers considered that, when the e ber volume fraction exceeds a critical e ber volume fraction, this effective piezoelectric coefe cient becomes signie cantly larger than the corresponding coefe cient of the piezoelectric material of the e ber. The methods also show the excellent matching of the predictions of the effective elastic constants and the dielectric constant of the PFRC in the useful range of e ber volume fraction.

Low-Reynolds-Number Turbulent Flows in Locally Constricted Conduits: A Comparison Study

Abstract: In numerous internal flow systems the velocity field can undergo all flow regimes, that is, from laminar, via transitional, to fully turbulent. Considering two test conduits with local constrictions, four turbulence models, with an emphasis on low-Reynolds-number (LRN) turbulence models, were compared and evaluated. The objective was to identify a readily available LRN turbulence model with which incompressible laminar-to-turbulent velocity and pressure fields in complex three-dimensional conduits can be directly computed. The comparison study revealed that the renormalization group (RNG) κ-e and Menter κ-ω models amplify the flow instabilities after tubular constrictions and hence fail to capture the laminar flow behavior at low Reynolds numbers

Flow-Tagging Velocimetry for Hypersonic Flows Using Fluorescence of Nitric Oxide

Abstract: We demonstrate a new variation of molecular-tagging velocimetry for hypersonic e ows based on laser-induced e uorescence. A thin line of nitric-oxide molecules is excited with a laser beam and then, after a time delay, a e uorescence image of the displaced line is acquired. One component of velocity is determined from the time of e ight.Thismethodisappliedtomeasurethevelocityproe leinaMach8.5laminar,hypersonicboundarylayerinthe Australian National University’ s T2 free-piston shock tunnel. The single-shot velocity measurement uncertainty in the freestream was found to be 3.5%, based on 90% cone dence. The method is also demonstrated in the separated e ow region forward of a blunt e n attached to a e at plate in a Mach 7.4 e ow produced by the Australian National University’ s T3 free-piston shock tunnel. The measurement uncertainty in the blunt e n experiment is approximately 30%, owing mainly to low e uorescence intensities, which could be improved signie cantly in future experiments. This velocimetry method is applicableto very high-speed e ows that have low collisional quenching of the e uorescing species. It is particularly convenient in facilities where planar laser-induced e uorescence is already being performed.

Analysis of Flow Conditions in Freejet Experiments for Studying Airfoil Self-Noise

Abstract: A new set of mean wall pressure data has been collected on a controlled diffusion airfoil at a chord Reynolds number of 1.2 £ £105 in a freejet anechoic wind tunnel. Comparisons of the experimental data with Reynoldsaveraged Navier‐ Stokes (RANS) simulationsin freeairshow signie cant e owe eld and pressure loading differences, indicatingsubstantialjetinterferenceeffects.Toanalyzetheseeffects,asystematicRANS-basedcomputationale uid dynamicsstudyoftheexperimentale owconditionshasbeencarriedout,whichquantie esthestrongine uenceofthe e nite jet (nozzle) width on the aerodynamic loading and e ow characteristics. When the jet width is not sufe ciently large compared to the frontal wetted area of the airfoil, the airfoil pressure distribution is found to be closer to the distribution on a cascade than that of an isolated proe le. The airfoil lift is signie cantly reduced. Accounting for the actual wind-tunnel setup recovers the wall pressure distribution on the airfoil without further empirical angle-of-attack corrections. These jet interference effects could be responsible for the discrepancies among some earlier experimental and computational studies of airfoil self-noise. They should be accounted for in future noise computations to ensure that the experimental e ow conditions are simulated accurately.

Control of supersonic impinging jet flows using supersonic microjets

Abstract: Supersonic impinging jets, such as those occurring in the next generation of short takeoff and vertical landing aircraft, generate a highly oscillatory e ow with very high unsteady loads on the nearby aircraft structures and the landing surfaces. These high-pressure and acoustic loads are also accompanied by a dramatic loss in lift during hover.Previousstudies of supersonic impinging jets suggestthatthehighly unsteady behavioroftheimpinging jets is due to a feedback loop between the e uid and acoustic e elds, which leads to these adverse effects. A unique active control technique was attempted with the aim of disrupting the feedback loop, diminishing the e ow unsteadiness, and ultimatelyreducing theadverseeffectsofthise ow.Flowcontrolwasimplementedbyplacingacirculararray of 400-πm-diamsupersonicmicrojetsaroundtheperipheryofthemainjet.Thiscontrolapproachwasverysuccessful in disrupting the feedback loop in that the activation of the microjets led to dramatic reductions in the lift loss (40%), unsteady pressure loads (11 dB), and near-e eld noise (8 dB). This relatively simple and highly effective control technique makes it a suitable candidate for implementation in practical aircraft systems. NUNDERSTANDINGoftheimpingingjete owe eld isnecessary for the design of efe cient short takeoff and vertical landing (STOVL) aircraft. When such STOVL aircraft are operating in hovermode,thatis,in closeproximityto theground,thedownwardpointing lift jets produce high-speed, hot e ow that impinges on the landing surface and generates the direct lift force. It is well known that in this cone guration several e ow-induced effects can emerge, which substantially diminish the performance of the aircraft. In particular, a signie cant lift loss can be induced due to e ow entrainment bytheliftingjetsfromtheambientenvironmentinthe vicinityofthe airframe. Other adverse phenomena include severe ground erosion on the landing surface and hot gas ingestion into the engine inlets. In addition, the impinging e owe eld usually generates signie cantly highernoiselevelsrelativetothatofafreejetoperatingundersimilar conditions. Increased overall sound pressure levels (OASPL) associated with the high-speed impinging jets can pose an environment pollution problem and adversely affect the integrity of structural elements in the vicinity of the nozzle exhaust due to acoustic loading. Moreover, the noise and the highly unsteady pressure e eld are frequently dominated by high-amplitude discrete tones, which may match the resonant frequencies of the aircraft panels, thus further exacerbating the sonic fatigue problem. These problems become more pronounced when the impinging jets are supersonic, the operating regime of the STOVL version of the future joint strike e ghter. In addition, the presence of multiple impinging jets can potentially further aggravate these effects due to the strong coupling between the jets and the emergence of an upward-moving fountain e ow e owing opposite to the lift jets. 1 A

Turbulence modeling applied to flow over a sphere

Abstract: Numerical simulations of the subcritical flow over a sphere are presented. The primary aim is to compare prediction of some of the main physics and flow parameters from solutions of the unsteady Reynolds-averaged Navier-Stokes (URANS) equations, large-eddy simulation (LES), and detached-eddy simulation (DES). URANS predictions are obtained using two-layer κ-e, κ-ω, ν 2 -f, and the Spalart-Allmaras model. The dynamic eddy viscosity model is used in the LES. DES is a hybrid technique in which the closure is a modification to the Spalart-Allmaras model, reducing to RANS near solid boundaries and LES in the wake. The techniques are assessed by evaluating simulation results against experimental measurements, as well as through their ability to resolve time-dependent features of the flow related to vortex shedding. Simulation are performed at a Reynolds number of 10 4 , where laminar boundary-layer separation occurs at approximately 83 deg

Symmetric State-Space Method for a Class of Nonviscously Damped Systems

Abstract: Multiple-degree-of-freedom linear nonviscously damped systems are considered. It is assumed that the nonviscousdampingforcesdependonthepasthistoryofvelocitiesviaconvolutionintegralsoverexponentiallydecaying kernel functions. The traditional state-space approach, well known for viscously damped systems, is extended to such nonviscously damped systems using a set of internal variables. Suitable numerical examples are provided to illustrate the proposed approach. I. Introduction V ISCOUSdampingisthemostcommonmodelforthemodeling of vibration damping in linear systems. This model, e rst introduced by Rayleigh, 1 assumes that the instantaneous generalized velocities are the only relevant variables that determine damping. Viscous damping models are used widely for their simplicity and mathematical convenience, even though the behavior of real structural materials is, at best, poorly mimicked by simple viscous models.Forthisreason,itiswellrecognizedthat,ingeneral,aphysically realistic model of damping will not be viscous. Damping models in whichthedissipativeforcesdependonanyquantityotherthantheinstantaneous generalized velocities are nonviscous damping models. Mathematically, any causal model that makes the energy dissipation functional nonnegative is a possible candidate for a nonviscous damping model. Clearly, a wide range of choice is possible, either based on the physics of the problem or by selecting a model a priori and e tting its parameters from experiments. Here, we will use a particular type of damping model that is not viscous, and throughout the paper the terminology nonviscous damping will refer to this specie c model only. Possiblythemostgeneralwaytomodeldampingwithinthelinear range is to use nonviscous damping models that depend on the past history of motion via convolution integrals over kernel functions. 2 The equations of motion of an N-degree-of-freedom linear system with such damping can be expressed by

Aeroacoustic Instability in Rockets

Abstract: Current solid-propellant rocket instability calculations (e.g., Standard Stability Prediction Program ) account only for the evolution of acoustic energy with time. However, the acoustic component represents only part of the total unsteady system energy; additional kinetic energy resides in the shear waves that naturally accompany the acousticoscillations. Becausemost solid-rocketmotor combustion chambercone gurationssupport gas oscillations parallel to the propellant grain, an acoustic representation of the e ow does not satisfy physically correct boundary conditions. It is necessary to incorporate corrections to the acoustic wave structure arising from generation of vorticity at the chamber boundaries. Modie cations of the classical acoustic stability analysis have been proposed that partially correct this defect by incorporating energy source/sink terms arising from rotational e ow effects. One of these is Culick’ s e ow-turning stability integral; related terms that are not found in the acoustic stability algorithm appear. A more complete representation of the linearized motor aeroacoustics is utilized to determine the growth or decay of the system energy with rotational e ow effects accounted for already. Signie cant changes in the motor energy gain/loss balance result; these help to explain experimental e ndings that are not accounted for in the present acoustic stability assessment methodology.

Leading-Edge Vortex Structure of Nonslender Delta Wings at Low Reynolds Number

Abstract: The velocity field near the apex region of moderately swept delta wings was measured in a water tunnel, using a version of stereoscopic digital particle imaging velocimetry. Flow visualization was also used to verify these results. In contrast to most recent studies, low angles of attack were emphasized, with most data in the range of 5–20 deg. Delta wings of 50- and 65-deg leading-edge sweep and 30-deg windward-side bevels were tested at Reynolds numbers of 6x10^3 –1.5x10^4. At these low Reynolds numbers, secondary leading-edge vortices were weak, giving way to essentially stagnant flow outboard of the primary leading-edge vortices at the higher angles of attack. Otherwise, velocity data for the 65-deg wing were consistent with well-known observations for slender delta wings. The 50-deg wing exhibited unexpectedly strong primary leading-edge vortices at low angles of attack, with a generally conical velocity field. Upstream progression of vortex breakdown with increasing angle of attack exhibited extensive regions of streamwise undulation. Leading-edge shear-layer rollup was observed in crossflow planes well downstream of the breakdown region, but with an increased occurrence of paired vortical structures of opposite sign inside the shear layer itself.

Large-Eddy Simulation of Flow Around Low-Pressure Turbine Blade with Incoming Wakes

Abstract: The flow around a low-pressure turbine rotor blade with periodically incoming wakes at a realistic Reynolds number is computed by means of large-eddy simulation (LES). The computed results are discussed in terms of phase-averaged and mean quantities. A comparison is made with an existing direct numerical simulation (DNS) for the same geometry and operating conditions. Particular attention is devoted to flow structures associated with the incoming wakes and their effect on the boundary layers. The analysis of the flowfield reveals patterns similar to those encountered in DNS and in LES of flow in the same geometry at a lower Reynolds number. Noticeable differences occur in the suction-side boundary layer, which exhibits a complete transition to turbulence for the present case

Experimental Investigations of Hypersonic Flow over Highly Blunted Cones with Aerospikes

Abstract: Effectiveness of aerospikes/aerodisk assemblies as retractable drag-reduction devices for large-angle blunt cones flying at hypersonic Mach numbers is investigated experimentally in hypersonic shock tunnel HST2 using a 120-deg apex-angle blunt cone. An internally mounted accelerometer balance system has been used for measuring the aerodynamic drag on the blunt cone with and without forward-facing aerospikes at various angles of attack. The measurements indicate around 55% reduction in drag for the blunt cone with flat-disk spike at zero degree angle of attack for a freestream Mach number of 5.75. Surface convective heat-transfer rate measurements have been carried out on the blunt cone with a flat-disk tipped spike of varying length in order to locate the shock reattachment point on the blunt-cone surface. The measured heat-transfer rates fluctuate by about ±20% in the separated flow region as well as near the reattachment point indicating the unsteady flowfleld around the spiked blunt cone. The shock structure around the 120-deg apex-angle blunt cone with a 12-mm-long flat-tipped aerospike has also been visualized using the electric discharge technique. The visualized shock structure and the measured drag on the blunt cone with aerospikes agree well with the axisymmetric numerical simulations

Computational Fluid Dynamics-Based Drag Prediction and Decomposition

Abstract: A method for the computation and breakdown of the aerodynamic drag into viscous and wave components is proposed. Given a numerical solution of the Reynolds averaged Navier-Stokes equations, the method, based on a Taylor's series expansion of the far-field drag expression, allows for the determination of the drag related to entropy variations in the flow. The identification of a spurious contribution, due to the numerical dissipation and discretization error of the flow solver algorithm, allows for drag computations weakly dependent on mesh size. Therefore, accurate drag evaluations are possible even on moderately sized grids. Results are presented for transonic flows around an airfoil and a wing-body configuration

Jet Aeroacoustic Testing: Issues and Implications

Abstract: Issues that are important for jet aeroacoustic tests and the critical role of good data in the development of jet noise technology are reviewed and discussed. A major effort for improving the quality of aeroacoustic data acquired at the Boeing Low Speed Aeroacoustic Facility has been carried out. This extensive undertaking targeted all aspects of model-scale testing and acquisition of good quality data and covered issues of flow quality, nozzle performance, and acoustics. Significant improvements have been made in all of the named categories. Simultaneous measurement of nozzle aerodynamic performance and noise is important, especially for the development of noise suppression devices. The capabilities of a jet rig incorporated with a six-component force balance are described. It is clearly demonstrated that the measured thrust with the current rig is in excellent agreement with that obtained using a dedicated force balance over a wide range of nozzle pressure ratios. Results of the efforts at rig refurbishment, carried out over the last few years, are presented. The high quality of noise measurements is established through good spectral agreement with data obtained with a blowdown jet, for a wide range of nozzle conditions. An extensive study of available jet noise data from various jet noise facilities has been completed. Implications of contaminated data from most tests and the obligations of the experimental community for the advancement of jet noise technology are discussed.

Lessons from LESFOIL Project on Large-Eddy Simulation of Flow Around an Airfoil

Abstract: A synopsis of important results obtained by seven partners in the Brite ‐Euram project LESFOIL sponsored by the European Union is presented. This project was devoted to assessing the feasibility of large-eddy simulations (LES)forthecomputationofthee owaroundanairfoil.Asatestcase,theAerospatialeA-airfoilat Re=2£10 6 and 13.3-deg angle of attack was chosen. All partners performed simulations of this e ow with various methods, most of which employed e nite volume schemes and various subgrid-scale models of eddy-viscosity type. The key results of theindividual partners’ publicationsarepresented ina comparativeway.Itis demonstratedthatthiscone guration with realistic conditions for aeronautical e ows is extremely demanding for LES. One reason is the need to achieve a threshold resolution for the adequate discretization of the attached and mildly separating boundary layer. The second and even more demanding requirement is the need for an adequate treatment of transition, which in the present computations was achieved by increased resolution.

Development of Comprehensive Detailed and Reduced Reaction Mechanisms for Combustion Modeling

Abstract: ChungK. Law received aB.Sc. in Physics from theUniversity ofAlberta in 1968,anM.A.Sc. inAerospace Studies from the University of Toronto in 1970, and a Ph.D. in Engineering Physics from the University of California at San Diego in 1973. Since graduation he has been associated with the General Motors Research Laboratories, Princeton University, Northwestern University, and the University of California at Davis. In 1988 he returned to Princeton University, where he is the Robert H. Goddard Professor of Mechanical and Aerospace Engineering. Law’s research interests cover various physical and chemical aspects of fundamental combustion phenomena. He is a Fellow of AIAA and the American Society of Mechanical Engineers, a member of the National Academy of Engineering, the President of the Combustion Institute (2000–2004), and a recipient of a number of professional awards for technical contributions. He is author or coauthor of over 300 journal publications.

Energy-Recycling Semi-Active Method for Vibration Suppression with Piezoelectric Transducers

Abstract: *A novel energy-recycling method is studied that enables effective semi-active vibration suppression with piezoelectric transducers embedded or bonded to a structure. In this method, the energy converted from the mechanical energy of a vibrating structure is collected in the capacitor of a piezoelectric transducer as an electric charge, and to suppress vibration, rather than dissipate the energy, the polarity of the charge is changed according to the state of vibration. With this method, no energy is supplied to the total system of the structure and transducers with shunt circuit, which means that the system is stable. A simple electric circuit and a control law for multiple-degree-offreedom systems with multiple piezoelectric transducers are proposed for this method based on energy recycling. Numerical simulation of vibration suppression of a truss structure shows that this method is more effective in suppressing vibration than both a semi-active method without energy-recycling and that based on the use of an optimally tuned passive system. A preliminary experiment with a truss structure also shows that this method can effectively suppress vibration in an actual structure. However, there was some discrepancy in the experimental results compared to the results of the numerical simulation performed assuming ideal linear characteristics of the piezoelectric transducers estimated from a static test. Nomenclature

Laser Energy Deposition in Quiescent Air

Abstract: Laser energy deposition in quiescent air has been studied experimentally and numerically. The study is focused on the gasdynamic effects of the laser energy spot on the e ow structure. A Gaussian proe le for initial temperature distribution is proposed to model the energy spot assuming the density is initially uniform. A e ltered Rayleigh scattering technique has been used for obtaining the experimental results. These consisted of e ow visualization of the blast wave, and simultaneous pressure, temperature, and velocity measurements. Good agreement has been achieved between numerical and experimental results for shock radius vs time. The comparison of computed and experimental density, pressure, temperature, and velocity outside the laser spot show good agreement as well.