Showing papers in "AIAA Journal in 1995"

Preconditioning Applied to Variable and Constant Density Flows

Abstract: A time-derivative preconditioning of the Navier-Stokes equations, suitable for both variable and constant density fluids, is developed. The ideas of low-Mach-number preconditioning and artificial compressibility are combined into a unified approach designed to enhance convergence rates of density-based, time-marching schemes for solving flows of incompressible and variable density fluids at all speeds. The preconditioning is coupled with a dual time-stepping scheme implemented within an explicit, multistage algorithm for solving time-accurate flows. The resultant time integration scheme is used in conjunction with a finite volume discretization designed for unstructured, solution-adaptive mesh topologies. This method is shown to provide accurate steady-state solutions for transonic and low-speed flow of variable density fluids. The time-accurate solution of unsteady, incompressible flow is also demonstrated.

Separated flow computations with the k-e-v2 model

Abstract: Tlirbulent separated flows over a backstep, in a plane diffuser and around a triangular cylinder, are computed with the k-e-v2 model. These provide examples of massive separation, of smooth separation, and of unsteady vortex shedding. It is shown that to accurately predict the time-averaged properties of bluff body flow, it is necessary to resolve the coherent vortex shedding. The near-wall treatment of the v2-/22 system of equations is able to cope with both the massive and smooth separations. Good agreement between experiment and prediction is found in all

Computational aeroacoustics - Issues and methods

Abstract: Computational fluid dynamics (CFD) has made tremendous progress especially in aerodynamics and aircraft design over the past 20 years. An obvious question to ask is "why not use CFD methods to solve aeroacoustics problems?" Most aerodynamics problems are time independent, whereas aeroacoustics problems are, by definition, time dependent. The nature, characteristics, and objectives of aeroacoustics problems are also quite different from the commonly encountered CFD problems. There are computational issues that are unique to aeroacoustics. For these reasons computational aeroacoustics requires somewhat independent thinking and development. The objectives of this paper are twofold. First, issues pertinent to aeroacoustics that may or may not be relevant to computational aerodynamics are discussed. The second objective is to review computational methods developed recently that are designed especially for computational aeroacoustics applications. Some of the computational methods to be reviewed are quite different from traditional CFD methods. They should be of interest to the CFD and fluid dynamics communities.

Numerical/experimental study of a wingtip vortex in the near field

Abstract: The near-field behavior of a wingtip vortex flow has been studied computationally and experimentally in an interactive fashion. The computational approach involved using the method of artificial compressibility to solve the three-dimensional, incompressible, Navier-Stokes equations with experimentally determined boundary conditions and a modified Baldwin-Barth turbulence model. Inaccuracies caused by the finite difference technique, grid resolution, and turbulence modeling have been explored. The complete geometry case was computed using 1.5 million grid points and compared with mean velocity measurements on the suction side of the wing and in the near wake. Good agreement between the computed and measured flowfields has been obtained. The velocity distribution in the vortex core compares to within 3% of the experiment.

Engineering application of experimental uncertainty analysis

Abstract: Publication in late 1993 by the International Organization for Standardization (ISO) of the Guide to the Expression of Uncertainty in Measurement in the name of ISO and six other international organizations has, in everything but name only, established a new international experimental uncertainty standard. In this article, an analysis of the assumptions and approximations used in the development of the methods in the ISO guide is presented, and a comparison of the resulting equation with previously published uncertainty analysis approaches is made. Also discussed are the additional assumptions necessary to achieve the less complex large sample methodology that is recommended in AIAA Standard S-071-1995, Assessment of Wind Tunnel Data Uncertainty, issued in 1995. It is shown that these assumptions are actually less restrictive than those associated with some previously accepted methodologies. The article concludes with a discussion of some practical aspects of implementing experimental uncertainty analysis in engineering testing.

Unstructured grid generation using iterative point insertion and local reconnection

Abstract: A procedure is presented for efficient generation of high-quality two- or three-dimensional unstructured grids of triangular or tetrahedral elements. The present procedure uses an iterative point creation and insertion scheme wherein points are created using advancing-front type point placement. Initially, the connectivity for these generated points is obtained by directly subdividing the elements which contain them, without regard to quality. This connectivity is then improved by iteratively using local reconnection subject to a quality criterion. For two dimensions, a min-max criterion is used and for three dimensions, a Delaunay in-sphere criterion followed by a min-max type criterion is used. The overall procedure is applied repetitively until a complete field grid is generated with a desired point distribution. Grid quality and performance statistics are presented for a variety of two- and three-dimensional configurations. The combined quality and efficiency attributes of this procedure appear to be a substantial improvement over existing methods.

Thermoacoustic oscillations in combustion chambers of gas turbines

Abstract: This paper presents an overview of thermoacoustic oscillations in modern gas turbine combustors with premixed combustion. In conventional combustors a substantial percentage of air enters downstream of the primary zone of combustion. As a consequence, the liners of conventional combustors are very powerful sound attenuators. In a modern premixing combustor effectively all of the air enters through the burners, and there is almost no sound attenuation downstream of the primary zone of combustion. The combination of this problem with the difficulties to stabilize premixed flames has led to a situation where thermoacoustic stability has become the key issue of modern combustion technology. The relative importance of a variety of self-excited oscillations and oscillations that are forced by aerodynamic instabilities is investigated. It is found that the two leading excitation mechanisms are associated with periodic (lean) extinction and with vortex rollup in the primary zone of combustion. For the latter case a simple combustor model is used to study the amplification effects due to resonant wave motion and coincidence of mechanical eigenfrequencies of combustor walls with excitation frequencies.

Convergence acceleration of a Navier-Stokes solver for efficient static aeroelastic computations

Abstract: New capabilities have been developed for a Navier-Stokes solver to perform steady-state simulations more efficiently. The flow solver for solving the Navier-Stokes equations is based on a combination of the lower-upper factored symmetric Gauss-Seidel implicit method and the modified Harten-Lax-van Leer-Einfeldt upwind scheme. A numerically stable and efficient pseudo-time-marching method is also developed for computing steady flows over flexible wings. Results are demonstrated for transonic flows over rigid and flexible wings.

Fuzzy finite element approach for the analysis of imprecisely defined systems

Abstract: Many engineering systems are too complex to be defined in precise mathematical terms. They often contain information and features that are vague, imprecise, qualitative, linguistic, or incomplete. The traditional deterministic and probabilistic techniques are not adequate to analyze such systems. This paper aims at developing a fuzzy finite element approach for the analysis of imprecisely defined systems. The development of the methodology starts from the basic concepts of fuzzy numbers of fuzzy arithmetic and implements suitably defined fuzzy calculus concepts such as differentiation and integration for the derivation, manipulation, and solution of the finite element equations. Simple stress analysis problems involving vaguely defined geometry, material properties, external loads, and boundary conditions are solved to establish and to illustrate the new procedure. The approach developed is applicable to systems that are described in linguistic terms as well as those that are described by incomplete information. If complete data are known, the method handles the information similar to that of a probabilistic approach. The present approach represents a unique methodology that enables us to handle certain types of imprecisely known data more realistically compared with the existing procedures.

Vortex shedding and shear-layer instability of wing at low-Reynolds numbers

Abstract: Flow patterns and characteristics of vortex shedding and shear-layer instability of a NACA 0012 cantilever wing are experimentally studied. Smoke-wire and surface oil-flow techniques are employed to visualize the flow patterns and evolution of vortex shedding. Hot-wire anemometers are used to characterize the frequency domain of the unsteady flow structures. Several characteristic flow modes are classified in the domain of chord Reynolds number and root angle of attack. Effects of the juncture and wing tip are discussed. Vortex shedding can be classified into four characteristic modes. Vortex shedding at low and high angles of attack are found to have different dominant mechanisms. Effects of the juncture and wing tip on the vortex shedding are discussed. Shear-layer instabilities are found to be closely related to the behaviors of the vortex shedding. Behaviors of the shear-layer instabilities can be traced back to the characteristics of the boundary layer on the suction surface of the airfoil.

Investigation of Natural Frequencies and Mode Shapes of Buckled Beams

Abstract: The linear modes of vibration of buckled beams are investigated analytically and experimentally. Assuming a static buckled shape corresponding to the nth buckling mode, an exact solution is obtained for the linear modes and associated frequencies of initially buckled beams with fixed-fixed, fixed-hinged, and hinged-hinged boundary conditions. The analytical solution for a first-mode buckled beam is validated experimentally for the fixed-fixed case. The analytically obtained natural frequencies are in excellent agreement with those obtained experimentally. These exact linear modes provide a basis from which to study the nonlinear vibrations of buckled beams.

Direct numerical simulation of turbulent transport with uniform wall injection and suction

Abstract: A direct numerical simulation (DNS) of the fully developed turbulent channel flow and heat transfer with uniform wall injection and suction was carried out. The Reynolds number, which was based on the channel half-width and the friction velocity averaged on the two walls, was set to be 150, whereas the Prandtl number was 0.71. The walls were assumed to be kept at isothermal but different temperatures. With any buoyancy effect neglected, temperature was considered as a passive scalar. The computation was executed on sufficiently dense grid points by using a spectral method. The statistics obtained include the mean velocity and temperature, Reynolds stresses, and turbulent heat fluxes. Each term in the budget equations of the second-order velocity and temperature correlations and of their destruction rates was also calculated. It is found that the injection decreases the friction coefficient, but tends to stimulate the near-wall turbulence activity so that the Reynolds stresses and turbulent heat fluxes are increased, whereas the suction has an inverse influence.

A perspective on unstructured grid flow solvers

Abstract: This survey paper assesses the status of compressible Euler and Navier-Stokes solvers on unstructured grids. Different spatial and temporal discretization options for steady and unsteady flows are discussed. The integration of these components into an overall framework to solve practical problems is addressed. Issues such as grid adaptation, higher order methods, hybrid discretizations and parallel computing are briefly discussed. Finally, some outstanding issues and future research directions are presented.

Vibrational relaxation and dissociation behind shock waves. Part 1 - Kinetic rate models.

Abstract: We address the validation of the analytical nonperturbative semiclassical vibration-translation and vibration-vibration-translation rate models for the atom-diatom and diatom-diatom vibrational energy transfer molecular collisions. These forced harmonic oscillator rate models are corrected and validated by comparison with three-dimensional semiclassical trajectory calculations for nitrogen, which are widely considered to be the most reliable theoretical data available. A remarkably good agreement is shown between the two models, for both the temperature and quantum number dependence of single-quantum and double-quantum vibration-vibration-translation jumps in the temperature range 200 < T < 8000 K and for vibrational quantum numbers 0 < v < 40. The simplicity of the theory, as well as the agreement shown, make the forced harmonic oscillator rate model attractive for master equation and direct simulation Monte Carlo modeling of nonequilibrium gas flows at very high temperatures, when the first-order, vibration-vibration-translation rate models are not applicable, and where use of the three-dimensional trajectory calculations is very cumbersome and time consuming. The forced harmonic oscillator model is also applied to obtain the probability of collision-induced dissociation of diatomics.

Behavior of linear reconstruction techniques on unstructured meshes

Abstract: : This report presents an assessment of a variety of reconstruction schemes on meshes with both quadrilateral and triangular tessellations. The investigations measure the order of accuracy, absolute error and convergence properties associated with each method. Linear reconstruction approaches using both Green-Gauss and least squares gradient estimation are evaluated against a structured MUSCL scheme wherever possible. In addition to examining the influence of polygon degree and reconstruction strategy, results with three limiters are examined and compared against unlimited results when feasible. The methods are applied on quadrilateral, right triangular, and equilateral triangular elements in order to facilitate an examination of the scheme behavior on a variety of element shapes. The numerical test cases include well known internal and external inviscid examples and also a supersonic vortex problem for which there exists a closed form solution to the 2-D compressible Euler equations. Such investigations indicate that the least squares gradient estimation provides significantly more reliable results on poor quality meshes. Furthermore, limiting only the face normal component of the gradient can significantly increase both accuracy and convergence while still preserving the integral cell average, and maintaining monoticity. The first order method performs poorly on stretched triangular meshes, and analysis shows that such meshes result in poorly aligned left and right states for the Riemann problem. The higher average valence of a vertex in the triangular tessellations does not appear to enhance the wave propagation, accuracy, or convergence properties of the method. Unstructured, Upwind, Inviscid, Reconstruction, Limiters, Riemann problems.

Large Eddy Simulation of Reacting Flows Applied to Bluff Body Stabilized Flames

Abstract: The objective of this paper is to present a large eddy simulation model for chemically reacting flows. The large eddy simulation model, founded on a physical model based on modern continuum mechanics, includes a complete treatment of the subgrid stresses and fluxes including both backscatter and diffusion. To investigate the predictive capabilities of the large eddy simulation model, numerical simulations of a configuration corresponding to a rig consisting of a rectilinear channel with a triangular-shaped bluff body have been performed. Both nonreacting and reacting flows have been examined under a variety of operating conditions. This paper focuses on the reacting case, which is characterized as lean and premixed. The simulation results are compared to experimental measurements of temperature, constituent mass fraction, and velocity fields in the test rig. The results indicate that the large eddy simulation technique works well and mimics most of the significant flow features, including the typical unsteady flow structures. The results from the large eddy simulations are furthermore used to investigate the mechanisms responsible for the typical flowfield in a bluff body stabilized flame.

Improved two-point function approximations for design optimization

Abstract: Two-point approximations are developed by utilizing both the function and gradient information of two data points. The objective of this work is to build a high-quality approximation to realize computational savings in solving complex optimization and reliability analysis problems. Two developments are proposed in the new twopoint approximations: 1) calculation of a correction term by matching with the previous known function value and supplementing it to the first-order approximation for including the effects of higher order terms and 2) development of a second-order approximation without the actual calculation of second-order derivatives by using an approximate Hessian matrix. Several highly nonlinear functions and structural examples are used for demonstrating the new two-point approximations that improved the accuracy of existing first-order methods. EVELOPMENTof new and improved constraint approximations has been an active area of research in the mathematical optimization community over the last two decades.1"3 More accurate function approximations reduce the finite element analysis cost and increase the search domain in each iteration of an optimization run. Recent developments in constraint approximations addressed methods for eigenvalues, dynamic response, static forces, structural reliability, etc. Some of the typical methods are summarized in the following. Kirsch4 developed a scaling factor for a stiffness matrix and approximated the displacements, stresses, and forces with respect to sizing and topology variables. Vanderplaats and Salajegheh5 used two levels of approximations for discrete sizing and shape design of structures, first for solving the continuous problem and next the discrete one using a dual theory. Canfield6 developed an approach for eigenvalues by approximating the modal strain and kinetic energies independently. Thomas et al.7 presented an approximation for the frequency response of damped structures by using the concept of Ref. 6. A multivariate spline approximation was developed by Wang and Grandhi8 making use of the previously generated exact finite element analyses. Similarly, a Hermite approximation for n-dimensional problems used several data points and had the property of reproducing the function and gradient values at the known data points.9 Canfield10 used multipoint data in building an approximate Hessian matrix and constructed a second-order approximation for improving the accuracy. Toropov et al.11 approximated functions as a linear combination or products of variables, and the coefficients of the polynomial were computed by applying a least squared approach on multiple data points. Fadel et al. 12 considered intermediate variables in terms of exponentials and they were computed by matching the approximate function gradients with the previous point's exact values. This approach did not compare the function values of the previous point. Wang and Grandhi13 used a single exponential for all of the variables and calculated it by matching the approximate function with the previous point exact value, and the gradient information of the previous point was not utilized. The proposed paper develops an improved two-point approximation utilizing both function and gradient information of two data points. In the new two-point approximation, two developments are proposed: 1) calculation of a correction term based on two function values and

Kelvin-Voigt versus fractional derivative model as constitutive relations for viscoelastic materials

Abstract: Three simple constitutive relationships are studied for application to viscoelastic materials. Experimental results for both a rubbery and a glassy viscoelastic material are fit by the three schemes. The Kelvin-Voigt scheme is shown to be adequate in only limited frequency ranges. A three-parameter fractional order constitutive relationship provides a substantially better model over a much larger bandwidth. A four-parameter fractional model improves on the accuracy in materials with significant glassy regions

Compressible cavity flow oscillation due to shear layer instabilities and pressure feedback

Abstract: A computational analysis was performed on a compressible flow oscillation due to shear layer instabilities over a cavity and pressure feedback in a cavity of length-to-depth ratio 3 at Mach 1.5 and 2.5. The mass-averaged NavierStokes equations were solved. Turbulence closure was achieved using a k-u> model with compressibility corrections. Self-sustained oscillations were produced. Negative form drag coefficient was observed within an oscillatory cycle due to mass ejection from the cavity near the trailing edge and vortex production near the leading edge. The shock wave-expansion wave interaction patterns, modes of the oscillation, sound pressure level, and time-averaged surface pressure were compared with experimental results of previous investigations and good agreement was achieved, particularly the time-averaged pressure. The prediction showed a marked improvement over earlier analysis.

Free Vibration Analysis of Anisotropic Thin-Walled Closed-Section Beams

Abstract: The equations of motion for the free vibration analysis of anisotropic thin-walled closed-section beams are derived using a variational asymptotic approach and Hamilton's principle. The analysis is applied to two laminated composite constructions. The circumferentially uniform stiffness produces extension-twist coupling and the circumferentially asymmetric stiffness produces bending-twist coupling. The effect of the elastic coupling mechanisms on the vibration behavior of thin-walled composite beams is evaluated analytically. The influence of stacking orientation on the frequencies associated with coupled vibration modes is investigated. The predictions are validated by comparison with a finite element simulation and test data.

Integration of Navier-Stokes Equations Using Dual Time Stepping and a Multigrid Method

Abstract: Efficient acceleration techniques typical of explicit steady-state solvers are extended to tune-accurate calcu- lations. Stability restrictions are greatly reduced by means of a fully implicit time discretization. A four-stage Runge-Kutta scheme with local tune stepping, residual smoothing, and multigridding is used instead of traditional computationally expensive factorizations. Two applications to natural unsteady viscous flows are presented to check for the capability of the procedure. ECENT progress in computational fluid dynamics along with the evolution of computer performance encourages scientists to look in to details of flow physics. There are practical applications where the unsteadiness of the problem can not be neglected (i.e., vortex shedding, natural unsteadiness, forced unsteadiness, aeroe- lasticity, turbomachinery rotor-stator interaction). Up to now, most of the analysis and designing tools are based on a steady or quasi- steady assumption, even if the flow is known to be unsteady. Today, due to the improvement in computer resources, there is a strong interest in developing methodologies for efficient and reliable sim- ulation of unsteady flow features. It is a common experience, while using time-accurate explicit schemes, to be forced to choose the time step on the basis of stability restrictions. As a consequence, unless the problem is a very high frequency one, the number of time steps to be performed is much higher than the one required for time accuracy. By means of some implicit factorization, stability restrictions can be relaxed, but the work required at each time step grows rapidly with grid dimension and complexity of the flow equations. In addition, application of boundary conditions in a fully implicit manner is difficult. In viscous flow calculations, the grid is clustered close to the shear layer and the characteristic time step varies several orders of mag- nitude inside the computational domain. Even if in several practical applications the characteristic time step of the core-flow region is comparable with the one required by time accuracy, close to the walls the time step restrictions become extremely severe. There- fore, highly vectorizable schemes with less stability restrictions on the allowable time step would be an interesting combination. Explicit schemes combined with acceleration techniques have proven to be very effective for solving steady problems.1"3 Unfortu- nately, the computational efficiency of those time-marching solvers is achieved by sacrificing the accuracy in time. In this paper, it is shown that the conventional steady-state acceleration techniques, specifically the multigrid techniques, can still be applied to unsteady viscous problems. The basic idea is to introduce a dual time stepping and to reformulate the governing equations so that they can be han- dled by an explicit accelerated scheme.4 If the time discretization is made implicit, stability restrictions are removed and accelera- tion techniques can be used instead of traditional time-consuming factorizations (i.e., alternate directional implicit and lower/upper).

ONERA Three-Dimensional Icing Model

Abstract: A three-dimensional icing model has been developed at ONERA to calculate ice accretion shapes for aerodynamic components that can not be predicted using conventional two-dimensional codes. It is described, emphasizing the original parts with respect to the two-dimensional existing models. The model includes Euler inviscid flow calculation. Droplet trajectories are calculated in a three-dimensional grid. The remesh on the leading edge is adapted to follow aerodynamics singularities. The boundary layer is calculated using a mixing length formulation to model the wall roughness influence on convective heat transfer. Runback paths are integrated. The heat balance is calculated in a grid created along the runback paths. The domain of validity of the three-dimensional icing code is described; compared with the two-dimensional model this domain is wider, especially for high speeds. The three-dimensional model is shown to simulate well a uniform ice deposit on a three-dimensional rotor blade tip. Then, a comparison of the three- and two-dimensional codes on an infinite swept wing shows that the corrected two-dimensional code predicts the catch efficiency but not the ice shape. Finally, it is shown that the continuum flux hypothesis prevents the three-dimensional model from simulating correctly the "lobster tail" ice shape (nonuniform ice deposit).

Evaluation of boundary conditions for computational aeroacoustics

Abstract: The performance of three boundary conditions for aeroacoustics were investigated, namely, (1) Giles-1990; (2) Tam and Webb-1993, and (3) Thompson-1987. For each boundary condition, various implementations were tested to study the sensitivity of their performance to the implementation procedure. Details of all implementations are given. Results are shown for the acoustic field of a monopole in a uniform freestream.

Prediction of Nonequilibrium Turbulent Flows with Explicit Algebraic Stress Models

Abstract: An explicit algebraic stress equation, developed by Gatski and Speziale, is used in the framework of the K-UJ and K-e formulations to predict two-dimensional separated turbulent flows. The nonequilibrium effects are modeled through coefficients that depend nonlinearly on both rotational and irrotational strains. The proposed models were implemented in the Courant-Friedrichs-Lewy three-dimensional Navier-Stokes code. Comparisons with the experimental data are presented, which clearly demonstrate that explicit algebraic stress models improve the response of standard two-equation models to nonequilibrium effects. I. Introduction

Prediction of supersonic inlet unstart caused by freestream disturbances

Abstract: The objective of this article is to report progress toward the development of an Euler analysis procedure for predicting the unstart tolerance of supersonic inlets. As an aid to understanding boundary condition issues, a one-dimensional, linear-analysis procedure was developed and used to analyze inlet unstart behavior. Using these results as a guide, an Euler analysis procedure was extended through the addition of a new bleed boundary condition, a new compressor face boundary condition, and an engine demand model for the simulation of unsteady inlet flows caused by freestream flow disturbances. Five unstart conditions were identified with the Euler analysis of the axisymmetric inlet for both 20- and 90-deg throat bleed configurations. Results show that both increases and decreases in temperature or velocity will unstart the inlet, whereas only pressure decreases will unstart the inlet. It was also found that 90-deg throat bleed improves the unstart tolerance relative to 20-deg throat bleed for freestream pressure decreases, temperature increases, and changes in velocity.

Suppression of nonlinear panel flutter with piezoelectric actuators using finite element method

Abstract: An optimal control design is presented to actively suppress large-amplitude, limit-cycle flutter motions of rectangular isotropic plates at supersonic speeds using piezoelectric actuators. The nonlinear panel flutter equations based on the finite element method are derived for isotropic plates with piezoelectric layers subjected to aerodynamic and thermal loads. A model reduction is performed to the finite element system equations of motion for the control design and the time domain simulation. An optimal controller is developed based on the linearized modal equations, and the norms of the feedback control gain are employed to provide the optimal shape and location of the piezoelectric actuators. Numerical simulations based on the reduced nonlinear panel flutter model show that the critical dynamic pressure can be increased three to four times by the piezoelectric actuation. Within the increased critical dynamic pressure, the limit-cycle motions can be completely suppressed. The results demonstrate that piezoelectric materials are effective in panel flutter suppression.

Quiet-flow Ludwieg tube for high-speed transition research

Abstract: Low-noise supersonic wind tunnels are required for unambiguous experimental research into high-speed laminar-flow instability and transition. The experience of the successful NASA Langley quiet-tunnel development program has been used to design and construct a new kind of low-cost, short-duration, quiet-flow tunnel. Measurements of the flow quality in the 9.7 x 10.9 cm Mach 4 test section were obtained using fast response pressure transducers mounted in the tip of a pitot tube. When the rms pitot pressure is approximately 0.05-0.10% of the mean pitot pressure, bursts of noise appear in the pitot-pressure signals. These bursts appear to be the radiated signature of turbulent spots in the boundary layers on the nozzle walls. Their appearance confirms the presence of laminar nozzle-wall boundary layers and quiet flow when the rms pitot pressure is about 0.06% or less. Based on this criterion, quiet flow is achieved to Reynolds numbers based on the axial length of the quiet-flow test region of more than 400,000 at unit Reynolds numbers of approximately 40,000 per cm. This performance is sufficient for research into receptivity, roughness, and instability effects at high speeds.

Using transfer function parameter changes for damage detection of structures

Abstract: A novel approach is presented for damage detection of large flexible structures by using the parameter change of the transfer function. First, an interval modeling technique, which represents the system uncertainty under the environmental change via the intervals of transfer function parameters, is used to distinguish the structural damage from the environmental change. In this paper a coherence approach is developed for locating the damage position when the structural damage is observed. Only a few sensors are required in using the proposed coherence approach for damage detection. This has the advantage of using the proposed algorithm for practical applications. Also this approach has the flexibility of using the multi-input and multi-output system. A nine-bay truss example is used to demonstrate and verify the approach developed. Nomenclature a, b = denominator and numerator parameters of transfer function C = coherence of the tested system to damage G = interval model g = transfer function k = number of modes m = number of outputs n = number of tests for environmental change p = parameter vector of transfer function R = magnitude ratio of the tested system to damage R, JR = magnitude ratio bounds W = weight of parameter change Wa, W}} = weights of intervals for interval model x,y = unit vectors of Cartesian coordinate system Ap = parameter change vector Superscripts

Sound Generation by Flow over a Two-Dimensional Cavity

Abstract: Sound generated by flow over a cavity at a Mach number of 0.1 and a Reynolds number based on cavity length of 5000 is calculated. The computation utilizes a two part technique where the time-dependent incompressible flow is first obtained and then a second calculation is performed for the compressible aspects of the flow. This second calculation utilizes a grid and numerical scheme designed for resolution of acoustic waves. The cavity flowfield is observed to oscillate quite regularly at the Strouhal number of 0.58 which produces an acoustic source of the same frequency. Time histories, spectra, and directivity of the sound radiation are computed.

Flux-limited schemes for the compressible Navier-Stokes equations

Abstract: Several high-resolution schemes are formulated with the goal of improving the accuracy of solutions to the full compressible Navier-Stokes equations. Calculations of laminar boundary layers at subsonic, transonic, and supersonic speeds are carried out to validate the proposed schemes. It is concluded that these schemes, which were originally tailored for nonoscillatory shock capturing, yield accurate solutions for viscous flows. The results of this study suggest that the formulation of the limiting process is more important than the choice of a particular flux splitting technique in determining the accuracy of computed viscous flows. Symmetric limited positive and upstream limited positive schemes hold the promise of improving the accuracy of the results, especially on coarser grids. HE calculation of compressible flows at transonic, supersonic, and hypersonic Mach numbers requires the implementation of nonoscillatory discrete schemes which combine high accuracy with high resolution of shock waves and contact discontinuities. These schemes must also be formulated in such a way that they facilitate the treatment of complex geometric shapes. In the past decade numerous schemes have been developed to meet these requirements in conjunction with the solution of the Euler equations.l More recently, the application of such schemes to the Navier-Stokes equations has produced algorithms which have progressively gained acceptance as analysis tools in the aerospace industry. There remains, however, a need to understand and improve Navier-Stokes schemes beyond the current state of the art. The most compelling reason for this rests on the fact that shock capturing requires the construction of schemes which are numerically dissipative, a requirement which could affect the global accuracy of the solution of the physical viscous problem. In a recent paper2 Jameson has shown that a theory of nonoscillatory schemes can be developed for scalar conservation laws based upon the local extremum diminishing (LED) principle that maxima should not increase and minima should not decrease. Moreover, although it is equivalent to the total variation diminishing principle (TVD) for one-dimensional problems, the LED principle can be applied naturally to multidimensional problems on both structured and unstructured meshes. This recent development has shed new light on the principles underlying the construction of both high-resolution switched and flux-limited dissipation schemes. In particular, it allowed the new formulation of two families of flux-limited schemes denominated, symmetric limited positive (SLIP) and upstream limited positive (USLIP), respectively. The present work merges several dissipation schemes based on both the SLIP and USLIP construction with a well-developed cell-centered, finite-volume formulation for solving the two-dimensional Navier-Stokes equations.3 The aim is to analyze and validate these new discretizations for the solution of viscous flow problems. In Sec. II the design principles of nonoscillator y discrete approximations to a scalar convection equation are reviewed together