Showing papers in "AIAA Journal in 1996"

Physical programming - Effective optimization for computational design

Abstract: A new effective and computationally efficient approach for design optimization, hereby entitled physical programming, is developed. This new approach is intended to substantially reduce the computational intensity of large problems and to place the design process into a more flexible and natural framework. Knowledge of the desired attributes of the optimal design is judiciously exploited. For each attribute of interest to the designer (each criterion), regions are defined that delineate degrees of desirability : unacceptable, highly undesirable, undesirable, tolerable, desirable, and highly desirable. This approach completely eliminates the need for iterative weight setting, which is the object of the typical computational bottleneck in large design optimization problems. Two key advantages of physical programming are 1) once the designer's preferences are articulated, obtaining the corresponding optimal design is a noniterative process-in stark contrast to conventional weight-based methods and 2) it provides the means to reliably employ optimization with minimal prior knowledge thereof. The mathematical infrastructure that supports the physical programming design optimization framework is developed, and a numerical example provided. Physical programming is a new approach to realistic design optimization that may be appealing to the design engineer in an industrial setting.

Two-layer approximate boundary conditions for large-eddy simulations

Abstract: A new method to model the effect of the solid boundaries on the rest of the flowfield in large-eddy simulations is proposed The filtered Navier-Stokes equations are solved up to the first computational point From there to the wall, a simplified set of equations is solved, and an estimate of the instantaneous wall shear stress required to impose boundary conditions is obtained Computations performed for the plane channel, square duct, and the rotating channel flow cases gave improved results compared with existing models The additional computing time required by the model is on the order of 10-15% of the overall computing time The mean flow quantities and low-order statistics, which are of primary interest in engineering calculations, are in very good agreement with the reference data available in the literature

Three-dimensional unstructured viscous grids by the advancing-layers method

Abstract: A method is presented for generating three-dimensional viscous unstructured grids on complex configurations. The approach stems from a natural extension of the advancing-layers method (ALM) that has been successfully applied to two-dimensional problems in prior work. High-aspect-ratio tetrahedral cells are constructed in viscous dominated flow regions by the ALM, with the remaining isotropic cells generated by the conventional advancing- front method. Relying on a totally unstructured grid-generation strategy, the method benefits from a high degree of flexibility and automation required for generating grids around complex geometries. Sample three-dimensional grids around complex configurations are presented to show the capability of the method. NSTRUCTURED grid methodology has demonstrated consid- erable success in computational fluid dynamics (CFD) mainly due to its inherent flexibility for discretization of geometrically com- plex domains. The growing number of new techniques and publi- cations imply the importance and increased interest in this class of grids. A thorough survey of the subject is given in Ref. 1. However, despite their remarkable effectiveness in computation of complex inviscid flow fields, unstructured grids have yet to provide for the routine computation of the Navier-Stokes equations in three dimen- sions. The lack of a robust unstructured grid strategy for generating highly stretched cells has been a major obstacle to applying such methodology to complex three-dimensional viscous-flow problems. Among the numerous references on the unstructured grid method- ology in the literature, there are only a few that discuss the problem of three-dimensi onal viscous flow on unstructured grids. Notable among the references that address unstructured viscous grid gener- ation are those cited in Refs. 2-10. Although most of the reported techniques have provided appropriate results for the specific applica- tions shown, many lack the desired generality, flexibility, efficiency, automation, and robustness. Semiunstructured techniques, for example, retain some of the limitations of structured grid-generation methods that impair the re- quired flexibility and robustness of the method to handle arbitrary three-dimensional complex configurations. Prismatic grids are ef- ficient in terms of computer memory requirement and are effec- tive as long as the configuration under consideration is relatively simple. For geometrically complex domains, this class of grid- generation techniques requires sophisticated schemes, to ensure the integrity of generated grids, which makes the method computation- ally intensive.3 Alternatively, prisms have been coupled with more flexible grids (e.g., tetrahedral) to form hybrid grids.5 This type of grid requires a special flow solution strategy to treat different cell types in the field. Furthermore, to form a one-to-one connection between the prismatic and tetrahedral cells, an identical number of prism layers should be maintained globally to obscure the quadri- lateral faces of prisms that obviously do not match with triangular faces of tetrahedral cells. This may limit the extent and flexibility of the prismatic portion of hybrid grids and, thus, reduce the capa- bility of the method for complex problems. Realistic configurations involving sharp edges, integrated components, gaps between close surfaces, etc., are examples of such complexities that require extra careful attention and enhanced capabilities that the existing semi- unstructured techniques may lack.

Quadrature-free implementation of discontinuous galerkin method for hyperbolic equations

Abstract: A discontinuous Galerkin formulation that avoids the use of discrete quadrature formulas is described and applied to linear and nonlinear test problems in one and two space dimensions. This approach requires less computational time and storage than conventional implementations but preserves the compactness and robustness inherent in the discontinuous Galerkin method. Test problems include the linear and nonlinear one-dimensional scalar advection of smooth initial value problems that are discretized by using unstructured grids with varying degrees of smoothness and regularity, and two-dimensional linear Euler solutions on unstructured grids.

Chemical laser modeling with genetic algorithms

Abstract: A genetic algorithm technique was implemented to determine a set of unknown parameters that best matched the Blaze II chemical laser model predictions with experimental data. This is the first known application of the genetic algorithm technique for modeling lasers, chemically reacting flows, and chemical lasers. Overall, the genetic algorithm technique worked exceptionally well for this chemical laser modeling problem in a cost effective and time efficient manner. Blaze II was baselined to existing chemical oxygen-iodine laser data taken with the research assessment and device improvement chemical laser device with very good agreement. Mixing calculations for the research assessment and device improvement chemical laser nozzle indicate that higher iodine flow rates are necessary to maintain a significant fraction of the nominal performance as the total pressure is increased by the addition of helium ; this agrees with research assessment and device improvement chemical laser experimental data. It may be possible to implement this genetic algorithm technique to optimize the performance of any chemical laser as a function of any of the flow rates, mirror location, mirror size, nozzle configuration, injector sizes, and other factors. This modeling procedure can be used as a method to guide experiments to improve chemical laser performance.

Comparison of eddy viscosity-transport turbulence models for three-dimensional, shock-separated flowfields

Abstract: An evaluation of four one-equation eddy viscosity-transport turbulence closure models as applied to three-dimensional shock wave/boundary-layer interactions is presented herein. Comparisons of two versions of the Baldwin-Barth model, an approach of Edwards and McRae, and a modified form of the Spalart-Allmaras model are presented for two test cases, one involving Mach 8 flow over a flat plate/sharp fin apparatus and the other involving Mach 3 flow over a cylinder-offset-cone geometry. Strengths and weaknesses of the one-equation approaches are highlighted through direct comparison with experimental data, and the effect of grid refinement is examined.

Damage Localization in Structures Without Baseline Modal Parameters

Abstract: A methodology to localize and estimate the severity of damage in structures for which only postdamage modal parameters are available for a few vibrational modes is presented. First, a theory of damage localization and severity estimation that utilizes only changes in mode shapes of the structures is outlined. Next, a system identification method that combines the experimental modal data and the modal parameters of a finite element model of the structure under investigation is developed to yield estimates of the baseline (i.e., predamage) modal parameters for the structure. Finally, the feasibility and practicality of the complete damage detection algorithm are demonstrated by localizing and sizing damage in a continuous beam for which only postdamage modal parameters for a single vibrational mode is available.

Optimized Compact Finite Difference Schemes with Maximum Resolution

Abstract: Direct numerical simulations and computational aeroacoustics require an accurate finite difference scheme that has a high order of truncation and high-resolution characteristics in the evaluation of spatial derivatives. Compact finite difference schemes are optimized to obtain maximum resolution characteristics in space for various spatial truncation orders. An analytic method with a systematic procedure to achieve maximum resolution characteristics is devised for multidiagonal schemes, based on the idea of the minimization of dispersive (phase) errors in the wave number domain, and these are applied to the analytic optimization of multidiagonal compact schemes. Actual performances of the optimized compact schemes with a variety of truncation orders are compared by means of numerical simulations of simple wave convections, and in this way the most effective compact schemes are found for tridiagonal and pentadiagonal cases, respectively. From these comparisons, the usefulness of an optimized high-order tridiagonal compact scheme that is more efficient than a pentadiagonal scheme is discussed. For the optimized high-order spatial schemes, the feasibility of using classical high-order Runge-Kutta time advancing methods is investigated.

Low-Reynolds-number separation on an airfoil

Abstract: Unsteady boundary-layer separation from an Eppler 387 airfoil at low Reynolds number is studied numerically. Through a series of computations, the effects of Reynolds number and angle of attack are investigated. For all cases, vortex shedding is observed from the separated shear layer. From linear stability analysis, a KelvinHelmholtz instability is identified as causing shear layer unsteadiness. The low-turbulence wind-tunnel tests of the Eppler 387 airfoil are used to compare with the time-averaged results of the present unsteady computations. The favorable comparison between computational and experimental results strongly suggests that the unsteady largescale structure controls the low-Reynolds-number separation bubble reattachment with small-scale turbulence playing a secondary role. Nomenclature C = chord length CD - drag coefficient CL = lift coefficient Cp = pressure coefficient / = shedding frequency Re = chord Reynolds number R P - reattachment point S P = separation point Sr = Strouhal number U = velocity 9 = momentum thickness Subscripts sep = conditions at separation oo = freestream conditions

Efficient Foil Propulsion Through Vortex Control

Abstract: We investigate the problem of a heaving and pitching hydrofoil in an inflow that consists of a uniform velocity field and a staggered array of vortices. The foil can exploit the energy in such a Karman vortex street for efficient propulsion of animals or submarines. Through a two-dimensional inviscid analysis, we find that the phase between foil motion and the arrival of inflow vortices is a critical parameter. Everything else being equal, the highest efficiency is seen when this phase is such that the foil moves in close proximity to the oncoming vortices. Different modes of vortex interaction in the wake results from a variation in this phase, and we show that the flow downstream of the foil is related to the foil input power.

Time-Domain Impedance Boundary Conditions for Computational Aeroacoustics

Abstract: It is an accepted practice in aeroacoustics to characterize the properties of an acoustically treated surface by a quantity known as impedance. Impedance is a complex quantity. As such, it is designed primarily for frequency-domain analysis. Time-domain boundary conditions that are the equivalent of the frequency-domain impedance boundary condition are proposed. Both single frequency and model broadband time-domain impedance boundary conditions are provided. It is shown that the proposed boundary conditions, together with the linearized Euler equations, form well-posed initial boundary value problems. Unlike ill-posed problems, they are free from spurious instabilities that would render time-marching computational solutions impossible.

Analysis of piezoelastic structures with laminated piezoelectric triangle shell elements

Abstract: In the recent development of active structural systems and microelectromechanical systems, piezoelectrics are widely used as sensors and actuators. Because of the limitations of theoretical and experimental models in design applications, finite element development and analysis are proposed and presented in this paper. A new laminated quadratic C° piezoelastic triangular shell finite element is developed using the layerwise constant shear angle theory. Element and system equations are also derived. The developed piezoelastic triangular shell element is used to model 1) a piezoelectric bimorph pointer and 2) a semicircular ring shell. Finite element (triangular shell finite element) solutions are compared closely with the theoretical, experimental, and finite element (thin solid finite element) results in the bimorph pointer case. Natural frequencies and distributed control effects of the ring shell with piezoelectric actuators of various length are also studied. Finite element analyses suggested that the inherent piezoelectric effect has little effect on natural frequencies of the ring shell. Vibration control effect increases as the actuator length increases, and it starts leveling off at the seven-patch (70%) actuator. Coupling and control spillover of lower natural modes are also observed.

Data-parallel lower-upper relaxation method for the navier-stokes equations

Abstract: The lower-upper symmetric Gauss-Seidel method is modified for the simulation of viscous flows on massively parallel computers. The resulting diagonal data-parallel lower-upper relaxation (DP-LUR) method is shown to have good convergence properties on many problems. However, the convergence rate decreases on the high cell aspect ratio grids required to simulate high Reynolds number flows. Therefore, the diagonal approximation is relaxed, and a full matrix version of the DP-LUR method is derived. The full matrix method retains the data-parallel properties of the original and reduces the sensitivity of the convergence rate to the aspect ratio of the computational grid. Both methods are implemented on the Thinking Machines CM-5, and a large fraction of the peak theoretical performance of the machine is obtained. The low memory use and high parallel efficiency of the methods make them attractive for large-scale simulation of viscous flows.

Demonstration and characterization of filtered Rayleigh scattering for planar velocity measurements

Abstract: Filtered Rayleigh scattering is an optical diagnostic technique that allows for simultaneous planar measurement of velocity, temperature, and pressure in unseeded flows. An overview of the major components of a filtered Rayleigh scattering system is presented. In particular, a detailed theoretical model is developed and discussed with associated model parameters and related uncertainties. Based on this model, results for two experimental conditions are presented: ambient room air and a Mach 2 freejet. These results include two-dimensional, spatially resolved measurements of velocity, temperature, and pressure derived from time-averaged spectra. ILTERED Rayleigh scattering (FRS), a recently developed flow diagnostic technique,1'2 achieves large suppression of background scattering allowing planar flowfield visualization and obtains quantitative measurements of velocity, temperature, and density in unseeded gaseous flows. This technique makes use of Rayleigh scattering from molecules in the flow and is driven by a high-power, narrow linewidth, tunable, injection seeded laser. When imaging the scattered light onto a charge-coupled device (CCD) camera, unwanted background scattering from stationary objects may be filtered out by tuning the frequency of the narrow linewidth laser to coincide with an atomic or molecular absorption line and by placing a cell containing the atomic or molecular species between the camera and the flow. This cell acts as a notch filter, absorbing all background scatter at the laser frequency. Scattered light that is Doppler shifted, however, passes through the filter and is imaged on the camera. Quantitative measure of flow properties is achieved by measuring the total intensity, Doppler shift, and spectral profile of the Rayleigh scattered light. The total intensity is directly proportional to density; the Doppler shift is directly proportional to velocity; and the spectral profile is a function of temperature and pressure. The scattering intensity, Doppler shift, and spectral profile are determined by passing the scattered light through the notch absorption filter and then by imaging it onto an intensified CCD camera. Because the filter absorbs light in a narrow frequency band, it converts the spectral information contained in the Doppler shift and Rayleigh profile into intensity information at the camera. By collecting data (camera pixel intensity) for varying conditions, v, T, and P may be determined. Previous work has concentrated on the use of this technique for background suppression when visualizing flows and for the measurement of velocity. The background suppression feature of FRS has been used to image flowfields that otherwise would be completely obscured by the strong scattering from wind-tunnel surfaces. The authors have used this technique to image the flowfield inside a Mach 3 inlet and to generate volumetric images of the crossing shocks and boundary layer.3 Elliott et al.4 have also used this technique to observe structures in compressible mixing layers. The use of FRS to measure velocity was initially demonstrated using scattering

Thrust Generation due to Airfoil Flapping

Abstract: Thrust generation on a single flapping airfoil and a flapping/stationary airfoil combination in tandem is studied parametrically. A multiblock Navier-Stokes solver is employed to compute unsteady flowfields. The unsteady flowfield around a single flapping airfoil is also computed by an unsteady potential flow code. The numerical solutions predict thrust generation in flapping airfoils and a significant augmentation of thrust in flapping/stationary airfoil combinations in tandem. The propulsive efficiency is found to be a strong function of reduced frequency and the amplitude of the flapping motion. At a flapping amplitude of 0.40 chord lengths and a reduced frequency of 0.10, the propulsive efficiency of a single NACA 0012 airfoil was computed to be more than 70 %. For the airfoil combination in tandem, the propulsive efficiency was augmented more than 40% at a reduced frequency of 0.75 and a flapping amplitude of 0.20 chord lengths when the airfoils are separated by about two chord lengths.

Solution-Adaptive Cartesian Cell Approach for Viscous and Inviscid Flows

Abstract: A Cartesian cell-based approach for adaptively refined solutions of the Euler and Navier-Stokes equations in two dimensions is presented. Grids about geometrically complicated bodies are generated automatically, by the recursive subdivision of a single Cartesian cell encompassing the entire flow domain. Where the resulting cells intersect bodies, polygonal cut cells are created using modified polygon-clipping algorithms. The grid is stored in a binary tree data structure that provides a natural means of obtaining cell-to-cell connectivity and of carrying out solution-adaptive mesh refinement. The Euler and Navier-Stokes equations are solved on the resulting grids using a finite volume formulation. The convective terms are upwinded: A linear reconstruction of the primitive variables is performed, providing input states to an approximate Riemann solver for computing the fluxes between neighboring cells. The results of a study comparing the accuracy and positivity of two classes of cell-centered, viscous gradient reconstruction procedures is briefly summarized. Adaptively refined solutions of the Navier-Stokes equations are shown using the more robust of these gradient reconstruction procedures, where the results computed by the Cartesian approach are compared to theory, experiment, and other accepted computational results for a series of low and moderate Reynolds number flows.

Computation of turbulent axisymmetric and nonaxisymmetric jet flows using the K-epsilon model

Abstract: It is known that the standard K-e model does not provide an accurate prediction of the mean flow of turbulent jets. This is so even when the Pope and Sarkar correction terms are included. It is suggested that the K-e model, together with the Pope and Sarkar terms for nonplanar and high convective Mach number flow corrections, does contain the essential ingredients of turbulence physics for adequate jet mean flow prediction. The problem lies in the standard coefficients that were calibrated by using boundary-layer and low Mach number plane mixing layer data. By replacing these coefficients by a new set of empirical coefficients, it is demonstrated that the model can offer good predictions of axisymmetric, rectangular, and elliptic jet mean flows over the Mach number range of 0.4-2.0 and jet total temperature to ambient temperature ratio of 1.0-4.0. The present result conveys the message that it is possible that there is no universally applicable turbulence model. The reason is that although the characteristics and dynamics of fine-scale turbulence may be the same for all turbulent flows, the large turbulence structures, having dimensions comparable to the local length scale of the flow, are significantly influenced by local boundary conditions and geometry. Thus overall turbulence dynamics are somewhat problem type dependent.

Identification of Impact Force and Location Using Distributed Sensors

Abstract: An impact load identification method using distributed built-in sensors for detecting foreign object impact is proposed. The identification system consists of a system model and a response comparator. The system model characterizes the dynamic response of the structure subject to a known impact force. The comparator compares the measured sensor outputs with the estimated measurements from the model and predicts the location and force history of the impact. The method has been developed for beams containing distributed piezoelectric sensors. The identification system was tested using simulated conditions. For all of the cases considered, the estimation errors fell well within the prespecified limit.

Analysis of cracked aluminum plates repaired with bonded composite patches

Abstract: Bonded composite repair has been recognized as an efficient and economical method to extend the service life of cracked aluminum components. An accurate tool for investigating the stress intensity factor in the cracked aluminum structure after repair is needed. The use of three-dimensional finite elements is computationally expensive. A simple analysis method using Mindlin plate theory is presented. Specifically, the aluminum plate and composite patch are modeled separately by the Mindlin plate finite element, whereas the adhesive layer is modeled with effective springs connecting the patch and aluminum plate. Constraint equations are used to enforce compatibility of the patch-adhesive and adhesive-aluminum plate interfaces. Comparison of the present stress intensity factors for the aluminum crack with a plane boundary element analysis and a three-dimensional finite element analysis are made. A procedure for calculating the strain energy release rate along the debond front at the aluminum-adhesive interface is proposed.

Hybrid Prismatic/Tetrahedral Grid Generation for Viscous Flows Around Complex Geometries

Abstract: A method for the generation of hybrid prismatic/tetrahedral grids is described for complex three-dimensional geometries, including multibody domains, and employed for viscous flows around an aircraft. The prisms cover the region close to each body's surface, while tetrahedra are created elsewhere. A special method is presented that allows the generation of single-block, nonoverlapping prismatic meshes even in complex geometries that contain narrow gaps and cavities. Examples of cases treated by this method include wing-engine configurations as well as multi-element wings. The second development is a combined octree/advancing front method for the generation of the tetrahedra of the hybrid mesh. The main feature of this octree-based tetrahedra generator is that it does not require the creation of a background mesh by the user for the determination of the grid-spacing and stretching parameters. These are determined via an automatically generated octree. The hybrid grid generator is robust, geometry independent, and requires no user intervention. The suitability of hybrid meshes for capturing viscous flow phenomena is demonstrated by simulation of viscous flows around a high-speed civil transport type of aircraft configuration. Reduction in computer resources has been substantial, allowing flow simulations to be performed on workstations rather than supercomputers.

Computation of quadrupole noise using acoustic analogy

Abstract: Acoustic analogy computations of vortex shedding noise were carried out in the context of a two-dimensional, low-Mach-number laminar flow past a NACA 0012 airfoil at chord Reynolds number of 10 4 . The incompressible Navier-Stokes equations were solved numerically to give an approximate description of the near-field flow dynamics and the acoustic source functions. The radiated far-field noise was computed based on Curle's extension to the Lighthill analogy. This study emphasizes an accurate evaluation of the Reynolds stress quadrupoles in the presence of an extensive wake. An effective method for separating the physical noise source from spurious boundary contributions caused by eddies crossing a permeable computational boundary is presented. The effect of retarded-time variations across the source region is also examined. Computational solutions confirm that the quadrupole noise is weak compared with the noise due to lift and drag dipoles when the freestream Mach number is small. The techniques developed in this study are equally applicable to flows in which the volume quadrupoles act as a prominent noise source.

Genetic optimization of target pressure distributions for inverse design methods

Abstract: A genetic algorithm has been applied to optimize target pressure distributions for inverse design methods. Pressure distributions around airfoils are parameterized by B-spline polygons, and the airfoil drag is minimized under constraints on lift, airfoil thickness, and other design principles. Once target pressure distributions are obtained, corresponding airfoil/wing geometries can be computed by an inverse design code coupled with a Navier-Stokes solver. Successful design results were obtained for transonic cases with and without a shock wave.

Experimental and numerical investigations of dynamic stall on a pitching airfoil

Abstract: The dynamic stall process on a pitching NACA 0012 airfoil was investigated by two experimental techniques-particle image velocimetry (PIV) and laser-sheet visualizations-and a numerical code based on the Navier-Stokes equations. The freestream velocity was 28 m/s, leading to a Reynolds number (based on airfoil chord) of 3.73 X 10 5 . The airfoil motion was a sinusoidal function between 5 and 25 deg of incidence, with a frequency of 6.67 Hz corresponding to a reduced frequency (based on airfoil half-chord) of 0.15. The out-of-plane component of the vorticity could be derived from the PIV velocity fields. The comparison between experimental and numerical results was conducted for the four main phases of the dynamic stall process, i.e., attached flow, development of the dynamic stall vortex, poststall vortex shedding, and reattachment. In general, the computational results agreed very well with the experimental results. However, some discrepancies were observed and discussed. The cycle-to-cycle nonreproducibility of the flowfield during the phase of massive separation is also mentioned.

Unsteady aerodynamics associated with a horizontal-axis wind turbine

Abstract: The wind turbine industry is currently facing many difficulties constructing efficient wind turbine machines caused by the inability to adequately predict structural loading and power output. Available evidence from wind turbines in a field environment suggests that formation of complex unsteady separated flowfields may be responsible for many aspects of wind turbine component failure. To examine this possibility in more detail, the Combined Experiment was developed. A full-scale wind turbine was constructed and operated in a field environment. The environment chosen was subject to wide variations in wind speed and direction and subsequently generated an extensive set of data for a variety of inflow test conditions. A single wind turbine blade was instrumented with pressure transducers and strain gauges with several data sets collected across a wide spectrum of typical and limiting wind turbine operating conditions. Surface pressure data taken at various spanwise locations along the blade demonstrated highly transient and spatially complex aerodynamic behavior for even the most basic operating conditions. Integrated normal force coefficient data showed enhanced lift values significantly beyond that predicted from steady-state two-dimensional wind-tunnel test data. Surface pressure data and integrated moment coefficient data suggested formation of coherent vortices consistent with the dynamic stall process observed in wind-tunnel tests for pitching wings. The unsteady, three-dimensional aerodynamic behavior for this wind turbine was then discussed and summarized.

Scanned- and fixed-wavelength absorption diagnostics for combustion measurements using multiplexed diode lasers

Abstract: A multiplexed diode-laser sensor system comprising two diode lasers and fiber-optic components has been developed to nonintrusively monitor temperature and species mole fraction over a single path using both scanned-and fixed-wavelength laser absorption spectroscopy techniques. In the scanned-wavelength method, two InGaAsP lasers were current tuned at a 2-kHz rate across H 2 O transitions near 1343 nm and 1392 nm in the 2v 1 and v 1 + v 3 bands. Gas temperature was determined from the ratio of single-sweep integrated line intensities. Species mole fraction was determined from the measured line intensity and the calculated line strength at the measured temperature. In the fixed-wavelength method, the wavelength of each laser was fixed near the peak of each absorption feature using a computer-controlled laser line-locking scheme. Rapid measurements of gas temperature were obtained from the determination of peak line-intensity ratios. The system was applied to measure temperature and species concentration in the postflame gases of an H 2 -O 2 flame. The good agreement between the laser-based measurements obtained using scanned- and fixed-wavelength methods with those recorded with thermocouples demonstrates the flexibility and utility of the multiplexed diode-laser sensor system and the potential for rapid, continuous measurements of gasdynamic parameters in high-speed or transient flows with difficult optical access.

Recirculation zones of unconfined and confined annular swirling jets

Abstract: An experimental study is presented for flowfields behind an axially mounted cylindrical bluff body of annular swirling jet flows. Both the confined and the unconfined cases are examined. The controlled parameters included the Reynolds number Re and the swirl number S. Smoke streaks were used to observe the dynamic flow structures of the recirculation zone behind the bluff body. Laser-Doppler anemometry was used for velocity measurements at the jet exit and downstream of the annular jet flows. The results indicate that the recirculation zones for both the unconfined and the confined cases can be classified into seven typical flow patterns based on the Reynolds number and swirl number : stable flow, vortex shedding, transition, prepenetration, penetration, vortex breakdown, and attachment. The flow patterns and their domains in (Re, S) space for unconfined and confined cases were basically the same. In the attachment regime, the central recirculation zone for the confined case was larger than that for the unconfined case. The boundaries of the recirculation zone for various flow conditions were investigated. Scaling analysis was used to correlate the lengths of the recirculation zone with the swirl number.

Three-dimensional aerodynamic shape optimization using discrete sensitivity analysis

Abstract: An aerodynamic shape optimization procedure based on discrete sensitivity analysis is extended to treat three-dimensional geometries. The function of sensitivity analysis is to directly couple computational fluid dynamics (CFD) with numerical optimization techniques, which facilitates the construction of efficient direct-design methods. The development of a practical three-dimensional design procedures entails many challenges, such as: (1) the demand for significant efficiency improvements over current design methods; (2) a general and flexible three-dimensional surface representation; and (3) the efficient solution of very large systems of linear algebraic equations. It is demonstrated that each of these challenges is overcome by: (1) employing fully implicit (Newton) methods for the CFD analyses; (2) adopting a Bezier-Bernstein polynomial parameterization of two- and three-dimensional surfaces; and (3) using preconditioned conjugate gradient-like linear system solvers. Whereas each of these extensions independently yields an improvement in computational efficiency, the combined effect of implementing all the extensions simultaneously results in a significant factor of 50 decrease in computational time and a factor of eight reduction in memory over the most efficient design strategies in current use. The new aerodynamic shape optimization procedure is demonstrated in the design of both two- and three-dimensional inviscid aerodynamic problems including a two-dimensional supersonic internal/external nozzle, two-dimensional transonic airfoils (resulting in supercritical shapes), three-dimensional transport wings, and three-dimensional supersonic delta wings. Each design application results in realistic and useful optimized shapes.

Boundary-layer stability measurements in a hypersonic quiet tunnel

Abstract: Hypersonic boundary-layer measurements were conducted over a flared cone in a quiet wind tunnel. The flared cone was tested at a freestream unit Reynolds number of 2.82 x 10 6 /ft in a Mach 6 flow. This Reynolds number provided laminar-to-transitional flow over the model in a low-disturbance environment. Point measurements with a single hot wire using a novel constant voltage anemometry system were used to measure the boundary-layer disturbances. Surface temperature and schlieren measurements were also conducted to characterize the laminar-to-transitional state of the boundary layer and to identify instability modes. Results suggest that the second mode is the dominant mode of instability. The integrated growth rates of the second mode compared well with linear stability theory in the linear stability regime. Furthermore, the existence of higher harmonics of the fundamental suggests that nonlinear disturbances are not associated with high freestream disturbance levels.

Thick beam theory and finite element model with zig-zag sublaminate approximations

Abstract: A new technical theory and associated finite element model is introduced for thick laminated and sandwich beams. The theory can be cast as a layerwise theory with high-order zig-zag sublaminate approximations, thus greatly reducing the number of degrees of freedom required to accurately describe the bending and transverse shear kinematics in thick laminates. Furthermore, the theory is adaptable, allowing the user to choose the number of sublaminate approximations to achieve the desired accuracy. Based on this theory, a simple, efficient, and robust finite element model is developed that has the nodal topology of a four-noded planar element yet has the advantages of beam-type kinematics and a special interpolation scheme that obviates locking. The element contains a single zig-zag sublaminate approximation. If desired, multiple elements can be used through the thickness of a laminate to increase accuracy.