Showing papers in "AIAA Journal in 1988"

Reassessment of the scale-determining equation for advanced turbulence models

Abstract: A comprehensive and critical review of closure approximations for two-equation turbulence models has been made. Particular attention has focused on the scale-determining equation in an attempt to find the optimum choice of dependent variable and closure approximations. Using a combination of singular perturbation methods and numerical computations, this paper demonstrates that: 1) conventional A:-e and A>w formulations generally are inaccurate for boundary layers in adverse pressure gradient; 2) using "wall functions'' tends to mask the shortcomings of such models; and 3) a more suitable choice of dependent variables exists that is much more accurate for adverse pressure gradient. Based on the analysis, a two-equation turbulence model is postulated that is shown to be quite accurate for attached boundary layers in adverse pressure gradient, compressible boundary layers, and free shear flows. With no viscous damping of the model's closure coefficients and without the aid of wall functions, the model equations can be integrated through the viscous sublayer. Surface boundary conditions are presented that permit accurate predictions for flow over rough surfaces and for flows with surface mass addition.

Lower-upper Symmetric-Gauss-Seidel method for the Euler and Navier-Stokes equations

Abstract: On developpe un schema de relaxation multigrille. Application a des ecoulements transsoniques

Near-wall turbulence models for complex flows including separation

Abstract: Results of a computational experiment designed to investigate the performance of different near-wall treatments in a single turbulence model with a common numerical method are reported. The complete fully elliptic, Reynoldsaveraged Navier-Stokes equations have been solved using a low-Reynolds-number model, a new two-layer model, and a two-point wall-function method, in thek-s turbulence model, for the boundary layer and wake of two axisymmetric bodies. These tests enable the evaluation of the performance of the different approaches in flows involving longitudinal and transverse surface curvatures, streamwise and normal pressure gradients, viscous-inviscid interaction, and separation. The two-layer approach has been found to be quite promising for such flows and can be extended to other complex flows.

Pressure based calculation procedure for viscous flows at all speeds in arbitrary configurations

Abstract: A calculation procedure for viscous flows at all Mach numbers has been described. The scheme has been developed for a nonorthogonal coordinate system. In order to handle both incompressible and compressible flows, pressure has been selected as a primary dependent variable in preference to density. The pressure field is evaluated using the compressible form of the SIMPLER algorithm. The accuracy of the proposed scheme has been assessed by comparing the results with experimental data or other numerical results available in the literature.

Ram accelerator - A new chemical method for accelerating projectilesto ultrahigh velocities

Abstract: A new method for accelerating projectiles from velocities of ~0.7 km/s up to -12 km/s using chemical energy is presented in this paper. The concept, called the "ram accelerator," is based on gasdynamic principles similar to those of an airbreathing ramjet but operates in a different manner. The projectile, which resembles the center body of a ramjet, travels through a tube filled with a premixed gaseous fuel and oxidizer mixture. The tube becomes the outer cowling of the ramjet, and the energy release process travels with the projectile. By tailoring the propellant mixture along the tube, a nearly constant acceleration can be achieved. In principle, the ram accelerator can be scaled for projectile masses ranging from grams to hundreds of kilograms and is capable of ballistic efficiencies as high as 30%. A straightforwar d, quasisteady, one-dimensional approach is used to model the acceleration process. The experimental facility developed to investigate the concept is described, and the results of recent experiments are presented. The velocity range of 690-1500 m/s has been explored in a 4.88-m long, 38-mm bore accelerator tube. Using methane, oxygen, and various diluents, accelerations of up to 16,000 g have been achieved with 75 gm projectiles and gas fill pressures of 20 atm. Proof of concept has been demonstrated, and agreement between theory and experiment has been found to be very good.

Two new approximate methods for analyzing free vibration of structural components

Abstract: Two approximate methods, which have not previously been used for structural dynamics problems, are applied to the free vibration analysis of various structural components. The first method is a new version of the complementary energy method. It is shown to be considerably more accurate than the conventional Rayleigh and Rayleigh-Schmidt methods when applied to spatially one-dimensional free vibration problems: prismatic and tapered bars, prismatic beams, and axisymmetric motion of circular membranes. The second method is the differential quadrature method introduced by Bellman and his associates. It is applied successfully here to all of the problems mentioned plus square membranes and circular and square plates.

Absolute instability in hot jets

Abstract: The linear inviscid stability of heated compressible axisymmetric jets is re-examined, focusing on the impulse response of the flow. The case is of particular interest in which a transient grows exponentially in place, i.e., at the location at which it was generated. Such a situation is commonly termed absolutely unstable as opposed to correctively unstable. Limiting the analysis to locally parallel mean velocity and density profiles, infinite Froude number, and zero Eckert number, the absolute instability boundaries, i.e., the boundaries between absolute and convective instability, are explored in the mean flow parameter space. The influence of mean flow profiles, exit Mach number, exit temperature ratio, and exit velocity ratio are considered and discussed separately. It is shown that heated jets, i.e, jets with lower than ambient density, can, depending on operating conditions, develop a region of local absolute instability in the potential core region if the jet density is less than 0.72 times the ambient density. The results suggest that a self-excited global resonance may dominate the heated jet response in situations of practical interest.

Finite-volume method for the calculation of compressible chemically reacting flows

Abstract: Integration numerique des equations d'Euler et Navier-Stokes. On developpe une technique d'acceleration basee sur le preconditionnement des equations de conservation

Calculation of eigenvector derivatives for structures with repeated eigenvalues

Abstract: In structural optimization and system identification, eigenvector derivatives provide important information for updating design/model parameters. When the current parameter values yield repeated eigenvalues, it has not been possible previously to calculate unique eigenvector derivatives. Recent work has provided a method for determining unique eigenvalue derivatives for this case, but methods for calculating the eigenvector sensitivities have been incomplete. In this work, a complete method for calculation of repeated-root eigenvector derivatives is shown for the real, symmetric structural eigenproblem. The derivation is completed by using information from the second derivative of the eigen problem and is limited to the case of distinct eigenvalue sensitivities. As an example, the repeated-root eigenvector sensitivities are calculated for a simple three degree-of-freedom beam grillage. Comparisons of linear approximations (using these derivatives) to the calculated eigenvectors support the accuracy of the formulation.

Measurements in turbulent swirling flow through an abrupt axisymmetric expansion

Abstract: Mesure a l'aide d'un anemometre laser Doppler des vitesses moyenne et quadratique moyenne dans un ecoulement d'eau

Buckling and Postbuckling of Delaminated Composites Under Compressive Loads Including Transverse Shear Effects

Abstract: The deformation of delaminated composites under axial compression is analyzed by a one-dimensional beam-plate model. In this model, a formulation that accounts for the transverse shear effects is also presented. With the perturbation technique, analytical solutions for the critical instability load and the postbuckling deflections are obtained. All possible instability modes, namely, local delamination buckling, global plate buckling, and coupled global and local (mixed) buckling, are considered. Specific emphasis is placed on studying the transverse shear effects on both the critical load and the postcritical characteristics, as well as the influence of the geometry such as that of the location of the delamination across the thickness. The postbuckling solution is used in conjunction with a /-integral formulation to study the postcritical characteristics with respect to possible quasistatic extension of the delamination and the energy absorption capacity of a beam.

On-orbit damage assessment for large space structures

Abstract: The need for monitoring the dynamic characteristics of large structural systems, for the purpose of assessing the potential degradation of structural properties, has been established. This paper develops a theory for assessing the occurrence, location, and extent of potential damage utilizing on-orbit response measurements. The feasibility of the method is demonstrated using a simple structural system as an example.

Structural optimization with frequency constraints

Abstract: This paper presents a design optimization algorithm for structural weight minimization with multiple frequency constraints. An optimality criterion method based on uniform Lagrangian density for resizing and a scaling procedure to locate the constraint boundary were used in optimization. Multiple frequency constraints of equality and inequality types were addressed. The effectiveness of the algorithm was demonstrated by designing a number of truss structures with as many as 489 design variables. No attempt was made to reduce the number of design variables by such procedures as linking and/or invoking symmetry conditions. The design examples include a 10-bar truss, 200-bar truss, a modified ACOSS-II, and COFS (Control of Flexible Structures) mast truss. All the structures contain nonstructural mass besides their own mass. The algorithm is extremely stable and, in all cases, the optimum designs were obtained in less than 20 iterations regardless of the size of the structure and the number of design variables.

Dynamic pressure loads associated with twin supersonic plume resonance

Abstract: The phenomenon of twin supersonic plume resonance is defined and studied as it pertains to high level dynamic loads in the inter-nozzle region of aircraft like the F-15 and B1-A. Using a 1/40th scale model twin jet nacelle with powered choked nozzles, it is found that intense internozzle dynamic pressures are associated with the synchrophased coupling of each plume's jet flapping mode. This condition is found most prevalent when each plume's jet flapping mode has constituent elements composed of the B-type helical instability. Suppression of these fatigue bearing loads was accomplished by simple geometric modifications to only one plume's nozzle. These modifications disrupt the natural selection of the B-type mode and thereby decouple the plumes.

Visualization and wake surveys of vortical flow over a delta wing

Abstract: An experimental investigation of vortex breakdown on delta wings at high angles of attack is presented. Smoke flow visualization and the laser light sheet technique were used to obtain cross-sectional views of the leading-edge vortices as they break down for a series of flat-plate delta wings having sweep angles of 70, 75, 80, and 85 deg. At low tunnel speeds (as low as 3 m/s), details of the flow that are usually imperceptible or blurred at higher speeds can be clearly seen. A combination of lateral and longitudinal cross-sectional views provides information on the three-dimensional nature of the vortex structure before, during, and after breakdown. Whereas details of the flow are identified in still photographs, the dynamic characteristics of the breakdown process have been recorded using high-speed movies. Velocity measurements have been obtained using a laser Doppler anemometer with the 70 deg delta wing at 30 deg angle of attack. The measurements show that, when breakdown occurs, the core flow is transformed from a jet-like to a wake-like flow.

Variational method for design sensitivity analysis in nonlinear structural mechanics

Abstract: This paper describes a unified variational theory of design sensitivity analysis of linear and nonlinear structures for shape, nonshape, and material selection problems. The concepts of reference volume and adjoint structure are used to develop the unified viewpoint. A general formula for design sensitivity analysis is derived. Several analytical linear and nonlinear examples are used to interpret various terms of the formula and demonstrate its use. Such analytical examples give insight into the application of the general formula to more complex problems that must be treated numerically. Adjoint state fields are also interpreted as sensitivities for the functional related to the primary structure.

Finite-Element Model for Composite Beams with Arbitrary Cross-Sectional Warping

Abstract: A finite-element formulation has been developed to take into account the warping effect of composite beams. This formulation is to be used to model combined bending, torsional, and extensional behavior of composite beams. The new approach can model thin-walled beams with complicated cross sections, tapers, and arbitrary planforms. The strain is assumed to vary linearly through the wall thickness. V^arping effects are properly incorporated in the formulation by assuming small warping displacements superimposed over cross sections in the deformed configuration. Transverse shear deformations are included. Numerical tests of example problems demonstrate the validity and effectiveness of the present approach. Comparisons of the present formulation with a shell element formulation and an experimental observation show excellent agreement.

Comparison of several methods for calculating vibration mode shape derivatives

Abstract: Four methods for the calculation of derivatives of vibration mode shapes (eigenvectors) with respect to design parameters are described. These are finite-difference method, modal method, a modified modal method and Nelson's method. The methods are implemented in a general-purpose commercial finite-element program and applied to the following test problems: a cantilever beam and a stiffened cylinder with a cutout. Design variables are a beam tip mass, a beam root height, and specific dimensions of the cylinder model. The methods are compared on the basis of central processor (CP) seconds required to obtain the derivatives, and two of the methods are also evaluated for the rapidity of convergence. Data is presented showing the amount of CP time used to compute the first four eigenvector derivatives for each example problem. A scalar measure of the error in the mode shape derivative is defined, and numerical results illustrating the rapidity of convergence of the approximate derivative to the exact derivative are presented. Results indicate an advantage in using Nelson's method because this method is exact and requires less CP time, especially when derivatives with respect to several design variables are computed.

Boundary Integral Equations for Recovery of Design Sensitivities in Shape Optimization

Abstract: A new formulation for obtaining design sensitivities in shape optimization has been developed. The formulation is based on a direct application of the material derivative concept to the appropriate boundary integral equations for displacements and stresses in an elastic solid. As a check on accuracy, the approach was applied to a uniformly loaded infinite plate containing an elliptical hole. In this case, the availability of an analytical solution made it possible to calculate errors exactly. Furthermore, a wide range of stress states could be considered simply by varying the aspect ratio of the elliptical hole. For convenience, the design sensitivities were calculated with respect to changes in the major axis. Numerical convergence was established by comparing results based on four successive boundary meshes. In general, the predictions for both displacement and stress sensitivities were remarkably accurate.

Game theory approach for the integrated design of structures and controls

Abstract: The problem of design of actively controlled structures subject to constraints on the damping parameters of the closed-loop system is formulated as a multiobjeetive optimization problem. The structural weight and the controlled system energy are considered as objective functions for minimization with cross-sectional areas of members as design variables. A computational procedure is developed for solving the multiobjeetive optimization problem using cooperative game theory. The feasibility of the procedure is demonstrated through the design of two truss structures.

Scaling of impact-loaded carbon-fiber composites

Abstract: The application of scaling laws to fiber composite laminates is discussed Particular emphasis is placed on the case of impact loading Scaling difficulties and conflicts are identified and illustrated in an experimental program based on impacted carbon-fiber composite beams It is shown that the lay-up of laminates is important in assessing the likely validity of scale-model tests for such composites It is also observed that significant size effects may dominate strength modeling

Transient response of graphite/epoxy and Kevlar/epoxy laminates subjected to impact

Abstract: The influence of different parameters on the impact behavior of laminated composite plates is considered analytically. A Rayleigh-Ritz energy method was used to spatially discretize the time-varying boundary value problem and a set of coupled, ordinary differential equations in time were obtained based on the discretized system Lagrangian. The effects of shearing deformation, bending-twisting coupling, and nonlinear contact behavior were included in the model. The resulting equations were integrated using the implicit Newmark beta method without the effects of rotary inertia. The results indicate that the effective mass of the plate is often an important effect in the response to impact events. In general, the influence of the constitutive behavior dominates for very low velocity impact, whereas the target mass properties become more important as the impactor velocity increases. This importance of mass clearly shows that impactor kinetic energy is not sufficient to characterize the impactor as the impactor mass is shown to have a large influence on the resulting dynamic behavior. In addition to these parameters, the effects of preload and material properties are considered and discussed.

Experimental investigation of the aerodynamic breakup of liquid drops

Abstract: An experimental investigation was made of the deformation and the mechanism of stripping-type breakup of liquid drops. Experiments were conducted in a 60 X 150 mm cross-sectional shock tube equipped with pulsed laser holographic interferometry. Water drops having diameters of 1030 and 4300 fim were examined for shock wave Mach numbers from 1.3 to 1.5 in atmospheric air. The Weber and Reynolds numbers under these conditions were in the range of 600 to 7600 and 1.38 to 10.4 x 10, respectively. In previous works, the experimental data of the stripping-type breakup were obtained by spark shadowgraphs and streak schlieren methods and high-speed movies. However, in previous works, due to the effect of light scattering through the micromist, the structure of disintegrating drops could not be visualized. The purpose of the present work is, by using holographic interferometry, to re-examine the classical problem of the stripping-type breakup of liquid drops. As a result, a four-stage mechanism of the stripping-type breakup of liquid drops was established.

Multigrid acceleration of the flux split Euler equations

Abstract: Multigrid acceleration is applied to a flux-split algorithm for solving the Euler equations in three dimensions. The basic algorithm is an implicit spatially split approximate factorization method. The stability and performance of the scheme in comparison to other factorizations are examined. Results are presented for three-dimensional flows that demonstrate substantially unproved convergence with the multigrid algorithm.

Efficient computation of modal sensitivities for systems with repeated frequencies

Abstract: The physical origin and ambiguity of repeated frequency modes is discussed, followed by a mathematical derivation of the governing equations for the partial derivatives of repeated eigenvalues with respect to changes in the system parameters. This is followed by a derivation for the associated eigenvector derivative equations. Physical interpretations are included to help explain the analytical results. An efficient computation procedure, which preserves the bandwidth for very large systems, is also proposed for systems with repeated or closely spaced eigenvalues. Two small examples to illustrate the solution procedure are presented. 8 references.

Structure of Turbulent Sonic Underexpanded Free Jets

Abstract: Turbulent mixing in adapted and weakly underexpanded (underexpansion ratios less than 1.4) round jets, involving fully developed pipe flows injected into still air, was studied experimentally. Measurements included mean and fluctuating concentrations and mean static pressures using laser-induced iodine flourescence and mean and fluctuating streamwise velocities using laser Doppler anemometry. Predictions were used to help interpret the measurements and to initiate evaluation of methods for analyzing these processes. The predictions were based on k-e turbulence models, including a proposed extension to treat compressibility effects at high convection Mach numbers. In conjunction with other measurements, the results show that the near-field region of underexpanded jets is influenced by compressibility, which tends to reduce turbulent mixing rates at high convective Mach numbers, and high turbulence levels at the jet exit, which tends to increase turbulent mixing rates. Predictions based on effective-ada pted-jet exit conditions yielded reasonably good estimates of mixing levels near the exit of underexpanded jets for both fully developed and slug flow jet exit conditions; however, such methods provide no information concerning the near-field region containing the shock waves. Predictions based on solution of parabolized Navier-Stokes governing equations, using the SCIPVIS algorithm, were encouraging for slug flow exit conditions, but this approach must be extended to treat fully developed flow at the jet exit.

Flow structure in the near-wall zone of a turbulent separated flow

Abstract: Les mesures effectuees montrent que la structure de la region proche de la paroi d'un ecoulement decolle est differente de celle d'une couche limite turbulente normale