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NASA NTRS1985

Nonlinear adhesive behavior effects in a cracked orthotropic sheet stiffened by a semi-infinite orthotropic sheet

The stress-intensity factors are determined for a cracked orthotropic sheet adhesively bonded to an orthotropic stringer where the adhesive layer is modeled with a nonlinear stress-strain curve. By the use of Green's functions and the complex variable theory of orthotropic elasticity, a set of integral equations is obtained. The integral equations are replaced by an equivalent set of algebraic equations, which are solved to obtain the shear stress distribution in the adhesive layer, with which the crack-tip stress-intensity factors are found. When the adhesive was modeled with a nonlinear stress-strain curve, the peak shear stresses in the adhesive were considerably reduced in comparison to the solution for the linear elastic adhesive. This resulted in increases in the stress-intensity factors for the nonlinear adhesive solution compared to the linear adhesive solution. The nonlinear adhesive has no significant effect on the stress-intensity factor unless the near crack tip is beneath the stringer. It is assumed that the adhesive bond remains intact and it is predicted that onset of adhesive failure occurs at decreasing levels of applied stress as the crack propagates beneath the stringer.

Bigelow, C. A.↗

NASA NTRS1984

Measurements of large scale disturbances in rectangular wall jets

Detailed measurements of the velocity field and wall surface pressure distribution of a tone-generating wall jet were obtained in the Ames 7- by 10-foot Wind Tunnel and anechoeic chamber. The jet issued from a 0.508 cm by 10.16 cm nozzle adjacent to a 3.81 cm wall, with an initially parabolic velocity profile at the exit. The jet was operated at Reynolds numbers, R, of 4910, 7740, and 9410. At R = 4910 and 7740, the flow appeared as a double-rowed vortex array which formed upstream of the edge and convected downstream without change in structure. At R = 9410, apparent vortex pairing occurred at the edge location. These flow characteristics were observed in both velocity field measurements and flow visualization records. The unsteady vorticity field of the jet was computed from phase-averaged hot-wire anemometer measurements of the velocity field. The associated acoustic field was estimated by the method of Green's functions, and was found to be similar to measurements of the sound field. These observations tend to support theoretical predictions that vortex shedding at the edge tends to cancel the effect of acoustic sources in other parts of the flow field.

Horne, W. C.↗

NASA NTRS1983

Impedance properties of circular microstrip antenna

A moment method solution to the input impedance of a circular microstrip antenna excited by either a microstrip feed or a coaxial probe is presented. Using the exact dyadic Green's function and the Fourier transform the problem is formulated in terms of Richmond's reaction integral equation from which the unknown patch current can be solved for. The patch current is expanded in terms of regular surface patch modes and an attachment mode (for probe excited case) which insures continuity of the current at probe/patch junction, proper polarization and p-dependance of patch current in the vicinity of the probe. The input impedance of a circular microstrip antenna is computed and compared with earlier results. Effect of attachment mode on the input impedance is also discussed.

Deshpande, M. D.↗

NASA NTRS1985

A new look at sound generation by blade/vortex interaction

As a preliminary attempt to understand the dynamics of blade/vortex interaction, the two-dimensional problem of a rectilinear vortex filament interacting with a Joukowski airfoil is analyzed in both the lifting and nonlifting cases. The vortex velocity components could be obtained analytically and integrated to determine the vortex trajectory. With this information, the aeroacoustic low-frequency Green's function approach could then be employed to calculate the sound produced during the encounter. The results indicate that the vortex path deviates considerably from simple convection due to the presence of the airfoil and that a reasonably sharp sound pulse is radiated during the interaction whose fundamental frequency is critically dependent upon whether the vortex passes above or below the airfoil. Determination of this gross parameter of the interaction is shown to be highly nonlinearly dependent upon airfoil circulation, vortex circulation, and initial position.

Hardin, J. C.↗

NASA NTRS1985

Harmonic response of cylindrical and toroidal shells to an internal acoustic field. I - Theory. II - Results

In the first part of the present work concerning the coupled elastic and acoustic response of a system of cylindrical and toroidal shells enclosing an acoustic medium, the theoretical model for the cylindrical and toroidal segments' elastodynamics incorporates an elastic simulation based on transfer matrices, while the acoustic simulation adapts Green's function and curved surface elements. The coupled response is determined by the equilibrium of the acoustic pressure and internal elastic reactions of the shell, and compatibility between acoustic and elastic accelerations at the shell-fluid interface. The second part of this work obtains and discusses numerical results based on the theory for the cases of shell configurations having a plane of symmetry, as well as asymmetric ones. The inadequacy of beam theory in modeling the response of short, thin shell configurations at frequencies above the fundamental elastic resonance is demonstrated.

El-Raheb, M.↗

NASA NTRS1985

Analysis of elliptical and circular microstrip antennas using moment method

A method of calculating the input impedance of either a circular or a slightly elliptical microstrip antenna excited by a coaxial probe is presented. Using the reaction integral equation and the exact dyadic Green's function, the finite substrate thickness is taken into account in the formulation. Good agreement with experimental results for an elliptical patch is obtained and a design procedure for a circularly polarized antenna is presented.

Bailey, M. C.↗

NASA NTRS1985

Remote sensing of earth terrain

Progress on the investigation of the anisotropy of the terrain media, such as vegetation canopy and sea ice, and the study of the fluctuation-dissipation theorem in conjunction with the application of strong fluctuation theory for passive remote sensing of snowpacks is reported. The Feynman diagrammatic technique is used to derive the Dyson equation for the mean field and the Bethe-Salpeter equation for the correlation or the covariance of the field for electromagnetic wave propagation and scattering in an anisotropic random medium. With the random permittivity expressed in a general form, the bilocal and the nonlinear approximations are employed to solve the Dyson equation and the ladder approximation to the Bethe-Salpeter equation. The mean dyadic Green's function for a two layer anisotropic random medium with arbitrary three dimensional correlation function was investigated with the zeroth-order solutions to the Dyson equation under the four characteristic waves associated with the coherent vector fields propagating in an anisotropic random medium layer, which are the ordinary and extraordinary waves with upward and downward propagating vectors.

Kong, J. A.↗

NASA NTRS1985

A General-applications Direct Global Matrix Algorithm for Rapid Seismo-acoustic Wavefield Computations

A new matrix method for rapid wave propagation modeling in generalized stratified media, which has recently been applied to numerical simulations in diverse areas of underwater acoustics, solid earth seismology, and nondestructive ultrasonic scattering is explained and illustrated. A portion of recent efforts jointly undertaken at NATOSACLANT and NORDA Numerical Modeling groups in developing, implementing, and testing a new fast general-applications wave propagation algorithm, SAFARI, formulated at SACLANT is summarized. The present general-applications SAFARI program uses a Direct Global Matrix Approach to multilayer Green's function calculation. A rapid and unconditionally stable solution is readily obtained via simple Gaussian ellimination on the resulting sparsely banded block system, precisely analogous to that arising in the Finite Element Method. The resulting gains in accuracy and computational speed allow consideration of much larger multilayered air/ocean/Earth/engineering material media models, for many more source-receiver configurations than previously possible. The validity and versatility of the SAFARI-DGM method is demonstrated by reviewing three practical examples of engineering interest, drawn from ocean acoustics, engineering seismology and ultrasonic scattering.

Schmidt, H.↗

NASA NTRS1986

Theoretical models of single-element LTSA antenna elements

The spectral domain computations for slotline guide wavelength and characteristic impedance provide the basic parameters that are need to calculate the radiation pattern of an Linearly Tapered Slot Antenna (LTSA) by using the half-plane Green's function and a stepped approximation to the LTSA structure. However, the spectral domain computations are somewhat time consuming and not suitable for computer aided design application. Therefore, the results of several computation were fitted by formulas that are readily evaluated on a calculator or computer. Antenna measurements were continued, and an attempt was made to predict the truncation effect by three different methods.

Source record↗

NASA NTRS1985

Active microwave remote sensing of an anisotropic random medium layer

A two-layer anisotropic random medium model has been developed to study the active remote sensing of the earth. The dyadic Green's function for a two-layer anisotropic medium is developed and used in conjunction with the first-order Born approximation to calculate the backscattering coefficients. It is shown that strong cross-polarization occurs in the single scattering process and is indispensable in the interpretation of radar measurements of sea ice at different frequencies, polarizations, and viewing angles. The effects of anisotropy on the angular responses of backscattering coefficients are also illustrated.

Lee, J. K.↗

NASA NTRS1985

Time-dependent radiation transport in a one-dimensional medium

An analytic solution of the time-dependent radiation transport problem in a one-dimensional, stationary and homogeneous medium of finite thickness is presented. The solution is found by the method of images, and is compared with an eigenfunction expansion. Previous conjectures about the structure of such an expansion are clarified. The Green's function of this problem is also expanded in scattering orders.

Nagel, W.↗

NASA NTRS1985

Electromagnetic wave scattering in a two-layer anisotropic random medium

For electromagnetic wave propagation and scattering in an anisotropic random medium, the Dyson equation for the mean field and the Bethe-Salpeter equation for the correlation or the covariance of the field were derived. With the random permittivity expressed in a general anisotropic form, the bilocal and the nonlinear approximations are employed to solve the Dyson equation, and the ladder approximation to solve the Bethe-Salpeter equation. The mean dyadic Green's function for a two-layer anisotropic random medium with arbitrary three-dimensional correlation functions has been investigated with the zeroth-order solutions to the Dyson equation under the nonlinear approximation. The effective propagation constants are calculated for the four characteristic waves associated with the coherent vector fields, propagating in an anisotropic random-medium layer, which are the ordinary and extraordinary waves with upward- and downward-propagating vectors.

Lee, J. K.↗

NASA NTRS1986

The crack-inclusion interaction problem

The general plane elastostatic problem of interaction between a crack and an inclusion is considered. The Green's functions for a pair of dislocations and a pair of concentrated body forces are used to generate the crack and the inclusion. Integral equations are obtained for a line crack and an elastic line inclusion having an arbitrary relative orientation and size. The nature of stress singularity around the end points of rigid and elastic inclusions is described and three special cases of this intersection problem are studied. The problem is solved for an arbitrary uniform stress state away from the crack-inclusion region. The nonintersecting crack-inclusion problem is considered for various relative size, orientation, and stiffness parameters, and the stress intensity factors at the ends of the inclusion and the crack are calculated. For the crack-inclusion intersection case, special stress intensity factors are defined and are calculated for various values of the parameters defining the relative size and orientation of the crack and the inclusion and the stiffness of the inclusion.

Liu, X.-H.↗

NASA NTRS1986

Sensitivity analysis of discrete structural systems

Methods for calculating sensitivity derivatives for discrete structural systems are surveyed, primarily covering literature published during the past two decades. Methods are described for calculating derivatives of static displacements and stresses, eigenvalues and eigenvectors, transient structural response, and derivatives of optimum structural designs with respect to problem parameters. The survey is focused on publications developed in nonstructural fields such as electronics, controls, and physical chemistry which are directly applicable to structural problems. Most notable among the nonstructural-based methods are the adjoint variable technique from control theory, and the Green's function and FAST methods from physical chemistry.

Adelman, H. M.↗

NASA NTRS1986

Use of speckle for determining the response characteristics of Doppler imaging radars

An optical model is developed for imaging optical radars such as the SAR on Seasat and the Shuttle Imaging Radar (SIR-B) by analyzing the Doppler shift of individual speckles in the image. The signal received at the spacecraft is treated in terms of a Fresnel-Kirchhoff integration over all backscattered radiation within a Huygen aperture at the earth. Account is taken of the movement of the spacecraft along the orbital path between emission and reception. The individual points are described by integration of the point source amplitude with a Green's function scattering kernel. Doppler data at each point furnishes the coordinates for visual representations. A Rayleigh-Poisson model of the surface scattering characteristics is used with Monte Carlo methods to generate simulations of Doppler radar speckle that compare well with Seasat SAR data SIR-B data.

Tilley, D. G.↗

NASA NTRS1984

Structural sensitivity analysis: Methods, applications, and needs

Some innovative techniques applicable to sensitivity analysis of discretized structural systems are reviewed. These techniques include a finite-difference step-size selection algorithm, a method for derivatives of iterative solutions, a Green's function technique for derivatives of transient response, a simultaneous calculation of temperatures and their derivatives, derivatives with respect to shape, and derivatives of optimum designs with respect to problem parameters. Computerized implementations of sensitivity analysis and applications of sensitivity derivatives are also discussed. Finally, some of the critical needs in the structural sensitivity area are indicated along with Langley plans for dealing with some of these needs.

Adelman, H. M.↗

NASA NTRS1987

Origin and evolution of osmoregulatory mechanisms in blue-green algae (cyanobacteria) as a function of metabolic and structural complexity: Reflections of Precambrian paleobiology

Major accomplishments underlying the basic understanding of cyanobacterial resistance to salt tolerance and osmotic stress were made. The methodology proposed included: the tracing of the pathways of formation of osmoregulatory solutes by traditional methods involving C-14 labelled substrates; gas chromatography; amino acid analysis; X-ray analysis using scanning transmission electron microscopy; and most importantly, C-13 labelled substrates, followed by Nuclear Magnetic Resonance (NMR) spectroscopy. It was found that the cyanobacteria employ a diversity of organic, osmoregulatory solutes. Osmoregulatory solutes were found to serve four functions: adjustment of water activity, noninhibition of enzymes; lowering of K sub m of enzymes to allow functioning at normal levels when the intracellular salt accumulates, and extending the pH optimum of enzymes as intracellular pH rises due to proton-potassium ion pump action during osmoregulation. Differences in osmoregulatory solutes may, but are not always, be attributed to differences in nutritional capabilities. The mechanism of osmoregulation and concomitant salt tolerance in halophilic cyanobacteria was elucidated. The activities of betaine and S-Adenosylhomocysteine hydrolase are discussed.

Yopp, John H.↗

NASA NTRS1986

Radiation pattern analysis of the tapered slot antenna, appendix 1

A theoretical model for the tapered slot antenna is presented. The model is valid for any smooth taper of the slot. The problem is solved by modeling the slot electric field and using the half plane Green's function to compute the far fields. The aperture field is obtained by affecting a stepped approximation to the continuous taper and utilizing the uniform wide slot line data. The uniform wide slot line is solved by the spectral Galerkin's technique and closed form experssions are developed for the slot wave length and the slot characteristic impedance. Numerous comparisons with measurement are made to demonstrate the versatility of the model in treating an arbitrary slot taper.

Janaswamy, Ramakrishna↗