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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Gravitational microlensing and gamma-ray bursts

Models of gravitational microlensing at small and large optical depth are presented, with a special emphasis on the time delays between microimages. The time distribution of microimage arrival times is random when the microobjects responsible for the lensing are randomly distributed. If the microobjects are clustered in space, then microimages are clustered in time. In some models there are many pairs of microimages with a very small time separation. It is proposed that the recently discovered recurrent soft gamma-ray burster GB 790107, for which more than 50 bursts were detected, is a single event multiply imaged due to gravitational microlensing by an ensemble of nonrandomly distributed microobjects, perhaps a rich cluster of galaxies. Some observational tests of this hypothesis are proposed. If correct, the hypothesis requires the burster to be at a cosmological distance, perhaps farther away than the most distant quasars.

Paczynski, Bohdan↗
NASA NTRS1989

Response of moderately thick laminated cross-ply composite shells subjected to random excitation

This study deals with the dynamic response of transverse shear deformable laminated shells subjected to random excitation. The analysis encompasses the following problems: (1) the dynamic response of circular cylindrical shells of finite length excited by an axisymmetric uniform ring loading, stationary in time, and (2) the response of spherical and cylindrical panels subjected to stationary random loadings with uniform spatial distribution. The associated equations governing the structural theory of shells are derived upon discarding the classical Love-Kirchhoff (L-K) assumptions. In this sense, the theory is formulated in the framework of the first-order transverse shear deformation theory (FSDT).

Elishakoff, Isaak↗
NASA NTRS1988

Probabilistic methods for structural response analysis

This paper addresses current work to develop probabilistic structural analysis methods for integration with a specially developed probabilistic finite element code. The goal is to establish distribution functions for the structural responses of stochastic structures under uncertain loadings. Several probabilistic analysis methods are proposed covering efficient structural probabilistic analysis methods, correlated random variables, and response of linear system under stationary random loading.

Wu, Y.-T.↗
NASA NTRS1973

Computer based analyses of the response of box type structures to random pressures.

Analysis of the vibration of the internal structure of a relatively simple flight vehicle structural model consisting of a multicellular box in which two parallel outer face plates (skins) are connected together by several internal plates (ribs) which are parallel to each other and normal to the face plates. Such a box is representative of a wing or stabilizer structure. Three representations of the structure were considered in the computation. The simplest model which can account for the coupling between the face plates and the ribs is a two-dimensional assembly of beams. A more realistic, yet still simple, three-dimensional model has been developed using the Rayleigh-Ritz method. The third computational method employed was the NASTRAN finite-element displacement method. The computational results are compared with experimental data.

Clarkson, B. L.↗
NASA NTRS1976

Response of linear dynamic systems with random coefficients

Numerous models of physical systems contain parameters whose values are not known exactly. The physical and mathematical complexities arising in the prediction of the statistical behavior of such systems are discussed. Although the discussions are far from providing a satisfactory solution to such problems, they perhaps, by utilization of simple examples, will create a greater awareness of the statistical effect of random parameters.

Dickerson, J.↗
NASA NTRS1978

Low frequency cabin noise reduction based on the intrinsic structural tuning concept: The theory and the experimental results, phase 2

Low frequency cabin noise and sonically induced stresses in an aircraft fuselage may be reduced by intrinsic tuning of the various structural members such as the skin, stringers, and frames and then applying damping treatments on these members. The concept is also useful in identifying the key structural resonance mechanisms controlling the fuselage response to broadband random excitation and in developing suitable damping treatments for reducing the structural response in various frequency ranges. The mathematical proof of the concept and the results of some laboratory and field tests on a group of skin-stringer panels are described. In the so-called stiffness-controlled region, the noise transmission may actually be controlled by stiffener resonances, depending upon the relationship between the natural frequencies of the skin bay and the stiffeners. Therefore, cabin noise in the stiffness-controlled region may be effectively reduced by applying damping treatments on the stiffeners.

Sengupta, G.↗
NASA NTRS1986

Structural dynamic measurement practices for turbomachinery at the NASA Lewis Research Center

Methods developed for measuring blade and rotor-shaft system response include optical systems, transient instruments, and special digital data processing equipment. Optical methods offer some distinct benefits for blade vibration measurement. Transient and steady state measurements of the response of rotor-shaft systems strongly affect analytical methods development. Digital computing systems allow processing of large volumes of high speed data from rotating blade sets. Also, digital systems develop useful vibration response signatures from randomly excited systems. Research facilities include the spin rig facility and the transient rotor response lab.

Kiraly, L. J.↗
DOE OSTI2023

Security component with fiber Bragg grating

A fiber Bragg grating (FBG) security component for single-party and multi-party monitoring is provided. The security component includes an optical fiber having a plurality of Bragg gratings. The Bragg gratings provide a spectral response that is randomized based on the manufacture of the security component. For single-party use, the spectral response provides a reproducible spectral signature when interrogated with an optical signal. For multi-party use, each party applies a known optical interrogation signal to the security component and applies an external stress known only to the respective monitoring party. The resulting shift in the spectral signature is unique to each monitoring party, making it extremely difficult to successfully counterfeit the security component's response for all such parties.

Ziock, Klaus-Peter↗
DOE OSTI2020

Specific features of fluorescence transfer in multiply scattering randomly inhomogeneous layers under intense laser pumping

Based on the analysis of experimental data on the effect of the pulsed laser pump intensity on the spectral properties and the size of the fluorescent response zone in randomly inhomogeneous fluorescent layers, we found that the amplification of spontaneous and stimulated emission significantly affects the statistical properties of the propagation lengths of the fluorescent field partial components in the layers. The experiments are performed with layers of SiO{sub 2} and TiO{sub 2} nanoparticles saturated with rhodamine 6G, pumped by 532-nm laser radiation in the intensity range corresponding to the transient regime from excitation of spontaneous fluorescence to random lasing in the layer. The experimental data are compared with the results of statistical modelling of fluorescence transfer. It is shown that, even at a pump intensity below the random lasing threshold, the spontaneous emission amplification in a layer leads to a significant increase in the contributions to the fluorescence response from partial components with propagation lengths much larger than the layer thickness. This can be interpreted as a manifestation of the quasi-waveguide effect, in which the probability of propagation of diffuse fluorescence components along the layer over distances many times greater than its thickness and the size of the pumped region increases significantly with a decrease in the characteristic radiation amplification length in the layer. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1982

A study of component design loads due to dynamic environment

The random vibration of the components of the space shuttle causing internal cargo bay acoustic impingement on the payload was investigated. The import factors to be considered in the design of acoustically loaded structures are the prediction of acoustic environment, prediction of structural response to this environment, and to determine the effect of the response on the structural strength requirements. Two basic responses were analyzed. The primary structure response due to random vibration is first analyzed. The output of primary structure response is used as an input to the secondary structure to calculate the component response.

Mehta, R. S.↗
NASA NTRS2001

Free Vibration of Uncertain Unsymmetrically Laminated Beams

Monte Carlo Simulation and Stochastic FEA are used to predict randomness in the free vibration response of thin unsymmetrically laminated beams. For the present study, it is assumed that randomness in the response is only caused by uncertainties in the ply orientations. The ply orientations may become random or uncertain during the manufacturing process. A new 16-dof beam element, based on the first-order shear deformation beam theory, is used to study the stochastic nature of the natural frequencies. Using variational principles, the element stiffness matrix and mass matrix are obtained through analytical integration. Using a random sequence a large data set is generated, containing possible random ply-orientations. This data is assumed to be symmetric. The stochastic-based finite element model for free vibrations predicts the relation between the randomness in fundamental natural frequencies and the randomness in ply-orientation. The sensitivity derivatives are calculated numerically through an exact formulation. The squared fundamental natural frequencies are expressed in terms of deterministic and probabilistic quantities, allowing to determine how sensitive they are to variations in ply angles. The predicted mean-valued fundamental natural frequency squared and the variance of the present model are in good agreement with Monte Carlo Simulation. Results, also, show that variations between plus or minus 5 degrees in ply-angles can affect free vibration response of unsymmetrically and symmetrically laminated beams.

Kapania, Rakesh K.↗
NASA NTRS2016

The Impact of Truth Surrogate Variance on Quality Assessment/Assurance in Wind Tunnel Testing

Minimum data volume requirements for wind tunnel testing are reviewed and shown to depend on error tolerance, response model complexity, random error variance in the measurement environment, and maximum acceptable levels of inference error risk. Distinctions are made between such related concepts as quality assurance and quality assessment in response surface modeling, as well as between precision and accuracy. Earlier research on the scaling of wind tunnel tests is extended to account for variance in the truth surrogates used at confirmation sites in the design space to validate proposed response models. A model adequacy metric is presented that represents the fraction of the design space within which model predictions can be expected to satisfy prescribed quality specifications. The impact of inference error on the assessment of response model residuals is reviewed. The number of sites where reasonably well-fitted response models actually predict inadequately is shown to be considerably less than the number of sites where residuals are out of tolerance. The significance of such inference error effects on common response model assessment strategies is examined.

DeLoach, Richard↗
NASA NTRS1956

The Dynamic Response of a Large Airplane to Continuous Random Atmospheric Disturbances

The statistical approach to the gust-loads problem, which consists in considering flight th1ough turbulent air to be a stationary random process, is extended by including the effect of lateral variations of the instantaneous gust intensity on the aerodynamic forces and on the resultant motions and stresses of rigid and flexible airplanes. By means of some calculations of normal and rolling accelerations, as well as of the root bending moment, it is shown that these effects may be significant for large airplanes.

Aeroplane↗
NASA NTRS2000

Demonstration of the Application of Composite Load Spectra (CLS) and Probabilistic Structural Analysis (PSAM) Codes to SSME Heat Exchanger Turnaround Vane

This report describes a probabilistic structural analysis performed to determine the probabilistic structural response under fluctuating random pressure loads for the Space Shuttle Main Engine (SSME) turnaround vane. It uses a newly developed frequency and distance dependent correlation model that has features to model the decay phenomena along the flow and across the flow with the capability to introduce a phase delay. The analytical results are compared using two computer codes SAFER (Spectral Analysis of Finite Element Responses) and NESSUS (Numerical Evaluation of Stochastic Structures Under Stress) and with experimentally observed strain gage data. The computer code NESSUS with an interface to a sub set of Composite Load Spectra (CLS) code is used for the probabilistic analysis. A Fatigue code was used to calculate fatigue damage due to the random pressure excitation. The random variables modeled include engine system primitive variables that influence the operating conditions, convection velocity coefficient, stress concentration factor, structural damping, and thickness of the inner and outer vanes. The need for an appropriate correlation model in addition to magnitude of the PSD is emphasized. The study demonstrates that correlation characteristics even under random pressure loads are capable of causing resonance like effects for some modes. The study identifies the important variables that contribute to structural alternate stress response and drive the fatigue damage for the new design. Since the alternate stress for the new redesign is less than the endurance limit for the material, the damage due high cycle fatigue is negligible.

Rajagopal, Kadambi R.↗
NASA NTRS1990

Assessment of computational prediction of tail buffeting

Assessments of the viability of computational methods and the computer resource requirements for the prediction of tail buffeting are made. Issues involved in the use of Euler and Navier-Stokes equations in modeling vortex-dominated and buffet flows are discussed and the requirement for sufficient grid density to allow accurate, converged calculations is stressed. Areas in need of basic fluid dynamics research are highlighted: vorticity convection, vortex breakdown, dynamic turbulence modeling for free shear layers, unsteady flow separation for moderately swept, rounded leading-edge wings, vortex flows about wings at high subsonic speeds. An estimate of the computer run time for a buffeting response calculation for a full span F-15 aircraft indicates that an improvement in computer and/or algorithm efficiency of three orders of magnitude is needed to enable routine use of such methods. Attention is also drawn to significant uncertainties in the estimates, in particular with regard to nonlinearities contained within the modeling and the question of the repeatability or randomness of buffeting response.

Edwards, John W.↗