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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

A parametric analysis of visual approaches for helicopters

A flight investigation was conducted to determine the characteristic shapes of the altitude, ground speed, and deceleration profiles of visual approaches for helicopters. Two hundred thirty-six visual approaches were flown from nine sets of initial conditions with four types of helicopters. Mathematical relationships were developed that describe the characteristic visual deceleration profiles. These mathematical relationships were expanded to develop equations which define the corresponding nominal ground speed, pitch attitude, pitch rate, and pitch acceleration profiles. Results are applicable to improved helicopter handling qualities in terminal area operations.

Moen, G. C.↗
NASA NTRS1981

A hybrid state vector approach to aeroelastic analysis with application to composite lifting surfaces

A computational technique has been developed for performing preliminary design aeroelastic analyses of large aspect ratio lifting surfaces. This technique, applicable to both fixed and rotating wing configurations, is based upon a formulation of the structural equilibrium equations in terms of a hybrid state vector containing generalized force and displacement variables. An integrating matrix is employed to solve these equations for divergence and flutter eigenvalues and steady aeroelastic deformation. Results are presented for simple examples which verify the technique and demonstrate how it can be applied to analyze lifting surfaces, including those constructed from composite materials.

Lehman, L. L.↗
NASA NTRS1984

Slave finite elements: The temporal element approach to nonlinear analysis

A formulation method for finite elements in space and time incorporating nonlinear geometric and material behavior is presented. The method uses interpolation polynomials for approximating the behavior of various quantities over the element domain, and only explicit integration over space and time. While applications are general, the plate and shell elements that are currently being programmed are appropriate to model turbine blades, vanes, and combustor liners.

Gellin, S.↗
NASA NTRS1983

Control structure interactions in large space structures Analysis using energy approach

A simple energy approach to study the problem of control structure interactions in large space structures is presented. For the illustrative cases of free-free beam and free rectangular plate, the vibrational energy imparted during operation of constant and pulsed thrusters is found in a nondimensional form. Then based on a parametric study, suggestions are made on the choice of the thruster location and parameters to minimize the control structure interactions.

Shrivastava, S. K.↗
NASA NTRS1984

An approach for automated analysis of particle holograms

A simple method for analyzing droplet holograms is proposed that is readily adaptable to automation using modern image digitizers and analyzers for determination of the number, location, and size distributions of spherical or nearly spherical droplets. The method determines these parameters by finding the spatial location of best focus of the droplet images. With this location known, the particle size may be determined by direct measurement of image area in the focal plane. Particle velocity and trajectory may be determined by comparison of image locations at different instants in time. The method is tested by analyzing digitized images from a reconstructed in-line hologram, and the results show that the method is more accurate than a time-consuming plane-by-plane search for sharpest focus.

Stanton, A. C.↗
NASA NTRS1990

Characteristic-Wave Approach Complements Modal Analysis

Aspects of estimation of unmodeled dynamics discussed. Report discusses solution of nonhomogeneous governing matrix equation for dynamics of short vibrational pulses propagating as characteristic waves in large structure. Applied to analyze response, to repeated pulses, of beam clamped at one end and free at other. Shows all qualitative characteristics occuring under arbitrary periodic excitations of beam and those of quasi-periodic excitations, in as much as such excitations obtained by linear superpositions of periodic excitations.

Zak, Michail↗
NASA NTRS1990

A new approach to the analysis of Mira light curves

Two different but complementary methods for predicting Mira luminosities are presented. One method is derived from a Fourier analysis, it requires performing deconvolution, and its results are not certain due to the inherent instability of deconvolution problems. The other method is a learning method utilizing artificial intelligence techniques where a light curve is presented as an ordered sequence of pseudocycles, and rules are learned by linking the characteristics of several consecutive pseudocycles to one characteristic of the future cycle. It is observed that agreement between these methods is obtainable when it is possible to eliminate similar false frequencies from the preliminary power spectrum and to improve the degree of confidence in the rules.

Mennessier, M. O.↗
NASA NTRS1992

The SURE approach to reliability analysis

The SURE computer program, a reliability-analysis tool for ultrareliable computer-system architectures, provides rapid computational capability for semi-Markov models useful in describing the fault-handling behavior of fault-tolerant computer systems. The basic mathematics of SURE and its user interface are described, including a sample interactive session. The basis of the SURE model-pruning capability is presented, and the SURE loop-truncation method is justified. The techniques used to develop a semi-Markov model of a fault-tolerant computer system are also reviewed.

Butler, Ricky W.↗
NASA NTRS1992

Finite element approach for transient analysis of multibody systems

A three-dimensional, finite element based formulation for the transient dynamics of constrained multibody systems with trusslike configurations is presented. A convected coordinate system is used to define the rigid-body motion of individual elements in the system. Deformation of each element is defined relative to its convected coordinate system. The formulation is oriented toward joint-dominated structures. Through a series of sequential transformations, the joint degree of freedom is built into the equations of motion of the element to reduce geometric constraints. Based on the derivation, a general-purpose code has been developed. Two examples are presented to illustrate the application of the code.

Wu, Shih-Chin↗
NASA NTRS1992

Finite-difference algorithms for the time-domain Maxwell's equations - A numerical approach to RCS analysis

The applications of two CFD-based finite-difference methods to computational electromagnetics are investigated. In the first method, the time-domain Maxwell's equations are solved using the explicit Lax-Wendroff scheme and in the second method, the second-order wave equations satisfying the Maxwell's equations are solved using the implicit Crank-Nicolson scheme. The governing equations are transformed to a generalized curvilinear coordinate system and solved on a body-conforming mesh using the scattered-field formulation. The induced surface current and the bistatic radar cross section are computed and the results are validated for several two-dimensional test cases involving perfectly-conducting scatterers submerged in transverse-magnetic plane waves.

Vinh, Hoang↗
NASA NTRS1991

Analysis of SSME inspection imagery using AI approaches

The automated analysis of SSME injector assemblies has been investigated for the cases of LOX post surface defects and injector-baffle deterioration. Defects are isolated via 2D feature extraction from borescope and camera images; temporal-frequency transforms are then used to create a multiresolution set of feature vectors representing image contents. The potential flaws thus discriminated are then segmented and classified according to known categories. AI is applied in the form of a blackboard architecture that is controlled by a rule-based production system.

Finegan, Michael K., Jr.↗
NASA NTRS1993

A Monte Carlo approach to the analysis of control system robustness

Stochastic robustness, a simple technique used to estimate the stability and performance robustness of linear, time-invariant systems, is described. The scalar probability of instability is introduced as a measure of stability robustness. Examples are given of stochastic performance robustness measures based on classical time-domain specifications. The relationship between stochastic robustness measures and control system design parameters is discussed. The technique is demonstrated by analyzing an LQG/LTR system designed for a flexible robot arm. It is concluded that the analysis of stochastic robustness offers a good alternative to existing robustness metrics.

Ray, Laura R.↗
NASA NTRS1990

A unified approach to the analysis and design of elasto-plastic structures with mechanical contact

With structural design in mind, a new unified variational model has been developed which represents the mechanics of deformation elasto-plasticity with unilateral contact conditions. For a design problem formulated as maximization of the load carrying capacity of a structure under certain constraints, the unified model allows for a simultaneous analysis and design synthesis for a whole range of mechanical behavior.

Bendsoe, Martin P.↗
NASA NTRS1995

An evaluation of a coupled microstructural approach for the analysis of functionally graded composites via the finite-element method

A comparison is presented between the predictions of the finite-element analysis and a recently developed higher-order theory for functionally graded materials subjected to a thorough-thickness temperature gradient. In contrast to existing micromechanical theories that utilize classical (i.e., uncoupled) homogenization schemes to calculate micro-level and macro-level stress and displacement fields in materials with uniform or nonuniform fiber spacing (i.e., functionally graded materials), the new theory explicitly couples the microstructural details with the macrostructure of the composite. Previous thermo-elastic analysis has demonstrated that such coupling is necessary when: the temperature gradient is large with respect to the dimension of the reinforcement; the characteristic dimension of the reinforcement is large relative to the global dimensions of the composite and the number of reinforcing fibers or inclusions is small. In these circumstances, the standard micromechanical analyses based on the concept of the representative volume element used to determine average composite properties produce questionable results. The comparison between the predictions of the finite-element method and the higher-order theory presented herein establish the theory's accuracy in predicting thermal and stress fields within composites with a finite number of fibers in the thickness direction subjected to a thorough-thickness thermal gradient.

Pindera, Marek-Jerzy↗