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At least 91 records · Page 5

The Effect of Fiber-Angle Fidelity on the Linear Response of Tow-steered Composite Plates

Composite laminate tailoring is traditionally performed by uniformly changing the in-plane ply orientation to obtain the desired mechanical performance. The emergence of tow-steered plies, where the fibers follow a prescribed curvilinear path, have increased the tailorability of composite laminates. However, characterizing the behavior of tow-steered laminates using finite element analysis is challenging because additional, and often numerous, orientation definitions may be required. The additional orientation definitions detrimentally increase the computational cost and hinder the use of advanced analysis techniques, such as Monte Carlo or uncertainty quantification, in the design process. To reduce computational cost without adversely affecting mechanics-based performance, the results of a parametric study that was used to investigate the effects of fiber-angle fidelity, element size, and tow-steered radius-of-curvature on various loading scenarios for tow-steered composite plates are presented. The three loading scenarios that were analyzed using finite element analysis included an axial tension load, an applied constant through-thickness-direction pressure load, and an axial compression load. Results for mechanics-based metrics of interest are presented and discussed for each loading scenario. Little sensitivity (less than one percent difference) to the effect of fiber-angle fidelity is observed in the mechanics-based metrics until the coarse-end of the considered range. Sensitivity to element size generally dominates the observed results for the mechanics-based metrics. Notable reductions in the preprocessing time are observed for increasingly coarse element size and fiber-rounding parameters. The preprocessing times decreased up to three orders of magnitude from a few thousand seconds to a few seconds without loss of accuracy in the mechanics-based metrics. Such increased computational performance is of particular interest to the structural design and analysis communities that may be conducting large counts of finite-element analyses, such as in other parametric studies, Monte-Carlo analyses, uncertainty quantification, or tow-steered optimization.

tow-steered composites↗

A Numerical and Experimental Study of Compression-Loaded Composite Panels With Cutouts

Results from a numerical and experimental study on the effects of laminate orthotropy and circular cutout size on the response of compression-loaded composite curved panels are presented. Several 60-in-radius composite panels with four different laminate configurations were tested with cutout diameters that range from 10% to 60% of the panel width. Finite-element analyses were performed for each panel in order to identify the effects boundary conditions, measured initial geometric imperfections and thickness variations had on the nonlinear and buckling behavior of the panels. The compression-loaded panels considered herein exhibited two separate types of behavior depending on the laminate stacking sequence and cutout size. More specifically, some of the panels exhibited the classical snap-through type buckling response; however, some of the panels exhibited a monotonically increasing stable response and achieved compressive loads in excess of twice the predicted linear bifurcation buckling load. In general, the finite-element analyses were able to predict accurately the nonlinear response and buckling loads of the panels and the prebuckling and postbuckling out-of-plane deformations and strains.

Thornburgh, Robert P.↗

Statistics of arcs in clusters of galaxies

Samples of gravitational lens events in clusters show many large arcs compared to arclets, relative to what can be obtained by idealized singular lens models. We describe the probability of image magnification for point sources and for simple but more realistic gravitational lensing models that include a finite core size and an ellipticity. In addition, we explore the changes in the probability distribution of image magnifications, distortions, and angular extents for sources of different sizes as the parameters of the lenses are varied. A finite core in spherically symmetric lens models introduces a discontinuity in the probability distribution at which the relative number of highly magnified images is increased. In elliptical lenses, this discontinuity and its effect are replaced by a continuous increase in the probability of obtaining high-magnification images relative to singular spherically symmetric models. We also find that the finite size of the source causes a further increase in the expected number of images just below the maximum possible magnification.

Bergmann, Anton G.↗

Finite Element Analysis of Reverberation Chambers

The primary motivating factor behind the initiation of this work was to provide a deterministic means of establishing the validity of the statistical methods that are recommended for the determination of fields that interact in -an avionics system. The application of finite element analysis to reverberation chambers is the initial step required to establish a reasonable course of inquiry in this particularly data-intensive study. The use of computational electromagnetics provides a high degree of control of the "experimental" parameters that can be utilized in a simulation of reverberating structures. As the work evolved there were four primary focus areas they are: 1. The eigenvalue problem for the source free problem. 2. The development of a complex efficient eigensolver. 3. The application of a source for the TE and TM fields for statistical characterization. 4. The examination of shielding effectiveness in a reverberating environment. One early purpose of this work was to establish the utility of finite element techniques in the development of an extended low frequency statistical model for reverberation phenomena. By employing finite element techniques, structures of arbitrary complexity can be analyzed due to the use of triangular shape functions in the spatial discretization. The effects of both frequency stirring and mechanical stirring are presented. It is suggested that for the low frequency operation the typical tuner size is inadequate to provide a sufficiently random field and that frequency stirring should be used. The results of the finite element analysis of the reverberation chamber illustrate io-W the potential utility of a 2D representation for enhancing the basic statistical characteristics of the chamber when operating in a low frequency regime. The basic field statistics are verified for frequency stirring over a wide range of frequencies. Mechanical stirring is shown to provide an effective frequency deviation.

Bunting, Charles F.↗

Experimental determination of in situ utilization of lunar regolith for thermal energy storage

A Lunar Thermal Energy from Regolith (LUTHER) experiment has been designed and fabricated at the NASA Lewis Research Center to determine the feasibility of using lunar soil as thermal energy storage media. The experimental apparatus includes an alumina ceramic canister which contains simulated lunar regolith, a heater, nine heat shields, a heat transfer cold jacket, and 19 type-B platinum rhodium thermocouples. The simulated lunar regolith is a basalt that closely resembles the lunar basalt returned to earth by the Apollo missions. The experiment will test the effects of vacuum, particle size, and density on the thermophysical properties of the regolith, which include melt temperature, specific heat thermal conductivity, and latent heat of storage. Two separate tests, using two different heaters, will be performed to study the effect of heating the system using radiative and conductive heat transfer. A finite differencing SINDA model was developed at NASA Lewis Research Center to predict the performance of the LUTHER experiment. The code will predict the effects of vacuum, particle size, and density has on the heat transfer to the simulated regolith.

Richter, Scott W.↗

Cloud field reflectance variations traceable to finite cloud effects

Variations in cloud field reflectances resulting from the organization of clouds within a cloud field are examined using a cloud field optical simulator. The problem of changing spot size is studied as a function of view angle for a fixed field of view detector. Also, problems associated with the assumption of isotropy are discussed and the effect of cloud interactions within cloud fields is investigated. The results show that radiance variation increases when the photodiodes measure radiances from portions of cloud fields which move in and out of the fields of view as the position of the photodiode changes. In addition, the experiments reveal several factors that are important in the determination of accurate reflectances.

Alberta, Timothy L.↗

Quasi-One-Dimensional Modeling of Pulse Detonation Rocket Engines

Pulsed detonation rocket engines (PDREs) have generated considerable research interest in recent years as a chemical propulsion system potentially offering improved performance and reduced complexity compared to conventional rocket engines. The detonative mode of combustion employed by these devices offers a thermodynamic advantage over the constant-pressure deflagrative combustion mode used in conventional rocket engines and gas turbines. However, while this theoretical advantage has spurred a great deal of interest in building PDRE devices, the unsteady blowdown process intrinsic to the PDRE has made realistic estimates of the actual propulsive performance problematic. The recent review article by Kailasanath highlights some of the difficulties in comparing the available experimental measurements with numerical models. In a previous paper by the author, parametric studies of the performance of a single, straight-tube PDRE were reported. A 1-D, unsteady method of characteristics code, employing a constant-gamma assumption behind the detonation front, was developed for that study. Models of this type are computationally inexpensive, and are particularly useful for parametric performance comparisons. For example, a plot showing the specific impulse of various PDRE and steady-state rocket engine (SSRE) configurations as a function of blowdown pressure ratio. The performance curves clearly indicate that a straight-tube PDRE is superior in specific impulse to a SSRE with a sonic nozzle over the entire range of pressure ratios. Note, however, that a straight-tube PDRE in general does not compare favorably to a SSRE fitted with an optimized de Laval supersonic nozzle, particularly at the high pressure ratios typical for boost or in-space rocket applications. However, the calculations also show that if a dynamically optimized, supersonic de Laval nozzle could be could be fitted to a PDRE, then the specific impulse of the device would exceed that of a comparable SSRE. While such a nozzle is a considerable idealization, it is clear that nozzle design and optimization will play a critical role in whether the performance potential of PDREs can be effectively realized in practice. In order to study PDRE nozzle issues with greater accuracy, a quasi-one-dimensional, finite-rate chemistry CFD code has been developed by the author. Comparisons of the code with both the previous MOC model and experimental data from Stanford University are reported. The effect of constant-gamma and finite-rate chemistry assumptions on the flowfield and performance is examined. Parametric studies of the effect of nozzle throat size and expansion ratio, at various blowdown pressure ratios, are reported.

Morris, Christopher I.↗

Microcomputer system for medium-sized and experimental finite element analysis

The development of a microcomputer system is described. A series of finite element analysis programs are evaluated in terms of their cost effective application within the microcomputer system. It is shown that the system is practically maintenance free and can be sustained by individual laboratories of standard scale in the educational or academic environment.

Yamada, Y.↗

Can the Lateral Proximity Effect Be Used to Create the Superconducting Transition of a Micron-Sized TES?

Recent measurements of micron-sized Mo/Au bilayer Transition Edge Sensors (TESs) have demonstrated that the TES can behave like an S-S'-S weak link due to the lateral proximity effect from superconducting leads. In this regime the Tc is a function of bias current, and the effective Tc shifts from the bilayer Tc towards the lead Tc. We explore the idea that a micron-sized S-N-S weak link could provide a new method to engineer the TES Tc. This method would be particularly useful when small size requirements for a bilayer TES (such as for a hot-electron microbolometer) lead to undesirable shifts in the bilayer Te. We present measurements of a variety of micron-sized normal Au 'TES' devices with Nb leads. We find no evidence of a superconducting transition in the Au film of these devices, in dramatic contrast to the strong lateral proximity effect seen in micron-sized Mo/Au bilayer devices. The absence of a transition in these devices is also in disagreement with theoretical predictions for S-N-S weak links. We hypothesize that a finite contact resistance between the Nb and Au may be weakening the effect. We conclude that the use of the lateral proximity effect to create a superconducting transition will be difficult given current fabrication procedures.

Barrentine, E. M.↗

Stellar occultation of polarized light from circumstellar electrons. I - Flat envelopes viewed edge on

The depolarizing and occultation effects of a finite spherical light source on the polarization of light Thomson-scattered from a flat circumstellar envelope seen edge-on are analyzed. The analysis shows that neglect of the finite size of the light source leads to a gross overestimate of the polarization for a given disk geometry. By including occultation and depolarization, it is found that B-star envelopes are necessarily highly flattened disk-type structures. For a disk viewed edge-on, the effect of occultation reduces the polarization more than the inclusion of the depolarization factor alone. Analysis of a one-dimensional plume leads to a powerful technique that permits the electron density distribution to be explicitly obtained from the polarimetric data.

Brown, John C.↗

Characterization of Surface Reflectance Variation Effects on Remote Sensing

The use of Monte Carlo radiative transfer codes to simulate the effects on remote sensing in visible and infrared wavelengths of variables which affect classification is examined. These variables include detector viewing angle, atmospheric aerosol size distribution, aerosol vertical and horizontal distribution (e.g., finite clouds), the form of the bidirectional ground reflectance function, and horizontal variability of reflectance type and reflectivity (albedo). These simulations are used to characterize the sensitivity of observables (intensity and polarization) to variations in the underlying physical parameters both to improve algorithms for the removal of atmospheric effects and to identify techniques which can improve classification accuracy. It was necessary to revise and validate the simulation codes (CTRANS, ARTRAN, and the Mie scattering code) to improve efficiency and accommodate a new operational environment, and to build the basic software tools for acquisition and off-line manipulation of simulation results. Initial calculations compare cases in which increasing amounts of aerosol are shifted into the stratosphere, maintaining a constant optical depth. In the case of moderate aerosol optical depth, the effect on the spread function is to scale it linearly as would be expected from a single scattering model. Varying the viewing angle appears to provide the same qualitative effect as modifying the vertical optical depth (for Lambertian ground reflectance).

Pearce, W. A.↗

MTF and point-spread function for a large-area CCD imager

The MTF degradation due to lateral diffusion is calculated for a back illuminated CCD imager for typical device parameters. The discrete nature of the CCD and finite size of the photosensitive elements result in an additional MTF degradation. The Fourier transform approach is utilized to calculate the effective point spread function for these processes in the time domain. Experimental data are presented on the point spread function for a three phase, double level anodized aluminum 160 x 100 thinned and back illuminated CCD imager and compared with the theoretical results. A simple modification of the Crowell and Labuda model suggested by these results is presented.

Ando, K. J.↗

Solar-neutrino oscillations

The theory of oscillations of solar neutrinos is developed as it applies to the electron-recoil spectrum in neutrino-electron scattering. The spectral information obtained by such measurements (as opposed to counting total event rates) is crucial for allowing observation of neutrino oscillations for masses down to 500 neV. In this regard, the effects of different masses and mixing angles, as well as such subtleties as thermal and pressure broadening, finite solar-core size, and variable indices of refraction are investigated.

Krauss, L.↗

Analysis of solar receiver flux distributions for US/Russian solar dynamic system demonstration on the MIR Space Station

The United States and Russia have agreed to jointly develop a solar dynamic (SD) system for flight demonstration on the Russian MIR space station starting in late 1997. Two important components of this SD system are the solar concentrator and heat receiver provided by Russia and the U.S., respectively. This paper describes optical analysis of the concentrator and solar flux predictions on target receiver surfaces. The optical analysis is performed using the code CIRCE2. These analyses account for finite sun size with limb darkening, concentrator surface slope and position errors, concentrator petal thermal deformation, gaps between petals, and the shading effect of the receiver support struts. The receiver spatial flux distributions are then combined with concentrator shadowing predictions. Geometric shadowing patterns are traced from the concentrator to the target receiver surfaces. These patterns vary with time depending on the chosen MIR flight attitude and orbital mechanics of the MIR spacecraft. The resulting predictions provide spatial and temporal receiver flux distributions for any specified mission profile. The impact these flux distributions have on receiver design and control of the Brayton engine are discussed.

Kerslake, Thomas W.↗

Low Loss, Finite Width Ground Plane, Thin Film Microstrip Lines on Si Wafers

Si RFICs on standard, 2 Omega-cm. Si wafers require novel transmission lines to reduce the loss caused by the resistive substrate. One such transmission line is commonly called Thin Film Microstrip (TFMS), which is created by depositing a metallic ground plane, thin insulating layers, and the microstrip lines on the Si wafer. Thus, the electric fields are isolated from the Si wafer. In this paper, it is shown through experimental results that the ground plane of TFMS may be finite width and comparable to the strip width in size while still achieving low loss on 2 Omega-cm Si. Measured effective permittivity shows that the field interaction with the Si wafer is small.

Ponchak, George E.↗

Analysis of new composite architectures

Global/local method in conjunction with a special macro finite element is used to gain computational efficiency in the simulation of textile composites behavior. Results are included to demonstrate the effectiveness of the method. Also, 2D finite elements are used to study boundary effects in plain weave composite specimens subjected to extension, shear, and flexure loads. Effective extension, shear, and flexural moduli were found to be quite sensitive to specimen size. For extension and flexure loads stress distributions were affected by a free surface, but the free surface boundary effect did not appear to propagate very far into the interior. For shear load, the boundary effect appeared to propagate much further into the interior. The report is in the form of two technical articles, the first describes the global/local method while the second describes the use of the 2D finite elements.

Whitcomb, John D.↗

Measurement, data analysis and prediction of pyrotechnic shock from pin-pullers and separation joints

To evaluate the potential for spacecraft damage caused by activation of pyrotechnic mechanisms, pyrotechnic shock tests were conducted on three configurations: pin-pullers on an orthogonal double Hopkinson bar arrangement, pin-pullers on a mock-up of the HALOE (Halogen Occultation Experiment) structure, and a section of separation joint on a single Hopkinson bar. Strains and accelerations were measured. The strains were converted to output stresses, forces and moments. Acceleration shock response spectra are presented for both acceleration- and force-time signals. The devices were attached to the Hopkinson bars with adaptors, which are typical of attachments used in practice. In order to predict the effect of the adaptors, finite element models of the adaptor-bar combinations were analyzed and results are presented with regard to grid size, time interval, and frequency range required to predict the response to pulses of 10 and 100 microsecond duration.

Evans, Maria J.↗

Two-Dimensional Diffusion Theory Analysis of Reactivity Effects of a Fuel-Plate-Removal Experiment

Two-dimensional two-group diffusion calculations were performed on the NASA reactor simulator in order to evaluate the reactivity effects of fuel plates removed successively from the center experimental fuel element of a seven- by three-element core loading at the Oak Ridge Bulk Shielding Facility. The reactivity calculations were performed by two methods: In the first, the slowing-down properties of the experimental fuel element were represented by its infinite media parameters; and, in the second, the finite size of the experimental fuel element was recognized, and the slowing-down properties of the surrounding core were attributed to this small region. The latter calculation method agreed very well with the experimented reactivity effects; the former method underestimated the experimental reactivity effects.

Gotsky, Edward R.↗