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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Efficient and Accurate Explicit Integration Algorithms with Application to Viscoplastic Models

Several explicit integration algorithms with self-adative time integration strategies are developed and investigated for efficiency and accuracy. These algorithms involve the Runge-Kutta second order, the lower Runge-Kutta method of orders one and two, and the exponential integration method. The algorithms are applied to viscoplastic models put forth by Freed and Verrilli and Bodner and Partom for thermal/mechanical loadings (including tensile, relaxation, and cyclic loadings). The large amount of computations performed showed that, for comparable accuracy, the efficiency of an integration algorithm depends significantly on the type of application (loading). However, in general, for the aforementioned loadings and viscoplastic models, the exponential integration algorithm with the proposed self-adaptive time integration strategy worked more (or comparably) efficiently and accurately than the other integration algorithms. Using this strategy for integrating viscoplastic models may lead to considerable savings in computer time (better efficiency) without adversely affecting the accuracy of the results. This conclusion should encourage the utilization of viscoplastic models in the stress analysis and design of structural components.

Arya, Vinod K.↗
NASA NTRS2011

Fermi/GBM Observations of SGRJ0501 + 4516 Bursts

We present our temporal and spectral analyses of 29 bursts from SGRJ0501+4516, detected with the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope during the 13 days of the source activation in 2008 (August 22 to September 3). We find that the T(sub 90) durations of the bursts can be fit with a log-normal distribution with a mean value of approx. 123 ms. We also estimate for the first time event durations of Soft Gamma Repeater (SGR) bursts in photon space (i.e., using their deconvolved spectra) and find that these are very similar to the T(sub 90)s estimated in count space (following a log-normal distribution with a mean value of approx. 124 ms). We fit the time-integrated spectra for each burst and the time-resolved spectra of the five brightest bursts with several models. We find that a single power law with an exponential cutoff model fits all 29 bursts well, while 18 of the events can also be fit with two black body functions. We expand on the physical interpretation of these two models and we compare their parameters and discuss their evolution. We show that the time-integrated and time-resolved spectra reveal that E(sub peak) decreases with energy flux (and fluence) to a minimum of approx. 30 keV at F = 8.7 x 10(exp -6)erg/sq cm/s, increasing steadily afterwards. Two more sources exhibit a similar trend: SGRs J1550 - 5418 and 1806 - 20. The isotropic luminosity, L(sub iso), corresponding to these flux values is roughly similar for all sources (0.4 - l.5 x 10(exp 40) erg/s.

Lin, Lin↗
NASA NTRS1977

Computation of unsteady transonic flows by the indicial method

The indicial method is investigated for the computation of unsteady transonic force and moment coefficients for use in flutter analyses. This approach has the advantage that solutions for all reduced frequencies for a given mode of motion can be obtained from a single finite-difference flowfield computation. Comparisons of indicial and time-integration computations for oscillating airfoil and flap motions help define limits on the motion amplitude for the applicability of the indicial method to transonic flows. Within these limits, solutions for various motion modes can be superposed to obtain solutions for multiple-degree-of-freedom aeroelastic systems. Also, a simple aeroelastic problem is solved by an alternative approach in which the structural motion and flowfield equations are integrated simultaneously using a time-integration finite-difference procedure.

Ballhaus, W. F.↗
NASA NTRS2015

Development of the US3D Code for Advanced Compressible and Reacting Flow Simulations

Aerothermodynamics and hypersonic flows involve complex multi-disciplinary physics, including finite-rate gas-phase kinetics, finite-rate internal energy relaxation, gas-surface interactions with finite-rate oxidation and sublimation, transition to turbulence, large-scale unsteadiness, shock-boundary layer interactions, fluid-structure interactions, and thermal protection system ablation and thermal response. Many of the flows have a large range of length and time scales, requiring large computational grids, implicit time integration, and large solution run times. The University of Minnesota NASA US3D code was designed for the simulation of these complex, highly-coupled flows. It has many of the features of the well-established DPLR code, but uses unstructured grids and has many advanced numerical capabilities and physical models for multi-physics problems. The main capabilities of the code are described, the physical modeling approaches are discussed, the different types of numerical flux functions and time integration approaches are outlined, and the parallelization strategy is overviewed. Comparisons between US3D and the NASA DPLR code are presented, and several advanced simulations are presented to illustrate some of novel features of the code.

CFD↗
NASA NTRS1992

Rain rate and modeled fade distributions at 20 GHz and 30 GHz derived from five years of network rain gauge measurements

Five years of rain rate and modeled slant path attenuation distributions at 20 GHz and 30 GHz derived from a network of 10 tipping bucket rain gages was examined. The rain gage network is located within a grid 70 km north-south and 47 km east-west in the Mid-Atlantic coast of the United States in the vicinity of Wallops Island, Virginia. Distributions were derived from the variable integration time data and from one minute averages. It was demonstrated that for realistic fade margins, the variable integration time results are adequate to estimate slant path attenuations at frequencies above 20 GHz using models which require one minute averages. An accurate empirical formula was developed to convert the variable integration time rain rates to one minute averages. Fade distributions at 20 GHz and 30 GHz were derived employing Crane's Global model because it was demonstrated to exhibit excellent accuracy with measured COMSTAR fades at 28.56 GHz.

Goldhirsh, Julius↗
NASA NTRS1985

Optical detection of large meteoroids in space

CCD sensors placed across the focal plane of large Schmidt telescopes have great potential for detecting and measuring the very low flux in space of meteoroids with diameters larger than 1 meter. With the Palomar 'Big Schmidt', a detection rate of 1.4 per hour is obtained for meteoroids between 0.6 and 200 meters in diameter. For the Baker-Nunn 'Satellite Tracking Camera', the corresponding rate is about 0.8 per hour. The key to obtaining such high detection rates derives from approximately setting the sensor integration time equal to the time it takes a meteoroid to cross a pixel field of view. This minimizes signal to noise problems and is accomplished, in practice, by multiple summing of short integration time data records to obtain data records of longer effective integration times.

Zook, H. A.↗
NASA NTRS1975

Design Data Collection with Skylab Microwave Radiometer-Scatterometer S-193, Volume 1

The author has identified the following significant results. Observations with S-193 have provided radar design information for systems to be flown on spacecraft, but only at 13.9 GHz and for land areas over the United States and Brazil plus a few other areas of the world for which this kind of analysis was not made. Observations only extended out to about 50 deg angle of incidence. The value of a sensor with such a gross resolution for most overland resource and status monitoring systems seems marginal, with the possible exception of monitoring soil moisture and major vegetation variations. The complementary nature of the scatterometer and radiometer systems was demonstrated by the correlation analysis. Although radiometers must have spatial resolutions dictated by antenna size, radars can use synthetic aperture techniques to achieve much finer resolutions. Multiplicity of modes in the S-193 sensors complicated both the system development and its employment. An attempt was made in the design of the S-193 to arrange optimum integration times for each angle and type of measurement. This unnecessarily complicated the design of the instrument, since the gains in precision achieved in this way were marginal. Either a software-controllable integration time or a set of only two or three integration times would have been better.

Moore, R. K.↗
NASA NTRS1989

Photon statistical limitations for daytime optical tracking

Tracking of interplanetary spacecraft equipped with optical communication systems by using astrometric instruments is being investigated by JPL. Existing instruments are designed to work at night and, for bright sources, are limited by tropospheric errors. To provide full coverage of the solar system, astrometric tracking instruments must either be capable of daytime operation or be space-based. The integration times necessary for the ground-based daytime photon statistical errors to reach a given accuracy level (5 to 50 nanoradians) were computed for an ideal astrometric instrument. The required photon statistical integration times are found to be shorter than the tropospheric integrations times for the ideal detector. Since the astrometric need not be limited by photon statistics even under daytime conditions, it may be fruitful to investigate instruments for daytime optical tracking.

Folkner, W. M.↗
NASA NTRS2016

Characterizing Rocky and Gaseous Exoplanets with 2 m Class Space-based Coronagraphs

Several concepts now exist for small, space-based missions to directly characterize exoplanets in reflected light. While studies have been performed that investigate the potential detection yields of such missions, little work has been done to understand how instrumental and astrophysical parameters will affect the ability of these missions to obtain spectra that are useful for characterizing their planetary targets. Here, we develop an instrument noise model suitable for studying the spectral characterization potential of a coronagraph-equipped, space-based telescope. We adopt a baseline set of telescope and instrument parameters appropriate for near-future planned missions like WFIRST-AFTA, including a 2 m diameter primary aperture, an operational wavelength range of 0.4–1.0 μm, and an instrument spectral resolution of λ/Δλ =70, and apply our baseline model to a variety of spectral models of different planet types, including Earth twins, Jupiter twins, and warm and cool Jupiters and Neptunes. With our exoplanet spectral models, we explore wavelength-dependent planet–star flux ratios for main-sequence stars of various effective temperatures and discuss how coronagraph inner and outer working angle constraints will influence the potential to study different types of planets. For planets most favorable to spectroscopic characterization—cool Jupiters and Neptunes as well as nearby super-Earths—we study the integration times required to achieve moderate signal-to-noise ratio spectra. We also explore the sensitivity of the integration times required to either detect the bottom or presence of key absorption bands (for methane, water vapor, and molecular oxygen) to coronagraph raw contrast performance, exozodiacal light levels, and the distance to the planetary system. Decreasing detector quantum efficiency at longer visible wavelengths makes the detection of water vapor in the atmospheres of Earth-like planets extremely challenging, and also hinders detections of the 0.89 μm methane band. Additionally, most modeled observations have noise dominated by dark currents, indicating that improving CCD performance could substantially drive down requisite integration times. Finally, we briefly discuss the extension of our models to a more distant future Large UV-Optical-InfraRed (LUVOIR) mission.

Astrobiology↗
NASA NTRS2010

Adaptive Numerical Algorithms in Space Weather Modeling

Space weather describes the various processes in the Sun-Earth system that present danger to human health and technology. The goal of space weather forecasting is to provide an opportunity to mitigate these negative effects. Physics-based space weather modeling is characterized by disparate temporal and spatial scales as well as by different physics in different domains. A multi-physics system can be modeled by a software framework comprising of several components. Each component corresponds to a physics domain, and each component is represented by one or more numerical models. The publicly available Space Weather Modeling Framework (SWMF) can execute and couple together several components distributed over a parallel machine in a flexible and efficient manner. The framework also allows resolving disparate spatial and temporal scales with independent spatial and temporal discretizations in the various models. Several of the computationally most expensive domains of the framework are modeled by the Block-Adaptive Tree Solar wind Roe Upwind Scheme (BATS-R-US) code that can solve various forms of the magnetohydrodynamics (MHD) equations, including Hall, semi-relativistic, multi-species and multi-fluid MHD, anisotropic pressure, radiative transport and heat conduction. Modeling disparate scales within BATS-R-US is achieved by a block-adaptive mesh both in Cartesian and generalized coordinates. Most recently we have created a new core for BATS-R-US: the Block-Adaptive Tree Library (BATL) that provides a general toolkit for creating, load balancing and message passing in a 1, 2 or 3 dimensional block-adaptive grid. We describe the algorithms of BATL and demonstrate its efficiency and scaling properties for various problems. BATS-R-US uses several time-integration schemes to address multiple time-scales: explicit time stepping with fixed or local time steps, partially steady-state evolution, point-implicit, semi-implicit, explicit/implicit, and fully implicit numerical schemes. Depending on the application, we find that different time stepping methods are optimal. Several of the time integration schemes exploit the block-based granularity of the grid structure. The framework and the adaptive algorithms enable physics based space weather modeling and even forecasting.

Toth, Gabor↗
NASA NTRS1988

Multi-time-step integration using nodal partitioning

An algorithm is presented which integrates different groups of nodes of a finite element mesh with different time steps and different integrators. Since the nodal groups are updated independently no unsymmetric systems need be solved. Stability is demonstrated by showing that an energy norm of the solution decreases after every update if the time step is less than a given critical value. The element eigenvalue inequality theorem is used to give the critical time step in terms of element eigenvalues.

Smolinski, P.↗
NASA NTRS1999

Three-Dimensional Simulations of Electron Beams Focused by Periodic Permanent Magnets

A fully three-dimensional (3D) model of an electron beam focused by a periodic permanent magnet (PPM) stack has been developed. First, the simulation code MAFIA was used to model a PPM stack using the magnetostatic solver. The exact geometry of the magnetic focusing structure was modeled; thus, no approximations were made regarding the off-axis fields. The fields from the static solver were loaded into the 3D particle-in-cell (PIC) solver of MAFIA where fully 3D behavior of the beam was simulated in the magnetic focusing field. The PIC solver computes the time-integration of electromagnetic fields simultaneously with the time integration of the equations of motion of charged particles that move under the influence of those fields. Fields caused by those moving charges are also taken into account; thus, effects like space charge and magnetic forces between particles are fully simulated. The electron beam is simulated by a number of macro-particles. These macro-particles represent a given charge Q amounting to that of several million electrons in order to conserve computational time and memory. Particle motion is unrestricted, so particle trajectories can cross paths and move in three dimensions under the influence of 3D electric and magnetic fields. Correspondingly, there is no limit on the initial current density distribution of the electron beam, nor its density distribution at any time during the simulation. Simulation results including beam current density, percent ripple and percent transmission will be presented, and the effects current, magnetic focusing strength and thermal velocities have on beam behavior will be demonstrated using 3D movies showing the evolution of beam characteristics in time and space. Unlike typical beam optics models, this 3D model allows simulation of asymmetric designs such as non- circularly symmetric electrostatic or magnetic focusing as well as the inclusion of input/output couplers.

Kory, Carol L.↗
NASA NTRS1987

Response of joint dominated space structures

An approximate method is developed for estimating the transient response of nonlinear systems in terms of linearized modes of response. Its advantages are that it is computationally more efficient than the time integration method and that it is possible to view the design problem in the more traditional physical terms of modal response. The major drawback of the approximate method is loss of accuracy. It seems that both approximate methods and time integration have their roles in design. Approximate methods provide efficient tools for performing parametric studies and they supply physical insights into how to optimize system performance that are not easily inferred from strictly numerical methods. Time integration provides a method for assessing the accuracy of the approximate solution for key simulations and for fine tuning the final design. In the procedure presented the nonlinear system is approximated by an equivalent linear system in which the system parameters are constant over the range of transient response.

Source record↗
NASA NTRS2008

The Miniaturized Moessbauer Spectrometers MIMOS II on MER: Four Years of Operation - A Summary

The two Miniaturized Moessbauer Spectrometers (MIMOS II) on board the two Mars Exploration Rovers Spirit and Opportunity have now been collecting important scientific data for more than four years. The spectrometers provide information about Fe-bearing mineral phases and determine Fe oxidation states. The total amount of targets analized exceeds 600, the total integration time exceeds 260 days for both rovers. Since landing, more than five half-lives of the Co-57 MB sources have past (intensity at the time of landing approx. 150 mCi). Current integration times are about 50 hours in order to achieve reasonable statistics as opposed to 8 hours at the beginning of the mission. In total, 13 different mineral phases were detected: Olivine, pyroxene, hematite, magnetite and nanophase ferric oxide were detected at both landing sites. At Gusev, ilmenite, goethite, a ferric sulfate phase and a yet unassigned phase (in the rock Fuzzy Smith) were detected. At Meridiani, jarosite, metallic iron in meteoritic samples (kamacite), troilite, and an unassigned ferric phase were detected. Jarosite and goethite are of special interest, as these minerals are indicators for water activity. In this abstract, an overview of Moessbauer results will be given, with a focus on data obtained since the last martian winter. The MER mission has proven that Moessbauer spectroscopy is a valuable tool for the in situ exploration of extraterrestrial bodies and for the study of Febearing samples. The experience gained through the MER mission makes MIMOS II a obvious choice for future missions to Mars and other targets. Currently, MIMOS II is on the scientific payload of two approved future missions: Phobos Grunt (Russian Space Agency; 2009) and ExoMars (European Space Agency; 2013).

Fleischer, I.↗
NASA NTRS1979

A correlative study of magnetic flux transfer in the magnetosphere

The applied magnetic flux of the southward component of the IMF in GSM coordinates (Bz-) which impinges upon the sunward magnetopause is compared to the time integral of the auroral AL index during 56 intervals within a 3-month period in 1969 when interplanetary records from Heos 1 and Explorer 35 were available. The periods of magnetic activity were those for which Bz was greater than 0 and AL was less than 20gamma at the beginning and end of the interval. It was found that for these intervals, the time integral of the AL index was proportional to the applied magnetic flux with a correlation coefficient of 0.94. In addition, the empirical relationships between magnetic flux transfer, applied southward flux, and the time integral of AL arrived at by Holzer and Slavin (1978) on the basis of expansion and contractions of the forward magnetosphere observed with OGO 5 are reexamined and improved.

Holzer, R. E.↗
NASA NTRS2016

Low Frequency 1/f Noise on QWIPs, nBn, and Superlattice Focal Plane Array

Noise Equivalent Difference Temperature (NE△T) is a standard performance metric for most infrared focal plane array (FPA) systems. The frequency bandwidth and range associated with NE△T is at high frequency and it does not describe the long time noise behavior or the very low frequency noise of the FPA. Very low frequency noise measurement requires data capture that takes longer time duration and sampling interval. This study investigates low frequency noise in QWIP, nBn and LWIR n-type Complementary Barrier Infrared Detector (CBIRD) FPAs. The corner frequencies are extracted from the power spectral density (PSD) as function of frequency. The peak wavelength, quantum efficiency of QWIP detector are 6.2 µm and 2 %, respectively. QWIP FPA has a mean NE△T ~ 25 mK at an operating temperature of 65 K and an integration time of 16 msec. The mean QWIP PSD plot shows a corner frequency of < 0.5 mHz. The nBn FPA with 4 µm cut off and quantum efficiency of 67 % has NE△T ~ 15.6 mK at an integration time ~ 7.52 msec and an operating temperature of 120 K. The nBn has corner frequency of > 50 mHz. Lastly the n-type CBIRD FPA with a 50% cutoff at 8.8 µm and quantum efficiency of ~ 50 % has an NE△T ~ 18.6 mK at an integration time of 1.86 msec and operating temperature 120 K. The superlattice FPA has a corner frequency ~ 10 mHz. The investigation of the tail on the NE△T histogram reveals that its origin is not accounted for entirely by the high noise current, but also it needs the inclusion of lower responsivity for some pixels.

Pepper, B.↗
NASA NTRS1975

Instruction manual, optical effects module electronic controller and processor, model OEMCP

The OEM-1 electronic module is discussed; it is comprised of four subsystems: the signal processing and display; the stepper motor controls; the chopper controls; and the dc-dc invertor. The OEM-1 module controls the sample wheel so that the relative transmittance of the samples can be compared to the clear aperture position. The 3-1/2 digit digital voltmeter displays the clear aperture signal level as well as the ratio of the remaining sample positions relative to the clear aperture position. The sample wheel position is decoded so that the signals and ratios can be correlated to the data. The OEM is automatically reset to the I sub o on initial turn-on and can be reset to the '0' position by actuating a front panel switch. The sample wheel can be interrupted to change samples or induce a longer integration time if desired by a front panel command. Integration times from 1 - 50 seconds are provided at the front panel, and BCD data for external interfacing is provided.

Source record↗