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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Development and application of computational aerothermodynamics flowfield computer codes

Computations are presented for one-dimensional, strong shock waves that are typical of those that form in front of a reentering spacecraft. The fluid mechanics and thermochemistry are modeled using two different approaches. The first employs traditional continuum techniques in solving the Navier-Stokes equations. The second-approach employs a particle simulation technique (the direct simulation Monte Carlo method, DSMC). The thermochemical models employed in these two techniques are quite different. The present investigation presents an evaluation of thermochemical models for nitrogen under hypersonic flow conditions. Four separate cases are considered. The cases are governed, respectively, by the following: vibrational relaxation; weak dissociation; strong dissociation; and weak ionization. In near-continuum, hypersonic flow, the nonequilibrium thermochemical models employed in continuum and particle simulations produce nearly identical solutions. Further, the two approaches are evaluated successfully against available experimental data for weakly and strongly dissociating flows.

Venkatapathy, Ethiraj↗
NASA NTRS1975

Energy transfer by radiation in non-grey atomic gases in isothermal and non-isothermal slabs

A multiband model for the absorption coefficient of atomic hydrogen-helium plasmas is constructed which includes continuum and line contributions. Emission from 28 stronger lines of 106 that have been screened is considered, of which 21 are from hydrogen and 7 belong to helium, with reabsorption due to line-line, line-continuum overlap accurately accounted for. The model is utilized in the computation of intensities and fluxes from shock-heated slabs of 85% H2-15% He mixtures for slab thicknesses from 1 to 30 cm, temperature from 10,000 to 20,000 K, and for different densities. In conjunction with the multiband model, simple numerical schemes have been devised which provide a quick and comprehensive way of computing radiative energy transfer in nonisothermal and nongrey gases.

Poon, P. T. Y.↗
NASA NTRSDate not supplied

An Automated Meshing Framework for Progressive Damage Analysis of Fabrics Using CompDam

A meshing and model construction framework for progressive damage analysis of fabric composites is proposed. A mesoscale idealization is used such that the fiber tows are considered homogeneous and transversely isotropic continua, and the matrix is a homogeneous isotropic continuum. By using this idealization, relatively mature damage modeling techniques developed for laminates of unidirectional plies are transferrable to fabrics. Cohesive interface elements and a continuum damage mechanics (CDM) model implemented in the software CompDam are used to account for the key damage modes including cracking at the tow/matrix interface, transverse matrix cracking in the tows, and fiber breakage in the tows. Matrix degradation is accounted for by plasticity. The modeling approach is demonstrated for a 3D orthogonal woven fabric. The main novelty of the proposed model construction framework is in its flexibility resulting from a formulation that is agnostic to the fabric architecture and implementation using mesh based operations. The algorithm does not make any assumptions about the tow sizes, shape, path, or proximity to other tows, thus it is applicable to a broad range of fabric architectures. The matrix region mesh surrounding the tows is constructed from domain boundary and tow surface meshes (i.e. through a series of mesh-based operations) such that the model domain can be arbitrarily shaped.

Andrew Cole Bergan↗
NASA NTRS1993

A numerical analysis of high-temperature heat pipe startup from the frozen state

Continuum and rarefied vapor flows co-exist along the heat pipe length for most of the startup period. A two-region model is proposed in which the vapor flow in the continuum region is modeled by the compressible Navier-Stokes equations, and the vapor flow in the rarefied region is simulated by a self-diffusion model. The two vapor regions are linked with appropriate boundary conditions, and heat pipe wail, wick, and vapor flow are solved as a conjugate problem. The numerical solutions for the entire heat pipe startup process from the frozen state are compared with the corresponding experimental data with good agreement.

Cao, Y.↗
NASA NTRS1977

Spectrophotometry of the transient X-ray source A0620-00

Optical observations of the transient X-ray source A0620-00 over the wavelength range from about 3200 to 6600 A are analyzed which were made with the 5-m Hale telescope using a multichannel spectrometer, a digital spectrograph with a cooled integrating SIT vidicon, and an image photon-counting system attached to the camera of the coude spectrograph. Infrared observations at 1.65, 2.2, and 3.5 microns are also examined. Individual spectral lines of special interest are discussed, the E(B-V) color excess is estimated to be about 0.40, and a distance of 1200 pc is adopted. Analysis of the continuous spectrum yields several simple binary models. A specific model is proposed in which one of the continuum sources is an optically thick object characterized by a model atmosphere at a temperature of 25,000 to 30,000 K and the second source is either a cool black body at 4000 K or a partially optically thick source of bound-free plus free-free radiation from a hydrogen gas with an electron temperature of at least 100,000 K. Some possible parameters of these sources and of the accretion disk are considered.

Oke, J. B.↗
NASA NTRS1985

Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region

A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.

Tielens, A. G. G. M.↗
NASA NTRS1995

Ultraviolet imaging telescope and optical emission-line observations of H II regions in M81

Images of the type Sab spiral galaxy M81 were obtained in far-UV and near-UV bands by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 Spacelab mission of 1990 December. Magnitudes in the two UV bands are determined for 52 H II regions from the catalog of Petit, Sivan, & Karachentsev (1988). Fluxes of the H-alpha and H-beta emission lines are determined from CCD images. Extinctions for the brightest H II regions are determined from observed Balmer decrements. Fainter H II regions are assigned the average of published radio-H-alpha extinctions for several bright H II regions. The radiative transfer models of Witt, Thronson, & Capuano (1992) are shown to predict a relationship between Balmer Decrement and H-alpha extinction consistent with observed line and radio fluxes for the brightest 7 H II regions and are used to estimate the UV extinction. Ratios of Lyman continuum with ratios predicted by model spectra computed for initial mass function (IMF) slope equal to -1.0 and stellar masses ranging from 5 to 120 solar mass. Ages and masses are estimated by comparing the H-alpha and far-UV fluxes and their ratio with the models. The total of the estimated stellar masses for the 52 H II regions is 1.4 x 10(exp 5) solar mass. The star-formation rate inferred for M81 from the observed UV and H-alpha fluxes is low for a spiral galaxy at approximately 0.13 solar mass/yr, but consistent with the low star-formation rates obtained by Kennicutt (1983) and Caldwell et al. (1991) for early-type spirals.

Hill, Jesse K.↗
NASA NTRSDate not supplied

A Holistic DSMC Transport Database for Re-Entry and Ablation Modeling

Hybrid simulation frameworks combining Computational Fluid Dynamics (CFD) and Direct Simulation Monte Carlo (DSMC) are frequently employed to efficiently perform high-fidelity solutions of environments containing combined continuum/rarified flow. The use of DSMC, a stochastic, particle-based method, is necessary for high-Knudsen flow where continuum-based assumptions governing CFD break down. However, the DSMC methodology is generally very computationally inefficient to model the continuum regime. In a CFD/DSMC hybrid approach, obtaining an accurate, high-fidelity solution hinges on the consistent treatment of transport properties and the used thermo-chemical models employed within the two solvers. In principle, in regions where CFD and DSMC are both employed, the same gas mixture under the same conditions should have the same properties, regardless of simulation type. Observed differences should be due to non-equilibrium processes, rather than differences in physical models. While the transport models governing CFD and DSMC simulations are starkly different, they can effectively be linked via their use of reduced Chapman-Enskog collision integrals. In CFD, these integrals are typically stored as fitted polynomial expressions and used to directly compute gas transport properties via mixing rules or the full Chapman-Enskog formulation. In DSMC, they can be used to derive the collision parameters needed for the phenomenological collision cross-section models that govern particle interactions, via a Nelder-Mead optimization scheme. The goal of this work is to provide a unified DSMC transport database encompassing the vast majority of known gas species encountered during atmospheric entry, on Earth or any other Solar body. This goal is largely possible due to recently performed ab-initio quantum chemistry calculations. Combined with other high-fidelity literature sources, the planned database will consist of collision integral data for over 200 neutral and ionized species and over 17000 binary collisions. From these collision integrals, Nelder-Mead optimization is used to compute Variable Soft Sphere (VSS) collision model parameters for DSMC, fitted from 300 K to 20000 K. Initial comparisons of transport properties of relevant equilibrium gas mixtures show great agreement between CFD and DSMC-derived results. The completed database will be able to be readily applied to model binary collisions of any gas mixture containing the included species over the specified temperature range, making it a valuable tool for future planetary probe modeling efforts. An example is shown below. Equilibrium mixture transport properties for a 19-species Titan atmospheric model [4] are computed using both fitted VSS parameters and the original CFD collision integral values. Deviations in computed properties between the two approaches is less than 5% for the entire temperature range.

M R Gosma↗
NASA NTRS1973

Direct integration transmittance model

A transmittance model was developed for the 200-2000/cm region for interpretation of high spectral resolution measurements of laboratory absorption and of planetary thermal emission. The high spectral resolution requires transmittances to be computed monochromatically by summing the contribution of individual molecular absorption lines. A magnetic tape atlas of H2O,O3, and CO2 molecular line parameters serves as input to the transmittance model with simple empirical representations used for continuum regions wherever suitable laboratory data exist. The theoretical formulation of the transmittance model and the computational procedures used for the evaluation of the transmittances are discussed. Application is demonstrated of the model to several homogenous path laboratory absorption examples.

Kunde, V. G.↗
NASA NTRS1974

Direct integration transmittance model

A transmittance model has been developed for interpretation of high spectral resolution measurements of laboratory absorption and of planetary thermal emission. The high spectral resolution requires transmittances to be computed monochromatically by summing the contribution of individual molecular absorption lines. A magnetic tape atlas of H2O, O3, and CO2 molecular line parameters serves as input to the transmittance model with simple empirical representations used for continuum regions wherever suitable laboratory data exist. The theoretical formulation of the transmittance model and the computational procedures used for the evaluation of the transmittances are discussed, and application of the model to several homogeneous-path laboratory absorption examples is demonstrated.

Kunde, V. G.↗
NASA NTRS1992

Reaction zone structure for strong, weak overdriven, and weak underdriven oblique detonations

A simple dynamic systems analysis is used to give examples of strong, weak overdriven, and weak underdriven oblique detonations. Steady oblique detonations consisting of a straight lead shock attached to a solid wedge followed by a resolved reaction zone structure are admitted as solutions to the reactive Euler equations. This is demonstrated for a fluid that is taken to be an inviscid, calorically perfect ideal gas that undergoes a two-step irreversible reaction with the first step exothermic and the second step endothermic. This model admits solutions for a continuum of shock wave angles for two classes of solutions identified by a Rankine-Hugoniot analysis: strong and weak overdriven waves. The other class, weak underdriven, is admitted for eigenvalue shock-wave angles. Chapman-Jouguet waves, however, are not admitted. These results contrast those for a corresponding onestep model that, for detonations with a straight lead shock, only admits strong, weak overdriven, and Chapman-Jouguet solutions.

Powers, Joseph M.↗
NASA NTRS2001

CFD Simulation of Liquid Rocket Engine Injectors

Detailed design issues associated with liquid rocket engine injectors and combustion chamber operation require CFD methodology which simulates highly three-dimensional, turbulent, vaporizing, and combusting flows. The primary utility of such simulations involves predicting multi-dimensional effects caused by specific injector configurations. SECA, Inc. and Engineering Sciences, Inc. have been developing appropriate computational methodology for NASA/MSFC for the past decade. CFD tools and computers have improved dramatically during this time period; however, the physical submodels used in these analyses must still remain relatively simple in order to produce useful results. Simulations of clustered coaxial and impinger injector elements for hydrogen and hydrocarbon fuels, which account for real fluid properties, is the immediate goal of this research. The spray combustion codes are based on the FDNS CFD code' and are structured to represent homogeneous and heterogeneous spray combustion. The homogeneous spray model treats the flow as a continuum of multi-phase, multicomponent fluids which move without thermal or velocity lags between the phases. Two heterogeneous models were developed: (1) a volume-of-fluid (VOF) model which represents the liquid core of coaxial or impinger jets and their atomization and vaporization, and (2) a Blob model which represents the injected streams as a cloud of droplets the size of the injector orifice which subsequently exhibit particle interaction, vaporization, and combustion. All of these spray models are computationally intensive, but this is unavoidable to accurately account for the complex physics and combustion which is to be predicted, Work is currently in progress to parallelize these codes to improve their computational efficiency. These spray combustion codes were used to simulate the three test cases which are the subject of the 2nd International Workshop on-Rocket Combustion Modeling. Such test cases are considered by these investigators to be very valuable for code validation because combustion kinetics, turbulence models and atomization models based on low pressure experiments of hydrogen air combustion do not adequately verify analytical or CFD submodels which are necessary to simulate rocket engine combustion. We wish to emphasize that the simulations which we prepared for this meeting are meant to test the accuracy of the approximations used in our general purpose spray combustion models, rather than represent a definitive analysis of each of the experiments which were conducted. Our goal is to accurately predict local temperatures and mixture ratios in rocket engines; hence predicting individual experiments is used only for code validation. To replace the conventional JANNAF standard axisymmetric finite-rate (TDK) computer code 2 for performance prediction with CFD cases, such codes must posses two features. Firstly, they must be as easy to use and of comparable run times for conventional performance predictions. Secondly, they must provide more detailed predictions of the flowfields near the injector face. Specifically, they must accurately predict the convective mixing of injected liquid propellants in terms of the injector element configurations.

Farmer, Richard↗
NASA NTRS2008

Modeling the Non-Linear Response of Fiber-Reinforced Laminates Using a Combined Damage/Plasticity Model

The present work is concerned with modeling the non-linear response of fiber reinforced polymer laminates. Recent experimental data suggests that the non-linearity is not only caused by matrix cracking but also by matrix plasticity due to shear stresses. To capture the effects of those two mechanisms, a model combining a plasticity formulation with continuum damage has been developed to simulate the non-linear response of laminates under plane stress states. The model is used to compare the predicted behavior of various laminate lay-ups to experimental data from the literature by looking at the degradation of axial modulus and Poisson s ratio of the laminates. The influence of residual curing stresses and in-situ effect on the predicted response is also investigated. It is shown that predictions of the combined damage/plasticity model, in general, correlate well with the experimental data. The test data shows that there are two different mechanisms that can have opposite effects on the degradation of the laminate Poisson s ratio which is captured correctly by the damage/plasticity model. Residual curing stresses are found to have a minor influence on the predicted response for the cases considered here. Some open questions remain regarding the prediction of damage onset.

Schuecker, Clara↗
NASA NTRS1989

Flare model sensitivity of the Balmer spectrum

Careful studies of various chromospheric spectral signatures are very important in order to explore their possible sensitivity to the modifications of the thermodynamic quantities produced by the flare occurrence. Pioneer work of Canfield and co-workers have shown how the H alpha behavior is able to indicate different changes in the atmospheric parameters structure associated to the flare event. It was decided to study the behavior of the highest Balmer lines and of the Balmer continuum in different solar flare model atmospheres. These spectral features, originating in the deep photosphere in a quiet area, may have a sensitivity different from H alpha to the modification of a flare atmosphere. The details of the method used to compute the Stark profile of the higher Balmer line (n is greater than or equal to 6) and their merging were extensively given elsewhere (Donati-Falchi et al., 1985; Falchi et al., 1989). The models used were developed by Ricchiazzi in his thesis (1982) evaluating the chromospheric response to both the nonthermal electron flux, for energy greater than 20 kev, (F sub 20) and to the thermal conduction, (F sub c). The effect of the coronal pressure values (P sub O) at the apex of the flare loop is also included.

Falchi, A.↗
NASA NTRS1976

Helium emission from model flare layers

The emission of visible and UV He I and He II line radiation from a plane-parallel model flare layer characterized by electron temperatures of 10,000 to 50,000 K and electron densities of 10 to the 10th power to 10 to the 15th power per cu cm is analyzed by solving the statistical-equilibrium equations for a 30-level He I-II-III system, using parametric representations of the line and continuum radiation fields. The atomic model was chosen to provide accurate solutions for the first two resonance lines of He I and He II as well as for the D3 and 10,830-A lines of He I. Reaction rates are discussed, and sample solutions to the steady-state population equations are given for a generally optically thin gas assumed to be irradiated over 2pi sr by a blackbody spectrum at 6000 K. Specific results are examined for ionization equilibrium, level populations, approximate optical depths of a 1000-km-thick flare layer, line intensities, and upper-level population rates.

Kulander, J. I.↗
NASA NTRS1989

The Lyman edge test of the quasar emission mechanism

The Lyman edge region in the spectra of 11 high-redshift quasars is studied using data designed to detect broadened Lyman edge absorption predicted by thermal models of the 'big blue bump' continuum component, especially thin accretion disks. Three objects have broadened, partial edges near the systemic redshifts, as expected for thermal models. In two cases, however, narrow Lyamn lines with the edge absorption, ruling out the disk interpretation. Thus, the data are negative for thermal models, at least for opaque thin accretion disks.

Antonucci, R. R. J.↗
NASA NTRS2018

Plasticity and Damage Modeling of Stress Asymmetry and Dynamic Behavior of AFS Additive Manufactured Aluminum Alloy 2219

The Solid State Additive Manufacturing (AM) process referred as MELD that fabricated the samples in this study, provides a new path for repairing, coating, joining and additive manufacturing metals and metal matrix composites. This research will be the first application of a physics-based microstructure dependent internal state variable (ISV) plasticity and damage material model to capture the mechanical response of an AM Aluminum Alloy (AA) 2219 via the MELD process. In this research, a microstructure-based internal state variable (ISV) plasticity-damage model was used to capture the mechanical behavior of AFS 2219 aluminum alloy. Aeroprobe Corporation, creator and patent holder for the MELD process, fabricated the material by pushing a solid filler rod of AA2219-T861 material through a hollow rotating tool onto an AA2219 T851 plate substrate. As feedstock, solid or powder precursor metals are pushed through a nonconsumable rotating cylindrical tool. Herein, added layers are deposited and metallurgically bonded to substrate material or previously deposited layers by the heat generated from the rotating tool through plastic deformation of the filler material. Once a layer has been added, the tool height increases, and starts the deposition of the next layer. This process results in beneficial properties such as grain refinement, homogenization and reduced porosity (fully dense). This process will experience temperatures similar to those in the weld nugget zone (WNZ) in friction stir welding (FSW), ranging from 0.6-0.9 Tm, with Tm being the melting point of the material. MELD is highly scalable with AA deposition rates reaching over 1000 cm3/hr, which allows for MELD being used for repairs, coatings, and building components. A motivating factor driving the research for physics-based history dependent material modeling of MELD components is the ability to accurately capture the stress-state and strain rate dependence in the material caused by variations in material microstructure from the MELD processing of new or repaired components. The ISV model incorporates microstructural content and is consistent with continuum level kinematics, kinetics, and thermodynamics. These features allow the ISV model to capture large deformations at the structural scale using the kinematic and isotropic hardening, while microscale damage is obtained from the microstructural features. The benefits of the ISV model arise from the inclusion of structure-property relationships identified from microstructural characterization and experimentation. The Bauschinger effect (BE) is an important concept, vital in the accurate prediction of cyclic stress-strain response of ductile materials such as metals. The ISV model has been successfully used to capture the behavior and damage, and the BE of different aluminum alloys and steels. The ISV model uses kinematic and isotropic hardening to help capture deformations of the material at the macro scale. To understand this hardening relationship, calculating the kinematic and isotropic hardening relationship in the material is warranted for a high-fidelity model. Electron Backscattered Diffraction (EBSD) was used to characterize the as-fabricated microstructure, where a fully-dense equiaxed grain morphology with average grain size of 2.5 μm was observed. Microhardness mapping of the as-built structures, monotonic tension and compression experiments at both quasi-static (0.001/s) strain rates, tension-followed-by-compression and compression-followed-by-tension experiments were performed to obtain the set of plasticity and damage constants necessary to capture strain rate and stress state behavior of this additive material. To calibrate the plasticity-damage model, a single set of constants were determined to capture the different stress states the MELD AA2219. One set of the constants was determined from experimental true stress-strain curves for the tension and compression data. Additionally, microstructural information and data from the open literature were used as the other model constants. This research is a first of its kind for AFS AA2219, includes correlating the ISV model to the monotonic experimental results that capture the isotropic and kinematic plasticity mechanical response.

Rivera, O. G.↗
NASA NTRS1995

EUVE spectroscopy of epsilon Canis Majoris (B2 II) from 70 to 730 A

We present spectra of the brightest stellar source of extreme ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon CMa. These data were taken with the three spectrometers aboard the NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the continuum and line spectrum of the stellar photosphere in the 320 to 730 A region, and on the wind emission lines observed in the 170-375 A region. This is the first EUV spectrum of an early-type star, and thus makes epsilon CMa the most comprehensively observed B star from the X-ray to infrared regimes. The radiation in both the H Lyman continuum and He I continuum (shortward of 504 A) are found to be significantly greater than predicted by both Local Thermodynamic Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa also exhibits a mid-infrared excess, this points to the outer layers being warmer than the models indicate. The anomalously large Lyman continuum flux, combined with the very low column density measured in the direction toward this star implies that it is the dominant source of hydrogen ionization of the local interstellar medium in the immediate vicinity of the sun. All of the lines predicted to be strong from model atmospheres are present and several wind absorption features are also identified. We have detected emission lines from highly ionized iron that are consistent with the ROSAT Position Sensitive Proportional Counter (PSPC) observations if a multi-temperature emission model is used, and the assumption is made that there is significant absorption beyond that of the neutral phase of the ISM. The spectrum shows strong O III 374 A line emission produced by the Bowen flourescence mechanism, which has not previously been observed in the spectra of hot stars.

Cassinelli, J. P.↗