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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Computational results for 2-D and 3-D ramp flows with an upwind Navier-Stokes solver

An implicit, finite-difference, upwind, full Navier-Stokes solver was applied to supersonic/hypersonic flows over two-dimensional ramps and three-dimensional obstacle. Some of the computed results are presented. The numerical scheme used in the study is an implicit, spatially second order accurate, upwind, LU-ADI scheme based on Roe's approximate Reimann solver with MUSCL differencing of Van Leer. An algebraic grid generation scheme based on generalized interpolation scheme was used in generating the grids for the various 2-D and 3-D problems.

Venkatapathy, Ethiraj↗
NASA NTRS1993

Navier-Stokes flowfield computation of wing/rotor interaction for a tilt rotor aircraft in hover

The download on the wing produced by the rotor-induced downwash of a tilt rotor aircraft in hover is of major concern because of its severe impact on payload-carrying capability. A method has been developed to help gain a better understanding of the fundamental fluid dynamics that causes this download, and to help find ways to reduce it. In particular, the method is employed in this work to analyze the effect of a tangential leading edge circulation-control jet on download reduction. Because of the complexities associated with modeling the complete configuration, this work focuses specifically on the wing/rotor interaction of a tilt rotor aircraft in hover. The three-dimensional, unsteady, thin-layer compressible Navier-Stokes equations are solved using a time-accurate, implicit, finite difference scheme that employs LU-ADI factorization. The rotor is modeled as an actuator disk which imparts both a radical and an azimuthal distribution of pressure rise and swirl to the flowfield. A momentum theory blade element analysis of the rotor is incorporated into the Navier-Stokes solution method. Solution blanking at interior points of the mesh has been shown here to be an effective technique in introducing the effects of the rotor and tangential leading edge jet. Results are presented both for a rotor alone and for wing/rotor interaction. The overall mean characteristics of the rotor flowfield are computed including the flow acceleration through the rotor disk, the axial and swirl velocities in the rotor downwash, and the slipstream contraction. Many of the complex tilt rotor flow features are captured including the highly three-dimensional flow over the wing, the recirculation fountain at the plane of symmetry, wing leading and trailing edge separation, and the large region of separated flow beneath the wing. Mean wing surface pressures compare fairly well with available experimental data, but the time-averaged download/thrust ratio is 20-30 percent higher than the measured value. The discrepancy is due to a combination of factors that are discussed. Leading edge tangential blowing, of constant strength along the wing span, is shown to be effective in reducing download. The jet serves primarily to reduce the pressure on the wing upper surface. The computation clearly shows that, because of the three-dimensionality of the flowfield, optimum blowing would involve a spanwise variation in blowing strength.

Fejtek, Ian G.↗
NASA NTRS1993

An experimental study of trace element partitioning between perovskite, hibonite and melt: Equilibrium values

The presence of perovskite (CATiO3) and hibonite (Ca Al12O19) within different regions of Calcium-, Aluminum-rich Inclusions (CAI) and the trace element concentrations of these minerals in each circumstance, constrain models of precursor formation, nebular condensation, the thermal history of inclusions with relict perovskite and hibonite, and the formation of the Wark-Lovering rim. At present mineral/melt partition coefficient data for hibonite are limited to a few elements in simple experimental systems, or to those derived from hibonite-glass pairs in hibonite/glass microspherules. Similarly, there is only limited data on perovskite D that are applicable to meteorite compositions. Apart from the importance of partitioning studies to meteorite research, D values also are invaluable in the development of thermodynamic models, especially when data is available for a large number of elements that have different ionic charge and radii. In addition, study of the effect of rapid cooling on partitioning is crucial to our understanding of meteorite inclusions. To expand our knowledge of mineral/melt D for perovskite and hibonite, a study was instituted where D values are obtained in both equilibrium and dynamic cooling experiments. As an initial phase of this study mineral/melt D was measured for major elements (Ca, Mg, Al, Ti, and Si), 15 rare earth elements (La-Lu) and 8 other elements (Ba, Sr, U, Th, Nb, Zr, Hf, and Ge) in perovskite and hibonite grown under equilibrium conditions, in bulk compositions that are respectively similar to Compact Type A (CTA) CAI and to a hibonite/glass microspherule. Experimental mixes were doped with REE at 20-50x chondritic (ch) abundances, Ba at 50 ppm, Sr, Hf, Nb, and Zr at 100 ppm and, U and Th at 200 ppm. Trace element abundances were measured with the PANURGE ion microprobe. Major element compositions were obtained by electron microprobe analysis.

Kennedy, A. K.↗
NASA NTRS1993

CFD analyses of combustor and nozzle flowfields

The objectives of the research are to improve design capabilities for low thrust rocket engines through understanding of the detailed mixing and combustion processes. A Computational Fluid Dynamic (CFD) technique is employed to model the flowfields within the combustor, nozzle, and near plume field. The computational modeling of the rocket engine flowfields requires the application of the complete Navier-Stokes equations, coupled with species diffusion equations. Of particular interest is a small gaseous hydrogen-oxygen thruster which is considered as a coordinated part of an ongoing experimental program at NASA LeRC. The numerical procedure is performed on both time-marching and time-accurate algorithms, using an LU approximate factorization in time, flux split upwinding differencing in space. The integrity of fuel film cooling along the wall, its effectiveness in the mixing with the core flow including unsteady large scale effects, the resultant impact on performance and the assessment of the near plume flow expansion to finite pressure altitude chamber are addressed.

Tsuei, Hsin-Hua↗
NASA NTRS1994

Numerical study of the trailing vortex of a wing with wing-tip blowing

Trailing vortices generated by lifting surfaces such as helicopter rotor blades, ship propellers, fixed wings, and canard control surfaces are known to be the source of noise, vibration, cavitation, degradation of performance, and other hazardous problems. Controlling these vortices is, therefore, of practical interest. The formation and behavior of the trailing vortices are studied in the present research. In addition, wing-tip blowing concepts employing axial blowing and spanwise blowing are studied to determine their effectiveness in controlling these vortices and their effects on the performance of the wing. The 3D, unsteady, thin-layer compressible Navier-Stokes equations are solved using a time-accurate, implicit, finite difference scheme that employs LU-ADI factorization. The wing-tip blowing is simulated using the actuator plane concept, thereby, not requiring resolution of the jet slot geometry. Furthermore, the solution blanking feature of the chimera scheme is used to simplify the parametric study procedure for the wing-tip blowing. Computed results are shown to compare favorably with experimental measurements. It is found that axial wing-tip blowing, although delaying the rolling-up of the trailing vortices and the near-field behavior of the flowfield, does not dissipate the circulation strength of the trailing vortex farther downstream. Spanwise wing-tip blowing has the effect of displacing the trailing vortices outboard and upward. The increased 'wing-span' due to the spanwise wing-tip blowing has the effect of lift augmentation on the wing and the strengthening of the trailing vortices. Secondary trailing vortices are created at high spanwise wing-tip blowing intensities.

Lim, Hock-Bin↗
NASA NTRS1992

Electromagnetic scattering calculations on the Intel Touchstone Delta

During the first year's operation of the Intel Touchstone Delta system, software which solves the electric field integral equations for fields scattered from arbitrarily shaped objects has been transferred to the Delta. To fully realize the Delta's resources, an out-of-core dense matrix solution algorithm that utilizes some or all of the 90 Gbyte of concurrent file system (CFS) has been used. The largest calculation completed to date computes the fields scattered from a perfectly conducting sphere modeled by 48,672 unknown functions, resulting in a complex valued dense matrix needing 37.9 Gbyte of storage. The out-of-core LU matrix factorization algorithm was executed in 8.25 h at a rate of 10.35 Gflops. Total time to complete the calculation was 19.7 h-the additional time was used to compute the 48,672 x 48,672 matrix entries, solve the system for a given excitation, and compute observable quantities. The calculation was performed in 64-b precision.

Cwik, Tom↗
NASA NTRS1994

Multi-dimensional combustor flowfield analyses in gas-gas rocket engine

The objectives of the present research are to improve design capabilities for low thrust rocket engines through understanding of the detailed mixing and combustions processes. Of particular interest is a small gaseous hydrogen-oxygen thruster which is considered as a coordinated part of an on-going experimental program at NASA LeRC. Detailed computational modeling requires the application of the full three-dimensional Navier Stokes equations, coupled with species diffusion equations. The numerical procedure is performed on both time-marching and time-accurate algorithms and using an LU approximate factorization in time, flux split upwinding differencing in space. The emphasis in this paper is focused on using numerical analysis to understand detailed combustor flowfields, including the shear layer dynamics created between fuel film cooling and the core gas in the vicinity on the nearby combustor wall; the integrity and effectiveness of the coolant film; three-dimensional fuel jets injection/mixing/combustion characteristics; and their impacts on global engine performance.

Tsuei, Hsin-Hua↗
NASA NTRS1994

On Bi-Grid Local Mode Analysis of Solution Techniques for 3-D Euler and Navier-Stokes Equations

A procedure is presented for utilizing a bi-grid stability analysis as a practical tool for predicting multigrid performance in a range of numerical methods for solving Euler and Navier-Stokes equations. Model problems based on the convection, diffusion and Burger's equation are used to illustrate the superiority of the bi-grid analysis as a predictive tool for multigrid performance in comparison to the smoothing factor derived from conventional von Neumann analysis. For the Euler equations, bi-grid analysis is presented for three upwind difference based factorizations, namely Spatial, Eigenvalue and Combination splits, and two central difference based factorizations, namely LU and ADI methods. In the former, both the Steger-Warming and van Leer flux-vector splitting methods are considered. For the Navier-Stokes equations, only the Beam-Warming (ADI) central difference scheme is considered. In each case, estimates of multigrid convergence rates from the bi-grid analysis are compared to smoothing factors obtained from single-grid stability analysis. Effects of grid aspect ratio and flow skewness are examined. Both predictions are compared with practical multigrid convergence rates for 2-D Euler and Navier-Stokes solutions based on the Beam-Warming central scheme.

Ibraheem, S. O.↗
NASA NTRS1994

An application of a two-equation model of turbulence to three-dimensional chemically reacting flows

A numerical study of three dimensional chemically reacting and non-reacting flowfields is conducted using a two-equation model of turbulence. A generalized flow solver using an implicit Lower-Upper (LU) diagonal decomposition numerical technique and finite-rate chemistry has been coupled with a low-Reynolds number two-equation model of turbulence. This flow solver is then used to study chemically reacting turbulent supersonic flows inside combustors with synergetic fuel injectors. The reacting and non-reacting turbulent combustor solutions obtained are compared with zero-equation turbulence model solutions and with available experimental data. The hydrogen-air chemistry is modeled using a nine-species/eighteen reaction model. A low-Reynolds number k-epsilon model was used to model the effect of turbulence because, in general, the low-Reynolds number k-epsilon models are easier to implement numerically and are far more general than algebraic models. However, low-Reynolds number k-epsilon models require a much finer near-wall grid resolution than high-Reynolds number models to resolve accurately the near-wall physics. This is especially true in complex flowfields, where the stiff nature of the near-wall turbulence must be resolved. Therefore, the limitations imposed by the near-wall characteristics and compressible model corrections need to be evaluated further. The gradient-diffusion hypothesis is used to model the effects of turbulence on the mass diffusion process. The influence of this low-Reynolds number turbulence model on the reacting flowfield predictions was studied parametrically.

Lee, J.↗
NASA NTRS1990

Using Strassen's algorithm to accelerate the solution of linear systems

Strassen's algorithm for fast matrix-matrix multiplication has been implemented for matrices of arbitrary shapes on the CRAY-2 and CRAY Y-MP supercomputers. Several techniques have been used to reduce the scratch space requirement for this algorithm while simultaneously preserving a high level of performance. When the resulting Strassen-based matrix multiply routine is combined with some routines from the new LAPACK library, LU decomposition can be performed with rates significantly higher than those achieved by conventional means. We succeeded in factoring a 2048 x 2048 matrix on the CRAY Y-MP at a rate equivalent to 325 MFLOPS.

Bailey, David H.↗
NASA NTRS1994

Preconditioned conjugate gradient methods for the Navier-Stokes equations

A preconditioned Krylov subspace method (GMRES) is used to solve the linear systems of equations formed at each time-integration step of the unsteady, two-dimensional, compressible Navier-Stokes equations of fluid flow. The Navier-Stokes equations are cast in an implicit, upwind finite-volume, flux-split formulation. Several preconditioning techniques are investigated to enhance the efficiency and convergence rate of the implicit solver based on the GMRES algorithm. The superiority of the new solver is established by comparisons with a conventional implicit solver, namely line Gauss-Seidel relaxation (LGSR). Computational test results for low-speed (incompressible flow over a backward-facing step at Mach 0.1), transonic flow (trailing edge flow in a transonic turbine cascade), and hypersonic flow (shock-on-shock interactions on a cylindrical leading edge at Mach 6.0) are presented. For the Mach 0.1 case, overall speedup factors of up to 17 (in terms of time-steps) and 15 (in terms of CPU time on a CRAY-YMP/8) are found in favor of the preconditioned GMRES solver, when compared with the LGSR solver. The corresponding speedup factors for the transonic flow case are 17 and 23, respectively. The hypersonic flow case shows slightly lower speedup factors of 9 and 13, respectively. The study of preconditioners conducted in this research reveals that a new LUSGS-type preconditioner is much more efficient than a conventional incomplete LU-type preconditioner.

Ajmani, Kumud↗
NASA NTRS1994

Watson: A new link in the IIE iron chain

Watson, which was found in 1972 in South Australia, contains the largest single silicate rock mass seen in any known iron meteorite. A comprehensive study has been completed on this unusual meteorite: petrography, metallography, analyses of the silicate inclusion (whole rock chemical analysis, INAA, RNAA, noble gases, and oxygen isotope analysis) and mineral compositions (by electron microprobe and ion microprobe). The whole rock has a composition of an H-chondrite minus the normal H-group metal and troilite content. The oxygen isotope composition is that of the silicates in the IIE iron meteorites and lies along an oxygen isotope fractionation line with the H-group chondrites. Trace elements in the metal confirm Watson is a new IIE iron. Whole rock Watson silicate shows an enrichment in K and P (each approximately 2X H-chondrites). The silicate inclusion has a highly equilibrated igneous (peridotite-like) texture with olivine largely poikilitic within low-Ca pyroxene: olivine (Fa20), opx (Fs17Wo3), capx (Fs9Wo14)(with very fine exsolution lamellae), antiperthite feldspar (An1-3Or5) with less than 1 micron exsolution lamellae (An1-3Or greater than 40), shocked feldspar with altered stoichiometry, minor whitlockite (also a poorly characterized interstitial phosphate-rich phase) and chromite, and only traces of metal and troilite. The individual silicate minerals have normal chondritic REE patterns, but whitlockite has a remarkable REE pattern. It is very enriched in light REE (La is 720X C1, and Lu is 90X C1, as opposed to usual chonditic values of approximately 300X and 100-150X, respectively) with a negative Eu anomaly. The enrichment of whole rock K is expressed both in an unusually high mean modal Or content of the feldspar, Or13, and in the presence of antiperthite.

Olsen, Edward↗
NASA NTRS1994

ALH84001, a cumulate orthopyroxenite member of the Martian meteorite clan

ALH84001, originally classified as a diogenite, is a coarse-grained, cataclastic, orthopyroxenite meteorite related to the martian (SNC) meteorites. The orthopyroxene is relatively uniform in composition, with a mean composition of Wo3.3En69.4Fs27.3. Minor phases are euhedral to subhedral chromite and interstitial maskelynite, An31.1Ab63.2Or5.7, with accessory augite, Wo42.2En45.1Fs12.7, apatite, pyrite and carbonates, Cc11.5Mg58.0Sd29.4Rd1.1. The pyroxenes and chromites in ALH84001 are similar in composition to these phases in EETA79001 lithology a megacrysts but are more homogeneous. Maskelynite is similar in composition to feldspars in the nakhlites and Chassigny. Two generations of carbonates are present, early (pre-shock) strongly zoned carbonates and late (post-shock) carbonates. The high Ca content of both types of carbonates indicates that they were formed at moderately high temperature, possibly approximately 700 C. ALH84001 has a slightly LREE-depleted pattern with La 0.67x and Lu 1.85x CI abundances and with a negative Eu anomaly (Eu/Sm 0.56x CI). The uniform pyroxene composition is unusual for martian meteorites, and suggests that ALH84001 cooled more slowly than did the shergottites, nakhlites of Chassigny. The nearly monomineralic composition, coarse-grain size, homogeneous orthopyroxene and chromite compositions, the interstitial maskelynite and apatite, and the REE pattern suggest that ALH84001 is a cumulate orthopyroxenite containing minor trapped, intercumulus material.

Mittlefehldt, David W.↗
NASA NTRS1994

A thermosphere-ionosphere-mesosphere-electrodynamic general circulation model (time-GCM): Equinox solar cycle minimum simulations (30-500 km)

A new simulation model of the mesosphere, thermosphere, and ionosphere with coupled electrodynamics has been developed and used to calculate the global circulation, temperature and compositional structure between 30-500 km for equinox, solar cycle minimum, geomagnetic quiet conditions. The model incorporates all of the features of the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere- electrodynamics general circulation model (TIE-GCM) but the lower boundary has been extended downward from 97 to 30 km (10 mb) and it includes the physical and chemical processes appropriate for the mesosphere and upper stratosphere. The first simulation used Rayleigh friction to represent gravity wave drag in the middle atmosphere and although it was able to close the mesospheric jets it severely damped the diurnal tide. Reduced Rayleigh friction allowed the tide to penetrate to thermospheric heights but did not close the jets. A gravity wave parameterization developed by Fritts and Lu (1993) allows both features to exist simultaneously with the structure of tides and mean flow dependent upon the strength of the gravity wave source. The model calculates a changing dynamic structure with the mean flow and diurnal tide dominant in the mesosphere, the in-situ generated semi-diurnal tide dominating the lower thermosphere and an in-situ generated diurnal tide in the upper thermosphere. The results also show considerable interaction between dynamics and composition, especially atomic oxygen between 85 and 120 km.

Roble, R. G.↗
NASA NTRS1994

The nature of the F str lambda 4077 stars. 3: Spectroscopy of the barium dwarfs and other CP stars

The abundances of C, O, Al, Ca, iron-peak and s-process elements have been derived from high-resolution spectra for a sample of stars classified as F str lambda 4077 by Bidelman. Among the 20 stars mentioned by Bidelman, we have discovered 8 barium dwarfs (or CH subgiants, according to Bond's terminology), while a 9th star, HD 182274, was already known as a CH subgiant. In addition, we have analyzed three barium stars taken from the list of Lu et al. (1983) which are probably dwarfs rather than giants, and three CH subgiants. The other 11 F str lambda 4077 stars resemble either the delta Delphini stars, since their iron abundance is enhanced while Ca is normal, or are probably spectrum composites. A few Am, Ap, lambda Bootis and normal stars have been analyzed for comparison. In particular, we have included three lambda Boo candidates, selected from their photometric properties, and their iron deficiency is confirmed. The spectroscopic, photometric and statistical evidences concerning the Ba dwarfs, support the idea that these stars may be the main sequence counterparts, and possibly the progenitors of the Ba giants. The C/O ratio varies in these stars from normal values to a maximum of 1.5, but mostly within 0.6 and 1.2. Some of these objects may therefore be considered, in this sense, as carbon stars. On the other hand, the abundances of carbon and s-process elements relative to iron are inversely correlated with metallicity, and may even exceed significantly those of typical, solar-metallicity carbon stars. Metal-deficient C stars must therefore have (C/Fe) greater than or approximately equal to 1 and (s/Fe) greater than or approximately equal to 1.5 as soon as (Fe/H) less than or approximately equal to -1. The neutron exposure is shown to increase when the metallicity decreases, which is compatible with the C-13 (alpha, n) O-16 neutron source, but not with the Ne-22 (alpha, n) Mg-25 one. The evolutionary state (within the main sequence) of the Ba dwarfs, is rediscussed in relation with their photometric and spectroscopic surface gravity, but it remains unclear.

North, P.↗
NASA NTRS1993

Are PSR 0656+14, PSR 0950+08, and PSR 1822-09 gamma ray pulsars?

The possible discovery of three new gamma-ray pulsars PSR 0656+14, PSR 0950+08, and PSR 1822-09 (Ma, Lu, Yu, and Young, 1993) in data obtained with the COS-B experiment is reinvestigated using a refined technique for pulsar light curve analysis. The results of this study do not confirm the previously claimed gamma-ray pulsar nature of any of these pulsars. Even when using the standard epoch folding technique in conjunction with energy-dependent acceptance cones, we do not detect pulsed gamma-ray emission from these sources. We suspect that insufficient position accuracy is the cause for the discrepancy between our results and those of Ma et al. (1993). We do not rule out that any one of the three candidates, or all of them, is in fact a gamma-ray pulsar, but their spin properties must differ from those derived by Ma et al. (1993). More work is needed to determine the correct high-energy properties of these three sources.

Brown, Lawrence E.↗
NASA NTRS1995

Computational strategies for three-dimensional flow simulations on distributed computer systems

This research effort is directed towards an examination of issues involved in porting large computational fluid dynamics codes in use within the industry to a distributed computing environment. This effort addresses strategies for implementing the distributed computing in a device independent fashion and load balancing. A flow solver called TEAM presently in use at Lockheed Aeronautical Systems Company was acquired to start this effort. The following tasks were completed: (1) The TEAM code was ported to a number of distributed computing platforms including a cluster of HP workstations located in the School of Aerospace Engineering at Georgia Tech; a cluster of DEC Alpha Workstations in the Graphics visualization lab located at Georgia Tech; a cluster of SGI workstations located at NASA Ames Research Center; and an IBM SP-2 system located at NASA ARC. (2) A number of communication strategies were implemented. Specifically, the manager-worker strategy and the worker-worker strategy were tested. (3) A variety of load balancing strategies were investigated. Specifically, the static load balancing, task queue balancing and the Crutchfield algorithm were coded and evaluated. (4) The classical explicit Runge-Kutta scheme in the TEAM solver was replaced with an LU implicit scheme. And (5) the implicit TEAM-PVM solver was extensively validated through studies of unsteady transonic flow over an F-5 wing, undergoing combined bending and torsional motion. These investigations are documented in extensive detail in the dissertation, 'Computational Strategies for Three-Dimensional Flow Simulations on Distributed Computing Systems', enclosed as an appendix.

Sankar, Lakshmi N.↗
NASA NTRS1996

NASA flow fields analysis

The objectives of the present research are to improve design capabilities for low thrust rocket engines through understanding the detailed mixing and combustion processes in a representative combustor. Of particular interest is a small gaseous hydrogen-oxygen thruster which is considered as a coordinated part of an on-going experimental program at NASA LERC. Detailed computational modeling involves the solution of both the two- and three-dimensional Navier Stokes equations, coupled with chemical reactions and the species diffusion equations. Computations of interest include both steady state and time-accurate flowfields and are obtained by means of LU approximate factorization in time and flux split upwinding differencing in space. The emphasis in the research is focused on using numerical analysis to understand detailed combustor flowfields, including the shear layer dynamics created between fuel film cooling and the core gas in the vicinity on the nearby combustor wall; the integrity and effectiveness of the coolant film; and three-dimensional fuel and oxidizer jet injection/mixing/combustion characteristics in the primary combustor along with their joint impacts on global engine performance.

Merkle, Charles L.↗