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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Semi-implicit and fully implicit shock-capturing methods for hyperbolic conservation laws with stiff source terms

Some numerical aspects of finite-difference algorithms for nonlinear multidimensional hyperbolic conservation laws with stiff nonhomogenous (source) terms are discussed. If the stiffness is entirely dominated by the source term, a semi-implicit shock-capturing method is proposed provided that the Jacobian of the soruce terms possesses certain properties. The proposed semi-implicit method can be viewed as a variant of the Bussing and Murman point-implicit scheme with a more appropriate numerical dissipation for the computation of strong shock waves. However, if the stiffness is not solely dominated by the source terms, a fully implicit method would be a better choice. The situation is complicated by problems that are higher than one dimension, and the presence of stiff source terms further complicates the solution procedures for alternating direction implicit (ADI) methods. Several alternatives are discussed. The primary motivation for constructing these schemes was to address thermally and chemically nonequilibrium flows in the hypersonic regime. Due to the unique structure of the eigenvalues and eigenvectors for fluid flows of this type, the computation can be simplified, thus providing a more efficient solution procedure than one might have anticipated.

Yee, H. C.↗
NASA NTRS1986

Calculation of secondary electron trajectories in multistage depressed collectors for microwave amplifiers

Computational procedures are reported for treating power losses due to secondary electrons in multistage depressed collectors (MDC) for traveling wave tubes (TWT) and other O-type electron tubes. The MDC is modeled with an advanced, multidimensional computer program. Representative beams of secondary electrons are then injected at the points of impact of the primary beams. Separate programs are used to calculate representative beams of high-energy primary electron beams and of low-energy true secondaries. The recomputation of the MDC model including the true secondary beam allows determination of the secondary emission losses, and, if necessary, redesign of the MDC to improve performance. Recomputation of the MDC model including the primary beams is used to check on possible backstreaming from the MDC to the RF interaction structure of the tube. A comparison with experimentally measured values of TWT and MDC efficiencies is made.

Force, Dale A.↗
NASA NTRS1987

Error control techniques for satellite and space communications

During the period June 1, 1986 through November 30, 1986, progress was made in the following areas: undetected error probability and throughput analysis of a concatenated coding scheme; capacity and cutoff rate analysis of concatenated codes; concatenated codes using bandwidth efficient trellis inner codes; bounds on the minimum free Euclidean distance of bandwidth efficient trellis codes; and construction of multidimensional bandwidth efficient trellis codes for use as inner codes in a concatenated coding system.

Costello, Daniel J., Jr.↗
NASA NTRS1986

Working group report on beam plasmas, electronic propulsion, and active experiments using beams

The JPL Workshop addressed a number of plasma issues that bear on advanced spaceborne technology for the years 2000 and beyond. Primary interest was on the permanently manned space station with a focus on identifying environmentally related issues requiring early clarification by spaceborne plasma experimentation. The Beams Working Group focused on environmentally related threats that platform operations could have on the conduct and integrity of spaceborne beam experiments and vice versa. Considerations were to include particle beams and plumes. For purposes of definition it was agreed that the term particle beams described a directed flow of charged or neutral particles allowing single-particle trajectories to represent the characteristics of the beam and its propagation. On the other hand, the word plume was adopted to describe a multidimensional flow (or expansion) of a plasma or neutral gas cloud. Within the framework of these definitions, experiment categories included: (1) Neutral- and charged-particle beam propagation, with considerations extending to high powers and currents. (2) Evolution and dynamics of naturally occurring and man-made plasma and neutral gas clouds. In both categories, scientific interest focused on interactions with the ambient geoplasma and the evolution of particle densities, energy distribution functions, waves, and fields.

Dawson, J. M.↗
NASA NTRS1987

Analytical and experimental performance of a dual-mode traveling wave tube and multistage depressed collector

A computational procedure for the design of traveling-wave-tube(TWT)/refocuser/multistage depressed collector (MDC) systems was used to design a short, permanent-magnet refocusing system and a highly efficient MDC for a medium-power, dual-mode, 4.8- to 9.6-GHz TWT. The computations were carried out with advanced, multidimensional computer programs which model the electron beam and follow the trajectories of representative charges from the radiofrequency (RF) input of the TWT, through the slow-wave structure and refocusing section, to their points of impact in the depressed collector. Secondary emission losses in the MDC were treated semiquantitatively by injecting representative secondary-electron-emission current into the MDA analysis at the point of impact of each primary beam. A comparison of computed and measured TWT and MDC performance showed very good agreement. The electrodes of the MDC were fabricated from a particluar form of isptropic graphite that was selected for its low secondary electron yield, ease of machinability, and vacuum properties.

Ramins, Peter↗
NASA NTRS1987

Comparative study of high-resolution shock-capturing schemes for a real gas

Recently developed second-order explicit shock-capturing methods, in conjunction with generalized flux-vector splittings, and a generalized approximate Riemann solver for a real gas are studied. The comparisons are made on different one-dimensional Riemann (shock-tube) problems for equilibrium air with various ranges of Mach numbers, densities and pressures. Six different Riemann problems are considered. These tests provide a check on the validity of the generalized formulas, since theoretical prediction of their properties appears to be difficult because of the non-analytical form of the state equation. The numerical results in the supersonic and low-hypersonic regimes indicate that these produce good shock-capturing capability and that the shock resolution is only slightly affected by the state equation of equilibrium air. The difference in shock resolution between the various methods varies slightly from one Riemann problem to the other, but the overall accuracy is very similar. For the one-dimensional case, the relative efficiency in terms of operation count for the different methods is within 30%. The main difference between the methods lies in their versatility in being extended to multidimensional problems with efficient implicit solution procedures.

Montagne, J.-L.↗
NASA NTRS1987

Measurement of pilot workload

A multistage process for evaluating the workload of a five-minute segment of flight including approach and landing for a typical transport aircraft was described. The goal of the analysis was to compare the workload of the two pilots. Four types of measurement techniques were suggested: Analytic (a preliminary task and time line analysis identified task requirements and target performance levels); Performance (flight path control, communications, and interval production); Physiological (heart rate and heart rate variability); and Subjective ratings (a multidimensional technique developed at NASA Ames).

Hart, Sandra G.↗
NASA NTRS1986

A search for narrow band signals with Serendip II - A progress report

Commensal programs for SETI, carried out concurrently with conventional radio astronomical observing programs, can be an attractive and cost-effective means of exploring the large multidimensional search space intrinsic to this effort. An automated commensal system, Serendip II, searches for 0.49-Hz signals in sequential 64,700 Hz bands of the IF of a radio telescope being used for an astronomical observation. Upon detection of a signal with power above a preset threshold, the frequency, power, time, and telescope direction are recorded for later study. The system has been tested at the Hat Creek Radio Astronomy Observatory 85-ft telescope and the NASA-JPL Deep Space Station (DSS 14) 64-m telescope.

Werthimer, D.↗
NASA NTRS1987

On the absence of convective stability in materials preparation on earth

The yield in the production and the performance of electronic and optical solid state devices is frequently limited by nonuniformities in the composition and structure of the underlying crystalline material. The formation of such inhomogeneities can be favored by convective heat and mass transfer during the preparation. Hence, it is often desirable to establish purely diffusive conditions in materials preparation. It has been thought that buoyancy-driven convection can be readily excluded by appropriate orientation of the density gradient with respect to the gravity vector. Such expectations overlook, however, that materials preparation processes are inherently associated with multidimensional density gradients. Hence, it is concluded, there will always be convective contributions to transport in materials processing on earth.

Rosenberger, Franz↗
NASA NTRS1987

Implicit flux-corrected transport scheme for low-speed flow

A new high-order nonlinear Eulerian convection algorithm, the barely implicit correction (BIC)-FCT method, is presented. The BIC-FCT makes it possible to perform accurate, multidimensional, slow-flow calculations in which the fluid expansion is important. Owing to its low cost even in two dimensions, fairly detailed chemistry models and other physical processes can be included. Areas of BIC-FCT applicability include premixed flames, diffusion flames, and turbulent jet flames.

Patnaik, G.↗
NASA NTRS1986

A two-dimensional numerical study of the flow inside the combustion chamber of a motored rotary engine

A numerical study was performed to investigate the unsteady, multidimensional flow inside the combustion chambers of an idealized, two-dimensional, rotary engine under motored conditions. The numerical study was based on the time-dependent, two-dimensional, density-weighted, ensemble-averaged conservation equations of mass, species, momentum, and total energy valid for two-component ideal gas mixtures. The ensemble-averaged conservation equations were closed by a K-epsilon model of turbulence. This K-epsilon model of turbulence was modified to account for some of the effects of compressibility, streamline curvature, low-Reynolds number, and preferential stress dissipation. Numerical solutions to the conservation equations were obtained by the highly efficient implicit-factored method of Beam and Warming. The grid system needed to obtain solutions were generated by an algebraic grid generation technique based on transfinite interpolation. Results of the numerical study are presented in graphical form illustrating the flow patterns during intake, compression, gaseous fuel injection, expansion, and exhaust.

Shih, T. I-P.↗
NASA NTRS1987

Time dependent boundary conditions for hyperbolic systems

Nonreflecting boundary conditions are defined for multidimensional fluid dynamics problems where waves enter and leave the interior of a domain modeled by hyperbolic equations. Separate equations are defined for each type of incoming and outgoing wave. Temporally varying problems are considered in terms of a nonreflecting boundary condition which permit the amplitude of incoming waves to remain constant over time. Conservative expressions are presented that include dissipative terms. Applications of the computational techniques are illustrated with sample results for a traveling shock wave, a shock tube, a spherical explosion and expansion problems on one- and two-dimensions.

Thompson, Kevin W.↗
NASA NTRS1987

Thermochemical erosion

A quasi-steady multidimensional Stefan problem is treated by means of similarity, asymptotic methods, and numerical analysis. The continuous wearing away of a solid 'erodee' is effected by gradual consumption of a solid 'eroder'. In the process under study, the erodee melts and the (liquid) melt diffuses to the eroder, where an exothermic, indefinitely rapid, diffusion-controlled surface reaction occurs. The products of reaction convey some of the chemically-generated heat to the surface of the erodee, so that further melting may occur, and the process may proceed. The analysis seeks to estimate the quasi-steady rates of consumption of the erodee and eroder, for the (convenient) self-similar case of confocal parabolas defining the boundaries of the melt layer. The analysis is simplified by observing that the flow is of lubrication-theory type: creeping, primarily unidirectional motion owing to the thinness of the melt layer between the erodee and eroder, with diffusion of heat and mass predominantly perpendicular to the principal motion. The analysis is completed by consideration of the force balance on the eroder.

Butler, Gerald↗
NASA NTRS1987

Response of discretely stiffened structures

This paper presents a theoretical study on dynamic response of complex discretely stiffened and interconnected structures to random loads. Modal and transfer matrix techniques are utilized to analyze these multidimensional structures. Application of this formulation for response and noise generation studies in space station and fuselage-wing design is discussed. The response spectral densities are calculated for several typical discretely stiffened and interconnected structural systems. It is found that this approach allows for easy parametric evaluation and that by proper selection of structural parameters and loading conditions, the vibration response can be reduced.

Vaicaitis, R.↗
NASA NTRS1986

Analysis of crew functions as an aid in Space Station interior layout

The Space Station must be designed to facilitate all of the functions that its crew will perform, both on-duty and off-duty, as efficiently and comfortably as possible. This paper examines the functions to be performed by the Space Station crew in order to make inferences about the design of an interior layout that optimizes crew productivity. Twenty-seven crew functions were defined, as well as five criteria for assessing relationships among all pairs of those functions. Hierarchical clustering and multidimensional scaling techniques were used to visually summarize the relationships. A key result was the identification of two dimensions for describing the configuration of crew functions: 'Private-Public' and 'Group-Individual'. Seven specific recommendations for Space Station interior layout were derived from the analyses.

Steinberg, A. L.↗
NASA NTRS1987

Semi-implicit and fully implicit shock-capturing methods for hyperbolic conservation laws with stiff source terms

Some numerical aspects of finite-difference algorithms for nonlinear multidimensional hyperbolic conservation laws with stiff nonhomogeneous (source) terms are discussed. If the stiffness is entirely dominated by the source term, a semi-implicit shock-capturing method is proposed provided that the Jacobian of the source terms possesses certain properties. The proposed semi-implicit method can be viewed as a variant of the Bussing and Murman point-implicit scheme with a more appropriate numerical dissipation for the computation of strong shock waves. However, if the stiffness is not solely dominated by the source terms, a fully implicit method would be a better choice. The situation is complicated by problems that are higher than one dimension, and the presence of stiff source terms further complicates the solution procedures for alternating direction implicit (ADI) methods. Several alternatives are discussed. The primary motivation for constructing these schemes was to address thermally and chemically nonequilibrium flows in the hypersonic regime. Due to the unique structure of the eigenvalues and eigenvectors for fluid flows of this type, the computation can be simplified, thus providing a more efficient solution procedure than one might have anticipated.

Yee, H. C.↗
NASA NTRS1987

Extension of a semi-implicit shock-capturing algorithm for 3-D fully coupled, chemically reacting flows in generalized coordinates

A semiimplicit high-resolution shock-capturing method for multidimensional systems of hyperbolic conservation laws with stiff source terms has been developed by Yee and Shinn (1987). The goal of this work is to extend this method to solve the three-dimensional fully coupled Navier-Stokes equations for a hypersonic chemically reacting flow in generalized coordinates. In this formulation, the global continuity equation was replaced by all the species continuity equations. The shock-capturing technique is a second-order-accurate, symmetric total-variation-diminishing method which accounts fully and directly for the coupling among the fluid and all the species. To verify the current approach, it was implemented into an existing computer code which contained the MacCormack method. Test results for a five-species reacting flow are shown to be oscillation-free around the shock, and the time spent per iteration only doubles when compared to the result using classical way of supplying numerical dissipation. The extra computation is more than justified by the elimination of spurious oscillation and nonlinear instability associated with the classical shock-capturing schemes in computing hypersonic reacting flows.

Shinn, J. L.↗
NASA NTRS1987

Navier-Stokes analysis of two- and three-dimensional flow field in solid rocket motors with segment joints

A multidimensional implicit Navier-Stokes analysis which uses numerical solution of ensemble-averaged Navier-Stokes equations in a nonorthogonal bodyfitted cylindrical-polar coordinate system has been applied to simulation of the internal flow field in solid-propellant rocket motor chambers with segment joints. The calculation procedure incorporates a two-equation (k-epsilon) turbulence model and utilizes a consistently split, linearized block-implicit algorithm for numerical solution of the governing equations. Computations performed to simulate the axisymmetric flow field in the vicinity of the aft field joint in the Space Shuttle SRB using 14,725 grid points show the presence of a region of reversed axial flow near the downstream edge of the slot. Calculations were also performed for two cases involving asymmetric three-dimensional flow in the vicinity of the aft field joint in the SRB using 721,525 grid points to estimate circumferential velocities and pressure gradients at the joint.

Sabnis, J. S.↗