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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Numerical algorithm conjugating steady and transient, separated, compressible flow and a solid body having arbitrarily distributed heat sources

A numerical algorithm that is second-order accurate in time has been developed for the conjugated problem of a separated, compressible flow field and a conductive solid body. The full two-dimensional time-dependent Navier-Stokes equations are coupled with the time-dependent energy equation for the solid body and are solved simultaneously. using implicit algorithms. The energy equation for the solid body may include arbitrarily distributed heat sources. The algorithm has been exmined for the case of two-dimensional supersonic compression-corner interaction, with a heat source embedded in the wall in the vicinity of the separation bubble and the attached boundary layer. The effect of the heat source on the flow field is studied for steady and transient cases.

Degani, D.↗
NASA NTRS1986

Compensation of reflector surface distortions using conjugate field matching

The feasibility of compensating for reflector surface distortions has been investigated. The performance characteristics (gain, sidelobe level, null location, beamwidth, etc.) of space communication reflector antenna systems degrade as the reflector surface distorts due to thermal effects from a varying solar flux. The technique reported here will maintain the design radiation performance independently of thermal effects on the reflector surface. With the advent of monolithic microwave integrated circuits (MMIC), a greater flexibility in array-fed reflector system design can be achieved. MMIC arrays provides independent control of amplitude and phase for each of many radiating elements of the feed array. The conjugate field matching technique provide a basis for obtaining the required element excitations under surface distortion for maintaining the design radiation performance. It is assumed that the surface characteristics (x, y, z, first derivatives, and second derivatives) under distortion are known.

Acosta, R. J.↗
NASA NTRS1985

Finite elements and the method of conjugate gradients on a concurrent processor

An algorithm for the iterative solution of finite element problems on a concurrent processor is presented. The method of conjugate gradients is used to solve the system of matrix equations, which is distributed among the processors of a MIMD computer according to an element-based spatial decomposition. This algorithm is implemented in a two-dimensional elastostatics program on the Caltech Hypercube concurrent processor. The results of tests on up to 32 processors show nearly linear concurrent speedup, with efficiencies over 90 percent for sufficiently large problems.

Lyzenga, G. A.↗
NASA NTRS1986

Preconditioned conjugate residual methods for the solution of spectral equations

Conjugate residual methods for the solution of spectral equations are described. An inexact finite-difference operator is introduced as a preconditioner in the iterative procedures. Application of these techniques is limited to problems for which the symmetric part of the coefficient matrix is positive definite. Although the spectral equation is a very ill-conditioned and full matrix problem, the computational effort of the present iterative methods for solving such a system is comparable to that for the sparse matrix equations obtained from the application of either finite-difference or finite-element methods to the same problems. Numerical experiments are shown for a self-adjoint elliptic partial differential equation with Dirichlet boundary conditions, and comparison with other solution procedures for spectral equations is presented.

Wong, Y. S.↗
NASA NTRS1987

Generation of spiral bevel gears with conjugate tooth surfaces and tooth contact analysis

A new method for generation of spiral bevel gears is proposed. The main features of this method are as follows: (1) the gear tooth surfaces are conjugated and can transform rotation with zero transmission errors; (2) the tooth bearing contact is localized; (3) the center of the instantaneous contact ellipse moves in a plane that has a fixed orientation; (4) the contact normal performs in the process of meshing a parallel motion; (5) the motion of the contact ellipse provides improved conditions of lubrication; and (6) the gears can be manufactured by use of Gleason's equipment.

Litvin, Faydor L.↗
NASA NTRS1986

Near-conjugate observations of polar cusp electron precipitation using DE 1 and DE 2

The DE 1/2 plasma measurements of the dayside polar cusp at high and low altitudes have been examined for six near-conjugate passes. The plasma moments of downward electrons are computed, including energy flux, near flux, number density, and average energy from plasma data from both the high-altitude plasma instrument and the low-altitude plasma instrument. The DE 1/2 plasma measurements at the peaks of energy flux are compared, and the downward energy flux expected at the DE 2 altitude is modelled according to the convergence of magnetic flux tubes and using the DE 1 plasma data.

Lin, C. S.↗
NASA NTRS1987

Method for generation of spiral bevel gears with conjugate gear tooth surfaces

A method for generation of spiral bevel gears is proposed that provides conjugate gear tooth surfaces. This method is based on a new principle for the performance of parallel motion of a straight line that slides along two mating ellipses with related dimensions and parameters of orientation. The parallel motion of the straight line, that is, the contact normal, is performed parallel to the line which passes through the foci of symmetry of the related ellipses. The manufacturing of gears can be performed with the existing Gleason's equipment.

Litvin, F. L.↗
NASA NTRS1987

Preconditioned conjugate gradient technique for the analysis of symmetric anisotropic structures

An efficient preconditioned conjugate gradient (PCG) technique and a computational procedure are presented for the analysis of symmetric anisotropic structures. The technique is based on selecting the preconditioning matrix as the orthotropic part of the global stiffness matrix of the structure, with all the nonorthotropic terms set equal to zero. This particular choice of the preconditioning matrix results in reducing the size of the analysis model of the anisotropic structure to that of the corresponding orthotropic structure. The similarities between the proposed PCG technique and a reduction technique previously presented by the authors are identified and exploited to generate from the PCG technique direct measures for the sensitivity of the different response quantities to the nonorthotropic (anisotropic) material coefficients of the structure. The effectiveness of the PCG technique is demonstrated by means of a numerical example of an anisotropic cylindrical panel.

Noor, Ahmed K.↗
NASA NTRS1988

End-effector - joint conjugates for robotic assembly of large truss structures in space: A second generation

Current designs, a first generation intended for robotic assembly, have given priority to the ease and certainty of the assembly process under less than ideal conditions with a minimum of sensory feedback. As a consequence they are either heavy or expensive and all exhibit a relatively low packaging density. Low packaging density is caused by extensive scars applied to the node, increasing its envelope diameter by as much as 150 percent. Strut envelopes are violated to a lessor extent with diameters increased by 25 percent or more. This smaller percentage is still a significant problem owing to a much higher fraction of the packaged volume represented by struts. As structures in space become larger, packaging density becomes an important consideration. The objective is to develop end-effector-joint conjugates that do not violate the envelopes of a 2.5 inch diameter node or a 1.0 inch diameter strut.

Brewer, W. V.↗
NASA NTRS1989

Conjugating binary systems for spacecraft thermal control

The materials search was directed to liquid pairs which can form hydrogen bonds of just the right strength, i.e., strong enough to give a high heat of mixing, but weak enough to enable phase change to occur. The cursory studies performed in the area of additive effects indicate that Conjugating Binary (CB) performance can probably be fine-tuned by this means. The Fluid Loop Test Systems (FLTS) tests of candidate CBs indicate that the systems Triethylamine (TEA)/water and propionaldehyde/water show close to the ideal, reversible behavior, at least initially. The Quick Screening Tests QSTs and FLTS tests, however, both suffer from rather severe static due either to inadequate stirring or temperature control. Thus it is not possible to adequately evaluate less than ideal CB performers. Less than ideal performers, it should be noted, may have features that make them better practical CBs than ideal performers. Improvement of the evaluation instrumentation is thus indicated.

Grodzka, Philomena G.↗
NASA NTRS1989

Phase conjugation by four-wave mixing in inhomogeneous plasmas

The effects of density, temperature, and velocity gradients on four-wave mixing (FWM) in a plasma are investigated. A fluid model is used in which the stimulated Brillouin terms are included, but pump depletion is neglected. The steady state phase conjugate reflectivity and signal transmission coefficients are calculated and discussed for both degenerate and resonant FWM. The substantial effects of inhomogeneity on the use of FWM as a plasma diagnostic are discussed.

Williams, Edward A.↗
NASA NTRS1989

Influence of gap states on the nonresonant second hyperpolarizabilities of conjugated organic polymers

The presence of conjugation and substitution defects introduces gap states in finite polyenes that are shown to influence the size and sign of the second molecular hyperpolarizability (SMH). Using a one-electron tight-binding model, the dependence of SMH on the defect-state occupancy and energy in finite polyenes is calculated. Defects can cause a significant decrease or enhancement of SMH by impeding charge delocalization or by creating partly filled bands (mimicking the one-band limit), respectively. Concomitant sign changes in SMH are predicted. Calculation results suggest strategies for designing molecules that can be either photochemically or electrochemically switched between states with considerably different SMHs.

Beratan, David N.↗
NASA NTRS1989

A fast, preconditioned conjugate gradient Toeplitz solver

A simple factorization is given of an arbitrary hermitian, positive definite matrix in which the factors are well-conditioned, hermitian, and positive definite. In fact, given knowledge of the extreme eigenvalues of the original matrix A, an optimal improvement can be achieved, making the condition numbers of each of the two factors equal to the square root of the condition number of A. This technique is to applied to the solution of hermitian, positive definite Toeplitz systems. Large linear systems with hermitian, positive definite Toeplitz matrices arise in some signal processing applications. A stable fast algorithm is given for solving these systems that is based on the preconditioned conjugate gradient method. The algorithm exploits Toeplitz structure to reduce the cost of an iteration to O(n log n) by applying the fast Fourier Transform to compute matrix-vector products. Matrix factorization is used as a preconditioner.

Pan, Victor↗
NASA NTRS1990

Preconditioned conjugate gradient methods for the compressible Navier-Stokes equations

The compressible Navier-Stokes equations are solved for a variety of two-dimensional inviscid and viscous problems by preconditioned conjugate gradient-like algorithms. Roe's flux difference splitting technique is used to discretize the inviscid fluxes. The viscous terms are discretized by using central differences. An algebraic turbulence model is also incorporated. The system of linear equations which arises out of the linearization of a fully implicit scheme is solved iteratively by the well known methods of GMRES (Generalized Minimum Residual technique) and Chebyschev iteration. Incomplete LU factorization and block diagonal factorization are used as preconditioners. The resulting algorithm is competitive with the best current schemes, but has wide applications in parallel computing and unstructured mesh computations.

Venkatakrishnan, V.↗
NASA NTRS1989

A numerical analysis of the effects of conjugate heat transfer, vapor compressibility, and viscous dissipation in heat pipes

The effects of conjugate heat transfer, vapor compressibility, and viscous dissipation in heat pipes are discussed. The accuracy of the partially parabolic versus the elliptic presentation of the governing equations is also examined. The results show that the axial wall conduction has a tendency to make the temperature distribution more uniform for heat pipes with large ratios of pipe wall to effective liquid-wick thermal conductivity. The compressible and incompressible models show very close agreement for the total pressure drop, while the local pressure variations along the heat pipe are quite different for these two models when the radial Reynolds number at the interface is high.

Faghri, Amir↗
NASA NTRS1991

Conjugate Gradient Algorithms For Manipulator Simulation

Report discusses applicability of conjugate-gradient algorithms to computation of forward dynamics of robotic manipulators. Rapid computation of forward dynamics essential to teleoperation and other advanced robotic applications. Part of continuing effort to find algorithms meeting requirements for increased computational efficiency and speed. Method used for iterative solution of systems of linear equations.

Fijany, Amir↗
NASA NTRS1989

Conjugate (solid/fluid) computational fluid dynamics analysis of the space shuttle solid rocket motor nozzle/case and case field joints

Three-dimensional, conjugate (solid/fluid) heat transfer analyses of new designs of the Solid Rocket Motor (SRM) nozzle/case and case field joints are described. The main focus was to predict the consequences of multiple rips (or debonds) in the ambient cure adhesive packed between the nozzle/case joint surfaces and the bond line between the mating field joint surfaces. The models calculate the transient temperature responses of the various materials neighboring postulated flow/leakpaths into, past, and out from the nozzle/case primary O-ring cavity and case field capture O-ring cavity. These results were used to assess if the design was failsafe (i.e., no potential O-ring erosion) and reusable (i.e., no excessive steel temperatures). The models are adaptions and extensions of the general purpose PHOENICS fluid dynamics code. A non-orthogonal coordinate system was employed and 11,592 control cells for the nozzle/case and 20,088 for the case field joints are used with non-uniform distribution. Physical properties of both fluid and solids are temperature dependent. A number of parametric studies were run for both joints with results showing temperature limits for reuse for the steel case on the nozzle joint being exceeded while the steel case temperatures for the field joint were not. O-ring temperatures for the nozzle joint predicted erosion while for the field joint they did not.

Doran, D.↗