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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 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

A first order theory of the p/+/-n-n/+/ edge-illuminated silicon solar cell at very high injection levels

A first order theory of the edge-illuminated p(+)-n-n(+) silicon solar cell under very high injection levels has been derived. The very high injection level illuminated J-V characteristic is derived for any general base width to diffusion length (W/L) ratio and it includes the minority carrier reflection by the n-n(+) high-low junction. The beneficial effects of the high-low junction are shown to be significant until extremely high injection levels are reached. The theoretical dependencies of Jsc and Voc on temperature, incident intensity, and base resistivity are derived and discussed in detail. Some experimental results are given and these are discussed in relation to the theory.

Goradia, C.↗
NASA NTRS1977

Stability of higher-order Bragg interactions in active periodic media

The stability of waves in unbounded, longitudinally periodic media is studied for index and gain coupling. Time-independent periodic media are found to support both stable and absolutely unstable waves. The wave characteristics depend upon average gain or loss, coupling type, and Bragg order. The extended coupled waves equations provide explicit values of threshold, frequency, and temporal growth rate for instabilities at all Bragg resonances through the dispersion relation. Applications to multiharmonic periodicities and complex couplings are briefly discussed with particular note taken of possible reductions of the stability thresholds and removal of threshold degeneracies. Comparisons are made to the longitudinally bounded case of distributed feedback lasers.

Jaggard, D. L.↗
NASA NTRS1977

A critical evaluation of invariant second-order closure models for subsonic boundary layers

Computations based on several second-order turbulence models, including full Reynolds stress and two-equation models, are compared with a number of boundary-layer experiments. In general, the models represent the data reasonably well, with skin friction tending to be somewhat overpredicted in the far downstream region of the adverse pressure gradient experiments. A discussion of the behavior of the ARAP full Reynolds stress model in predicting the components of the Reynolds stress tensor is given. It is concluded that compatibility at the wall may necessitate the use of more than one length scale.

Rubesin, M. W.↗
NASA NTRS1977

Lowest-order average effect of turbulence on atmospheric profiles derived from radio occultation

Turbulence in planetary atmospheres and ionospheres causes changes in angles of refraction of radio waves used in occultation experiments. Atmospheric temperature and pressure profiles, and ionospheric electron concentration profiles, derived from radio occultation measurements of Doppler frequency contain errors due to such angular offsets. The lowest-order average errors are derived from a geometrical-optics treatment of the radio-wave phase advance caused by the addition of uniform turbulence to an initially homogeneous medium. It is concluded that the average profile errors are small and that precise Doppler frequency measurements at two or more wavelengths could be used to help determine characteristics of the turbulence, as well as accuracy limits and possible correction terms for the profiles. However, a more detailed study of both frequency and intensity characteristics in radio and optical occultation measurements of turbulent planetary atmospheres and ionospheres is required to realize the full potential of such measurements.

Eshleman, V. R.↗
NASA NTRS1978

Application of second-order turbulent modeling to the prediction of radiated aerodynamic sound

The Ribner formulation of the generation of aerodynamic sound is coupled with predictions of second-order velocity correlations and integral scale to estimate the sound radiated from several complicated jet flows. In particular, it is shown that the sound radiated from a cold swirling jet is greater than from its nonswirling equal thrust counterpart. The noise radiated from the flow field of a multitube suppressor was estimated and compared with an equal thrust diameter Gaussian jet. It is shown that the multitube concept is indeed quieter.

Bilanin, A. J.↗
NASA NTRS1978

A high order accurate finite element algorithm for high Reynolds number flow prediction

A Galerkin-weighted residuals formulation is employed to establish an implicit finite element solution algorithm for generally nonlinear initial-boundary value problems. Solution accuracy, and convergence rate with discretization refinement, are quantized in several error norms, by a systematic study of numerical solutions to several nonlinear parabolic and a hyperbolic partial differential equation characteristic of the equations governing fluid flows. Solutions are generated using selective linear, quadratic and cubic basis functions. Richardson extrapolation is employed to generate a higher-order accurate solution to facilitate isolation of truncation error in all norms. Extension of the mathematical theory underlying accuracy and convergence concepts for linear elliptic equations is predicted for equations characteristic of laminar and turbulent fluid flows at nonmodest Reynolds number. The nondiagonal initial-value matrix structure introduced by the finite element theory is determined intrinsic to improved solution accuracy and convergence. A factored Jacobian iteration algorithm is derived and evaluated to yield a consequential reduction in both computer storage and execution CPU requirements while retaining solution accuracy.

Baker, A. J.↗
NASA NTRS1977

Infinite order sudden approximation for rotational energy transfer in gaseous mixtures

Rotational energy transfer in gaseous mixtures is analyzed within the framework of the infinite order sudden (IOS) approximation, and a new derivation of the IOS from the coupled states Lippman-Schwinger equation is presented. This approach shows the relation between the IOS and coupled state T matrices. The general IOS effective cross section can be factored into a finite sum of 'spectroscopic coefficients' and 'dynamical coefficients'. The evaluation of these coefficients is considered. Pressure broadening for the systems HD-He, HCl-He, CO-He, HCl-Ar, and CO2-Ar is calculated, and results based on the IOS approximation are compared with coupled state results. The IOS approximation is found to be very accurate whenever the rotor spacings are small compared to the kinetic energy, provided closed channels do not play too great a role.

Goldflam, R.↗
NASA NTRS1978

Invariants of optimal minimal-order observer-based compensators

The relations characterizing the optimal minimal-order observer-based compensators for a linear time-invariant multivariable system with a random initial state (or, equivalently, known initial state and white plant-driving noise) have been reported by Miller (1973). In this note we establish in general that the plant transfer function uniquely determines the optimal compensator transfer function, and that this characterizes precisely the degrees of freedom in the compensator design; computational implications of this result are indicated.

Blanvillain, P. J.↗
NASA NTRS1978

A preference-ordered discrete-gaming approach to air-combat analysis

An approach to one-on-one air-combat analysis is described which employs discrete gaming of a parameterized model featuring choice between several closed-loop control policies. A preference-ordering formulation due to Falco is applied to rational choice between outcomes: win, loss, mutual capture, purposeful disengagement, draw. Approximate optimization is provided by an active-cell scheme similar to Falco's obtained by a 'backing up' process similar to that of Kopp. The approach is designed primarily for short-duration duels between craft with large-envelope weaponry. Some illustrative computations are presented for an example modeled using constant-speed vehicles and very rough estimation of energy shifts.

Kelley, H. J.↗
NASA NTRS1978

Specific-optimal control with a dual minimal-order observer-based compensator

The steady-state optimal control of a linear time-invariant stochastic system by means of a minimal-order dual-observer-based compensator is considered in this paper. The structure of the compensator is fixed while the associated gains are to be chosen so as to minimize a quadratic penalty on the plant state. Necessary and sufficient conditions for optimality are given, and an explicit solution is displayed. Salient features pertaining to the optimal system are: a decoupling property, a projection property, and an innovation property. Finally, it is shown that this design corresponds to a singular LQG problem, which is precisely the dual of another singular LQG problem: namely Newmann's problem. A complete picture is then given showing clearly the correspondence between the two designs.

Blanvillain, P. J.↗
NASA NTRS1978

An improved higher order panel method for linearized supersonic flow

An improved higher order panel method for linearized supersonic flow is described. Each panel, defined by four points on the surface, is divided into eight subpanels in such a way that all subpanel and panel edges are contiguous. By prescribing a quadratic distribution of the doublet on each subpanel, the doublet strength is made strictly continuous on the paneled surface. A linear source distribution is also used. Numerical results are smoother and in better agreement with experiment than the previous method with less strict continuity. A brief discussion of superinclined panels used to eliminate interior interference in nacelles is included.

Ehlers, F. E.↗
NASA NTRS1979

A high order accurate numerical solution algorithm for turbulent boundary layer flow

A fourth-order accurate numerical solution algorithm is derived using finite element interpolation theory for the non-linear parabolic equations governing turbulent boundary layer flow including a two-equation turbulence closure model. The results of carefully controlled numerical experiments firmly quantize for the first time performance differences between finite element and finite difference solution methodology for this type of equation. The developed algorithm takes advantage of the apparent semi-analytical formulational procedure, in establishment of a single, retarded-evaluation Jacobian matrix iterative solution algorithm. Numerical results document performance of solution economy features in terms of computer requirements and solution accuracy. The developed algorithm should find wide application in aerodynamics analysis.

Soliman, M. O.↗
NASA NTRS1979

Rotational excitation of symmetric top molecules by collisions with atoms. II - Infinite order sudden approximation

The infinite order sudden (IOS) approximation is extended to rotational excitation of symmetric tops by collisions with atoms. After development of a formalism for 'primitive' or 'one-ended' tops, proper parity-adapted linear combinations describing real rotors are considered and modifications needed for asymmetric rigid rotors are noted. The generalized spectroscopic relaxation cross sections are discussed. IOS calculations for NH3-He and H2CO-He are performed and compared with more accurate calculations, and the IOS approximation is found to provide a reasonably accurate description.

Green, S.↗
NASA NTRS1978

Computational test of the infinite order sudden approximation for excitation of linear rigid rotors by collisions with atoms

The infinite order sudden approximation for excitation of linear rigid rotors by collisions with atom is tested by comparing integral state-to-state cross sections with accurate close coupling and coupled states results. The systems studied are HCl-Ar, HCl-He, CO-He, HCN-He, CS-H2 and OCS-H2. With the exception of diatomic hydrides (e.g., HCl) which have atypically large rotational constants the method is found to be very accurate to remarkably low collision energies. This approximation should generally be extremely useful for thermal energy collisions.

Green, S.↗
NASA NTRS1979

Second order nonlinear equations of motion for spinning highly flexible line-elements

A formulation is given for the second order nonlinear equations of motion for spinning line-elements having little or no intrinsic structural stiffness. Such elements have been employed in recent studies of structural concepts for future large space structures such as the Heliogyro solar sailer. The derivation is based on Hamilton's variational principle and includes the effect of initial geometric imperfections (axial, curvature, and twist) on the line-element dynamics. For comparison with previous work, the nonlinear equations are reduced to a linearized form frequently found in the literature. The comparison has revealed several new spin-stiffening terms that have not been previously identified and/or retained. They combine geometric imperfections, rotary inertia, Coriolis, and gyroscopic terms.

Salama, M.↗
NASA NTRS1979

Second-order estimates in singular perturbation of output regulator problems

A method of order reduction in singular perturbation problems due to Chow and Kokotovic is studied. The method is used for an output regulator problem in control theory. The quadratic term in the asymptotic expansion of Chow and Kokotovic is calculated and it is shown by some examples how this term can give evidence of the accuracy of the procedure.

Rublein, G.↗
NASA NTRS1979

Vibrational dependence of pressure induced spectral linewidths and line shifts - Application of the infinite order sudden scattering approximation

The infinite order sudden (IOS) approximation to molecular rotation is applied to simplify the theory of linewidths and shifts in vibration-rotation spectra. This approximation is expected to be most accurate for hard, short-range collisions and is therefore complementary to Anderson theory which is best for weak, glancing collisions. The IOS approximation predicts identical linewidths and shifts for P- and R-branch transitions with the same line number. It also predicts zero line shifts for pure rotational spectra. The dependence of linewidths and shifts on vibrational band is seen to be due mainly to variation in diagonal vibrational matrix elements of the intermolecular potential. Calculations are performed for the 0-0, 0-1, and 0-2 bands of CO perturbed by He, using a theoretical interaction potential with no semiempirical or adjustable parameters; results are in satisfactory accord with experimental data.

Green, S.↗