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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

New results on mean time-to-first-slip for a first-order loop

Approximate results are derived from a new expression for the mean time-to-first-slip for a first-order loop with arbitrary periodic equivalent phase detector characteristic in the presence of frequency detuning. They are accurate for loop signal-to-noise ratios greater than 3 dB or less than 0 dB. The approximations found serve to provide insight into the mean first slip time dependence on the shape of the equivalent phase detector characteristic and the frequency detuning, and dependence on loop parameters. The new expression itself is also of theoretical interest, since it allows one to explicitly express the stationary phase error probability density function and probability current in terms of the mean first slip time. This sheds further light on the interconnections between the conventional modulo-2 pi approach and the renewal theory approach to the Fokker-Planck method. Extensions to nonperiodic equivalent phase detector characteristics are also considered.

Chie, C. M.↗
NASA NTRS1991

From differential to difference equations for first order ODEs

When constructing an algorithm for the numerical integration of a differential equation, one should first convert the known ordinary differential equation (ODE) into an ordinary difference equation. Given this difference equation, one can develop an appropriate numerical algorithm. This technical note describes the derivation of two such ordinary difference equations applicable to a first order ODE. The implicit ordinary difference equation has the same asymptotic expansion as the ODE itself, whereas the explicit ordinary difference equation has an asymptotic that is similar in structure but different in value when compared with that of the ODE.

Freed, Alan D.↗
NASA NTRS2000

First-Order Model Management With Variable-Fidelity Physics Applied to Multi-Element Airfoil Optimization

First-order approximation and model management is a methodology for a systematic use of variable-fidelity models or approximations in optimization. The intent of model management is to attain convergence to high-fidelity solutions with minimal expense in high-fidelity computations. The savings in terms of computationally intensive evaluations depends on the ability of the available lower-fidelity model or a suite of models to predict the improvement trends for the high-fidelity problem, Variable-fidelity models can be represented by data-fitting approximations, variable-resolution models. variable-convergence models. or variable physical fidelity models. The present work considers the use of variable-fidelity physics models. We demonstrate the performance of model management on an aerodynamic optimization of a multi-element airfoil designed to operate in the transonic regime. Reynolds-averaged Navier-Stokes equations represent the high-fidelity model, while the Euler equations represent the low-fidelity model. An unstructured mesh-based analysis code FUN2D evaluates functions and sensitivity derivatives for both models. Model management for the present demonstration problem yields fivefold savings in terms of high-fidelity evaluations compared to optimization done with high-fidelity computations alone.

Alexandrov, N. M.↗
NASA NTRS1977

First-order torques and solid-body spinning velocities in intense sound fields

The letter reports an observation of first-order nonzero time-averaged torques and solid-body spinning velocities in intense acoustic fields. The experimental apparatus consisted of a vertical cylindrical rod supported on an air bearing and passing through a box with two loudspeakers centered on adjoining vertical sides. The rim velocity of the cylinder and the torque on the cylinder are measured as functions of air-particle velocity and the phase difference between the x and y components of the particle velocity. It is found that both rim velocity and torque are linear functions of particle velocity. Difficulties in constructing a proper theoretical description of the observed effects are discussed.

Wang, T. G.↗
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 NTRS2014

First-Order Hyperbolic System Method for Time-Dependent Advection-Diffusion Problems

A time-dependent extension of the first-order hyperbolic system method for advection-diffusion problems is introduced. Diffusive/viscous terms are written and discretized as a hyperbolic system, which recovers the original equation in the steady state. The resulting scheme offers advantages over traditional schemes: a dramatic simplification in the discretization, high-order accuracy in the solution gradients, and orders-of-magnitude convergence acceleration. The hyperbolic advection-diffusion system is discretized by the second-order upwind residual-distribution scheme in a unified manner, and the system of implicit-residual-equations is solved by Newton's method over every physical time step. The numerical results are presented for linear and nonlinear advection-diffusion problems, demonstrating solutions and gradients produced to the same order of accuracy, with rapid convergence over each physical time step, typically less than five Newton iterations.

Mazaheri, Alireza↗
NASA NTRS1990

First-order Fermi particle acceleration by relativistic shocks

Monte Carlo calculations of test particle spectra and acceleration times are presented from first-order Fermi particle acceleration for parallel shocks with arbitrary flow velocities and compression ratios r up to seven, shock velocities u1 up to 0.98c, and injection energies ranging from thermal to highly superthermal. Far above the injection energy, the spectra are well-approximated by a power law and the spectra are always harder than for nonrelativistic shocks. Approximate analytic expression are given for the spectral slope as a function of u1 and r. The acceleration time as a function of particle energy is less than for nonrelativistic shocks by a factor that increases with u1 and is about three for u1 = 0.98c. It is confirmed that the spectrum for pitch-angle diffusion is considerably steeper than for large-angle scattering for the same shock parameters.

Ellison, Donald C.↗
NASA NTRS1984

Finite element methods for first-order hyperbolic systems with particular emphasis on the compressible Euler equations

A Petrov-Galerkin finite element formulation is presented for first-order hyperbolic systems of conservation laws with particular emphasis on the compressible Euler equations. Applications of the methodology are made to one- and two-dimensional steady and unsteady flows with shocks. Results obtained suggest the potential of the type of methods developed.

Hughes, T. J. R.↗