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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1970

A rough earth scattering model for multipath prediction

The most important phenomena to be considered in a model of radio wave communication between earth satellites are scattering from the surface of the earth. A model is derived and implemented on a computer to predict the field received after reflection from a rough, spherical earth. The scattering integrals are computed numerically; the domain of integration is the appropriate region on the surface of the earth. Calculations have been performed at VHF frequencies and for terrain which could be described as marshy land. Rough surface scattering calculations must be performed over a spherical earth when satellites are involved. There is a definite dependence on the values of the roughness, and the correlation length.

Page, L. J.↗
NASA NTRS1973

On electromagnetic field problems in inhomogeneous media

Analysis of electromagnetic fields in inhomogeneous media is of practical interest in general scattering and propagation problems and in the study of lenses. For certain types of inhomogeneities, the fields may be represented in terms of two scalars. In a general orthogonal coordinate system, these potentials satisfy second order differential equations. Exact solutions of these equations are known only for a few particular cases and in general, an approximate or numerical technique must be employed. The present work reviews and generalizes some of the main methods of attack of the problem. The results are presented in a form appropriate for numerical computation.

Mohsen, A.↗
NASA NTRS1973

Rate effects on instabilities in slightly ionized plasmas

Results are presented of numerical computer calculations of dispersive wave growth rates in slightly ionized plasmas. The effects of employing a plasma model with detailed consideration of rate and radiative processes involving the electron quantum states were studied. Acoustic and electrothermal waves in unseeded argon are found to be frozen with the latter damped. Rate thermal modes can exhibit large growth. Mode mixing and switching in both seeded and unseeded argon are demonstrated.

Harstad, K. G.↗
NASA NTRS1972

The transmission of sonic boom signals into rooms through open windows.

This paper is the second of two companion papers. In the first paper, expressions for the acoustic field generated inside a room with an open window due to incoming transient or periodic signals have been presented. In this paper, the technique has been applied to the specific example of an N-wave type signal. Detailed mathematical analysis is followed by its approximate version and numerical computation. Results of controlled experiments, using simulated sonic booms, have been compared with theoretical predictions.

Vaidya, P. G.↗
NASA NTRS1974

Dynamic analysis of a system of hinge-connected rigid bodies with nonrigid appendages

Equations of motion are derived for use in simulating a spacecraft or other complex electromechanical system amenable to idealization as a set of hinge-connected rigid bodies of tree topology, with rigid axisymmetric rotors and nonrigid appendages attached to each rigid body in the set. In conjunction with a previously published report on finite-element appendage vibration equations, this report provides a complete minimum-dimension formulation suitable for generic programming for digital computer numerical integration.

Likins, P. W.↗
NASA NTRS1974

Nonstationary homogeneous nucleation

The theory of homogeneous condensation is reviewed and equations describing this process are presented. Numerical computer solutions to transient problems in nucleation (relaxation to steady state) are presented and compared to a prior computation.

Harstad, K. G.↗
NASA NTRS1974

On the design of optimal input signals in system identification

The problem of designing optimal inputs in the identification of linear systems with unknown random parameters is considered using a Bayesian approach. The information matrix, which is positive definite for the class of systems analyzed, gives a measure of performance for the system inputs. The computation of the optimal closed-loop input mappings is shown to be a nontrivial exercise in adaptive control. Deterministic optimal inputs are shown to be easily computable. Numerical examples are given. A Kalman filter is used to estimate the parameters. A necessary condition for the Kalman filter not to diverge when applying linear feedback is also given.

Lopez-Toledo, A. A.↗
NASA NTRS1974

Thermal relaxation of a two dimensional plasma in a dc magnetic field. Part 2: Numerical simulation

The thermal relaxation process for a spatially uniform two dimensional plasma in a uniform dc magnetic field is simulated numerically. Thermal relaxation times are defined in terms of the time necessary for the numerically computer Boltzman H-function to decrease through a given part of the distance to its minimum value. Dependence of relaxation time on two parameters is studied: number of particles per Debye square and ratio of gyrofrequency to plasma frequency.

Hsu, J. Y.↗
NASA NTRS1973

Dynamic analysis of a system of hinge-connected rigid bodies with nonrigid appendages

Equations of motion are derived for use in simulating a spacecraft or other complex electromechanical system amenable to idealization as a set of hinge-connected rigid bodies of tree topology, with rigid axisymmetric rotors and nonrigid appendages attached to each rigid body in the set. In conjunction with a previously published companion paper on finite-element appendage vibration equations, this paper provides a complete minimum-dimension formulation suitable for generic programming for digital computer numerical integration.

Likins, P. W.↗
NASA NTRS1973

A study of the reflection effect in close spectroscopic binary stars

Four W Ursae Majoris-type binaries were investigated using the simple model of blackbody solid spheres. Numerical computations were carried out for the rotationally broadened profiles of spectral lines Sr II (4077.7) and Ca I (4226.7) and the mean radial velocities. These mean velocities at different phases outside eclipses were compared with sinusoidal values of the radial velocity curve for point masses. Within the present accuracy of observations, the reflection effect is small and may be neglected in the analysis of radial velocity curves.

Chen, K.-Y.↗
NASA NTRS1973

Aircraft maneuver optimization of reduced-order approximation

Review of recent work in recasting the energy type of approximation to aircraft flight in terms of singular perturbation theory and in extending this theory to three-dimensional maneuvers. Singular perturbations for differential equations arising in optimal control are first examined for a system of fairly general form but low order. The attitude dynamics for optimal flight of a rocket in vacuum is then studied as an introductory, and fairly transparent, example. The question of the choice of variables is then discussed. Finally, optimal aircraft flight in various reduced-order approximations is investigated. In particular, the problem of three-dimensional aircraft flight is formulated for singular perturbation treatment, and possibilities for decoupling into several lower-order problems are illustrated. The use of reduced-order approximation facilitates numerical computations by reducing the number of multiplier initial values that must be determined simultaneously and by improving the conditioning of the differential equations.

Kelley, H. J.↗
NASA NTRS1974

Theoretical and observational analysis of spacecraft fields

In order to investigate the dipolar and nondipolar contributions of spacecraft magnetic fields, a simple magnetic field model is proposed. This model consists of a number N of randomly oriented dipoles of strength M sub k in a given volume. Two sets of formulas are presented that give the rms multipole field components, first, for isotropic orientations of the dipoles at given positions and second, for isotropic orientations of the dipoles distributed uniformly throughout a cube or sphere. The statistical results for a cube (8 cu m in size) together with individual examples computed numerically show the following features. Beyond about 2- to 3-m distance from the center of the cube the field is dominated by an equivalent dipole. The magnitude of the magnetic moment of the dipolar part is approximated by N to the 1/2 power times M for equal magnetic moments or generally by the Pythagorean sum of the dipole moments. The radial component tend to be greater than either of the transverse components for the dipole portion as well as for the nondipolar field contributions.

Neubauer, F. M.↗
NASA NTRS1974

On the shock-on-shock interaction problem

The unsteady, inviscid flowfield that results when a supersonic vehicle strikes a planar oblique shock wave, though difficult to simulate experimentally, is quite easy to model and compute numerically. The complicated flowfield, which contains multiple shock wave interactions, is determined using a second-order, shock-capturing, finite-difference approach which solves the time-dependent Euler equations under a self-similar transformation. A series of numerical results for a simple two-dimensional wedge is presented which describes the entire disturbed region, including the wave structure, and shows good agreement with the available two- and three-dimensional experimental data.-

Kutler, P.↗
NASA NTRS1974

Scattering and absorption from poly-dispersed aerosols

Results from numerical computations of the diffuse sky radiances and degree of polarization are given. These results were obtained by solving the equation of radiative transfer in the formulation of Eschelbach. In these computations, multiple scattering and absorption by aerosol particles were considered. In addition, results from experimental measurements of the complex refractive index of aerosol samples at different relative humidities are presented. From the radiation flux divergences which were computed based on the determined properties of atmospheric aerosol particles, atmospheric heating rates were derived which were found to be comparable to the heating rates by water vapor.

Bullrich, K.↗
NASA NTRS1975

Vaporization response of evaporating drops with finite thermal conductivity

A numerical computing procedure was developed for calculating vaporization histories of evaporating drops in a combustor in which travelling transverse oscillations occurred. The liquid drop was assumed to have a finite thermal conductivity. The system of equations was solved by using a finite difference method programmed for solution on a high speed digital computer. Oscillations in the ratio of vaporization of an array of repetitivity injected drops in the combustor were obtained from summation of individual drop histories. A nonlinear in-phase frequency response factor for the entire vaporization process to oscillations in pressure was evaluated. A nonlinear out-of-phase response factor, in-phase and out-of-phase harmonic response factors, and a Princeton type 'n' and 'tau' were determined. The resulting data was correlated and is presented in graphical format. Qualitative agreement with the open literature is obtained in the behavior of the in-phase response factor. Quantitatively the results of the present finite conductivity spray analysis do not correlate with the results of a single drop model.

Agosta, V. D.↗
NASA NTRS1975

Terminal guidance and navigation for comet and asteroid rendezvous

A terminal guidance and navigation scheme developed in earlier work was modified and evaluated for a solar electric propulsion rendezvous mission to comet Encke. The scheme is intended for autonomous, on-board use. The guidance algorithm is based on optimal control theory and minimizes the time integrated square of thrust acceleration. The navigation algorithm employs a modified Kalman filter set in measurement variables. Random sequences were generated to simulate measurement errors, and the evaluation was conducted with detailed numerical computations which include actual motions of spacecraft and comet. The evaluations showed that the scheme attains rendezvous and maintains station after rendezvous within less than 10 km for estimated best measurements and within less than 100 km for estimated worst measurements. The measurements required are angles, range, and range rate. Angles and range appear to be absolutely necessary; range rate is not as strong a measurement type, and further modifications of the filter will allow a scheme that does not require the rate measurements.

Bennett, A. G.↗
NASA NTRS1975

Theory of Noise Generation from Moving Bodies with an Application to Helicopter Rotors

Several expressions for the determination of the acoustic field of moving bodies are presented. The analysis is based on the Ffowcs Williams-Hawkings equation. Applying some proposed criteria, one of these expressions is singled out for numerical computation of acoustic pressure signature. The compactness of sources is not assumed and the main results are not restricted by the observer position. The distinction between compact and noncompact sources on moving surfaces is discussed. Some thickness noise calculations of helicopter rotors and comparison with experiments are included which suggest this mechanism as the source of high-speed blade slap of rotors.

Farassat, F.↗
NASA NTRS1975

Use of graphics in the design office at the Military Aircraft Division of the British Aircraft Corporation

The CAD/CAM interactive computer graphics system was described; uses to which it has been put were shown, and current developments of the system were outlined. The system supports batch, time sharing, and fully interactive graphic processing. Engineers using the system may switch between these methods of data processing and problem solving to make the best use of the available resources. It is concluded that the introduction of on-line computing in the form of teletypes, storage tubes, and fully interactive graphics has resulted in large increases in productivity and reduced timescales in the geometric computing, numerical lofting and part programming areas, together with a greater utilization of the system in the technical departments.

Coles, W. A.↗