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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Calculation of vertical and ramp-assisted takeoffs for supersonic cruise fighters

A procedure that allows rapid preliminary evaluations of the vertical, short, and normal takeoff performance of supersonic cruise aircraft concepts was developed into a numerical computer program. The program is used to determine the effects on takeoff performance of various parameters, such as thrust-weight ratio, wing loading, thrust vector angle, and flap setting. Ramp-assisted takeoffs for overloaded configurations typical of a ground-attack mission are included. The effects of wind on the takeoff performance are also considered.

Liu, G. C.↗
NASA NTRS1984

Identification of aerosol composition from multi-wavelength lidar measurements

This paper seeks to develop the potential of lidar for the identification of the chemical composition of atmospheric aerosols. Available numerical computations suggest that aerosols can be identified by the wavelength dependence of aerosol optical properties. Since lidar can derive the volume backscatter coefficient as a function of wavelength, a multi-wavelength lidar system may be able to provide valuable information on the composition of aerosols. This research theoretically investigates the volume backscatter coefficients for the aerosol classes, sea-salts, and sulfates, as a function of wavelength. The results show that these aerosol compositions can be characterized and identified by their backscatter wavelength dependence. A method to utilize multi-wavelength lidar measurements to discriminate between compositionally different thin aerosol layers is discussed.

Wood, S. A.↗
NASA NTRS1983

Turbulence and surface heat transfer near the stagnation point of a circular cylinder in turbulent flow

A turbulent boundary layer flow analysis of the momentum and thermal flow fields near the forward stagnation point due to a circular cylinder in turbulent cross flow is presented. Turbulence modeling length scale, anisotropic turbulence initial profiles and boundary conditions were identified as functions of the cross flow turbulence intensity and the boundary layer flow far field velocity. These parameters were used in a numerical computational procedure to calculate the mean velocity, mean temperature, and turbulence double correlation profiles within the flow field. The effects of the cross flow turbulence on the stagnation region momentum and thermal flow fields were investigated. This analysis predicted the existing measurements of the stagnation region mean velocity and surface heat transfer rate with cross flow Reynolds number and turbulence intensity less than 250,000 and 0.05, respectively.

Wang, C. R.↗
NASA NTRS1984

Analytical description of an imperfect Fabry-Perot etalon

A novel method is presented for the investigation of Fabry-Perot etalon defects, incorporating the effects of microsurface imperfection, bowing, and departure from parallelism, as well as the effect of finite aperture on the etalon, into the perturbation function. The tolerance imperfections of reflective layers can be incorporated into the transfer coefficients of a noncorrugated thin-film system, and any defect can be investigated either separately or in combination with others through the use of the appropriate analytical or numerical simulation of the corrugated function. In the context of numerical computation, the present method permits the simulation and evaluation of different specific situations. The expressions derived for the transfer coefficients of a corrugated, equidistant thin film system may be applicable to other branches of applied optics.

Prikryl, I.↗
NASA NTRS1984

Pitch angle diffusion in the Jovian magnetodisc

Using a linear approximation to the field variation and a mathematical technique for distorting the orbit integration into the complex gyrophase plane, a formula is derived for the nonadiabatic change in magnetic moment. This change varies as the cosine of the particle gyrophase at the equator, so that if a particle traverses the equator several times with different values of this gyrophase, it suffers randomly correlated values for the nonadiabatic change in magnetic moment. The algebraic formula for this nonadiabatic change is found to be in excellent agreement with previous numerical computations. The phase space density, averaged over several bounce periods, is found to satisfy a diffusion equation in pitch angle. The diffusion coefficient is dependent on particle energy, pitch angle, and the field line along which the particles are moving.

Birmingham, T. J.↗
NASA NTRS1983

A user's view of optical disc applications

An overall space information system is considered. It is found that data storage is required in all elements of the system with the possible exception of the wideband communication link. The data capture facility is the first ground facility to handle space-acquired data. A typical nine-day Spacelab mission is expected to generate up to 10 to the 13th bits of data. The current storage medium for this data is approximately 4000 magnetic tapes. It is pointed out that optical disk storage provides an attractive alternative to magnetic disk and tape storage. Options to install an optical disk storage system in the GSFC data capture facility in the 1986 time frame are evaluated by NASA. Attention is given to data capture and data base applications, onboard data storage applications, and numerical computation applications.

Holcomb, L. B.↗
NASA NTRS1984

Gain optimization with non-linear controls

An algorithm has been developed for the analysis and design of controls for non-linear systems. The technical approach is to use statistical linearization to model the non-linear dynamics of a system by a quasi-Gaussian model. A covariance analysis is performed to determine the behavior of the dynamical system and a quadratic cost function. Expressions for the cost function and its derivatives are determined so that numerical optimization techniques can be applied to determine optimal feedback laws. The primary application for this paper is centered about the design of controls for nominally linear systems but where the controls are saturated or limited by fixed constraints. The analysis is general, however, and numerical computation requires only that the specific non-linearity be considered in the analysis.

Slater, G. L.↗
NASA NTRS1984

Effect of inhomogeneous radiation fields on free-free scattering processes

The apparent collision cross section of an inverse-bremsstrahlung (free-free) collision process is investigated analytically for the case of laser-field inhomogeneity. Numerical computations are performed for typical experimental conditions involving high-intensity laser fields, and the results are presented in a table: significant effects due to the field inhomogeneity are found, and it is determined that free-free processes are less likely in the TEM(01) mode than in the TEM(00) mode for given values of the parameters p and n.

Curry, P. J.↗
NASA NTRS1984

MMIC devices for active phased array antennas

The study of printed circuit discontinuities is necessary in order to design, for example, transitions between rectangular waveguides and printed circuits. New developments with respect to the analytical approaches to this problem are discussed. A summary of the progress in the experimental approach is presented. The accurate solution for the modes in various millimeter-wave waveguides is essential in the analysis of many integrated circuit components, such as filters and impedance transformers. Problems associated with the numerical computation of these modes in two frequently used waveguide forms, namely, the finline and microstrip, are presented. The spectral domain method of formulation, with a moment method solution, is considered. This approach can be readily extended to analyze an arbitrary configuration of dielectric and metallized regions in a shielded enclosure. Galerkin's method is used, where the testing and basic functions are the same. It is shown that the mode functions, or eigenfunctions, are more sensitive to errors than the phase constants, or eigenvalues. The approximate mode functions do not satisfy the orthogonality relationship well, resulting in difficulties when these modal solutions are used to form an approximate Green's function or are used in a mode matching analysis.

Mittra, R.↗
NASA NTRS1985

Models of Angular Momentum Input to a Circumterrestrial Swarm from Encounters with Heliocentric Planetesimals

Models of lunar origin in which the Moon accretes in orbit about the Earth from material approaching the Earth from heliocentric orbits must overcome a fundamental problem: the approach orbits of such material would be, in the simplest approximation, equally likely to be prograde or retrograde about the Earth, with the result that accretion of such material adds mass but not angular momentum to circumterrestrial satellites. Satellite orbits would then decay due to the resulting drag, ultimately impacting onto the Earth. One possibility for adding both material and angular momentum to Earth orbit is investigated: imbalance in the delivered angular momentum between pro and retrograde Earth passing orbits which arises from the three body dynamics of planetesimals approaching the Earth from heliocentric space. In order to study angular momentum delivery to circumterrestrial satellites, the near Earth velocities were numerically computed as a function of distance from the Earth for a large array of orbits systematically spanning heliocentric phase space.

Davis, D. R.↗
NASA NTRS1985

Generation of capillary instabilities by external disturbances in a liquid jet

The receptivity problem in a circular liquid jet is considered. A time harmonic axial pressure gradient is imposed on the steady, parallel flow of a jet of liquid emerging from a circular duct. Using a technique developed in plasma physics a casual solution to the forced problem is obtained over certain ranges of Weber number for a number of mean velocity profiles. This solution contains a term which grows exponentially in the downstream direction and can be identified with a capillary instability wave. Hence, it is found that the externally imposed disturbances can indeed trigger instability waves in a liquid jet. The amplitude of the instability wave generated relative to the amplitude of the forcing is computed numerically for a number of cases.

Leib, S. J.↗
NASA NTRS1984

The large-time behavior of the scalar, genuinely nonlinear Lax-Friedrichs scheme

The Lax-Friedrichs scheme, approximating the scalar, genuinely nonlinear conservation law u sub t + f sub x (u) = O where f(u) is, say, strictly double dot f dot a sub asterisk O is studied. The divided differences of the numerical solution at time t do not exceed 2 (t dot a sub asterisk) to the -1. This one-sided Lipschitz boundedness is in complete agreement with the corresponding estimate one has in the differential case; in particular, it is independent of the initial amplitude in sharp contrast to linear problems. It guarantees the entropy compactness of the scheme in this case, as well as providing a quantitative insight into the large-time behavior of the numerical computation.

Tadmor, E.↗
NASA NTRS1984

Dynamic alignment and selective decay in MHD

Under some circumstances, incompressible magnetohydrodynamic turbulence will evolve toward a state in which the velocity fields and magnetic fields are aligned or anti-aligned. We propose a mechanism for this effect and illustrate with numerical computations. Under some other circumstances, the energy appears to decay selectively toward a minimum energy state in which the kinetic energy has disappeared. It has not been possible so far to identify a boundary in the phase space which divides the two regimes.

Matthaeus, W. H.↗
NASA NTRS1983

Simultaneous ocean cross-section and rainfall measurements from space with a nadir pointing radar

A modified version of the surface-target-attenuation radar described by Meneghini et al. (1983) is proposed which permits simultaneous measurement of ocean radar cross sections and path-average rain rates using a nadir-pointing satellite-borne microwave radar. The basic concept is explained and illustrated; the equations describing the data reduction are derived; some preliminary numerical computations based on a 7.5-m-diameter 10-kW 1.33-microsec-pulse radar operating at 1.87 cm from an altitude of 500 km are performed; and the major error sources (mismatches between rain scattering volumes and additional multipath contributions) and limitations (nadir pointing) are discussed. It is suggested that the system could provide a nadir calibration for wide-swath observing systems such as scanning microwave radiometers.

Atlas, D.↗
NASA NTRS1985

Estimating the weight of generally configured dual wing systems

Formulas available for the weight estimation of monoplane wings cannot be said to be appropriate for the estimation of generally configured dual wing systems. In the present paper a method is described which simultaneously generates a structural weight estimate and a fully stressed, quasi-optimal structure for a model of a dual wing system. The method is fast and inexpensive. It is ideally suited to preliminary design. To illustrate the method, a dual wing system and a conventional wing system are sized. Numerical computation is shown to be suitably fast for both cases and, for both cases, convergence to a final configuration is shown to be quite rapid. To illustrate the validity of the method, a conventional wing is sized and its weight obtained by the present method is compared to its weight determined by a reputable weight estimation formula. The results are shown to be very close.

Cronin, D. L.↗
NASA NTRS1985

Stimulated Compton conversion of Langmuir waves by relativistic electron beams

The scattering of a Langmuir wave by a relativistic electron to produce a transversely polarized high-frequency EM wave by an inverse Compton process is investigated theoretically. The results of numerical computations are presented in diagrams and graphs, and it is shown that although net growth rates are predicted for several classes of Langmuir spectra, they are at least several orders of magnitude too small to account for the intense high-frequency emission seen in recent beam-plasma experiments (Benford et al., 1980; Kato et al., 1983) unless some unknown coherence mechanism is focusing the inverse-Compton emission. The implications of the mechanism for astrophysical beam-plasma systems such as quasars and radio galaxies are indicated.

Newman, D. L.↗
NASA NTRS1985

A note on the loop thermosyphon with mixed boundary conditions

This paper presents a one-dimensional model for flow in a toroidal fluid loop in which the wall heat transfer coefficient is constant over one half of the loop, and zero over the other. A mathematically low-dimensional form of the model gives reasonably accurate predictions of the steady state mass flow, stability, and the nature of nonsteady motions. Numerical computations with a high-dimensional model illustrate the effects of mixed boundaries and axial heat conduction on the chaotic solutions. Comparisons are made with previous low-dimensional models that are exact only for a spatially uniform heat transfer coefficient.

Hart, J. E.↗