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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Estimation of geopotential from satellite-to-satellite range rate data: Numerical results

A technique for high-resolution geopotential field estimation by recovering the harmonic coefficients from satellite-to-satellite range rate data is presented and tested against both a controlled analytical simulation of a one-day satellite mission (maximum degree and order 8) and then against a Cowell method simulation of a 32-day mission (maximum degree and order 180). Innovations include: (1) a new frequency-domain observation equation based on kinetic energy perturbations which avoids much of the complication of the usual Keplerian element perturbation approaches; (2) a new method for computing the normalized inclination functions which unlike previous methods is both efficient and numerically stable even for large harmonic degrees and orders; (3) the application of a mass storage FFT to the entire mission range rate history; (4) the exploitation of newly discovered symmetries in the block diagonal observation matrix which reduce each block to the product of (a) a real diagonal matrix factor, (b) a real trapezoidal factor with half the number of rows as before, and (c) a complex diagonal factor; (5) a block-by-block least-squares solution of the observation equation by means of a custom-designed Givens orthogonal rotation method which is both numerically stable and tailored to the trapezoidal matrix structure for fast execution.

Thobe, Glenn E.↗
NASA NTRS1989

Viscosity measurements at high temperature and high pressure: A novel technique

The extensive numerical modelling of transport phenomena was performed for the melt growth of mercury cadmium telluride. To increase the fidelity of modelling the kinematic viscosity of liquid Hg(1-x)Cd(x)Te was determined at various compositions in the range of x between 0 and 0.2 and at temperatures around and below the respective melting point. The phase diagram of Hg(1-x)Cd(x)Te shows that for this range the melting point varies from 670 C for pure HgTe to 790 C at x=0.2. The vapor pressure above the melt varies correspondingly from 15 to about 40 atm. Hence, the measurement of viscosities in this system requires a technique that allows for combinations of high temperatures and pressures. In addition, a closed isothermal system is required. The high pressure melt container must also be inert to molten Hg(1-x)Cd(x)Te to avoid possible errors from contamination of the liquid. A novel technique that largely circumvents the above experimental problems is described. Its theory is also presented.

Jin, Wei-Qing↗
NASA NTRS1972

Analysis and design of three dimensional supersonic nozzles. Volume 2: Numerical program for analysis of nozzle-exhaust flow fields

The FORTRAN IV Program developed to analyze the flow field associated with scramjet exhaust systems is presented. The instructions for preparing input and interpreting output are described. The program analyzes steady three dimensional supersonic flow by the reference plane characteristic technique. The governing equations and numerical techniques employed are presented in Volume 1 of this report.

Kalben, P.↗
NASA NTRS1990

Prediction of blade-vortex interaction noise using airloads generated by a finite-difference technique

The present numerical finite-difference scheme for helicopter blade-load prediction during realistic, self-generated three-dimensional blade-vortex interactions (BVI) derives the velocity field through a nonlinear superposition of the rotor flow-field yielded by the full potential rotor flow solver RFS2 for BVI, on the one hand, over the rotational vortex flow field computed with the Biot-Savart law. Despite the accurate prediction of the acoustic waveforms, peak amplitudes are found to have been persistently underpredicted. The inclusion of BVI noise source in the acoustic analysis significantly improved the perceived noise level-corrected tone prediction.

Tadghighi, Hormoz↗
NASA NTRS1987

Application of artificial intelligence to impulsive orbital transfers

A generalized technique for the numerical solution of any given class of problems is presented. The technique requires the analytic (or numerical) solution of every applicable equation for all variables that appear in the problem. Conditional blocks are employed to rapidly expand the set of known variables from a minimum of input. The method is illustrated via the use of the Hohmann transfer problem from orbital mechanics.

Burns, Rowland E.↗
NASA NTRS1983

Accuracy and resolution of Earth Radiation Budget Estimates

A numerical filter inversion technique that reduces wide-angle satellite measurements to top-of-the-atmosphere radiant exitances has been proposed for the Earth Radiation Budget Experiment (ERBE). The matrix formulation of this technique is presented, and the design of the numerical filter is discussed. The filter is smoothed with a singular value decomposition. The inversion process is simulated by generating synthetic measurements from a 24 degree spherical harmonic radiation field derived from Nimbus 6 ERB data. The numerical filter is applied to these measurements after they are corrupted with instrument error. The results are curves of expected error versus resolution area.

Green, R. N.↗
NASA NTRS1990

A differential delay equation arising from the sieve of Eratosthenes

Consideration is given to the differential delay equation introduced by Buchstab (1937) in connection with an asymptotic formula for the uncanceled terms in the sieve of Eratosthenes. Maier (1985) used this result to show there is unexpected irreqularity in the distribution of primes in short intervals. The function omega(u) is studied in this paper using numerical and analytical techniques. The results are applied to give some numerical constants in Maier's theorem.

Cheer, A. Y.↗
NASA NTRS1973

Partial implicitization

The steady-state solution to the full Navier-Stokes equations for complicated flows is generally difficult to obtain. The Burgers (1948) equation is used as a model of the Navier-Stokes equations. The steady-state solution is obtained by a one-step explicit technique resulting from a partial implicitization of the difference equation. Stability analysis shows that the technique is unconditionally stable, and numerical tests show the technique to be accurate.

Graves, R. A., Jr.↗
NASA NTRS1987

Use of simulated satellite radiances from a mesoscale numerical model to understand kinematic and dynamic processes

Techniques by which mesoscale numerical weather prediction model output and radiative transfer codes are combined to simulate the radiance fields that a given passive temperature/moisture satellite sensor would see if viewing the evolving model atmosphere are introduced. The goals are to diagnose the dynamical atmospheric processes responsible for recurring patterns in observed satellite radiance fields, and to develop techniques to anticipate the ability of satellite sensor systems to depict atmospheric structures and provide information useful for numerical weather prediction (NWP). The concept of linking radiative transfer and dynamical NWP codes is demonstrated with time sequences of simulated radiance imagery in the 24 TIROS vertical sounder channels derived from model integrations for March 6, 1982.

Kalb, Michael↗
NASA NTRS1993

Extending model-based diagnosis for analog thermodynamical devices

The increasing complexity of process control applications have posed difficult problems in fault detection, isolation, and recovery. Deep knowledge-based approaches, such as model-based diagnosis, have offered some promise in addressing these problems. However, the difficulties of adapting these techniques to situations involving numerical reasoning and noise have limited the applicability of these techniques. This paper describes an extension of classical model-based diagnosis techniques to deal with sparse data, noise, and complex noninvertible numerical models. These diagnosis techniques are being applied to the External Active Thermal Control System for Space Station Freedom.

Rouquette, Nicolas↗
NASA NTRS1984

Direct numerical simulation of reacting flows

The objectives of this work are: (1) to extend the technique of direct numerical simulations to turbulent, chemically reacting flows, (2) to test the validity of the method by comparing computational results with laboratory data, and (3) to use the simulations to gain a better understanding of the effects of turbulence on chemical reactions. The effects of both the large scale structure and the smaller scale turbulence on the overall reaction rates are addressed. The relationship between infinite reaction rate and finite reaction rate chemistry is compared with some of the results of calculations with existing theories and laboratory data. The direct numerical simulation method involves the numerical solution of the detailed evolution of the complex turbulent velocity and concentration fields. Using very efficient numerical methods (e.g., pseudospectral methods), the fully nonlinear (possibly low pass filtered) equations of motion are solved and no closure assumptions or turbulence models are used. Statistical data are obtained by performing spatial, temporal, and/or ensemble averages over the computed flow fields.

Riley, J. J.↗
NASA NTRS1991

Shortwave wide-field-of-view results from the Earth Radiation Budget Experiment

The deconvolution (DCN) and numerical filter (NF) techniques of analyzing nonscanning radiometer measurements are evaluated by comparison with scanning radiometer results for monthly mean maps of albedo and absorbed solar radiation. Both techniques successfully enhance the resolution of the monthly mean product. The numerical filter albedo adheres closer to the scanning radiometer in the zonal average than does the DCN albedo. Zonal rms's, however, show that the DCN albedo is about 1 percent better than the NF in the Southern Hemisphere, but in the Northern Hemisphere the NF is better by up to 2 percent.

Rutan, David↗
NASA NTRS1982

Numerical generation of composite three dimensional grids by quasilinear elliptic systems

A three-dimensional grid generation technique designed to numerically construct a boundary-conforming grid within a three-dimensional region bounded by a closed surface is described. The boundary values are generated numerically by a modified elliptic system and are used to compute grid control parameters that are contained in the elliptic systems. The interior grid distribution is governed by the distribution of points on the boundary as well as by the boundary's geometric shape. The composite three-dimensional grid remains both continuous and smooth across the surface of juncture between any two adjoining subregions. The details of the method and its implementation are presented, displaying numerical results for three-dimensional composite grid for a wing-body combination and surface grids.

Thomas, P. D.↗
NASA NTRS1977

Fast but accurate techniques for calculating radiative terms in numerical atmospheric models and in remote sensing applications

Methods were developed for calculating radiative terms with relatively high accuracy but with sufficient speed, so that they can be used in numerical atmospheric models or in high volume processing of satellite measured radiances for remote sensing of atmospheric and surface parameters. Comparison with commonly used methods in both types of applications indicate improvements in calculating transmittances of factors between two and three, and in calculating radiances and cooling rates of factors between two and seven.

Arking, A.↗
NASA NTRS1994

Gradient drift eigenmodes in the equatorial electrojet

The problem of kilometer-scale irregularities in the daytime equatorial electrojet is revisited by means of an eigenmode analysis of the gradient drift instability. Realistic physical parameters are used, including the modeled altitude variations of ion and electron collision frequencies and mobilities. The full fourth-order system of two coupled differential equations (each of second order) for the denisty and electrostatic potential perturbations is solved numerically by a relaxation technique. Under some approximations, the fourth-order system can be shown to reduce to a second-order differential equation for the perturbed potential or density. The latter is solved using a shooting technique and provides initial guesses for numerical solutions to the full problem. It is shown that the linear growth rate peaks for kilometer-scale waves, contrary to the findings of recent initial-value studies. This occurs because the equilibrium velocity shear is much more effective as a damping mechanism for short-wavelength modes than it is for the longer, kilometer-scale modes. These results provide a natural qualitative explanation for the observed dominance of kilometer-scale structures in the daytime electrojet spectrum.

Wang, X.-H.↗