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At least 271 records · Page 15

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NASA NTRS1978

Martian atmosphere modeling between 0.4 and 3.5 microns - Comparison of theory and experiment

A model of the lower atmosphere of Mars has been constructed that combines aerosol absorption and scattering with a line-by-line analysis of CO2 and H2O in a multilayer radiative transfer program. Aerosol composition previously inferred from the NASA Lear Jet Observatory data was used to measure the optical complex indices of refraction of appropriate Martian analogs from 0.4 to 2.5 microns. The aerosol vertical particle density scale was deduced using the Viking camera observations of the soil and sky intensities between 0.4 and 1.0 microns in comparison with those modeled using a multilayer Mie scattering program. A comparison of observed Mars atmospheric absorptions was made with those obtained using Lorentz, Voigt, and Doppler line profiles in a multilayer model of the CO2 and H2O. The Voigt line profile of CO2 absorption at approximately 4976 kaysers was then combined in a multilayer aerosol model of the Martian atmosphere. An evaluation of the effect on the line shape was made using several aerosol loadings.

Egan, W. G.↗

NASA NTRS1978

Comparison of theoretical and experimental solar cell performance

This paper presents the results of a detailed comparison between the theoretically predicted performance of silicon solar cells and the experimentally observed performance. The comparison has included the dark I-V characteristics, the spectral response characteristics and the I-V characteristics under AM0 illumination. In general it has been found that the agreement between theory and experimental behavior is very good. This good agreement has been obtained for a variety of cells fabricated with junction depths ranging from 0.1 micron to more than 1 micron and for base layer resistivities from 10 to 0.1 ohm-cm.

Fang, C. R.↗

NASA NTRS1979

A comparison of GLAS SAT and NMC high resolution NOSAT forecasts from 19 and 11 February 1976

A subjective comparison of the Goddard Laboratory for Atmospheric Sciences (GLAS) and the National Meteorological Center (NMC) high resolution model forecasts is presented. Two cases where NMC's operational model in 1976 had serious difficulties in forecasting for the United States were examined. For each of the cases, the GLAS model forecasts from initial conditions which included satellite sounding data were compared directly to the NMC higher resolution model forecasts, from initial conditions which excluded the satellite data. The comparison showed that the GLAS satellite forecasts significantly improved upon the current NMC operational model's predictions in both cases.

Atlas, R.↗

NASA NTRS1980

Comparison of finite element analysis of a piping tee using NASTRAN and CORTES/SA

A comparison of finite element analyses of a piping tee was made using NASTRAN and CORTES/SA, a modified version of SAP3 having a special purpose input processor for generating geometries for a wide variety of tee joints. Four finite element models were subjected in force, moment, and pressure loadings. Flexibility factors and principal stresses were computed for each model and compared with results obtained experimentally by Combustion Engineering, Inc. Results from the NASTRAN analyses were in good agreement with experimental results for all loadings except internal pressure. The CORTES/SA analyses gave good results for the internal pressure loading, but poorer results for out of plane bending moments or forces resulting in out of plane bending. Two of the basic load cases in CORTES/SA were found to contain errors that could not be easily corrected. COST COMPARison of NASTRAN and CORTES/SA showed NASTRAN to be less expensive to two than CORTES/SA for identical meshes.

Quezon, A. J.↗

NASA NTRS1980

Comparison of inlet suppressor data with approximate theory based on cutoff ratio

This paper represents the initial quantitative comparison of inlet suppressor far-field directivity suppression with that predicted using an approximate liner design and evaluation method based upon mode cutoff ratio. The experimental data was obtained using a series of cylindrical point-reacting inlet liners on an Avco-Lycoming YF102 engine. The theoretical prediction program is based upon simplified sound propagation concepts derived from exact calculations. These indicate that all of the controlling phenomenon can be approximately correlated with mode cutoff ratio which itself is intimately related to the angles of propagation within the duct. The objective of the theory-data comparisons is to point out possible deficiencies in the approximate theory which may be corrected. After all theoretical refinements have been made, then empirical corrections can be applied.

Rice, E. J.↗

NASA NTRS1980

Experimental and analytical comparison of aerodynamic characteristics of a forward-control missile

Comparisons of analytical and experimental aerodynamic data for canard controlled missile configurations are presented. Recently, techniques to estimate the longitudinal, directional and lateral aerodynamic characteristics for cruciform missiles have been developed. Nielsen Engineering and Research, Inc. (NEAR, Inc.), supported by various governmental agencies, has been the originator of many of these new computational techniques. Two of these are major computer programs currently being implemented by several research organizations. Predicted data from these two programs are compared with experimental data recently obtained at the NASA Langley Research Center Unitary Plan wind tunnel facility. Comparisons cover the supersonic Mach number regime of 1.60 to 3.50, angles-of-attack from 0 to 20 degrees and roll angles of 0, 26.57 and 45 degrees. Major emphasis is on the roll characteristics due to aileron with limited longitudinal and directional characteristics addressed.

Blair, A. B.↗

NASA NTRS1980

A comprehensive comparison between experiment and prediction for a transonic turbulent separated flow

Attempts to predict surface pressure distributions on lifting surfaces have been relatively unsuccessful in the transonic regime when the shock wave is of sufficient strength to produce an extensive region of turbulent separated flow. For these conditions, the viscous flow behavior must be accurately described even to obtain reasonable predictions of surface pressure. The present paper addresses this problem. Detailed comparisons between prediction and experiment are made for a transonic, turbulent boundary-layer separation (freestream Mach number = 0.875) for which the turbulent flow properties (including the turbulent Reynolds stress) had been measured by the laser velocimeter technique from upstream of the separated region through reattachment. The flow was generated on an axisymmetric 'bump' model designed to simulate the flow on an airfoil at transonic conditions. The numerical methods used in the comparisons include the solution of the time-dependent, mass-averaged Navier-Stokes equations, and the solution of the compressible boundary-layer equations by the inverse method. Solutions were obtained for the well established Cebeci-Smith algebraic turbulence model and the more recently developed Wilcox-Rubesin two-equation turbulence model.

Johnson, D. A.↗

NASA NTRS1980

Comparison of surface wind stress measurements - Airborne radar scatterometer versus sonic anemometer

Sea surface wind stress measurements recorded by a sonic anemometer are correlated with airborne scatterometer measurements of ocean roughness (cross section of radar backscatter) to establish the accuracy of remotely sensed data and assist in the definition of geophysical algorithms for the scatterometer sensor aboard Seasat A. Results of this investigation are as follows: Comparison of scatterometer and sonic anemometer wind stress measurements are good for the majority of cases; however, a tendency exists for scatterometer wind stress to be somewhat high for higher wind conditions experienced in this experiment (6-9 m/s). The scatterometer wind speed algorithm tends to overcompute the higher wind speeds by approximately 0.5 m/s. This is a direct result of the scatterometer overestimate of wind stress from which wind speeds are derived. Algorithmic derivations of wind speed and direction are, in most comparisons, within accuracies defined by Seasat A scatterometer sensor specifications.

Brucks, J. T.↗

NASA NTRS1980

Synthetic aperture radar imaging of ocean waves - Comparison with wave measurements

Synthetic aperture radar images of ocean waves were obtained in conjunction with reference wave data near Marineland, Florida, December 14, 1975. Each of the various types of measurements were processed into a form that allowed direct comparisons with the others. Maxima of radar spectra occurred at the same frequencies as the maxima of reference wave height spectra. In a comparison of a radar spectrum with observed spectra of wave height, wave orbital velocity, and surface slope the high-frequency portion of the radar spectrum lay near and between the wave height and the orbital velocity spectra but differed significantly from the surface slope spectrum. The radar-derived mean directions and model-fitted directional spreads of wave energy were close to the values from a directional wave buoy and indicated the accuracy of radar measurements of wave direction. However, a directional plot of a radar spectrum near shore at the frequency of the maximum showed a sharper peak than such a plot of a fitted spectrum derived from reference data.

Mcleish, W.↗

NASA NTRS1980

Radar estimation of slant path rain attenuation at frequencies above 10 GHz and comparisons with measured multi-season results

Techniques and results pertaining to estimating Earth satellite path rain attenuation events and statistics using radar at nonattenuating wavelengths are presented. The radar formulation and beam scanning methods are described and the procedure for relating the radar measured Rayleigh reflectivity to the high frequency Mie attenuation coefficient are given. Examples of radar derived single terminal statistics and estimation criteria as they relate to path angle and frequency are reviewed. Radar derived space diversity statistics and their dependence on terminal spacing and frequency are described. Site diversity performance curves obtained by radar and radiometry are compared with each other demonstrating the utility of radar methods. Results of a multi-year experiment to test, refine, and establish accuracies of radar methods for arriving at estimates of rain attenuation along an Earth-satellite path are discussed. Comparisons of measured and radar estimated fade events are presented and found to be good. Comparisons of cumulative fade distributions show agreement to be excellent giving an rms deviation of 1 dB.

Goldhirsh, J.↗

NASA NTRS1980

Comparison of Space Shuttle Orbiter low-speed static stability and control derivatives obtained from wind-tunnel and approach and landing flight tests

Tests were conducted in the 8 foot transonic pressure tunnel to obtain wind tunnel data for comparison with static stability and control parameters measured on the space shuttle orbiter approach and landing flight tests. The longitudinal stability, elevon effectiveness, lateral directional stability, and aileron effectiveness derivatives were determined from the wind tunnel data and compared with the flight test results. The comparison covers a range of angles of attack from approximately 2 deg to 10 deg at subsonic Mach numbers of 0.41 to 0.56. In general the wind tunnel results agreed well with the flight test results, indicating the wind tunnel data is applicable to the design of entry vehicles for subsonic speeds over the angle of attack range studied.

Freeman, D. C., Jr.↗

NASA NTRS1980

Unimolecular decomposition reactions at low-pressure: A comparison of competitive methods

The lack of a simple rate coefficient expression to describe the pressure and temperature dependence hampers chemical modeling of flame systems. Recently developed simplified models to describe unimolecular processes include the calculation of rate constants for thermal unimolecular reactions and recombinations at the low pressure limit, at the high pressure limit and in the intermediate fall-off region. Comparison between two different applications of Troe's simplified model and a comparison between the simplified model and the classic RRKM theory are described.

Adams, G. F.↗

NASA NTRS1981

A space system for high-accuracy global time and frequency comparison of clocks

A Space Shuttle experiment in which a hydrogen maser clock on board the Space Shuttle will be compared with clocks on the ground using two-way microwave and short pulse laser signals is described. The accuracy goal for the experiment is 1 nsec or better for the time transfer and 10 to the minus 14th power for the frequency comparison. A direct frequency comparison of primary standards at the 10 to the minus 14th power accuracy level is a unique feature of the proposed system. Both time and frequency transfer will be accomplished by microwave transmission, while the laser signals provide calibration of the system as well as subnanosecond time transfer.

Decher, R.↗

NASA NTRS1981

A validation of LTRAN2 with high frequency extensions by comparisons with experimental measurements of unsteady transonic flows

A high frequency extension of the unsteady, transonic code LTRAN2 was created and is evaluated by comparisons with experimental results. The experimental test case is a NACA 64A010 airfoil in pitching motion at a Mach number of 0.8 over a range of reduced frequencies. Comparisons indicate that the modified code is an improvement of the original LTRAN2 and provides closer agreement with experimental lift and moment coefficients. A discussion of the code modifications, which involve the addition of high frequency terms of the boundary conditions of the numerical algorithm, is included.

Hessenius, K. A.↗

NASA NTRS1981

A reattaching free shear layer in compressible turbulent flow - A comparison of numerical and experimental results

An investigation of a two-dimensional, free turbulent shear layer reattaching on an inclined surface at Mach 2.92 and at a high Reynolds number is described. The test geometry is specifically designed to isolate the reattachment process of a high-speed separated flow. A numerical solution of the time-dependent, Reynolds-averaged, Navier-Stokes equations for the entire flow field, employing a two-equation eddy viscosity turbulence model, is presented. Detailed comparisons of prediction and experiment are made in the free shear layer, at reattachment, and in the developing boundary layer downstream. These comparisons include mean surface quantities as well as mean and fluctuating flowfield quantities. Although the overall features of this complex flow field are predicted, there are several deficiencies in the numerical solution, particularly in the region downstream of reattachment. Modifications of the turbulence model to correct these deficiencies are discussed.

Horstman, C. C.↗

NASA NTRS1981

Comparison of analytical and experimental supersonic aerodynamic characteristics of a forward control missile

Techniques to predict the aerodynamic characteristics of slender cruciform missiles have been developed and are constantly being updated and improved. This paper presents comparisons between analytical and experimental supersonic aerodynamic data for a class of canard-controlled missile configurations similar to the sidewinder missile. Three aerodynamic prediction computer codes, including program MISSILE2, a recently improved version of program MISSILE, are evaluated by comparison with the test data to assess their accuracy. The major emphasis is placed on the roll control characteristics. In addition, tail span optimization, longitudinal and lateral control, induced roll, and missile roll orientation effects are addressed.

Allen, J. M.↗

NASA NTRS1981

Qualitative comparison of calculated turbulence responses with wind-tunnel measurements for a DC-10 derivative wing with an active control system

This paper presents comparisons of analytically-predicted and experimental turbulence responses of a wind-tunnel model of a DC-10 derivative wing equipped with an active control system. The active control system was designed for the purpose of flutter suppression, but it had the additional benefit of alleviating gust loads (wing bending moment) by about 25 percent. Comparisons of various wing responses are presented for variations in active-control-system parameters and tunnel speed. The analytical turbulence responses were obtained using DYLOFLEX, a computer program for dynamic loads analyses of flexible airplanes with active controls. In general, the analytical predictions agreed reasonably well with the experimental data.

Perry, B., III↗

NASA NTRS1981

Comparison of analysis and flight test data for a drone aircraft with active flutter suppression

This paper presents a comparison of analysis and flight test data for a drone aircraft equipped with an active flutter suppression system. Emphasis is placed on the comparison of modal dampings and frequencies as a function of Mach number. Results are presented for both symmetric and antisymmetric motion with flutter suppression off. Only symmetric results are presented for flutter suppression on. Frequency response functions of the vehicle are presented from both flight test data and analysis. The analysis correlation is improved by using an empirical aerodynamic correction factor which is proportional to the ratio of experimental to analytical steady-state lift curve slope. In addition to presenting the mathematical models and a brief description of existing analytical techniques, an alternative analytical technique for obtaining closed-loop results is presented.

Newsom, J. R.↗