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At least 145 records · Page 8

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NASA NTRS1976

A photon counting array photometer using an intensified charge coupled device

An array photometer which uses an intensified charge coupled device was developed. This system, the University of Maryland Array Photometer (UMAP), has the ability to discriminate single photoelectron events in real time at a 4 Megahertz rate. The photosensor uses an S-20 photocathode and a Fairchild CCD202 with a 100 by 100 array of separate individual channels. The overall system has demonstrated a noise level which permits single photoelectron discrimination, a scan rate of 400 frames per second, a very linear response, and very low lag. The UMAP system has a very large dynamic range exceeding 10,000 in the photon counting mode. A special technique to extend the dynamic range to 1,000,000 was described. The performance parameters of this system, both in the laboratory and on the telescope were cited. Data from these observations were discussed to illustrate the system sensitivity, which is basically limited by the pixel size in the array, rather than the electrooptics.

Currie, D. G.↗

NASA NTRS1979

Simulation of liquid propellant rocket engine combustion instabilities

A simulation technique for studying the high frequency combustion instabilities of liquid propellant rocket engines has been developed and used to investigate various aspects of instability phenomena. Of importance was investigation of the significance of the method of coupling the combustion and the gas dynamics of the system. Two coupling processes were studied: linear response of the combustion process to pressure fluctuations, and the nature of the resulting instabilities; and nonlinear response of the combustion process to velocity fluctuations, and the nature of the resulting instabilities. For the combustion model studied, nonlinear (velocity) coupling was found to more closely characterize liquid propellant instabilities.

Ventrice, M. B.↗

NASA NTRS1978

FASOR - A second generation shell of revolution code

An integrated computer program entitled Field Analysis of Shells of Revolution (FASOR) currently under development for NASA is described. When completed, this code will treat prebuckling, buckling, initial postbuckling and vibrations under axisymmetric static loads as well as linear response and bifurcation under asymmetric static loads. Although these modes of response are treated by existing programs, FASOR extends the class of problems treated to include general anisotropy and transverse shear deformations of stiffened laminated shells. At the same time, a primary goal is to develop a program which is free of the usual problems of modeling, numerical convergence and ill-conditioning, laborious problem setup, limitations on problem size and interpretation of output. The field method is briefly described, the shell differential equations are cast in a suitable form for solution by this method and essential aspects of the input format are presented. Numerical results are given for both unstiffened and stiffened anisotropic cylindrical shells and compared with previously published analytical solutions.

Cohen, G. A.↗

NASA NTRS1978

Fiber optic rotation sensor /FORS/ laboratory performance evaluation

A single axis rotation sensor, utilizing a 200 m single mode optical fiber in a 30 cm diameter coil has been constructed and tested in the laboratory. A He-Ne laser is the light source for the Sagnac interferometer-mode sensor. The outputs of Si detectors viewing two complementary interference patterns are electronically differenced to obtain a rotation-dependent signal. Linear response over a rotation rate range of about 0.0001 to 0.1 rad/sec was obtained.

Goldstein, R.↗

NASA NTRS1979

Some recent results in quiescent prominence spectrometry

This paper describes some recent results of a quiescent-prominence spectrometry program. The observations were made with a 25-cm coronagraph/coude spectrograph system using a silicon vidicon detector. This detector consists of 500 contiguous channels covering approximately 6 or 80 A, depending on the grating used. The instrument is interfaced to a computer system and has the important advantages of wide spectral response, linearity, and signal averaging with real-time display. Its principal drawback is the relatively small target size. For the present work, the aperture was about 3 x 5 arcsec. Absolute intensity calibrations were made by measuring quiet regions near the center of the sun.

Landman, D. A.↗

NASA NTRS1979

The excitation of density waves at the Lindblad and corotation resonances by an external potential

The linear response of a differentially rotating two-dimensional gas disk, both with and without self-gravity, to a rigidly rotating external potential is calculated on the assumptions that the speed of sound is much smaller that the orbital velocity and that the external potential varies on the scale of the disk radius. The results show that: (1) the external potential exerts torques on the disk only at the Lindblad and corotation resonances; (2) the torque is positive at the outer Lindblad resonance and negative at the inner Lindblad resonance; (3) the torque at corotation has the sign of the radial vorticity gradient; and (4) the torques are of the same order of magnitude at both types of resonance and independent of the speed of sound in the disk. It is found that the external potential also excites density waves in the vicinity of the Lindblad and corotation resonances, that the long trailing wave is excited at a Lindblad resonance, and that short trailing waves are excited at the corotation resonance. The behavior of particle disks is briefly discussed, and the external torques on particle disks are proven to be identical to those on gas disks

Goldreich, P.↗

NASA NTRS1980

Collisional redistribution of radiation. III - The equation of motion for the correlation function and the scattered spectrum

Computations were made of the scattering of monochromatic radiation by a degenerate atom in the binary-collision approximation for field strengths whose products of the Rabi frequency for atomic transition and the duration of a strong collision are much less than 1. An expression of motion for the correlation function is derived which does not exclude the region where thermal correlations may be neglected; the equation is valid outside the quantum-regression regime, and has a straightforward solution for practical cases. Solutions for the weak-field linear response regime are presented in terms of generalized absorption and emission profiles which depend on the indices of the atomic multipoles.

Burnett, K.↗

NASA NTRS1982

Blade loss transient dynamic analysis of turbomachinery

This paper reports on work completed to develop an analytical method for predicting the transient non-linear response of a complete aircraft engine system due to the loss of a fan blade, and to validate the analysis by comparing the results against actual blade loss test data. The solution, which is based on the component element method, accounts for rotor-to-casing rubs, high damping and rapid deceleration rates associated with the blade loss event. A comparison of test results and predicted response show good agreement except for an initial overshoot spike not observed in test. The method is effective for analysis of large systems.

Stallone, M. J.↗

NASA NTRS1982

Aerodynamic characteristics of a large-scale, twin tilt-nacelle V/STOL model

A subsonic tilt-nacelle V/STOL aircraft configuration is described which is powered by two turbofan engines, mounted on a single carry-through structure that is designed to maintain the thrust axis close to the center of gravity at nacelle incidences of 5-95 deg. Control during V/STOL operation, often from small ship platforms, is achieved by means of a vane assembly that is immersed in each turbofan's exhaust. Wind tunnel test data have been obtained with an 11.2-m wing span model for the vertical-to-horizontal flight transition capability of the concept. It is found that the aircraft can operate over a broad transition corridor, with sufficient maneuver capability about the trim points. The control vane exhibited linear response characteristics over a large deflection range, with little influence from power, angle of attack or ground proximity in hover. The model induced a positive ground effect in hover that increased as wheel height was approached and strake angle increased.

Falarski, M. D.↗

NASA NTRS1982

Water Raman normalization of airborne laser fluorosensor measurements - A computer model study

The technique for normalizing airborne lidar measurements of chlorophyll fluoresence by the water Raman scattering signal is investigated for laser-excitation wavelengths of 480 and 532 nm using a semianalytic Monte Carlo methodology (SALMON). The signal-integration depth for chlorophyll fluorescence Z(90,F), is found to be insensitive to excitation wavelength and ranges from a maximum of 4.5 m in clearest waters to less than 1 m at a chlorophyll concentration of 20 microgram/liter. For excitation at 532 nm, the signal-integration depth for Raman scattering, Z(90,R), is comparable to Z(90,F). For excitation at 480 nm, Z(90,R) is four times as large as Z(90,F) in clearest waters but nearly equivalent at chlorophyll concentrations greater than 2-3 microgram/liter. Absolute signal levels are stronger with excitation at 480 nm than with excitation at 532 nm, but this advantage must be weighed against potential ambiguities resulting from different integration depths for the fluorescence and Raman scattering signals in clearer waters. To the precision of the simulations, Raman normalization produces effectively linear response to chlorophyll concentration for both excitation wavelengths.

Poole, L. R.↗

NASA NTRS1983

Bathymetric prediction from Seasat altimeter data

The linear response function technique is used to analyze two 1300 km tracks of Seasat altimeter data and corresponding bathymetry in the Musician Seamounts region north of Hawaii. A predictive filter is developed which can operate on Seasat altimetry in poorly surveyed oceanic regions to indicate the presence of major bathymetric anomalies. Modelling of the bathymetry-geoid correlation in the Musician region is attempted using the elastic plate model. The flexural rigidity of the plate is not well constrained by the data but appears to be in the range 5 x 10 to the 21st N m up to 5 x 10 to the 22nd N m at the time of loading. Since the Musician Seamounts and the crust on which they lie are both Late Cretaceous in age, this value represents the effective flexural rigidity of very young lithosphere that was 'frozen in' at the time of volcanism. The modelling indicates that the general form of a predictive filter will strongly depend on various geologic parameters, especially the effective flexural rigidity.

Dixon, T. H.↗

NASA NTRS1983

Ion-acoustic solitary waves in a magnetized plasma with arbitrary electron equation of state

The oblique propagation of fully nonlinear, slow ion-acoustic solitary waves in a collisionless, low-beta, magnetized plasma is examined. The analysis includes the effects of a finite ion pressure, electron trapping, and multicomponent particle populations. The existence of both compressive and rarefactive modes propagating obliquely to the magnetic field in a plasma with two distinct Boltzmann electron populations and cold ions is demonstrated. It is shown that paired electrostatic shocks observed over the earth's auroral zone may be closely related to the rarefactive modes. As a measure of the collisionless dissipation encountered by the solitary waves, the linear response of the plasma to slow ion-acoustic waves is also examined.

Witt, E.↗

NASA NTRS1984

On the dynamics of equatorial forcing of climate teleconnections

A number of observational, theoretical, and modeling studies have been conducted on the remote influence of the tropics on the midlatitudes. Teleconnection in the geopotential height field between the tropics and the extratropics associated with the Southern Oscillation in the so-called Pacific-North America (PNA) pattern, has often been cited as an example of such remote influence. Simmons et al. (1983) have shown that teleconnections such as PNA may arise both from barotropic instability of the extratropical zonal flow and from tropical forcing. The present investigation is only concerned with the dynamics of the part of the atmospheric teleconnection which arises from tropical forcing. Some fundamentals of Rossby wave theory in unsheared flow are discussed along with the linear response to equatorial forcing for horizontally, sheared zonal mean flow. A global shallow-water spectral model is employed in conjunction with a theoretical analysis to show the teleconnection patterns which develop under different large-scale mean circulation conditions.

Lau, K.-M.↗

NASA NTRS1984

Friction and particle-hole pairs

The effect induced by dissipation on quantum phenomena has recently been considered, taking into account as a starting point a phenomenological Hamiltonian in which the environment is simulated by an appropriately chosen set of harmonic oscillators. It is found that this approach should be adequate to describe the low-energy behavior of a wide class of environments. The present investigation is concerned with an analysis of the case in which the environment is a gas (or liquid) of fermions, and the relevant low-energy excitations are particle-hole pairs. A study is conducted regarding the extent to which the quantum results obtained for harmonic oscillators are also valid in the considered situation. Linear-response theory is used to derive an effective action which describes the motion of an external particle coupled to a normal Fermi fluid.

Guinea, F.↗

NASA NTRS1984

Photodiodes for Astronomical Stellar Radiometry

The theory of photodiodes and their application to radiometric measurements is reviewed. Some suggestions concerning photodiode detectors for use in stellar radiometric measurements are given. Quantum efficiency, linear response characteristics, and noise current values of silicon photodiodes are considered.

Schaefer, A. R.↗

NASA NTRS1984

Secular rotational motions and the mechanical structure of a dynamical viscoelastic earth

A survey is presented of analytical methods for computing the linear responses of the rotational axis of a layered viscoelastic earth to surface loading. Theoretical research in this area is first summarized, and the differences between the mechanical boundary conditions to be applied at the interface separating the upper and lower mantles for an adiabatically and chemically stratified mantle are discussed. Some examples of polar wander and secular variation of the spin rate from glacial excitation are presented for various types of chemical and viscosity stratifications. The effects of an artificial density jump at the base of the lithosphere in models are examined, and certain issues concerning the fluid tidal Love number for different types of density stratification are addressed. The meaning of effective plate thickness over geological time scales for rotational dynamics is discussed.

Yuen, D. A.↗

NASA NTRS1985

The exact thermal rotational spectrum of a two-dimensional rigid rotor obtained using Gaussian wave packet dynamics

The exact thermal rotational spectrum of a two-dimensional rigid rotor is obtained using Gaussian wave packet dynamics. The spectrum is obtained by propagating, without approximation, infinite sets of Gaussian wave packets. These sets are constructed so that collectively they have the correct periodicity, and indeed, are coherent states appropriate to this problem. Also, simple, almost classical, approximations to full wave packet dynamics are shown to give results which are either exact or very nearly exact. Advantages of the use of Gaussian wave packet dynamics over conventional linear response theory are discussed.

Reimers, J. R.↗

NASA NTRS1985

Observations on the propagation, growth, and predictability of Gulf Stream meanders

During a three-year period, Gulf Stream positions determined by satellite infrared imagery on a grid northeast of Cape Hatteras have been correlated against 'inlet' path parameters (displacement from the mean position, angle, and curvature) monitored by an array of inverted echo sounders. By cross-spectral calculations between these measurements, the downstream coherences, phase speeds, and spatial growth rates were determined. The downstream path predictability from these inlet parameters was tested in a multiple input linear response model. The most energetic meanders, with periods of 33-50 days, remained highly coherent for 300 km downstream, roughly the dominant wavelength, with marginal coherence again at 500-575 km. Phase speeds are frequency dependent and decrease with distance downstream. For the 50- (33) day meanders, phase speeds decreased from 10 (17) km/d at the inlet to 6 (8) km/d at a distance downstream of about 550 km. The average growth rate (spatial e-folding wave number) over the entire region is 1.3 x 10 to the -3rd per km. Within 300 km of the inlet, the multiple coherence accounts for 55-65 percent of the total path-displacement variance, with displacement and angle being the best individual predictors in the first and last halves of this region, respectively.

Vazquez, J.↗