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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
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NASA NTRS1991

The wavelength of supercritical surface tension driven Benard convection

The size or the wavelength of moderately supercritical surface tension driven Benard convection has been investigated experimentally in a thin fluid layer of large aspect ratio. It has been found that the number of the hexagonal convection cells increases with increased temperature differences, up to 1.3 times the critical temperature difference. That means that the wavelength of surface tension driven convection decreases after onset of the instability for moderately nonlinear conditions. This result is in striking contrast to the well-known increase of the wavelength of buoyancy driven Rayleigh-Benard convection.

Koschmieder, E. L.↗
NASA NTRS1990

The wavelength dependence of broadband linear polarization in cool stars

A model is developed to investigate the effect of wavelength dependence of magnetically generated broadband linear polarization (BLP) in late-type stars as a function of line blanketing, observation filters, and other variables. Linear polarization is assumed to be a single 'average' line and the line opacity, magnetic area filling factor, and continuum intensity are incorporated to establish an equation for disk-generated BLP. The method is investigated by computing special models for the flare star BY Dra and the sun which are compared to previous calculations. The model is shown to agree with the calculations of BLP, and the magnetic BLP dependent on wavelengths varies significantly from the simple power law dependences on wavelength. The model sets lower limits to the area occupied by magnetic regions, and an upper limit to the expected BLP also results from the formulation.

Saar, Steven H.↗
NASA NTRS1992

Multi-wavelength Airborne Lidar Intercomparisons of Aerosol and Cirrus Backscatter over the Pacific Ocean

Airborne CO2 and Nd:YAG lidar instruments were flown on the NASA DC-8 research aircraft during two extended missions over the Pacific. The objectives during these flights were to characterize aerosol backscatter levels at infrared wavelengths in relatively pristine regions of the troposphere, and to observe cloud effects on lidar signals. In addition, the three wavelength Nd:YAG data were processed using independent temperature profile information, in order to deduce the Rayleigh scattering component of the total backscatter at each wavelength. The data can be used to better understand the capabilities of various airborne or Earth-orbiting lidar designs to provide backscatter information from aerosols and clouds, or to provide Doppler or DIAL information from aerosol and cloud backscatter.

Menzies, Robert T.↗
NASA NTRS1992

Three-wavelength Lidar Measurements of Pinatubo Aerosol and Its Optical Properties

Enhanced stratospheric aerosols due to Mt. Pinatubo eruption have been measured using a YAG laser-based three wavelength lidar and a YAG laser-based large-scale lidar. Temporal variation of the integrated backscatter coefficient derived from the backscatter coefficient profiles were obtained. The present paper describes some results of optical properties analysis using lidar data obtained since Dec., 1991 when the main body of aerosols started to appear over Japan. The derived properties of the Pinatubo aerosols are extinction to backscatter ratios, wavelength dependencies of backscatter coefficients and extinction coefficients, and optical thickness. The analysis is based on the assumption of similarity in backscatter profiles for three wavelengths which are derived from lidar signals using the Fernald equation with assumed extinction to backscatter ratios.

Sasano, Y.↗
NASA NTRS1992

Multiple scattering wavelength dependent backscattering of kaolin dust in the IR: Measurements and theory

Knowing the optical properties of aerosol dust is important for designing electro-optical systems and for modeling the effect on propagation of light in the atmosphere. As CO2 lidar technology becomes more advanced and is used for multiwavelength measurements, information on the wavelength dependent backscattering of aerosol dust particles is required. The volume backscattering coefficient of aerosols in the IR is relatively small. Thus, only a few field measurements of backscattering, usually at only a few wavelengths, are reported in the literature. We present spectral field measurements of backscattering of kaolin dust in the 9-11 micron wavelength range. As the quantity of dust increases, multiple scattering contributes more to the measured backscattered signal. The measurements show the effect of the dust quantity of the spectral backscatter measurements. A simple analytical two stream radiative transfer model is applied to confirm the measurements and to give insight to the multiple scattering spectra of backscattering.

Ben-David, Avishai↗
NASA NTRS1991

Far-infrared and submillimeter wavelength observations of star-forming dense cores. II - Images

This paper reports on far-infrared and submillimeter wavelength observations of low-mass protostellar candidates. The data set comprises emission maps of nine sources observed over a wavelength range 100-800 microns. The emission is extended at all wavelengths longer than 100 microns. The apparent size of the emission regions is weakly correlated with the beam size and is consistent with an underlying specific intensity profile which is scale-free. The observed emission maps are not circular, but have a mean aspect ratio of 1.3, and the position angles are similar to those determined from maps of molecular emission. The observational results are used in conjunction with theoretical considerations to constrain the physical properties of the putative protostellar envelopes.

Ladd, E. F.↗
NASA NTRS1991

Four NASA submillimeter-wavelength space-astrophysics missions

For several years studies have been conducted at the NASA/Jet Propulsion Laboratory on four passively-cooled submillimeter-wavelength, space observatories. Two exploratory missions were studied: a 2.5-m Submillimeter Explorer (SMME) and a more ambitious 3.7-m Submillimeter Imager and Line Survey (SMILS). Only one of these missions would actually be flown, and its goal would be to perform a high-spectral-resolution survey of several hundreds of sources at wavelengths between 100 and about 750 microns with modest angular resolution. Following either SMME or SMILS, the Large Deployable Reflector (LDR) and/or the Synthesis Array for Lunar Submillimeter Astronomy (SALSA) would be flown. LDR is a 10- to 20-m diameter telescope with greatly increased sensitivity and imaging capabilities compared to the exploratory missions. SALSA is a lunar-based array consisting of twelve 3.5-m diameter telescopes with a maximum baseline of nearly 1-km. With operating wavelengths between 30 and 500 microns, SALSA would achieve 10 milliarcsecond angular resolution, and thus could explore source structure in much greater detail than the other missions. The purpose of this paper is to present the current conceptual designs for these missions, and to discuss the most recent payload analysis.

Mahoney, M. J.↗
NASA NTRS1991

Optical properties of aggregate particles whose outer diameter is comparable to the wavelength

Results are described of numerical calculations of the optical properties (extinction efficiency, single-scattering albedo, phase function, and linear polarization) of aggregate particles whose outer diameter is comparable with the wavelength. Results are presented for two types of particle, one composed of monomers whose radius is small compared with the wavelength and a second containing monomers with larger radii. The shape of the forward-scattered lobe of the phase function is diagnostic of the mean projected area (but differs from that for an equal-area sphere), while the linear polarization, phase function at large scattering angles, and single-scattering albedo depend on the monomer diameter. The wavelength dependence of the extinction efficiency differs markedly from that for equal-area spheres. These results can be used to infer particle properties from remotely sensed data.

West, Robert A.↗
NASA NTRS1992

Effects of long-wavelength dissipation on beam-driven Langmuir turbulence

The effects of long-wavelength dissipation on beam-driven Langmuir turbulence are investigated using numerical simulations that include both weak and strong turbulence effects. Strong-turbulence wave collapses occur concurrently with weak-turbulence energy cascades if the long-wavelength damping is sufficiently small relative to the growth rate of the beam-unstable waves. Above a threshold damping level, only the weak-turbulence backscatter cascade is observed, and it becomes increasingly truncated as the damping increases, eventually consisting of only a single backscatter. A simple Lotka-Volterra model gives an excellent description of the periodic evolution observed in the weak-turbulence regime. Suppression of the usual backscatter cascade by long-wavelength damping enables intense beam-aligned density troughs to form, which trap and duct Langmuir waves.

Robinson, P. A.↗
NASA NTRS1992

Silicon micromachined waveguides for millimeter and submillimeter wavelengths

The majority of radio receivers, transmitters, and components operating at millimeter and submillimeter wavelengths utilize rectangular waveguides in some form. However, conventional machining techniques for waveguides operating above a few hundred GHz are complicated and costly. This paper reports on the development of silicon micromachining techniques to create silicon-based waveguide circuits which can operate at millimeter and submillimeter wavelengths. As a first step, rectangular WR-10 waveguide structures have been fabricated from (110) silicon wafers using micromachining techniques. The waveguide is split along the broad wall. Each half is formed by first etching a channel completely through a wafer. Potassium hydroxide is used to etch smooth mirror-like vertical walls and LPCVD silicon nitride is used as a masking layer. This wafer is then bonded to another flat wafer using a polyimide bonding technique and diced into the U-shaped half wavelengths. Finally, a gold layer is applied to the waveguide walls. Insertion loss measurements show losses comparable to those of standard metal waveguides. It is suggested that active devices and planar circuits can be integrated with the waveguides, solving the traditional mounting problems. Potential applications in terahertz instrumentation technology are further discussed.

Yap, Markus↗
NASA NTRS1992

Determination of refractive properties of fluids for dual-wavelength interferometry

Methods to calculate the refractive properties of solutions at different wavelengths are described by using experimental data at just two wavelengths. The properties are the refractive index and its gradients with temperature and concentration. Cauchy's equation is used to determine the refractive indices. The gradients versus temperature and concentration are then determined by using the Murphy-Alpert and the Lorentz-Lorenz equation, respectively. Finally, the particular case of a triglycine sulfate aqueous solution is considered as an example. The approach should provide the desired information for fringe analysis when dual-wavelength holographic or other interferometry is used for solving heat and mass transfer problems in fluids during crystal-growth experiments.

Vikram, Chandra S.↗
NASA NTRS1993

Broad-band linear polarization in cool stars. II - Amplitude and wavelength dependence for magnetic and scattering regions

We have developed a model to estimate the amplitude and wavelength dependence of broad-band linear polarization (BLP) from magnetic regions on cool stars. The model includes corrections both for line blends and for the partial cancellation of polarization in the vector sum over the stellar disk. Our results agree with recent calculations of BLP in the red, but show larger amplitudes and a different wavelength dependence in the blue. We find that the detailed wavelength dependence of the polarization is complex and varies with effective temperature and gravity due to changes in line blanketing. The BLP amplitudes depend strongly on field strength, blanketing, and magnetic region filling factor and geometry. We make rough estimates of the maximum BLP for stars of various spectral types and demonstrate a method for deriving a lower limit to the filling factor from the maximum observed BLP. This lower limit is related to the fractional area covered by the spatially asymmetric component of magnetic regions.

Saar, Steven H.↗
NASA NTRS1991

Lithographic spiral antennas at short wavelengths

We have extended the high efficiency of lithographic antennas to mid-infrared wavelengths. Pattern measurements made at 9.5/zm wavelength on a 65 deg, self-complementary, spiral antenna exhibit a ratio of response to orthogonal linear polarizations of 1.35 dB, a beamwidth of 85 deg (3 dB full width), a directivity of 8.2 dB, and surprisingly, a close resemblance to the theoretical pattern for a 65 deg spiral in free space. Direct detection measurements made with an ambient temperature blackbody source yield an antenna efficiency of 52 +/- 7 percent, when corrected for incomplete filling of the antenna beam by the source, at a mean effective wavelength of 19 micron.

Grossman, E. N.↗
NASA NTRS1992

A liquid-helium-cooled absolute reference cold load for long-wavelength radiometric calibration

Design, radiometric and thermal performance, and operation of a large diameter (78 cm) liquid-helium-cooled blackbody absolute reference cold load (CL) for the calibration of microwave radiometers is described. CL provides an absolute calibration near the liquid-helium (LHe) boiling point, with total uncertainty in the radiometric temperature of less than 30 mK over the 2.5-23 cm wavelength operating range. CL was used at several wavelengths at the South Pole, Antarctica and the White Mountain Research Center, California. Results show that, for the instruments operated at 20-, 12-, 7.9-, and 4.0 cm wavelength at the South Pole, the total corrections to the LHe boiling-point temperature (about 3.8 K) were 48 +/-23, 18 +/-10, 10 +/-18, and 15 +/-mK.

Bensadoun, Marc↗
NASA NTRS1994

Wavelength-Division Multiplexing Of Bipolar Digital Signals

In system, bipolar digital data transmitted by use of wavelength-division multiplexing on single optical fiber. Two different wavelengths used to transmit pulses signifying "positive" or "negative" bipolar digital data. Simultaneous absence of pulses at both wavelengths signifies digital "zero."

Gibbons, Ronnie D.↗
NASA NTRS1994

Long-wavelength asymptotics of unstable crossflow modes, including the effect of surface curvature

Stationary vortex instabilities with wavelengths significantly larger than the thickness of the underlying three-dimensional boundary layer are studied with asymptotic methods. The long-wavelength Rayleigh modes are locally neutral and are aligned with the direction of the local inviscid streamline. For a spanwise wave number Beta much less than 1, the spatial growth rate of these vortices is O(Beta(exp 3/2)). When Beta becomes O(R(exp -1/7)), the viscous correction associated with a thin sublayer near the surface modifies the inviscid growth rate to the leading order. As Beta is further decreased through this regime, viscous effects assume greater significance and dominate the growth-rate behavior. The spatial growth rate becomes comparable to the real part of the wave number when Beta = O(R(exp -1/4)). At this stage, the disturbance structure becomes fully viscous-inviscid interactive and is described by the triple-deck theory. For even smaller values of Beta, the vortex modes become nearly neutral again and align themselves with the direction of the wall-shear stress. Thus, the study explains the progression of the crossflow-vortex structure from the inflectional upper branch mode to nearly neutral long-wavelength modes that are aligned with the wall-shear direction.

Choudhari, Meelan↗
NASA NTRS1993

The wavelength dependence of Martian atmospheric dust radiative properties

One of the key radiative agents in the atmosphere of Mars is the suspended dust particles. A new analysis of two data sets of the Martian atmosphere is being carried out in order to better evaluate the radiative properties of the atmospheric dust particles. The properties of interest are the size distribution, optical constants, and other radiative properties, such as the single-scattering albedo and phase function. Of prime importance is the wavelength dependence of these radiative properties throughout the visible and near-infrared wavelengths. Understanding the wavelength dependence of absorption and scattering characteristics will provide a good definition of the influence that the atmospheric dust has on heating of the atmosphere.

Pollack, J. B.↗