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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

FIRE aircraft observations of horizontal and vertical transport in marine stratocumulus

A major goal of research on marine stratocumulus is to try to understand the processes that generate and dissipate them. One approach to studying this problem is to investigate the boundary layer structure in the vicinity of a transition from a cloudy to a cloud-free region to document the differences in structure on each side of the transition. Since stratiform clouds have a major impact on the radiation divergence in the boundary layer, the transition from a cloudy to a clear boundary layer is a region of large horizontal inhomogeneity in air temperature and turbulence intensity. This leads to a considerable difference in horizontal and vertical transports between the cloudy and cloud-free regions. Measurements are used from the NCAR Electra aircraft during flights 5 (7 July 1987) and 10 (18 July 1987) of FIRE for this purpose. Flight 5 coincided with a LANDSAT overflight, and was designed to investigate the transition across a well-defined N-S cloud boundary, since the LANDSAT image can document the cloud cover in considerable detail. Turbulence legs were flown about 60 km on both sides of the cloud boundary. Flight 10 was flown at night in an area of scattered small cumuli and broken cloud patches.

Paluch, Ilga R.↗
NASA NTRS1992

Ocean interactions with the base of Amery Ice Shelf, Antarctica

Using a two-dimensional ocean themohaline circulation model, we varied the cavity shape beneath Amery Ice Shelf in an attempt to reproduce the 150-m-thick marine ice layer observed at the 'G1' ice core site. Most simulations caused melting rates which decrease the ice thickness by as much as 400 m between grounding line and G1, but produce only minor accumulation at the ice core site and closer to the ice front. Changes in the sea floor and ice topographies revealed a high sensitivity of the basal mass balance to water column thickness near the grounding line, to submarine sills, and to discontinuities in ice thickness. Model results showed temperature/salinity gradients similar to observations from beneath other ice shelves where ice is melting into seawater. Modeled outflow characteristics at the ice front are in general agreement with oceanographic data from Prydz Bay. We concur with Morgan's inference that the G1 core may have been taken in a basal crevasse filled with marine ice. This ice is formed from water cooled by ocean/ice shelf interactions along the interior ice shelf base.

Hellmer, Hartmut H.↗
NASA NTRS1982

Methane and nonmethane hydrocarbon concentrations in the Atlantic marine boundary layer

The reactions of reduced tropospheric trace gases with hydroxyl radicals are thought to play a significant role in global tropospheric photochemistry. Since tropospheric methane and nonmethane hydrocarbon (NMHC) concentrations will affect the tropospheric hydroxyl radical concentrations, accurate measurements of tropospheric CH4 and NMHC concentrations are necessary. Such measurements were, therefore, included in the first leg (from Hamburg, West Germany to Montevideo, Uruguay) of the 1980-1981 Antarctic Expedition. On the basis of the obtained data, a pronounced gradient between northern and southern hemispheric methane concentrations was observed in the marine boundary layer in the region of the intertropical convergence zone. Methane concentrations gradually decreased from about 1.70 ppmv at 40 deg N latitude to about 1.68 ppmv at 14 deg N latitude, then dropped sharply between 14 deg N and 10 deg N latitude to about 1.61 ppmv. NMHC data are also discussed.

Cofer, W. R., III↗
NASA NTRS1996

Methyl bromide: ocean sources, ocean sinks, and climate sensitivity

The oceans play an important role in the geochemical cycle of methyl bromide (CH3Br), the major carrier of O3-destroying bromine to the stratosphere. The quantity of CH3Br produced annually in seawater is comparable to the amount entering the atmosphere each year from natural and anthropogenic sources. The production mechanism is unknown but may be biological. Most of this CH3Br is consumed in situ by hydrolysis or reaction with chloride. The size of the fraction which escapes to the atmosphere is poorly constrained; measurements in seawater and the atmosphere have been used to justify both a large oceanic CH3Br flux to the atmosphere and a small net ocean sink. Since the consumption reactions are extremely temperature-sensitive, small temperature variations have large effects on the CH3Br concentration in seawater, and therefore on the exchange between the atmosphere and the ocean. The net CH3Br flux is also sensitive to variations in the rate of CH3Br production. We have quantified these effects using a simple steady state mass balance model. When CH3Br production rates are linearly scaled with seawater chlorophyll content, this model reproduces the latitudinal variations in marine CH3Br concentrations observed in the east Pacific Ocean by Singh et al. [1983] and by Lobert et al. [1995]. The apparent correlation of CH3Br production with primary production explains the discrepancies between the two observational studies, strengthening recent suggestions that the open ocean is a small net sink for atmospheric CH3Br, rather than a large net source. The Southern Ocean is implicated as a possible large net source of CH3Br to the atmosphere. Since our model indicates that both the direction and magnitude of CH3Br exchange between the atmosphere and ocean are extremely sensitive to temperature and marine productivity, and since the rate of CH3Br production in the oceans is comparable to the rate at which this compound is introduced to the atmosphere, even small perturbations to temperature or productivity can modify atmospheric CH3Br. Therefore atmospheric CH3Br should be sensitive to climate conditions. Our modeling indicates that climate-induced CH3Br variations can be larger than those resulting from small (+/- 25%) changes in the anthropogenic source, assuming that this source comprises less than half of all inputs. Future measurements of marine CH3Br, temperature, and primary production should be combined with such models to determine the relationship between marine biological activity and CH3Br production. Better understanding of the biological term is especially important to assess the importance of non-anthropogenic sources to stratospheric ozone loss and the sensitivity of these sources to global climate change.

NASA Discipline Exobiology↗
NASA NTRS2005

Planetary Geophysics and Tectonics

The broad objective of this work is to improve understanding of the internal structures and thermal and stress histories of the solid planets by combining results from analytical and computational modeling, and geophysical data analysis of gravity, topography and tectonic surface structures. During the past year we performed two quite independent studies in the attempt to explain the Mariner 10 magnetic observations of Mercury. In the first we revisited the possibility of crustal remanence by studying the conditions under which one could break symmetry inherent in Runcorn's model of a uniformly magnetized shell to produce a remanent signal with a dipolar form. In the second we applied a thin shell dynamo model to evaluate the range of intensity/structure for which such a planetary configuration can produce a dipole field consistent with Mariner 10 results. In the next full proposal cycle we will: (1) develop numerical and analytical and models of thin shell dynamos to address the possible nature of Mercury s present-day magnetic field and the demise of Mars magnetic field; (2) study the effect of degree-1 mantle convection on a core dynamo as relevant to the early magnetic field of Mars; (3) develop models of how the deep mantles of terrestrial planets are perturbed by large impacts and address the consequences for mantle evolution; (4) study the structure, compensation, state of stress, and viscous relaxation of lunar basins, and address implications for the Moon s state of stress and thermal history by modeling and gravity/topography analysis; and (5) use a three-dimensional viscous relaxation model for a planet with generalized vertical viscosity distribution to study the degree-two components of the Moon's topography and gravity fields to constrain the primordial stress state and spatial heterogeneity of the crust and mantle.

Zuber, Maria↗
NASA NTRS1991

The solar wind interaction with Mars - Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations of bow shock position and shape

An aggregate Mars bow shock data set using Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations has been analyzed. The results support the earlier conclusion that the mean distance to the subsolar shock at Mars is nearly 1.5 planetary radii, from which gas dynamic models predict an obstacle altitude of 500 km. The Martian bow shock does not appear to vary significantly in shape or altitude with the phase of the solar cycle. The unusually distant dayside bow shock crossings reported by Mars 2 and 3 also appear in the Phobos 3 observations, suggesting that the dayside obstacle can on rare occasions reach altitudes over 1000 km. The Martian bow shock differs from that of Venus in that its mean altitude is greater, it lacks a strong solar cycle variation, and its location is far more variable, including the occurrence of strong bow shocks over the dayside hemisphere at distances at least as great as the orbit of Phobos 2, i.e., 2.8 Mars radii.

Slavin, J. A.↗
NASA NTRS2006

Mode I Toughness Measurements of Core/Facesheet Bonds in Honeycomb Sandwich Structures

Composite sandwich structures will be used in many future applications in aerospace, marine and offshore industries due to the fact that the strength and stiffness to mass ratios surpass any other structural type. Sandwich structure also offers advantages over traditional stiffened panels such as ease of manufacturing and repair. During the last three decades, sandwich structure has been used extensively for secondary structure in aircraft (fuselage floors, rudders and radome structure). Sandwich structure is also used as primary structure in rotorcraft, the most common example being the trailing edge of rotor blades. As with other types of composite construction, sandwich structure exhibits several types of failure mode such as facesheet wrinkling, core crushing and sandwich buckling. Facesheet/core debonding has also been observed in the marine and aerospace industry. During this failure mode, peel stresses applied to an existing facesheet/core debond or an interface low in toughness, results in the facesheet being peeled from the core material, possibly leading to a significant loss in structural integrity of the sandwich panel. In an incident during a test on a liquid hydrogen fuel tank of the X-33 prototype vehicle, the outer graphite/epoxy facesheet and honeycomb core became debonded from the inner facesheet along significant areas, leading to failure of the tank. As a consequence of the accident; significant efforts were made to characterize the toughness of the facesheet/core bond. Currently, the only standardized method available for assessing the quality of the facesheet/core interface is the climbing drum peel test (ASTM D1781). During this test a sandwich beam is removed from a panel and the lip of one of the facesheets is attached to a drum, as shown in Fig. 1. The drum is then rotated along the sandwich beam, causing the facesheet to peel from the core. This method has two major drawbacks. First, it is not possible to obtain quantitative fracture data from the test and so the results can only be used in a qualitative manner. Second, only sandwich structure with thin facesheets can be tested (to facilitate wrapping of the facesheet around the climbing drum). In recognition of the need for a more quantitative facesheet/core fracture test, several workers have devised experimental techniques for characterizing the toughness of the facesheet/core interface. In all of these cases, the tests are designed to yield a mode I-dominated fracture toughness of the facesheet/core interface in a manner similar to that used to determine mode I fracture toughness of composite laminates. In the current work, a modified double cantilever beam is used to measure the mode I-dominated fracture toughness of the interface in a sandwich consisting of glass/phenolic honeycomb core reinforced with graphite epoxy facesheets. Two specimen configurations were tested as shown in Fig 2. The first configuration consisted of reinforcing the facesheets with aluminum blocks (Fig. 2a). In the second configuration unreinforced specimens were tested (Fig. 2b). Climbing drum peel tests were also conducted to compare the fracture behavior observed between this test and the modified double cantilever beam. This paper outlines the test procedures and data reduction strategies used to compute fracture toughness values from the tests. The effect of specimen reinforcement on fracture toughness of the facesheet/core interface is discussed.

Nettles, Alan T.↗