Search NASA⌕ Search

SEARCH · Search NASA

Results for “experimental studies”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1961

Experimental Study of Combined Forced and Free Laminar Convection in a Vertical Tube

An apparatus was built to verify an analysis of combined forced and free convection in a vertical tube with uniform wall heat flux and to determine the limits of the analysis. The test section was electrically heated by resistance heating of the tube wall and was instrumented with thermocouples in such a way that detailed thermal entrance heat-transfer coefficients could be obtained for both upflow and downflow and any asymmetry in wall temperature could be detected. The experiments showed that fully developed heat-transfer results, predicted by a previous analysis, were confirmed over the range of Rayleigh numbers investigated. The concept of "locally fully developed" heat transfer was established. This concept involves the assumption that the fully developed heat-transfer analysis can be applied locally even though the Rayleigh number is varying along the tube because of physical-property variations with temperature. Thermal entrance region data were obtained for pure forced convection and for combined forced and free convection. The analysis of laminar pure forced convection in the thermal entrance region conducted by Siegel, Sparrow, and Hallman was experimentally confirmed. A transition to an eddy motion, indicated by a fluctuation in wall temperature was found in many of the upflow runs. A stability correlation was found. The fully developed Nusselt numbers in downflow were below those for pure forced convection but fell about 10 percent above the analytical curve. Quite large circumferential variations in wall temperature were observed in downflow as compaired with those encountered in upflow, and the fully developed Nussalt numbers reported are based on average wall temperatures determined by averaging the readings of two diametrically opposite wall thermocouples at each axial position. With larger heating rates in downflow the wall temperature distributions strongly suggested a cell flow near the bottom. At still larger heating rates the wall temperatures varied in a periodic way.

Hallman, Theodore M.↗
NASA NTRS1999

Experimental Studies of Mars-Analog Brines

Evaporite deposits may represent significant sinks of mobile cations (e.g., those of Ca, N, Mg, and Fe) and anions (e.g., those of C, N, S, and Cl) among the materials composing the Martian surface and upper crust. Carbon and nitrogen are especially interesting because of their role as atmospheric gases which can become incorporated into crustal rocks. However, the nature of evaporite precursor brines formed under Martian conditions is poorly understood. To date, only a very limited number of laboratory investigations have been reported which have any bearing on a better understanding of various processes related to brine or evaporite formation on Mars. Here we report on preliminary laboratory experiments that exposed igneous minerals analogous to those in Martian (Shergottites, Nakhlites, and Chassigny (SNC) group) meteorites to a simulated Martian atmosphere and pure, deoxygenated water. Analysis of the water over intervals of time approaching 1 year showed that atmospheric gases dissolved to form carbonate and nitrate ions while minerals dissolved to form sulfate and chloride along with various cations. On an annual basis, ion formation gave a carbonate/sulfate ratio that is comparable to the ratio found among salts in SNC meteorites. The sulfate/chloride ratio of the experimental brines is higher than in SNC meteorites but lower than in surface soils measured at the Viking and Pathfinder landing sites.

Moore, Jeffrey M.↗
NASA NTRS2001

Rock Abrasion and Ventifact Formation on Mars from Field Analog, Theoretical, and Experimental Studies

Rocks observed by the Viking Landers and Pathfinder Lander/Sojourner rover exhibit a suite of perplexing rock textures. Among these are pits, spongy textures, penetrative flutes, lineaments, crusts, and knobs Fluvial, impact, chemical alteration, and aeolian mechanisms have been proposed for many of these. In an effort to better understand the origin and characteristics of Martian rock textures, abraded rocks in the Mojave Desert and other regions have been studied. We find that most Martian rock textures, as opposed to just a few, bear close resemblance to terrestrial aeolian textures and can most easily be explained by wind, not other, processes. Flutes, grooves, and some pits on Mars are consistent with abrasion by saltating particles, as described previously. However, many other rock textures probably also have an aeolian origin. Sills at the base of rocks that generally lie at high elevations, such as Half Dome, are consistent with such features on Earth that are related to moats or soil ramps that shield the basal part of the rock from erosion. Crusts consisting of fluted fabrics, such as those on Stimpy and Chimp, are similar to fluted crusts on Earth that spall off over time. Knobby and lineated rocks are similar to terrestrial examples of heterogeneous rocks that differentially erode. The location of specific rock textures on Mars also gives insight into their origin. Many of the most diagnostic ventifacts found at the Pathfinder site are located on rocks that lie near the crests or the upper slopes of ridges. On Earth, the most active ventifact formation occurs on sloped or elevated topography, where windflow is accelerated and particle kinetic energy and flux are increased. Integrated 0 together, these observations point to significant aeolian 0 modification of rocks on Mars and cast doubt on whether many primary textures resulting from other processes are preserved. Experimental simulations of abrasion in the presence of abundant sand indicate that rocks on Mars should erode at a rate of 7.7 to 210 micrometers/yr. These rates cannot have operated over the entire history of the Pathfinder site or elsewhere on Mars, because craters, knobs, and other obstacles would be quickly worn away. More likely, rock abrasion occurs over short time periods when sand supplies are sufficient and saltation friction speeds are frequently reached. Depletion or exhaustion of sand and a decline in wind fluxes at speeds greater than that of saltation friction will then act to reduce the rate of further abrasion. We are currently engaged in a new set of wind tunnel experiments coupled with theoretical models and field studies that address rock abrasion and ventifact formation on Mars and Earth. These studies have implications for the Noachian, when sand supplies were probably more plentiful and the threshold friction speed was possibly lower because of a more dense atmosphere. Under these conditions, erosion rates from the wind could have been much greater than to day, contributing, along with probable fluvial erosion, to the Noachian landscape that is in limited preservation today.

Bridges, N. T.↗
NASA NTRS1990

Analytical and experimental study of the vibration of bonded beams with a lap joint

A theoretical model to study the flexural vibration of a bonded lap joint system is described in this paper. First, equations of motion at the joint region are derived using a differential element approach. The transverse displacements of the upper and lower beam are considered to be different. The adhesive is assumed to be linearly viscoelastic and the widely used Kelvin-Voight model is used to represent the viscoelastic behavior of the adhesive. The shear force at the interface between the adhesive and the beam is obtained from the simple bending motion equations of the two beams. The resulting equations of motion are combined with the equations of transverse vibration of the beams in the unjointed regions. These are later solved as a boundary value problem to obtain the eigenvalues and eigenvectors of the system. The model can be used to predict the natural frequencies, modal damping ratios, and mode shapes of the system for free vibration. Good agreement between numerical and experimental results was obtained for a system of graphite epoxy beams lap-jointed by an epoxy adhesive.

Rao, M. D.↗
NASA NTRS2014

A Numerical and Experimental Study of Damage Growth in a Composite Laminate

The present study has three goals: (1) perform an experiment where a simple laminate damage process can be characterized in high detail; (2) evaluate the performance of existing commercially available laminate damage simulation tools by modeling the experiment; (3) observe and understand the underlying physics of damage in a composite honeycomb sandwich structure subjected to low-velocity impact. A quasi-static indentation experiment has been devised to provide detailed information about a simple mixed-mode damage growth process. The test specimens consist of an aluminum honeycomb core with a cross-ply laminate facesheet supported on a stiff uniform surface. When the sample is subjected to an indentation load, the honeycomb core provides support to the facesheet resulting in a gradual and stable damage growth process in the skin. This enables real time observation as a matrix crack forms, propagates through a ply, and then causes a delamination. Finite element analyses were conducted in ABAQUS/Explicit(TradeMark) 6.13 that used continuum and cohesive modeling techniques to simulate facesheet damage and a geometric and material nonlinear model to simulate core crushing. The high fidelity of the experimental data allows a detailed investigation and discussion of the accuracy of each numerical modeling approach.

McElroy, Mark↗
NASA NTRS2003

Experimental Study of Arcing on High-Voltage Solar Arrays

The main obstacle to the implementation of a high-voltage solar array in space is arcing on the conductor-dielectric junctions exposed to the surrounding plasma. One obvious solution to this problem would be the installation of fully encapsulated solar arrays which were not having exposed conductors at all. However, there are many technological difficulties that must be overcome before the employment of fully encapsulated arrays will turn into reality. An alternative solution to raise arc threshold by modifications of conventionally designed solar arrays looks more appealing, at least in the nearest future. A comprehensive study of arc inception mechanism suggests that such modifications can be done in the following directions: 1) To insulate conductor-dielectric junction from a plasma environment (wrapthrough interconnects); 2) To change a coverglass geometry (overhang); 3) To increase a coverglass thickness; 4) To outgas areas of conductor-dielectric junctions. The operation of high-voltage array in LEO produces also the parasitic current power drain on the electrical system. Moreover, the current collected from space plasma by solar arrays determines the spacecraft floating potential that is very important for the design of spacecraft and its scientific apparatus. In order to verify the validity of suggested modifications and to measure current collection five different solar array samples have been tested in a large vacuum chamber. Each sample (36 silicon based cells) consists of three strings containing 12 cells connected in series. Thus, arc rate and current collection can be measured on every string independently, or on a whole sample when strings are connected in parallel. The heater installed in the chamber provides the possibility to test samples under temperature as high as 80 C that stimulates the LEO operational temperature. The experimental setup is described below.

Vayner, Boris↗
NASA NTRS2005

Experimental Study of Arcing on High-voltage Solar Arrays

The main obstacle to the implementation of a high-voltage solar array in space is arcing on the conductor-dielectric junctions exposed to the surrounding plasma. One obvious solution to this problem would be the installation of fully encapsulated solar arrays which were not having exposed conductors at all. However, there are many technological difficulties that must be overcome before the employment of fully encapsulated arrays will turn into reality. An alternative solution to raise arc threshold by modifications of conventionally designed solar arrays looks more appealing, at least in the nearest future. A comprehensive study of arc inception mechanism [1-4] suggests that such modifications can be done in the following directions: i) to insulate conductor-dielectric junction from a plasma environment (wrapthrough interconnects); ii) to change a coverglass geometry (overhang); iii) to increase a coverglass thickness; iiii) to outgas areas of conductor-dielectric junctions. The operation of high-voltage array in LEO produces also the parasitic current power drain on the electrical system. Moreover, the current collected from space plasma by solar arrays determines the spacecraft floating potential that is very important for the design of spacecraft and its scientific apparatus. In order to verify the validity of suggested modifications and to measure current collection five different solar array samples have been tested in large vacuum chamber. Each sample (36 silicon based cells) consists of three strings containing 12 cells connected in series. Thus, arc rate and current collection can be measured on every string independently, or on a whole sample when strings are connected in parallel. The heater installed in the chamber provides the possibility to test samples under temperature as high as 80 C that simulates the LEO operational temperature. The experimental setup is described below.

Vayner, Boris↗
NASA NTRS1998

Experimental Study of Lunar and SNC Magmas

The research described in this progress report involved the study of petrological, geochemical and volcanic processes that occur on the Moon and the SNC parent body, generally accepted to be Mars. The link between these studies is that they focus on two terrestrial-type parent bodies somewhat smaller than earth, and the fact that they focus on the role of volatiles in magmatic processes and on processes of magma evolution on these planets. The work on the lunar volcanic glasses has resulted in some exciting new discoveries over the years of this grant. We discovered small metal blebs initially in the Al5 green glass, and determined the significant importance of this metal in fixing the oxidation state of the parent magma (Fogel and Rutherford, 1995). More recently, we discovered a variety of metal blebs in the Al7 orange glass. Some of these Fe-Ni metal blebs were in the glass; others were in olivine phenocrysts. The importance of these metal spheres is that they fix the oxidation state of the parent magma during the eruption, and also indicate changes during the eruption (Weitz et al., 1997) They also yield important information about the composition of the gas phase present, the gas which drove the lunar fire-fountaining. One of the more exciting and controversial findings in our research over the past year has been the possible fractionation of H from D during shock (experimental) of hornblende bearing samples (Minitti et al., 1997). This research is directed at explaining some of the low H2O and high D/H observed in hydrous phases in the SNC meteorites.

Rutherford, Malcolm J.↗
NASA NTRS1990

An experimental study of counter-rotating cores in elliptical galaxies

Recent observational studies (Franx and Illingworth 1987; Jedrzejewski and Schechter 1988; Bender 1988; Illingworth and Franx 1989) have shown that some elliptical galaxies have a small region near the center that rotates in the opposite direction from the outer parts of the galaxy. Often the rotation in the central part is much faster than that in the outer part. A few other galaxies show a small region near the center that rotates in the same direction as the rest of the galaxy, but much faster. Either way, the part near the center that shows a strange pattern of rotation (the 'core') has been interpreted as a distinct dynamical subsystem. Very briefly, the observational data seem to be that anomalies show up in rotation curves near the centers of some elliptical galaxies and that galaxies with these strange rotational properties do not show a photometric signature: there are no noticeable bumps in the brightness profile and no unusual shapes of isophotal contours that would suggest an excess of matter concentrated near the center. No strong color variations have been reported. The puzzle is to learn what we can about elliptical galaxies in general, and about galaxies with strange central regions in particular, from these observational facts. The authors' approach is experimental. They make a guess about the form of the dynamically distinct subsystem, and then build a galaxy model to test experimental consequences such as the amount of matter required to produce observable effects and the length of time over which these effects would remain observable. They sidestep questions about how the galaxy might have gotten to be that way in the first place. That gives them more freedom to explore a variety of suggestions about what kind of dynamical system might give rise to the observed rotational patterns.

Miller, R. H.↗