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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 1,549 records · Page 86
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NASA NTRS1990

Evaluation of two transport aircraft and several ground test vehicle friction measurements obtained for various runway surface types and conditions. A summary of test results from joint FAA/NASA Runway Friction Program

Tests with specially instrumented NASA Boeing 737 and 727 aircraft together with several different ground friction measuring devices were conducted for a variety of runway surface types and conditions. These tests are part of joint FAA/NASA Aircraft/Ground Vehicle Runway Friction Program aimed at obtaining a better understanding of aircraft ground handling performance under adverse weather conditions and defining relationships between aircraft and ground vehicle tire friction measurements. Aircraft braking performance on dry, wet, snow and ice-covered runway conditions is discussed as well as ground vehicle friction data obtained under similar runway conditions. For a given contaminated runway surface condition, the correlation between ground vehicles and aircraft friction data is identified. The influence of major test parameters on friction measurements such as speed, test tire characteristics, type and amount of surface contaminant, and ambient temperature are discussed. The effect of surface type on wet friction levels is also evaluated from comparative data collected on grooved and ungrooved concrete and asphalt surfaces.

Yager, Thomas J.↗
NASA NTRS1993

The effect of sub-surface volume scattering on the accuracy of ice-sheet altimeter retracking algorithms

The NASA and ESA retracking algorithms are compared with an algorithm based upon a combined surface and volume (S/V) scattering model. First, the S/V, NASA, and ESA algorithms were used to retrack over 400,000 altimeter return waveforms from the Greenland and Antarctic ice sheets. The surface elevations from the S/V algorithm were compared with the elevations produced by the NASA and ESA algorithms to determine the relative accuracy of these algorithms when subsurface volume-scattering occurs. The results show that the NASA algorithm produced surface elevations within 35 to 50 cm of the S/V algorithm, while the performance of the ESA algorithm was slightly worse. Next, by analyzing several thousand satellite crossover points from the Antarctic data set, we determined the retracking algorithm that produced the most repeatable surface elevations. The elevations derived from the S/V algorithm had the smallest RMS error for the region of the East Antarctic plateau examined here. The ESA algorithm produced erroneous estimates of elevation change when seasonal variations were present; it measured 0.7 to 1.6-m change in elevation over a 6-month period on the East Antarctic plateau where accumulation rates are only 10 cm/year.

Davis, Curt H.↗
NASA NTRS1994

An inquiry into the cirrus-cloud thermostat effect for tropical sea surface temperature

In this paper, we investigate the relative importance of local vs remote control on cloud radiative forcing using a cumulus ensemble model. It is found that cloud and surface radiation forcings are much more sensitive to the mean vertical motion assoicated with large scale tropical circulation than to the local SST (sea surface temperature). When the local SST is increased with the mean vertical motion held constant, increased surface latent and sensible heat flux associated with enhanced moisture recycling is found to be the primary mechanism for cooling the ocean surface. Large changes in surface shortwave fluxes are related to changes in cloudiness induced by changes in the large scale circulation. These results are consistent with a number of earlier empirical studies, which raised concerns regarding the validity of the cirrus-thermostat hypothesis (Ramanathan and Collins, 1991). It is argued that for a better understanding of cloud feedback, both local and remote controls need to be considered and that a cumulus ensemble model is a powerful tool that should be explored for such purpose.

Lau, K.-M.↗
NASA NTRS1998

On Pulsating and Cellular Forms of Hydrodynamic Instability in Liquid-Propellant Combustion

An extended Landau-Levich model of liquid-propellant combustion, one that allows for a local dependence of the burning rate on the (gas) pressure at the liquid-gas interface, exhibits not only the classical hydrodynamic cellular instability attributed to Landau but also a pulsating hydrodynamic instability associated with sufficiently negative pressure sensitivities. Exploiting the realistic limit of small values of the gas-to-liquid density ratio p, analytical formulas for both neutral stability boundaries may be obtained by expanding all quantities in appropriate powers of p in each of three distinguished wave-number regimes. In particular, composite analytical expressions are derived for the neutral stability boundaries A(sub p)(k), where A, is the pressure sensitivity of the burning rate and k is the wave number of the disturbance. For the cellular boundary, the results demonstrate explicitly the stabilizing effect of gravity on long-wave disturbances, the stabilizing effect of viscosity (both liquid and gas) and surface tension on short-wave perturbations, and the instability associated with intermediate wave numbers for negative values of A(sub p), which is characteristic of many hydroxylammonium nitrate-based liquid propellants over certain pressure ranges. In contrast, the pulsating hydrodynamic stability boundary is insensitive to gravitational and surface-tension effects but is more sensitive to the effects of liquid viscosity because, for typical nonzero values of the latter, the pulsating boundary decreases to larger negative values of A(sub p) as k increases through O(l) values. Thus, liquid-propellant combustion is predicted to be stable (that is, steady and planar) only for a range of negative pressure sensitivities that lie below the cellular boundary that exists for sufficiently small negative values of A(sub p) and above the pulsating boundary that exists for larger negative values of this parameter.

Margolis, Stephen B.↗
NASA NTRS2002

AFM Studies of Salt Concentration Effects on the (110) Surface Structure of Tetragonal Lysozyme Crystals

Previous high resolution AFM studies of the (110) surface of tetragonal chicken egg white lysozyme crystals had shown that only one of two possible molecular surfaces is present, those constituting the completed 43 helices. These suggested that the crystal growth process was by the solution-phase assembly of the growth units, which then attach to the surface. However, the best fit for the imaged surfaces, vs. those predicted based upon the bulk crystallographic coordinates, were obtained when the packing about the 43 helices was "tightened up", while maintaining the underlying crystallographic unit cell spacing. This results in a widening of the gap between adjacent helices, and the top- most layer(s) may no longer be in contact. We postulated that the tightened packing about the helices is a result of the high salt concentrations in the bulk solution, used to crystallize the protein, driving hydrophobic interactions. Once the crystal surface is sufficiently buried by subsequent growth layers the ratio of salt to protein molecules decreases and the helices relax to their bulk crystallographic coordinates. The crystal surface helix structure is thus a reflection of the solution structure, and the tightness of the packing about the 43 helices would be a function of the bulk salt concentration. AFM images of the (110) surface of tetragonal lysozyme crystals grown under low (2%) and high (5%) NaCl concentrations reveal differences in the packing about the 43 helices consistent with the above proposal.

Pusey, Marc Lee↗
NASA NTRS2008

Analysis of the Effects of Vitiates on Surface Heat Flux in Ground Tests of Hypersonic Vehicles

To achieve the high enthalpy conditions associated with hypersonic flight, many ground test facilities burn fuel in the air upstream of the test chamber. Unfortunately, the products of combustion contaminate the test gas and alter gas properties and the heat fluxes associated with aerodynamic heating. The difference in the heating rates between clean air and a vitiated test medium needs to be understood so that the thermal management system for hypersonic vehicles can be properly designed. This is particularly important for advanced hypersonic vehicle concepts powered by air-breathing propulsion systems that couple cooling requirements, fuel flow rates, and combustor performance by flowing fuel through sub-surface cooling passages to cool engine components and preheat the fuel prior to combustion. An analytical investigation was performed comparing clean air to a gas vitiated with methane/oxygen combustion products to determine if variations in gas properties contributed to changes in predicted heat flux. This investigation started with simple relationships, evolved into writing an engineering-level code, and ended with running a series of CFD cases. It was noted that it is not possible to simultaneously match all of the gas properties between clean and vitiated test gases. A study was then conducted selecting various combinations of freestream properties for a vitiated test gas that matched clean air values to determine which combination of parameters affected the computed heat transfer the least. The best combination of properties to match was the free-stream total sensible enthalpy, dynamic pressure, and either the velocity or Mach number. This combination yielded only a 2% difference in heating. Other combinations showed departures of up to 10% in the heat flux estimate.

Cuda, Vincent↗
NASA NTRS2015

Effect of Recent Sea Surface Temperature Trends on the Arctic Stratospheric Vortex

The springtime Arctic polar vortex has cooled significantly over the satellite era, with consequences for ozone concentrations in the springtime transition season. The causes of this cooling trend are deduced by using comprehensive chemistry-climate model experiments. Approximately half of the satellite era early springtime cooling trend in the Arctic lower stratosphere was caused by changing sea surface temperatures (SSTs). An ensemble of experiments forced only by changing SSTs is compared to an ensemble of experiments in which both the observed SSTs and chemically- and radiatively-active trace species are changing. By comparing the two ensembles, it is shown that warming of Indian Ocean, North Pacific, and North Atlantic SSTs, and cooling of the tropical Pacific, have strongly contributed to recent polar stratospheric cooling in late winter and early spring, and to a weak polar stratospheric warming in early winter. When concentrations of ozone-depleting substances and greenhouse gases are fixed, polar ozone concentrations show a small but robust decline due to changing SSTs. Ozone changes are magnified in the presence of changing gas concentrations. The stratospheric changes can be understood by examining the tropospheric height and heat flux anomalies generated by the anomalous SSTs. Finally, recent SST changes have contributed to a decrease in the frequency of late winter stratospheric sudden warmings.

Arctic↗