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At least 775 records · Page 43

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NASA NTRS1992

The optimization of force inputs for active structural acoustic control using a neural network

This paper investigates the use of a neural network to determine which force actuators, of a multi-actuator array, are best activated in order to achieve structural-acoustic control. The concept is demonstrated using a cylinder/cavity model on which the control forces, produced by piezoelectric actuators, are applied with the objective of reducing the interior noise. A two-layer neural network is employed and the back propagation solution is compared with the results calculated by a conventional, least-squares optimization analysis. The ability of the neural network to accurately and efficiently control actuator activation for interior noise reduction is demonstrated.

Cabell, R. H.↗

NASA NTRS2001

Exploring the Martian Highlands using a Rover-Deployed Ground Penetrating Radar

The Martian highlands record a long and often complex history of geologic activity that has shaped the planet over time. Results of geologic mapping and new data from the Mars Global Surveyor spacecraft reveal layered surfaces created by multiple processes that are often mantled by eolian deposits. Knowledge of the near-surface stratigraphy as it relates to evolution of surface morphology will provide critical context for interpreting rover/lander remote sensing data and for defining the geologic setting of a highland lander. Rover-deployed ground penetrating radar (GPR) can directly measure the range and character of in situ radar properties, thereby helping to constrain near-surface geology and structure. As is the case for most remote sensing instruments, a GPR may not detect water unambiguously on Mars. Nevertheless, any local, near-surface occurrence of liquid water will lead to large, easily detected dielectric contrasts. Moreover, definition of stratigraphy and setting will help in evaluating the history of aqueous activity and where any water might occur and be accessible. GPR data can also be used to infer the degree of any post-depositional pedogenic alteration or weathering, thereby enabling assessment of pristine versus secondary morphology. Most importantly perhaps, GPR can provide critical context for other rover and orbital instruments/data sets. Hence, rover-deployment of a GPR deployment should enable 3-D mapping of local stratigraphy and could guide subsurface sampling.

Grant, J. A.↗

NASA NTRS1988

Dynamic compensation in the central Pacific Ocean

The intermediate-wavelength geoid (lambda similar to 2000 km) and sea-floor topography fields in the central Pacific Ocean were studied in terms of static and dynamic compensation models. Topographic features on the sea-floor with lambda less than 1000 km were found to be compensated both regionally, by the elastic strength of the lithosphere, and locally, by displacing mantle material to reach isostatic adjustment. The larger-scale sea-floor topography and the corresponding geoid anomalies with lambda similar to 2000 km cannot be explained by either local or regional compensation. The topography and the resulting geoid anomaly at this wavelength were modeled by considering the dynamic effects arising from viscous stresses in a layer of fluid with a highly temperature-dependent viscosity for the cases of: (1) surface cooling, and (2) basal heating. In this model, the mechanical properties of the elastic part of the lithosphere were taken into account by considering an activation energy of about 520 kJ/mol in the Arrhenius law for the viscosity. Numerical predictions of the topography, total geoid anomaly, and admittance were obtained, and the results show that the thermal perturbation in the layer, which accounts for the mass deficit, must be located close to the surface to compensate the gravitational effect of the surface deformation. For the case of basal heating, the temperature dependence of viscosity results in a separation of the upper, quasi-rigid lid from the lower mobile fluid, hence inhibiting the development of a compensating thermal perturbation at shallow depths. The results clearly rule out small-scale, upper-mantle convection as the source of these anomalies. Instead, the geophysical observables can be well explained by a shallow, transient thermal perturbation.

Hinojosa, Juan Homero↗

NASA NTRS1972

Rechargeable Al/Cl2 battery with molten AlCl4/-/ electrolyte.

A molten salt system based on Al- and Cl2 carbon electrodes, with an AlCl3 alkali chloride eutectic as electrolyte, offers promise as a rechargeable, high energy density battery which can operate at a relatively low temperature. Electrode kinetic studies showed that the electrode reactions at the Al anode were rapid and that the observed passivation phenomena were due to the formation at the electrode surface of a solid salt layer resulting from concentration changes on anodic or cathodic current flow. It was established that carbon electrodes were intrinsically active for chlorine reduction in AlCl3-alkali chloride melts. By means of a rotating vitreous carbon disk electrode, the kinetic parameters were determined.

Holleck, G. L.↗

NASA NTRS1973

Arc jet tests of RSI materials: Screening and comparative evaluation

Cyclic convective heating test results show that all surface insulation materials tested exhibit a wide range of surface emittance and catalytic activity depending on their surface coatings. Coatings have a significant effect on the overall material response depending on boundary layer characteristics.

Schaefer, J. W.↗

NASA NTRS1976

Adsorption of oxygen on W/100/ - Adsorption kinetics and structure

The adsorption of oxygen on W(100) single-crystal surfaces is studied by Auger electron spectroscopy (AES), flash desorption, low-energy electron diffraction (LEED), and retarding-field work-function measurements. The AES results reveal stepwise changes in the sticking coefficients in the coverage range 0 to 1 and activated adsorption at higher coverages. Upon room-temperature adsorption, a series of complex LEED patterns is observed. In layers adsorbed at 1050 K and cooled to room temperature, the p(2 x 1) structure is the first ordered structure observed. This structure shows a reversible order-disorder transition between 700 and 1000 K and is characterized by a work function which is lower than that of the clean surface. Heating room-temperature adsorbates changes their structure irreversibly. At temperatures below 750 K, some new structures are observed.

Bauer, E.↗

NASA NTRS1978

Properties of the stratospheric aerosol layer studied with a one-dimensional computer model

Aerosol particle effects are often neglected in theoretical studies of stratospheric phenomena. In reality, the particulate matter normally found above the tropopause may influence the terrestrial radiation balance, catalyze heterogeneous chemical interactions, and serve as a tracer of atmospheric motions. The paper proposes a one-dimensional model of the stratospheric aerosol layer, and it is used to compare aerosol theory with observational data. The model considers gaseous sulfur photochemistry and the physical aerosol processes of nucleation, coagulation, sedimentation, and diffusion. Calculations of the effects on the aerosol layer of stratospheric injections of aluminum oxide particles by Space Shuttle engines and of sulfur dioxide molecules by volcanic activity are performed. The relation between measured aerosol variability and changes in stratospheric air temperatures and vertical transport rates are discussed.

Turco, R. P.↗

NASA NTRS1985

Sea breeze-induced mesoscale systems and severe weather

The relationship between thunderstorm activity during the summer months along coastal regions of the Atlantic and Gulf coasts and the dry sea breeze circulation was investigated. Satellite composites of thunderstorm activity for synoptically undisturbed conditions have been obtained for south Florida for a series of days in the summer of 1983. These data were catalogued into different low level synoptic flow regimes. Five synoptic flow regimes were found from the data. A three-dimensional mesoscale numerical model was used for each sysnoptic flow regime to quantitatively predict the location of enhanced thunderstorm activity. This model includes a parameterization of vegetation and soil moisture feedbacks as well as a sophisticated planetary boundary layer representation. Using the results of the satellite image composites, spatial and temporal characteristics of deep convective cloud patterns and their variation with synoptic flow are described. The results from the numerical model have provided explanations for the observed patterns.

Pielke, R. A.↗

NASA NTRS1985

Solar activity beyond the disk and variations of the cosmic ray gradient

Part of galactic cosmic rays (CR) observed near the Earth and on the Earth come from beyond-disk regions of circumsolar space. But CR of those energies which undergo substantial modulation cover too large a path across the lines of force of the interplanetary magnetic field (IMF) in order that they could provide an effective transfer of information about beyond-disk solar activity. And if it is still possible, the most probable channel for transferring such information must be a neutral layer of heliomagnetosphere in which the transverse CR transport is facilitated by their drift in an inhomogeneous magnetic field. A simple diffusion model for an expected CR variation in a neutral layer near the Earth is discussed. It is of importance that variations of the CR gradient are not at all always accompanied by considerable variations of IMF and solar wind velocity at the point of observation.

Belov, A. V.↗

NASA NTRS1985

Interannual variability of Martian weather

Pressure, temperature, imaging, and wind data from the Mutch Memorial Station, the Viking lander located in Mars' subtropics, are used to demonstrate the existence of two disctinct regimes of northern hemisphere winter weather on Mars. One of these regime is characterized by one or more intense global dust storms in which the optical depth reaches about 5 over most of the planet. During these events, traveling baroclinic waves of the winter hemisphere are suppressed. In the other regime, there are no global dust storms, but traveling baroclinic waves are active, and high winds associated with these storms raise large quantities of dust which remain confined to a shallow layer in the northern hemisphere. The strongest winds at the Mutch Memorial Station occur during intense storms in the latter regime.

Leovy, C. B.↗

NASA NTRS1988

Observations of the stratospheric conductivity and its variation at three latitudes

Measurements from nine high-altitude balloon flights at altitudes ranging from 10 to 30 km have been obtained in order to study atmospheric conductivity variations on local and global scales. The conductivity profiles are not correlated with solar or cosmic ray activity, but they exhibit variations which appear to be dominated by local effects. Evidence is provided of stratospheric aerosol layers with number densities of the order of 1000/cu cm. It is suggested that global conductivity models employing latitudinal variations based on cosmic ray ionization alone may be improved by including the effects of latitudinal variations of temperature and aerosols.

Byrne, G. J.↗

NASA NTRS1990

Control framework for hand-arm coordination

Coordination of multiple manipulators requires cooperation at several levels in the control hierarchy. A distributed processing environment with no hardware dependencies except at the motor servo level would provide a flexible architecture for coordination. A system on these lines is being built to control an articulated hand and an arm. The four levels of control envisaged include a task decomposition level, a planning level, a scheduling level, and a server level. The hand will carry both force and tactile sensors, feedback from these are used to provide adaptive control in grasping tasks. The processing of the sensory information is performed by independent processes, with analyzed information being sent to the relevant layer of the system. The manipulators are also controlled by individual processes. All process can open communications with an active process sending commands or data, or receiving them. The scope of the system and the current setup are described along with future lines of development.

Agrawal, Sanjay↗

NASA NTRS1992

Raman lidar measurements of water vapor and aerosol/clouds during the FIRE/SPECTRE field campaign

Water vapor is one of the most important constituents of the earth's atmosphere. It has a major impact on both atmospheric dynamics and radiative transfer. From a dynamic standpoint, the distribution of water vapor with height determines convective stability which is the major indicator of destructive storm development. Also, water vapor stored in the planetary boundary layer acts as the fuel to intensify severe weather. In regards to radiative transfer, water vapor is the most active IR molecule in the atmosphere. It is more effective in absorbing and emitting IR radiation than either carbon dioxide or methane, and thus plays an important role in global change. The main objective of FIRE (First ISSCCP (International Satellite Cloud Climatology Project) Regional Experiment) was to study the development and radiative characteristics of cirrus clouds. The SPECTRE (Spectral Radiation Experiment) project was designed to acquire the necessary atmospheric observations to compare radiative measurements with radiative transfer theory, with special emphasis on understanding the water vapor spectral continuum. The FIRE/SPECTRE field campaign was conducted during Nov. - Dec. 1991 in Coffeyville, Kansas. A complete understanding of water vapor, its distribution with height, and its temporal variation was important for both experiments.

Melfi, S. H.↗

NASA NTRS1992

Laminar flow control - Introduction and overview

A development history and a development-trends evaluation are presented for laminar flow controlled airfoil technologies and design concepts, including the search for 'natural' laminar flow and actively controlled flow via suction through small pores on the airfoil surface. While most NASA activities in this field have been concerned with subsonic aircraft, it has been projected that the control of boundary layer turbulence may be even more critical to the aerodynamic efficiency of supersonic aircraft. Developmental programs for these techniques have been conducted with several modified conventional aircraft.

Hefner, Jerry N.↗

NASA NTRS1992

Maneuvering technology for advanced fighter aircraft

The need for increased maneuverability has its genesis from the first aerial combat engagement when two adversaries entangled themselves in a deadly aerial dance trying to gain the advantage over the other. It has only been in the past two decades that technologies have been investigated to increase aircraft control at maneuver attitudes that are typically dominated by highly separated flows. These separated flow regions are aggravated by advanced fighter aircraft shapes required to defeat an electronic enemy. This paper discusses passive and active devices that can be used to enhance the maneuverability of advanced fighter aircraft through vortex flow control, boundary layer control, and innovative flow manipulation.

Alexander, Michael G.↗

NASA NTRS2009

Pulmonary Toxicity Studies of Lunar Dusts in Rodents

NASA will build an outpost on the Moon for prolonged human habitation and research. The lunar surface is covered by a layer of fine, reactive dust. Astronauts on the Moon will go in and out of the base for various activities, and will inevitably bring some dust into the living quarters. Depressurizing the airlock so that astronauts can exit for outdoor activities could also bring dust inside the airlock to the habitable area. Concerned about the potential health effects on astronauts exposed to airborne lunar dust, NASA directed the JSC Toxicology Laboratory to determine the pulmonary toxicity of lunar dust. The toxicity data also will be needed by toxicologists to establish safe exposure limits for astronauts residing in the lunar habitat and by environmental engineers to design an appropriate dust mitigation strategy. We conducted a study to examine biomarkers of toxicity (inflammation and cytotoxicity) in lung lavage fluids from mice intrapharyngeally instilled with lunar dust samples; we also collected lung tissue from the mice for histopathological examination 3 months after the dust instillation. Reference dusts (TiO2 and quartz) having known toxicities and industrial exposure limits were studied in parallel with lunar dust so that the relative toxicity of lunar dust can be determined. A 6-month histopathology study has been planned. These instillation experiments will be followed by inhalation studies, which are more labor intensive and technologically difficult. The animal inhalation studies will be conducted first with an appropriate lunar dust simulant to ensure that the exposure techniques to be used with actual lunar dust will be successful. The results of these studies collectively will reveal the toxicological risk of exposures and enable us to establish exposure limits on lunar dust for astronauts living in the lunar habitat.

Lam, C.-W.↗

NASA NTRS1970

Intumescent Coatings as Fire Retardants

The development of fire-retardant coatings to protect surfaces which may be exposed to fire or extreme heat is a subject of intense interest to many industries. A fire-retardant paint has been developed which represents a new chemical approach for preparing intumescent coatings, and potentially, is very important to fire-prevention authorities. The requirements for a superior coating include ease of application, suitability to a wide variety of surfaces and finishes, and stability over an extended period of time within a broad range of ambient temperature and humidity conditions. These innovative coatings, when activated by the heat of a fire, react to form a thick, low-density, polymeric coating or char layer. Water vapor and sulphur dioxide are released during the intumescent reaction. Two fire-protection mechanisms thus become available: (1) the char layer retards the flow of heat, due to the extremely low thermal conductivity; and (2) water vapor and sulfur dioxide are released, providing fire quenching properties. Still another mechanism functions in cases where the char, by virtue of its high oxidation resistance and low thermal conductivity, reaches a sufficiently high temperature to re-radiate much of the incident heat load. The coatings consist of dispersions of selective salts of a nitro-amino-arornatic compound. Specifically, para-nitroaniline bisulfate and the ammonium salt of para-nitroaniline-ortho sulphuric acid (2-amino-5-nitrobenzenesulphuric acid) are used. Suitable vehicles are cellulose nitrate of lacquer grade, a nitrite-phenolic modified rubber, or epoxy-polysulfide copolymer. Three separate formulations have been developed. A solvent is usually employed, such as methylethyl ketone, butyl acetate, or toluene, which renders the coatings suitably thin and which evaporates after the coatings are applied. Generally, the intumescent material is treated as insoluble in the vehicle, and is ground and dispersed in the vehicle and solvent like an ordinary coating pigment. The char found on intumescence is better in terms of yield and physical properties than chars obtained from many previously known intumescent materials. Prior to intumescence, the coating has a density of 85 pounds per cubic foot. After intumescence, the density is approximately 0.3 pounds per cubic loot. The linear expansion of the coatings ranges from 70 to 200 times the applied coating thickness.

Parker, J. A.↗

NASA NTRS2006

Turbulence Measurements on a 2D NACA 0036 with Synthetic Jet Flow Control

An active flow control experiment was conducted on a 2-ft chord NACA 0036 airfoil in a 3-ft by 4-ft Wind Tunnel at Re = 1 x 10(exp 6). The model was equipped with synthetic jet actuators at x/c = 0.30 and 0.65 that provided 120 Hz periodic excitation at a C(sub mu) 0.86% through 0.06-in wide slots. Three different slot con gurations were tested, including a baseline with no slots. Surface pressure data was collected to compare to previous tests and to combine with turbulence data to aid future CFD modeling efforts. Turbulence data, measured by hot-wire, was compared with and without flow control. Pressure data corroborates previous test data and provides more points for CFD validation. Hot-wire results showed ow control reduced the separated wake size and brought the high Reynolds stress shear layer closer to the airfoil surface. The position of this layer to the surface was altered more significantly than the magnitude of the peak stresses. Flow control was shown to increase turbulent energy in the attached boundary layer downstream of the slot but to have little effect upstream. These results provide further justification to continue assessing the potential of active flow control to reduce drag of helicopter airframe components.

Wilson, J. S.↗