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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Continuous monitoring of the lunar or Martian subsurface using on-board pattern recognition and neural processing of Rover geophysical data

The ultimate goal is to create an extraterrestrial unmanned system for subsurface mapping and exploration. Neural networks are to be used to recognize anomalies in the profiles that correspond to potentially exploitable subsurface features. The ground penetrating radar (GPR) techniques are likewise identical. Hence, the preliminary research focus on GPR systems will be directly applicable to seismic systems once such systems can be designed for continuous operation. The original GPR profile may be very complex due to electrical behavior of the background, targets, and antennas, much as the seismic record is made complex by multiple reflections, ghosting, and ringing. Because the format of the GPR data is similar to the format of seismic data, seismic processing software may be applied to GPR data to help enhance the data. A neural network may then be trained to more accurately identify anomalies from the processed record than from the original record.

Mcgill, J. W.↗
NASA NTRS1991

Airborne coherent continuous wave CO2 Doppler lidars for aerosol backscatter measurement

Two focused coherent, continuous wave (CW) lidars have been developed by the Marshall Space Flight Center (MSFC) for airborne and ground-based measurement of aerosol backscatter coefficients. The first of these instruments uses a mixture of CO2 and other gases, and measures backscatter at 10.6 m. The second lidar uses an isotope of carbon dioxide, which enables lasing at 9.1 m. The 10.6 m backscatter measurement serves as a reference to allow variations in backscatter due to aerosol concentration to be distinguished from variations due to spectral variability. The 10.6 m lidar has been used in airborne field programs since 1981. Development of the 9.1 m lidar was completed in early 1989. Recently, both lidars were flown on the NASA/Ames Research Center DC-8 research aircraft in the remote Pacific Basin as part of the NASA GLObal Backscatter Experiment (GLOBE) survey missions. The GLOBE program, of which the survey missions are the centerpieces, supports design and simulation studies for NASA's prospective Laser Atmospheric Wind Sounder (LAWS).

Rothermel, Jeffry↗
NASA NTRS1991

Mechanical behavior of a continuous fiber reinforced aluminum matrix composite subjected to transverse and thermal loading

The transverse properties of an aluminum alloy metal matrix composite reinforced by continuous alumina fibers were investigated. The composite is subjected to both mechanical and cyclic thermal loading. The results of an experimental program indicate that the shakedown concept of structural mechanics provides a means of describing the material behavior. When the loading conditions are within the shakedown region, the material finally responds in an elastic manner after initial plastic response, and for loading conditions outside the shakedown region, the material exhibits a rapid incremental plastic strain accumulation. The failure strain varies by an order of magnitude according to the operating conditions. Hence, for high mechanical and low thermal loading, the failure strains is small; for low mechanical and high thermal loading, the failure strain is large.

Jansson, S.↗
NASA NTRS1990

A continuous proportional low-thrust propulsion system

Helium gas flows continuously from a low-temperature dewar supplying propellant for satellite thrusters. Thrust is obtained by movement of a spool located between two opposing nozzles, thus differentially restricting the helium flow. The design procedure and test results of a high bandwidth, low power, linear electromagnetic actuator for the pool movement are presented. Two mathematical models are used to describe the helium flow through the thruster under different flow conditions. The experimental data measured in the vacuum chamber and the computational results from the mathematical models are compared. The models agree with the experiment to 3 percent except near a Knudsen number of 1.

Chen, J.-H.↗
NASA NTRS1990

Continuous monitoring of a changing sample by multiplex gas chromatography

Results are presented from a study in which a continuously changed gaseous sample was monitored by multiplex gas chromatography (MGC), using the exponential dilution (ED) technique of Ritter and Adams (1976) to change the composition and concentration of a gaseous mixture in such a way as to imitate changes in the atmospheric gases sampled by a descending aircraft. A calibration of the MGC system was performed with four different rates of sample dilution, and the errors resulting from various degrees of change in the sample concentration were determined.

Valentin, Jose R.↗
NASA NTRS1991

Asymptotic expansions of the kernel functions for line formation with continuous absorption

Asymptotic expressions are obtained for the kernel functions M2(tau, alpha, beta) and K2(tau, alpha, beta) appearing in the theory of line formation with complete redistribution over a Voigt profile with damping parameter a, in the presence of a source of continuous opacity parameterized by beta. For a greater than 0, each coefficient in the asymptotic series is expressed as the product of analytic functions of a and eta. For Doppler broadening, only the leading term can be evaluated analytically.

Hummer, D. G.↗
NASA NTRS1990

On the continuous spectra of the compressible boundary layer stability equations

The branch cuts in the complex frequency plane (omega) that correspond to the continuous spectrum of the two-dimensional compressible boundary layer stability equations are computed by looking for solutions that are pure oscillatory in the free-stream. In the complex omega-plane there are three compressible and one incompressible branch cuts. The compressible branch cuts are given as solutions of third order polynomials. Computations are made for a range of Reynolds and Mach numbers, and also for several streamwise wavenumbers.

Ashpis, David E.↗
NASA NTRS1991

Reliability analysis of continuous fiber composite laminates

This paper describes two methods, the maximum distortion energy (MDE) and the principle of independent action (PIA), developed for the analysis of the reliability of a single continuous composite lamina. It is shown that, for the typical laminated plate structure, the individual lamina reliabilities can be combined in order to produce the upper and the lower bounds of reliability for the laminate, similar in nature to the bounds on properties produced from variational elastic methods. These limits were derived for both the interactive and the model failure considerations. Analytical expressions were also derived for the sensitivity of the reliability limits with respect to changes in the Weibull parameters and in loading conditions.

Thomas, David J.↗
NASA NTRS1991

Calculation of aerosol backscatter from airborne continuous wave focused CO2 Doppler lidar measurements. I - Algorithm description

Since 1981 the Royal Signals and Radar Establishment and the Royal Aircraft Establishment, United Kindom, have made vertical and horizontal sounding measurements of aerosol backscatter coefficients at 10.6 microns, using an airborne continuous-wave-focused CO2 Doppler lidar, the Laser True Airspeed System (LATAS). In this paper, the heterodyne signal from the LATAS detector is spectrally analyzed. Then, in conjunction with aircraft flight parameters, the data are processed in a six-stage computer algorithm: set search window, search for peak signal, test peak signal, measure total signal, calculate signal-to-noise ratio, and calculate backscatter coefficient.

Rothermel, Jeffry↗
NASA NTRS1991

Calculation of aerosol backscatter from airborne continuous wave focused CO2 Doppler lidar measurements. II - Algorithm performance

This paper examines the performance of the Rothermel et al. (1991) algorithm for calculating aerosol backscatter coefficients from measurements by an airborne continuous wave focused CO2 Doppler lidar by using the algorithm to analyze measurements by the Laser True Airspeed System. The results show that, under optimum signal conditions, backscatter signals are detected unambiguously and the aerosol backscatter coefficients are determined accurately. However, under marginal or subthreshold conditions under which the system sensitivity is degraded, noise can be mistaken for signals. As determined from parametric studies, the 'false-alarm' rate under these conditions depends on the size of the search window and the SNR threshold for valid signals.

Rothermel, Jeffry↗
NASA NTRS1991

Continuous wave laser absorption techniques for gasdynamic measurements in supersonic flows

Line-of-sight measurements of velocity, temperature, pressure, density, and mass flux were performed in a transient shock tube flow using three laser absorption schemes. All methods employed an intracavity-doubled ring dye laser tuned to an OH transition at 306 nm. In the first scheme, the gas was labeled by 193.3-nm excimer photolysis of H2O, and the passage of the generated OH was detected downstream. In the second method, the laser was tuned at a rate of 3 kHz over the R1(7) and R1(11) line pair, and absorption was simultaneously monitored at 90 and 60 deg with respect to the flow. Velocity was determined from the Doppler shift of the profiles and the temperature from the intensity ratio of the lines. Pressure was determined from both the magnitude of absorption and the collisional broadening. In the third method, the laser wavelength was fixed at a single frequency, and a continuous measurement of velocity and pressure was obtained using the signals from the two beam paths. All methods gave results which compare favorably to calculated values.

Davidson, David F.↗
NASA NTRS1991

Gravitational lenses: The current sample, recent results, and continuing searches

Gravitational lensing is one of the topics in astrophysics that was quite extensively discussed over time before it was actually discovered. Ten years after the discovery of the first one, it is interesting to note how the field has developed. After an initial slow rate of discovery, the last few years have seen an explosion in the number of reported cases. Attention was drawn to the first few cases because quasars at the same red shift, with similar optical spectra, were observed with angular separations of only a few arc seconds. Most observational effort has been devoted to searching for new candidate lens systems and carefully measuring their properties, both to test whether they are indeed lensed and to provide constraints for modeling. A classification of the lenses is into rings, arcs, multiples, and doubles, where the progression is from sources close to the optical axis to far from the optical axis. The known candidate systems are listed. The searches for gravitational lenses are proving to be successful, and more lenses continue to be discovered serendipitously. Many searches are under way, along with instruments that will routinely increase the resolution of astronomical imaging.

Hewitt, Jacqueline N.↗
NASA NTRS1992

Discrete and continuous dynamics modeling of a mass moving on a flexible structure

A general discrete methodology for modeling the dynamics of a mass that moves on the surface of a flexible structure is developed. This problem was motivated by the Space Station/Mobile Transporter system. A model reduction approach is developed to make the methodology applicable to large structural systems. To validate the discrete methodology, continuous formulations are also developed. Three different systems are examined: (1) simply-supported beam, (2) free-free beam, and (3) free-free beam with two points of contact between the mass and the flexible beam. In addition to validating the methodology, parametric studies were performed to examine how the system's physical properties affect its dynamics.

Herman, Deborah Ann↗
NASA NTRS1991

Remote semi-continuous flow rate logging seepage meter

The movement of groundwater and its associated solutes from upland regions was implicated in the degradation of receiving surface water bodies. Current efforts to directly measure this influx of water incorporate manually operated seepage meters which are hindered by severe limitations. A prototype seepage meter was developed by NASA Langley Research Center and Virginia Polytechnic Institute and State University that will allow for the semi-continuous collection and data logging of seepage flux across the sediment water interface. The meter is designed to operate at depths to 40 meters, and alleviate or minimize all disadvantages associated with traditional methods while remaining cost effective. The unit was designed to operate independently for time periods on the order of weeks with adjustable sample sequences depending upon hydrologic conditions. When used in conjunction with commercially available pressure transducers, this seepage meter allows for correlations to be made between groundwater discharge and tidal/sea state conditions in coastal areas. Field data from the Chesapeake Bay and Florida Bay systems are presented.

Reay, William G.↗
NASA NTRS1990

Continuous wavelet analysis of coherent structures

We perform an analysis of planar cuts through three dimensional turbulent fields (planar channel flow and mixing layer) using the 2D continuous wavelet transform. We propose two new diagnostics: (1) a measure of intermittency I(r, vector x), which is the ratio of local energy and average energy at a given scale r; and (2) a local Reynolds number, defined on the local velocity contribution at a given scale, computed from the wavelet transform of the three velocity components, the scale of the transform, and molecular viscosity; this gives a representation of the local non-linearity of the flow viewed in both space and scale. We find, for the analyzed flows, strong small-scale intermittency located in the ejection regions for the channel flow and in the vortex core of the mixing layer.

Farge, M.↗
NASA NTRS1992

Continuation of tailored composite structures of ordered staple thermoplastic material

The search for the cost effective composite structure has motivated the investigation of several approaches to develop composite structure from innovative material forms. Among the promising approaches is the conversion of a planar sheet to components of complex curvature through sheet forming or stretch forming. In both cases, the potential for material stretch in the fiber direction appears to offer a clear advantage in formability over continuous fiber systems. A framework was established which allows the simulation of the anisotropic mechanisms of deformation of long discontinuous fiber laminates wherein the matrix phase is a viscous fluid. Predictions for the effective viscosities of a hyper-anisotropic medium consisting of collimated, discontinuous fibers suspended in viscous matrix were extended to capture the characteristics of typical polymers including non-Newtonian behavior and temperature dependence. In addition, the influence of fiber misorientation was also modeled by compliance averaging to determine ensemble properties for a given orientation distribution. A design tool is presented for predicting the effect of material heterogeneity on the performance of curved composite beams such as those used in aircraft fuselage structures. Material heterogeneity can be induced during manufacturing processes such as sheet forming and stretch forming of thermoplastic composites. This heterogeneity can be introduced in the form of fiber realignment and spreading during the manufacturing process causing radial and tangential gradients in material properties. Two analysis procedures are used to solve the beam problems. The first method uses separate two-dimensional elasticity solutions for the stresses in the flange and web sections of the beam. The separate solutions are coupled by requiring that forces and displacements match section boundaries. The second method uses an approximate Rayleigh-Ritz technique to find the solutions for more complex beams. Analyses are performed for curved beams of various cross-sections loaded in pure bending and with a uniform distributed load. Preliminary results show that the geometry of the beam dictates the effect of heterogeneity on performance. The role of heterogeneity is larger in beams with a small average radius-to-depth ration, R/t, where R is the average radius of the beam and t is the difference between the inside and outside radii. Results of the anlysis are in the form of stresses and displacements and are compared to both mechanics of materials and numerical solutions obtained using finite element analysis.

Santare, Michael H.↗
NASA NTRS1990

Interesting examples of supervised continuous variable systems

The authors analyze two simple deterministic flow models for multiple buffer servers which are examples of the supervision of continuous variable systems by a discrete controller. These systems exhibit what may be regarded as the two extremes of complexity of the closed loop behavior: one is eventually periodic, the other is chaotic. The first example exhibits chaotic behavior that could be characterized statistically. The dual system, the switched server system, exhibits very predictable behavior, which is modeled by a finite state automaton. This research has application to multimodal discrete time systems where the controller can choose from a set of transition maps to implement.

Chase, Christopher↗