Showing papers by "Ames Research Center published in 1992"
TL;DR: A modified version of the linear/energy model is presented in which striate cells mutually inhibit one another, effectively normalizing their responses with respect to stimulus contrast, and shows that the new model explains a significantly larger body of physiological data.
Abstract: Simple cells in the striate cortex have been depicted as half-wave-rectified linear operators. Complex cells have been depicted as energy mechanisms, constructed from the squared sum of the outputs of quadrature pairs of linear operators. However, the linear/energy model falls short of a complete explanation of striate cell responses. In this paper, a modified version of the linear/energy model is presented in which striate cells mutually inhibit one another, effectively normalizing their responses with respect to stimulus contrast. This paper reviews experimental measurements of striate cell responses, and shows that the new model explains a significantly larger body of physiological data.
1,840 citations
01 Oct 1992
TL;DR: In this article, two new versions of the k-omega two-equation turbulence model are presented, the baseline model and the Shear-Stress Transport model, which is based on the BSL model, but has the additional ability to account for the transport of the principal shear stress in adverse pressure gradient boundary layers.
Abstract: Two new versions of the k-omega two-equation turbulence model will be presented. The new Baseline (BSL) model is designed to give results similar to those of the original k-omega model of Wilcox, but without its strong dependency on arbitrary freestream values. The BSL model is identical to the Wilcox model in the inner 50 percent of the boundary-layer but changes gradually to the high Reynolds number Jones-Launder k-epsilon model (in a k-omega formulation) towards the boundary-layer edge. The new model is also virtually identical to the Jones-Lauder model for free shear layers. The second version of the model is called Shear-Stress Transport (SST) model. It is based on the BSL model, but has the additional ability to account for the transport of the principal shear stress in adverse pressure gradient boundary-layers. The model is based on Bradshaw's assumption that the principal shear stress is proportional to the turbulent kinetic energy, which is introduced into the definition of the eddy-viscosity. Both models are tested for a large number of different flowfields. The results of the BSL model are similar to those of the original k-omega model, but without the undesirable freestream dependency. The predictions of the SST model are also independent of the freestream values and show excellent agreement with experimental data for adverse pressure gradient boundary-layer flows.
1,709 citations
TL;DR: Two examples of jointly shiftable transforms that are simultaneously shiftable in more than one domain are explored and the usefulness of these image representations for scale-space analysis, stereo disparity measurement, and image enhancement is demonstrated.
Abstract: One of the major drawbacks of orthogonal wavelet transforms is their lack of translation invariance: the content of wavelet subbands is unstable under translations of the input signal. Wavelet transforms are also unstable with respect to dilations of the input signal and, in two dimensions, rotations of the input signal. The authors formalize these problems by defining a type of translation invariance called shiftability. In the spatial domain, shiftability corresponds to a lack of aliasing; thus, the conditions under which the property holds are specified by the sampling theorem. Shiftability may also be applied in the context of other domains, particularly orientation and scale. Jointly shiftable transforms that are simultaneously shiftable in more than one domain are explored. Two examples of jointly shiftable transforms are designed and implemented: a 1-D transform that is jointly shiftable in position and scale, and a 2-D transform that is jointly shiftable in position and orientation. The usefulness of these image representations for scale-space analysis, stereo disparity measurement, and image enhancement is demonstrated. >
1,448 citations
Proceedings Article•
12 Jul 1992TL;DR: An average-case analysis of the Bayesian classifier, a simple induction algorithm that fares remarkably well on many learning tasks, and explores the behavioral implications of the analysis by presenting predicted learning curves for artificial domains.
Abstract: In this paper we present an average-case analysis of the Bayesian classifier, a simple induction algorithm that fares remarkably well on many learning tasks. Our analysis assumes a monotone conjunctive target concept, and independent, noise-free Boolean attributes. We calculate the probability that the algorithm will induce an arbitrary pair of concept descriptions and then use this to compute the probability of correct classification over the instance space. The analysis takes into account the number of training instances, the number of attributes, the distribution of these attributes, and the level of class noise. We also explore the behavioral implications of the analysis by presenting predicted learning curves for artificial domains, and give experimental results on these domains as a check on our reasoning.
1,328 citations
TL;DR: Estimates of these sources for plausible end-member oxidation states of the early terrestrial atmosphere suggest that the heavy bombardment before 3.5 Gyr ago either produced or delivered quantities of organics comparable to those produced by other energy sources.
Abstract: The contribution of organic-rich comets, carbonaceous asteroids, and interplanetary dust particles and of impact shock-synthesized organics in the atmosphere to the origin of life on earth is studied and quantitatively compared with the principal non-heavy-bombardment sources of prebiotic organics. The results suggest that heavy bombardment before 3.5 Gyr ago either produced or delivered quantities of organics comparable to those produced by other energy sources.
1,127 citations
TL;DR: The generalized approximate-reasoning-based intelligent control (GARIC) architecture learns and tunes a fuzzy logic controller even when only weak reinforcement is available; introduces a new conjunction operator in computing the rule strengths of fuzzy control rules; and learns to produce real-valued control actions.
Abstract: A method for learning and tuning a fuzzy logic controller based on reinforcements from a dynamic system is presented. It is shown that: the generalized approximate-reasoning-based intelligent control (GARIC) architecture learns and tunes a fuzzy logic controller even when only weak reinforcement, such as a binary failure signal, is available; introduces a new conjunction operator in computing the rule strengths of fuzzy control rules; introduces a new localized mean of maximum (LMOM) method in combining the conclusions of several firing control rules; and learns to produce real-valued control actions. Learning is achieved by integrating fuzzy inference into a feedforward network, which can then adaptively improve performance by using gradient descent methods. The GARIC architecture is applied to a cart-pole balancing system and demonstrates significant improvements in terms of the speed of learning and robustness to changes in the dynamic system's parameters over previous schemes for cart-pole balancing. >
987 citations
TL;DR: In this article, the min-conflicts heuristic is used to minimize the number of constraint violations after each step in a value-ordering heuristic search, which can be used with a variety of different search strategies.
945 citations
TL;DR: HARP (Hybrid Automated Reliability Predictor) is a software package developed at Duke University and NASA Langley Research Center that can solve fault-tree models that frequently employ high levels of redundancy, dynamic redundancy management, and complex fault and error recovery techniques.
Abstract: Reliability analysis of fault-tolerant computer systems for critical applications is complicated by several factors. Systems designed to achieve high levels of reliability frequently employ high levels of redundancy, dynamic redundancy management, and complex fault and error recovery techniques. This paper describes dynamic fault-tree modeling techniques for handling these difficulties. Three advanced fault-tolerant computer systems are described: a fault-tolerant parallel processor, a mission avionics system, and a fault-tolerant hypercube. Fault-tree models for their analysis are presented. HARP (Hybrid Automated Reliability Predictor) is a software package developed at Duke University and NASA Langley Research Center that can solve those fault-tree models. >
730 citations
TL;DR: It is shown that, while straightforward application of the linearization theory to a non-minimum phase system results in a system with a linear input-output response but unstable internal dynamics, designing a feedback control based on a minimum phase approximation to the true system results with desirable properties such as bounded tracking and asymptotic stability.
661 citations
TL;DR: In this article, two new techniques for the study of the linear and nonlinear instability in growing boundary layers are presented, one of which employs partial differential equations of parabolic type exploiting the slow change of the mean flow, disturbance velocity profiles, wavelengths, and growth rates in the streamwise direction.
Abstract: Two new techniques for the study of the linear and nonlinear instability in growing boundary layers are presented. The first technique employs partial differential equations of parabolic type exploiting the slow change of the mean flow, disturbance velocity profiles, wavelengths, and growth rates in the streamwise direction. The second technique solves the Navier-Stokes equation for spatially evolving disturbances using buffer zones adjacent to the inflow and outflow boundaries. Results of both techniques are in excellent agreement. The linear and nonlinear development of Tollmien-Schlichting (TS) waves in the Blasius boundary layer is investigated with both techniques and with a local procedure based on a system of ordinary differential equations. The results are compared with previous work and the effects of non-parallelism and nonlinearity are clarified. The effect of nonparallelism is confirmed to be weak and, consequently, not responsible for the discrepancies between measurements and theoretical results for parallel flow.
555 citations
TL;DR: This article shows that the nonlinear equation describing the optical flow field can be split by an exact algebraic manipulation to form three sets of equations, and shows that depth and rotation need not be known or estimated prior to solving for translation.
Abstract: As an observer moves and explores the environment, the visual stimulation in his/her eye is constantly changing. Somehow he/she is able to perceive the spatial layout of the scene, and to discern his/her movement through space. Computational vision researchers have been trying to solve this problem for a number of years with only limited success. It is a difficult problem to solve because the optical flow field is nonlinearly related to the 3D motion and depth parameters. Here, we show that the nonlinear equation describing the optical flow field can be split by an exact algebraic manipulation to form three sets of equations. The first set relates the flow field to only the translational component of 3D motion. Thus, depth and rotation need not be known or estimated prior to solving for translation. Once the translation has been recovered, the second set of equations can be used to solve for rotation. Finally, depth can be estimated with the third set of equations, given the recovered translation and rotation. The algorithm applies to the general case of arbitrary motion with respect to an arbitrary scene. It is simple to compute, and it is plausible biologically. The results reported in this article demonstrate the potential of our new approach, and show that it performs favorably when compared with two other well-known algorithms.
TL;DR: In this article, a detailed model for the IR point-source sky is presented, which comprises geometrically and physically realistic representations of the Galactic disk, bulge, stellar halo, spiral arms (including the local arm), molecular ring, and the extragalactic sky.
Abstract: We present a detailed model for the IR point-source sky that comprises geometrically and physically realistic representations of the Galactic disk, bulge, stellar halo, spiral arms (including the 'local arm'), molecular ring, and the extragalactic sky. We represent each of the distinct Galactic components by up to 87 types of Galactic source, each fully characterized by scale heights, space densities, and absolute magnitudes at BVJHK, 12, and 25 microns. The model is guided by a parallel Monte Carlo simulation of the Galaxy at 12 microns. The content of our Galactic source table constitutes a good match to the 12 micron luminosity function in the simulation, as well as to the luminosity functions at V and K. We are able to produce differential and cumulative IR source counts for any bandpass lying fully within the IRAS Low-Resolution Spectrometer's range (7.7-22.7 microns as well as for the IRAS 12 and 25 micron bands. These source counts match the IRAS observations well. The model can be used to predict the character of the point source sky expected for observations from IR space experiments.
01 May 1992
TL;DR: The purpose of these notes is to present recent developments in the unstructured grid generation and flow solution technology.
Abstract: One of the major achievements in engineering science has been the development of computer algorithms for solving nonlinear differential equations such as the Navier-Stokes equations. In the past, limited computer resources have motivated the development of efficient numerical schemes in computational fluid dynamics (CFD) utilizing structured meshes. The use of structured meshes greatly simplifies the implementation of CFD algorithms on conventional computers. Unstructured grids on the other hand offer an alternative to modeling complex geometries. Unstructured meshes have irregular connectivity and usually contain combinations of triangles, quadrilaterals, tetrahedra, and hexahedra. The generation and use of unstructured grids poses new challenges in CFD. The purpose of this note is to present recent developments in the unstructured grid generation and flow solution technology.
TL;DR: Trends in the carbon isotope composition of sedimentary organic carbon and carbonate show that during the Proterozoic aeon the organic carbon reservoir grew in size, relative to the carbonate reservoir, mostly during episodes of global rifting and orogeny.
Abstract: The oxidation of the Earth's crust and the increase in atmospheric oxygen early in Earth history have been linked to the accumulation of reduced carbon in sedimentary rocks. Trends in the carbon isotope composition of sedimentary organic carbon and carbonate show that during the Proterozoic aeon (2.5-0.54 Gyr ago) the organic carbon reservoir grew in size, relative to the carbonate reservoir. This increase, and the concomitant release of oxidizing power in the environment, occurred mostly during episodes of global rifting and orogeny.
TL;DR: This paper introduces Bayesian techniques for splitting, smoothing, and tree averaging, which are similar to Quinlan's information gain, while smoothing and averaging replace pruning.
Abstract: Algorithms for learning classification trees have had successes in artificial intelligence and statistics over many years. This paper outlines how a tree learning algorithm can be derived using Bayesian statistics. This introduces Bayesian techniques for splitting, smoothing, and tree averaging. The splitting rule is similar to Quinlan's information gain, while smoothing and averaging replace pruning. Comparative experiments with reimplementations of a minimum encoding approach,c4 (Quinlanet al., 1987) andcart (Breimanet al., 1984), show that the full Bayesian algorithm can produce more accurate predictions than versions of these other approaches, though pays a computational price.
TL;DR: In this article, the gas phase chemistry of warm molecular material around protostars that is seeded with evaporating grain mantles is modeled, and the release of simple molecules into the gas drives ion-molecule and neutral chemistries which can account for many of the complex O-bearing and N-bearing molecules observed in hot cores.
Abstract: We have modeled the gas phase chemistry of warm molecular material around protostars that is seeded with evaporating grain mantles. We show that the release of simple molecules into the gas drives ion-molecule and neutral chemistries which can account for many of the complex O-bearing and N-bearing molecules observed in hot cores. Initial grain mantle components and secondary product molecules are identified, and the observational consequences are discussed.
TL;DR: It is suggested that, under appropriate conditions, spatial attention can be involuntarily drawn to abrupt-onset events despite the intention of subjects’ to ignore them.
Abstract: Five experiments were carried out to examine the extent to which brief abrupt-onset visual stimuli involuntarily capture spatial attention A fundumantal limitation on the conscious control of spatial attention is demonstrated Data obtained reveal conditions under which the control of spatial attention is completely involuntary: attention is captured by an irrelevant event despite subjects' intentions to ignore the event The paradigm used provided strong incentives to ignore the distracting abrupt onset, but these were insufficient to prevent capture Results suggest that voluntary control of attention is limited to focusing attention in advance on locations, objects, or properties of interest Under appropriate conditions, spatial attention can be involantarily drawn to abrupt-onset events despite the intention of subjects' to ignore them
TL;DR: Since humans drink when there is no apparent physiological stimulus, the psychological component should always be considered when investigating the total mechanisms for drinking.
Abstract: The phenomenon of involuntary dehydration, the delay in full restoration of a body water deficit by drinking, has been described extensively but relatively little is known about its physiological mechanism. It occurs primarily in humans when they are exposed to various stresses including exercise, environmental heat and cold, altitude, water immersion, dehydration, and perhaps microgravity, singly and in various combinations. The level of involuntary dehydration is approximately proportional to the degree of total stress imposed on the body. Involuntary dehydration appears to be controlled by more than one factor including social customs that influence what is consumed, the capacity and rate of fluid absorption from the gastrointestinal system, the level of cellular hydration involving the osmotic-vasopressin interaction with sensitive cells or structures in the central nervous system, and, to a lesser extent, hypovolemic-angiotensin II stimuli. Since humans drink when there is no apparent physiological stimulus, the psychological component should always be considered when investigating the total mechanisms for drinking.
07 Jul 1992
TL;DR: This paper presents an average-case analysis of a simple algorithm that induces one-level decision trees for concepts defined by a single relevant attribute, and derives the expected classification accuracy over the entire instance space.
Abstract: In recent years, researchers have made considerable progress on the worst-case analysis of inductive learning tasks, but for theoretical results to have impact on practice, they must deal with the average case. In this paper we present an average-case analysis of a simple algorithm that induces one-level decision trees for concepts defined by a single relevant attribute. Given knowledge about the number of training instances, the number of irrelevant attributes, the amount of class and attribute noise, and the class and attribute distributions, we derive the expected classification accuracy over the entire instance space. We then examine the predictions of this analysis for different settings of these domain parameters, comparing them to experimental results to check our reasoning.
TL;DR: In this paper, the Kelvin Helmholtz roll up of three dimensional, temporally evolving, plane mixing layers were simulated numerically, starting from a few low wavenumber disturbances, usually derived from linear stability theory, in addition to the mean velocity profile.
Abstract: The Kelvin Helmholtz roll up of three dimensional, temporally evolving, plane mixing layers were simulated numerically. All simulations were begun from a few low wavenumber disturbances, usually derived from linear stability theory, in addition to the mean velocity profile. The spanwise disturbance wavelength was taken to be less than or equal to the streamwise wavelength associated with the Kelvin Helmholtz roll up. A standard set of clean structures develop in most of the simulations. The spanwise vorticity rolls up into a corrugated spanwise roller, with vortex stretching creating strong spanwise vorticity in a cup shaped region at the vends of the roller. Predominantly streamwise rib vortices develop in the braid region between the rollers. For sufficiently strong initial three dimensional disturbances, these ribs collapse into compact axisymmetric vortices. The rib vortex lines connect to neighboring ribs and are kinked in the opposite direction of the roller vortex lines. Because of this, these two sets of vortex lines remain distinct. For certain initial conditions, persistent ribs do not develop. In such cases the development of significant three dimensionality is delayed. When the initial three dimensional disturbance energy is about equal to, or less than, the two dimensional fundamental disturbance energy, the evolution of the three dimensional disturbance is nearly linear (with respect to the mean and the two dimensional disturbances), at least until the first Kelvin Helmholtz roll up is completed.
TL;DR: In this article, a detailed chemical kinetic scheme is applied to stellar envelope profiles of gas density and temperature profiles in order to study the formation of PAH molecules in carbon-rich stellar outflows.
Abstract: A detailed chemical kinetic scheme is applied to stellar envelope profiles of gas density and temperature profiles in order to study the formation of PAH molecules in carbon-rich stellar outflows. Chemical concentration profiles are calculated for several envelope models by integrating the coupled continuity equations that include spherically expanding flows from an inner boundary at the shock formation radius. The influence of the 'inverse greenhouse' effect experienced by small PAHs is investigated and shown to increase the PAH yield by many orders of magnitude. It is shown that the route through propargyl radicals could be an important channel to produce benzene. PAH formation yields are found to be extremely sensitive to gas density and temperature and are much smaller than values inferred from the observed dust content of late-type carbon-rich stellar envelopes. It is therefore unlikely that aromatic molecules are generated in the stellar outflow itself.
TL;DR: A methanogen (or acetogen) and sulfur-based microbial ecology as one possible basis for microbial primary production and a possible deep subsurface microbial ecology similar to those discovered to depths of several kilometers below the surface of the Earth are suggested.
TL;DR: In this article, a method for generating stochastic inflow boundary conditions with a prescribed power spectrum was developed for numerical simulation of inhomogeneous turbulent flows, and the statistics of dilatation showed a significant departure from those obtained in the temporal simulation.
Abstract: For the numerical simulation of inhomogeneous turbulent flows, a method is developed for generating stochastic inflow boundary conditions with a prescribed power spectrum. Turbulence statistics from spatial simulations using this method with a low fluctuation Mach number are in excellent agreement with the experimental data, which validates the procedure. Turbulence statistics from spatial simulations are also compared to those from temporal simulations using Taylor’s hypothesis. Statistics such as turbulence intensity, vorticity, and velocity derivative skewness compare favorably with the temporal simulation. However, the statistics of dilatation show a significant departure from those obtained in the temporal simulation. To directly check the applicability of Taylor’s hypothesis, space‐time correlations of fluctuations in velocity, vorticity, and dilatation are investigated. Convection velocities based on vorticity and velocity fluctuations are computed as functions of the spatial and temporal separations. The profile of the space‐time correlation of dilatation fluctuations is explained via a wave propagation model.
01 Dec 1992
TL;DR: ATRAN as mentioned in this paper is a software tool that computes the transmittance of Earth's atmosphere at near and far-infrared wavelengths using water-vapor and ozone models.
Abstract: This report describes a new software tool, ATRAN, which computes the transmittance of Earth's atmosphere at near- and far-infrared wavelengths. We compare the capabilities of this program with others currently available and demonstrate its utility for observational data calibration and reduction. The program employs current water-vapor and ozone models to produce fast and accurate transmittance spectra for wavelengths ranging from 0.8 microns to 10 mm.
TL;DR: A preliminary parametric study shows that, although spatial frequency has little effect, relative orientation is important and the misperception of relative speed appears lessened when the stimuli to be matched are presented sequentially.
TL;DR: In this article, the locus-of-cognitive-slack method was used to determine whether dual-task interference occurs before or after the response selection stage, and two experiments used the overlapping tasks paradigm, where two signals, each requiring a different speeded choice response, are presented in rapid succession.
Abstract: Two experiments used the locus-of-cognitive-slack method to determine whether dual-task interference occurs before or after the response selection stage. The experiments used the overlapping tasks paradigm, in which two signals, each requiring a different speeded choice response, are presented in rapid succession. In Experiment 1, stimulus-response (S-R) compatibility was manipulated by varying whether Task 2 stimuli were mapped onto their responses by a rule or arbitrarily. Compatibility effects were additive with the effects of degree of task overlap, manipulated by varying the stimulus onset asynchrony between the signals. Experiment 2 examined 2 additional forms of S-R compatibility: symbolic compatability (arrows vs. letters) and spatial compatibility (the 'Simon' effect). Effects of symbolic compatibility were additive with effects of degree of task overlap, whereas the effects of spatial compatibility and degree of task overlap were underadditive. It is argued that only a central-bottleneck model provides a consistent account of these results. The nature of the central bottleneck is considered.
01 Jan 1992
TL;DR: The current understanding of the Martian atmospheric circulation and boundary layer is described in this paper, with particular attention given to the Viking Orbiter and Lander observations relating to the winds required for raising dust from the surface.
Abstract: The current understanding of the Martian atmospheric circulation and boundary layer is described The available meteorological data and the first-order constraints that can be derived from them are reviewed Pertinent aspects of atmospheric radiation on Mars are briefly presented; their main features are the short radiative time constraints on Mars, as compared to earth, and the important role of suspended dust in providing a potent thermal drive for the atmosphere The current understanding of the Martian atmospheric circulation and its various components - the zonal-mean, zonal, and meridional flows, stationary and traveling planetary waves, atmospheric thermal tides, topographic wind systems, free modes, and gravity waves - is examined Present-day knowledge of the PBL on Mars is reviewed, with particular attention given to the Viking Orbiter and Lander observations relating to the winds required for raising dust from the surface The theory of the origin and decay of great dust storms on Mars is reviewed
TL;DR: The detection of this band in the spectra of all four dense molecular clouds suggests that the carrier is ubiquitous in dense clouds, and is tentatively attribute this new feature to interstellar "diamonds."
Abstract: The paper presents high spectral resolution studies in the 3100-2600/cm range of the protostars NGC 7538 IRS9, W33A, W3 IRS 5, and S140 IRS 1. Well-resolved absorption bands at about 2825/cm and 2880/cm were found superposed on the LF wing of the strong O-H stretch feature. The 2880/cm band, previously detected toward W33A, is also in the spectrum of NGC 7538 IRS 9. The relative strength of these two bands varies, showing that they are associated with two different carriers. The new band at about 2880/cm falls near the position of C-H stretching vibrations in tertiary carbon atoms. The strength of this feature, in combination with the lack of strong features associated with primary and secondary carbon atoms, suggests that the carrier of the new feature has a diamondlike structure. This new feature is tentatively attributed to interstellar 'diamonds'. The detection of this band in the spectra of all four dense molecular clouds suggests that the carrier is ubiquitous in dense clouds.
TL;DR: Why virtual acoustic displays are useful, characterizes the abilities of such displays, reviews some recent approaches to their implementation and application, describes the research project at NASA Ames in some detail, and finally outlines some critical research issues for the future.
Abstract: This paper discusses the development of a particular spatial display medium, the virtual acoustic display. Although the technology can stand alone, it is envisioned ultimately to be a component of a larger multisensory environment and will no doubt find its greatest utility in that context. A general philosophy of the project has been that the development of advanced computer interfaces should be driven first by an understanding of human perceptual requirements, and secondarily by technological capabilities or constraints. In expanding on this view, the paper addresses why virtual acoustic displays are useful, characterizes the abilities of such displays, reviews some recent approaches to their implementation and application, describes the research project at NASA Ames in some detail, and finally outlines some critical research issues for the future.
TL;DR: In this paper, two studies on the potential effect of cockpit automation on the pilot's performance are presented, which provide data on pilots' difficulties with understanding and operating the flight management system (FMS) and show that design-related factors such as opaque interfaces contribute to these difficulties.
Abstract: Results are presented of two studies on the potential effect of cockpit automation on the pilot's performance, which provide data on pilots' difficulties with understanding and operating one of the core systems of cockpit automation, the Flight Management System (FMS). The results of both studies indicate that, although pilots do become proficient in standard FMS operations through ground training and subsequent flight experience, they still have difficulties tracking the FMS status and behavior in certain flight contexts and show gaps in the understanding of the functional structure of the system. The results suggest that design-related factors such as opaque interfaces contribute to these difficulties, which can affect the pilot's situation awareness.