Showing papers by "Ames Research Center published in 1988"

Development of NASA-TLX (Task Load Index): Results of Empirical and Theoretical Research

Abstract: The results of a multi-year research program to identify the factors associated with variations in subjective workload within and between different types of tasks are reviewed. Subjective evaluations of 10 workload-related factors were obtained from 16 different experiments. The experimental tasks included simple cognitive and manual control tasks, complex laboratory and supervisory control tasks, and aircraft simulation. Task-, behavior-, and subject-related correlates of subjective workload experiences varied as a function of difficulty manipulations within experiments, different sources of workload between experiments, and individual differences in workload definition. A multi-dimensional rating scale is proposed in which information about the magnitude and sources of six workload-related factors are combined to derive a sensitive and reliable estimate of workload.

Direct simulation of a turbulent boundary layer up to R sub theta = 1410

Abstract: The turbulent boundary layer on a flat plate, with zero pressure gradient, is simulated numerically at four stations between R sub theta = 225 and R sub theta = 1410. The three-dimensional time-dependent Navier-Stokes equations are solved using a spectra method with up to about 10 to the 7th power grid points. Periodic spanwise and stream-wise conditions are applied, and a multiple-scale procedure is applied to approximate the slow streamwise growth of the boundary layer. The flow is studied, primarily, from a statistical point of view. The solutions are compared with experimental results. The scaling of the mean and turbulent quantities with Reynolds number is examined and compared with accepted laws, and the significant deviations are documented. The turbulence at the highest Reynolds number is studied in detail. The spectra are compared with various theoretical models. Reynolds-stress budget data are provided for turbulence-model testing.

Eddies, streams, and convergence zones in turbulent flows

Abstract: Recent studies of turbulent shear flows have shown that many of their important kinematical and dynamical properties can be more clearly understood by describing the flows in terms of individual events or streamline patterns These events or flow regions are studied because they are associated with relatively large contributions to certain average properties of the flow, for example kinetic energy, Reynolds stress, or to particular processes in the flow, such as mixing and chemical reactions, which may be concentrated at locations where streamlines converge for fast chemical reactions (referred to as convergence or C regions), or in recirculating eddying regions for slow chemical reactions The aim of this project was to use the numerical simulations to develop suitable criteria for defining these eddying or vortical zones The C and streaming (S) zones were defined in order to define the whole flow field It is concluded that homogeneous and sheared turbulent flow fields are made up of characteristic flow zones: eddy, C, and S zones A set of objective criteria were found which describe regions in which the streamlines circulate, converge or diverge, and form high streams of high velocity flow

Lower-upper Symmetric-Gauss-Seidel method for the Euler and Navier-Stokes equations

Abstract: On developpe un schema de relaxation multigrille. Application a des ecoulements transsoniques

Learning From Noisy Examples

Abstract: The basic question addressed in this paper is: how can a learning algorithm cope with incorrect training examples? Specifically, how can algorithms that produce an “approximately correct” identification with “high probability” for reliable data be adapted to handle noisy data? We show that when the teacher may make independent random errors in classifying the example data, the strategy of selecting the most consistent rule for the sample is sufficient, and usually requires a feasibly small number of examples, provided noise affects less than half the examples on average. In this setting we are able to estimate the rate of noise using only the knowledge that the rate is less than one half. The basic ideas extend to other types of random noise as well. We also show that the search problem associated with this strategy is intractable in general. However, for particular classes of rules the target rule may be efficiently identified if we use techniques specific to that class. For an important class of formulas – the k-CNF formulas studied by Valiant – we present a polynomial-time algorithm that identifies concepts in this form when the rate of classification errors is less than one half.

Reynolds-stress and dissipation-rate budgets in a turbulent channel flow

Abstract: The budgets for the Reynolds stresses and for the dissipation rate of the turbulence kinetic energy are computed using direct simulation data of a turbulent channel flow. The budget data reveal that all the terms in the budget become important close to the wall. For inhomogeneous pressure boundary conditions, the pressure-strain term is split into a return term, a rapid term, and a Stokes term. The Stokes term is important close to the wall. The rapid and return terms play different roles depending on the component of the term. A split of the velocity pressure-gradient term into a redistributive term and a diffusion term is proposed, which should be simpler to model. The budget data is used to test existing closure models for the pressure-strain term, the dissipation rate, and the transport rate. In general, further work is needed to improve the models.

Progress in analysis and prediction of dynamic stall

Abstract: Nomenclature c = chord CD =drag coefficient, D/qA CL =lift coefficient, L/qa CP = pressure coefficient, P-PW /q CM = pitching-moment coefficient, M/qA D =drag, Ib k = reduced frequency, wc/2U L =lift, Ib M =Mach number; pitching moment, ft-lb P = pressure, lb/in q = dynamic pressure, 1 /2p U R, Re, Re, Rn = Reynolds number t time U,V = freestream velocity, ft/s x = distance a = angle of attack, deg f =nondimensional chord length, x/c $tr transition location co = rotational frequency, rad/s p = density, lb/ft A = sweep angle, deg

Human factors in aviation

Abstract: The fundamental principles of human-factors (HF) analysis for aviation applications are examined in a collection of reviews by leading experts, with an emphasis on recent developments. The aim is to provide information and guidance to the aviation community outside the HF field itself. Topics addressed include the systems approach to HF, system safety considerations, the human senses in flight, information processing, aviation workloads, group interaction and crew performance, flight training and simulation, human error in aviation operations, and aircrew fatigue and circadian rhythms. Also discussed are pilot control; aviation displays; cockpit automation; HF aspects of software interfaces; the design and integration of cockpit-crew systems; and HF issues for airline pilots, general aviation, helicopters, and ATC.

Model consistency in large eddy simulation of turbulent channel flows

Abstract: Combinations of filters and subgrid scale stress models for large eddy simulation of the Navier-Stokes equations are examined by a priori tests and numerical simulations. The structure of the subgrid scales is found to depend strongly on the type of filter used, and consistency between model and filter is essential to ensure accurate results. The implementation of consistent combinations of filter and model gives more accurate turbulence statistics than those obtained in previous investigations in which the models were chosen independently from the filter. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.

A multidimensional model for aerosols - Description of computational analogs

Abstract: The numerical algorithms used to simulate the advection, diffusion, sedimentation, coagulation, and condensational growth of atmospheric aerosols are described. The model can be used in one, two, or three spatial dimensions. The continuity equation in a generalized horizontal and vertical coordinate system is developed, which allows the model to be quickly adapted to a wide variety of dynamical models of global or regional scale. Algorithms are developed to treat the various physical processes, and the results of simulations are presented, which show the strengths and weaknesses of these algorithms. Although the emphasis is on the modeling of aerosols, the work is also applicable to the simulations of the transport of gases.

Climatological observations and predicted sublimation rates at Lake Hoare, Antarctica.

Abstract: In December 1985, an automated meteorological station was established at Lake Hoare in the dry valley region of Antarctica. Here, the first year-round observations available for any site in Taylor Valley are reported. The mean annual solar flux at Lake Hoare was 92 W/sq m during 1986, the mean air temperature -17.3 C, and the mean 3-m wind speed 3.3 m/s. The local climate is controlled by the wind regime during the 4-month sunless winter and by seasonal and diurnal variations in the incident solar flux during the remainder of the year.

Remote sensing of canopy chemistry and nitrogen cycling in temperate forest ecosystems

Abstract: The use of images acquired by the Airborne Imaging Spectrometer, an experimental high-spectral resolution imaging sensor developed by NASA, to estimate the lignin concentration of whole forest canopies in Wisconsin is reported. The observed strong relationship between canopy lignin concentration and nitrogen availability in seven undisturbed forest ecosystems on Blackhawk Island, Wisconsin, suggests that canopy lignin may serve as an index for site nitrogen status. This predictive relationship presents the opportunity to estimate nitrogen-cycling rates across forested landscapes through remote sensing.

Stochastic estimation of organized turbulent structure - Homogeneous shear flow

Abstract: A generalization of the conditional-eddy concept is proposed in which the conditional event specifies the local kinematic state in terms of the velocity and the deformation. Results are presented for stochastically estimated conditional eddies given the local kinematics. The equation governing the probability density function of a kinematic state has been derived for constant-property incompressible flow, providing a link between coherent flow structures corresponding to the conditional eddies and the modelling of turbulent transport. The primary contributions to the second-quadrant and fourth-quadrant Reynolds-stress events in homogeneous shear flow are shown to come from flow induced through the 'legs' and close to the 'heads' of upright and inverted 'hairpins', respectively.

Heating rates in tropical anvils

Abstract: The interaction of infrared and solar radiation with tropical cirrus anvils is addressed. Optical properties of the anvils are inferred from satellite observations and from high-altitude aircraft measurements. An infrared multiple-scattering model is used to compute heating rates in tropical anvils. Layer-average heating rates in 2 km thick anvils were found to be on the order of 20 to 30 K/day. The difference between heating rates at cloud bottom and cloud top ranges from 30 to 200 K/day, leading to convective instability in the anvil. The calculations are most sensitive to the assumed ice water content, but also are affected by the vertical distribution of ice water content and by the anvil thickness. Solar heating in anvils is shown to be less important than infrared heating but not negligible. The dynamical implications of the computed heating rates are also explored and it is concluded that the heating may have important consequences for upward mass transport in the tropics. The potential impact of tropical cirrus on the tropical energy balance and cloud forcing are discussed.

A case study of mobilization and transport of Saharan dust

Abstract: The mobilization and transport of the Saharan dust over the West Africa and the tropical Atlantic Ocean during the days of August 23-28, 1974, were investigated using the data set developed by GFDL, together with numerical models of the atmosphere and aerosol. It was found that the mobilization of dust during this period was related to the passage of a shallow easterly wave; mobilization was effected by dry convective mixing of low-level jets associated with the easterly wave. Model simulations show that the aerosol at any one point can be a complicated mixture of particles lifted at different times and different places; bimodal size distributions developed when dust was mobilized within a dust plume that was generated on a previous day.

The enzymes associated with denitrification.

Abstract: NASA: The enzymes involved in the reduction of nitrogenous oxides are thought to be intermediates in denitrification processes. This review examines the roles of nitrate reductase, nitrite reductases, nitric oxide reductase, mechanisms of N-N bond formation, and nitrous oxide reductases.

Oscillations of drops in zero gravity with weak viscous effects

Abstract: Nonlinear oscillations and other motions of large axially symmetric liquid drops in zero gravity are studied numerically by a boundary-integral method. The effect of small viscosity is included in the computations by retaining first-order viscous terms in the normal stress boundary condition. This is accomplished by making use of a partial solution of the boundary-layer equations which describe the weak vortical surface layer. Small viscosity is found to have a relatively large effect on resonant mode coupling phenomena.

Distribution of volatile organic chemicals in outdoor and indoor air

Abstract: The EPA volatile organic chemistry (VOC) national ambient data base (Shah, 1988) is discussed. The 320 chemicals included in the VOC data base are listed. The methods used to obtain the data are reviewed and the availability, accessibility, and operation of the data base are examined. Tables of the daily outdoor concentrations for 66 chemicals and the daily indoor concentrations for 35 chemicals are presented.

Chlorine-Hydrocarbon Photochemistry in the Marine Troposphere and Lower Stratosphere

Abstract: The role of chlorine atoms in the oxidation of methane and nonmethane hydrocarbons (NMHCs) in the marine troposphere and lower stratosphere was investigated using a one-dimensional photochemical model that incorporated the chemistry of CH4, NMHCs, NO(x), O(x), and HO(x), as well as organic and inorganic halogens in the troposphere and lower stratosphere. The model predicted that chlorine atoms are present in the marine troposphere at the concentrations of about 1000/cu cm, mostly as a product of the reaction between OH and HCl released from sea spray. The results indicate that Cl atoms cause 20 to 40 percent of NMHC oxidation in the troposphere and 40 to 90 percent in the lower stratosphere. At 15 km, the NMHC-Cl reactions account for nearly 80 percent of the PAN produced. Where available, experimental data confirmed the model predictions.

Laboratory studies of the infrared spectral properties of CO in astrophysical ices.

Abstract: The spectral properties of CO in various ices are presented. Included are peak positions, FWHMs, band profiles, and integrated absorbances of the CO fundamental at 2137/cm in different ices under various conditions. The results reveal that good quality moderate-resolution spectra of the interstellar CO feature can be used to unravel the composition, temperature, and thermal history of interstellar and cometary ices containing CO.

Numerical simulation of the integrated space shuttle vehicle in ascent

Abstract: A simulation of the flow about the integrated space shuttle vehicle in ascent mode has been undertaken for various flight conditions using the Chimera composite grid discretization approach. Overset body-conforming grids were used to represent each geometric component, and an implicit approximately factored finite-difference procedure was used to solve the three-dimensional thin-layer Navier-Stokes equations. The computational results have been compared with both wind tunnel and flight test data. Although relatively good agreement is obtained with the experimental data, further refinement and evaluation of numerical error is under way.

Nitrogen transformations in a range of tropical forest soils

Abstract: Nitrogen transformations were measured in soil from 33 sites arranged in 15 classes in five tropical areas. Sites sampled included forests on fertile volcanic and sedimentary soils, infertile oxisols and ultisols, extremely infertile white-sand soils, montane sites, and a primary and a secondary successional sere. In comparison with temperate forests, net N mineralization and nitrification were rapid in intact lowland tropical forests (other than white sands) and microbial 15 N uptake was relatively slow. In contrast, white-sand and upper montane sites had lower rates of mineralization and nitrification and greater microbial immobilization of 15 N; N could be in short supply in many such sites. These patterns of N transformations reinforce inferences about soil N supply based on patterns of litterfall N in tropical forests.

Bayesian classification

Abstract: This paper describes a Bayesian technique for unsupervised classification of data and its computer implementation, AutoClass. Given real valued or discrete data, AutoClass determines the most probable number of classes present in the data, the most probable descriptions of those classes, and each object's probability of membership in each class. The program performs as well as or better than other automatic classification systems when run on the same data and contains no ad hoc similarity measures or stopping criteria. AutoClass has been applied to several databases in which it has discovered classes representing previously unsuspected phenomena.

A simple formulation of the delta-four-stream approximation for radiative transfer parameterizations

Abstract: A systematic development of the delta-four-stream approximation for calculations of radiative fluxes in planetary atmosphere is presented. It is shown that an analytic solution for this approximation can be derived explicitly, with minimum computational effort for flux calculations. Relative accuracy checks for reflection, transmission, and absorption for numerous asymmetry factors, single-scattering albedos, optical depths, and solar zenith angles have been performed with respect to the 'exact' results computed from the adding method for radiative transfer. Overall, results from the delta-four-stream approximation yield relative accuracies within about 5 percent. This approximation is well suited to radiative transfer parameterizations involving flux and heating calculations in aerosol and cloudy atmospheres.