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At least 1,603 records · Page 89
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NASA NTRS1987

Global circuit variability from multiple stratospheric electrical measurements

The vertical component of the electric field, conductivity, and the derived current density from eight superpressure balloons are examined. Special emphasis was placed on the fair-weather, simultaneous measurements from widely spaced constant-altitude (26 km) balloons. The conductivity measurements were well organized by a simple ionization rate parameterization depending on the geomagnetic latitude. The variability of all the electrical parameters was found to be independent of the balloon separation. Much of the time the current density measurements were within 20 percent, for simultaneous flights, even though the balloons were up to 6000 km apart. Also, these simultaneous current density data show that the global current source was significantly variable on hourly and daily time scales. Finally, the use of the simultaneous current density data is discussed as a possible 'geoelectric index'. Advantages and limitations of such an index are discussed.

Norville, Kent↗
NASA NTRS1987

A multiple-time-scale turbulence model based on variable partitioning of turbulent kinetic energy spectrum

A multiple-time-scale turbulence model of a single point closure and a simplified split-spectrum method is presented. In the model, the effect of the ratio of the production rate to the dissipation rate on eddy viscosity is modeled by use of the multiple-time-scales and a variable partitioning of the turbulent kinetic energy spectrum. The concept of a variable partitioning of the turbulent kinetic energy spectrum and the rest of the model details are based on the previously reported algebraic stress turbulence model. Example problems considered include: a fully developed channel flow, a plane jet exhausting into a moving stream, a wall jet flow, and a weakly coupled wake-boundary layer interaction flow. The computational results compared favorably with those obtained by using the algebraic stress turbulence model as well as experimental data. The present turbulence model, as well as the algebraic stress turbulence model, yielded significantly improved computational results for the complex turbulent boundary layer flows, such as the wall jet flow and the wake boundary layer interaction flow, compared with available computational results obtained by using the standard kappa-epsilon turbulence model.

Kim, S.-W.↗
NASA NTRS1988

Variable spectra of active galaxies

The analysis of EXOSAT spectra of active galaxies are presented. The objects examined for X-ray spectral variability were MR 2251-178 and 3C 120. The results of these investigations are described, as well as additional results on X-ray spectral variability related to EXOSAT observations of active galaxies. Additionally, the dipping X-ray source 4U1624-49 was also investigated.

Halpern, Jules P.↗
NASA NTRS1988

An internal variable constitutive model for the large deformation of metals at high temperatures

The advent of large deformation finite element methodologies is beginning to permit the numerical simulation of hot working processes whose design until recently has been based on prior industrial experience. Proper application of such finite element techniques requires realistic constitutive equations which more accurately model material behavior during hot working. A simple constitutive model for hot working is the single scalar internal variable model for isotropic thermal elastoplasticity proposed by Anand. The model is recalled and the specific scalar functions, for the equivalent plastic strain rate and the evolution equation for the internal variable, presented are slight modifications of those proposed by Anand. The modified functions are better able to represent high temperature material behavior. The monotonic constant true strain rate and strain rate jump compression experiments on a 2 percent silicon iron is briefly described. The model is implemented in the general purpose finite element program ABAQUS.

Brown, Stuart↗
NASA NTRS1988

Estimating the cost of mental loading in a bimodal divided-attention task: Combining reaction time, heart-rate variability and signal-detection theory

Multiple approaches are necessary for understanding and measuring workload. In particular, physiological systems identifiable by employing cardiac measures are related to cognitive systems. One issue of debate in measuring cardiac output is the grain of analysis used in recording and summarizing data. Various experiments are reviewed, the majority of which were directed at supporting or contradicting Lacey's intake-rejection hypothesis. Two of the experiments observed heart rate in operational environments and found virtually no changes associated with mental load. The major problems facing researchers using heart rate variability, or sinus arrhthmia, as a dependent measure have been associated with valid and sensitive scoring and preventing contamination of observed results by influences unrelated to cognition. Spectral analysis of heart rate variability offers two useful procedures: analysis from the time domain and analysis from the frequency domain. Most recently, data have been collected in a divided attention experiment, the performance measures and cardiac measures of which are detailed.

Casper, Patricia A.↗
NASA NTRS1988

Brainstem response and state-trait variables

A series of investigations are summarized from a personality research program that have relevance for mental state estimation. Of particular concern are those personality variables that are believed to have either a biological or perceptual basis and their relationship to human task performance and psychophysiology. These variables are among the most robust personality measures and include such dimensions as extraversion-introversion, sensation seeking, and impulsiveness. These dimensions also have the most distinct link to performance and psychophysiology. Through the course of many of these investigations two issues have emerged repeatedly: these personality dimensions appear to mediate mental state, and mental state appears to influence measures of performance or psychophysiology.

Gilliland, Kirby↗
NASA NTRS1986

Variable Sweep Transition Flight Experiment (VSTFE)-Parametric Pressure Distribution Boundary Layer Stability Study and Wing Glove Design Task

The Variable Sweep Transition Flight Experiment (VSTFE) was initiated to establish a boundary-layer transition data base for laminar flow wing design. For this experiment, full-span upper-surface gloves will be fitted to a variable sweep F-14 aircraft. The results of two initial tasks are documented: a parametric pressure distribution/boundary-layer stability study and the design of an upper-surface glove for Mach 0.8. The first task was conducted to provide a data base from which wing-glove pressure distributions could be selected for glove designs. Boundary-layer stability analyses were conducted on a set of pressure distributions for various wing sweep angles, Mach numbers, and Reynolds number in the range of those anticipated for the flight-test program. The design procedure for the Mach 0.8 glove is described, and boundary-layer stability calculations and pressure distributions are presented both at design and off-design conditions. Also included is the analysis of the clean-up glove (smoothed basic wing) that will be flight-tested initially and the analysis of a Mach 0.7 glove designed at the NASA Langley Research Center.

Rozendaal, Rodger A.↗
NASA NTRS1987

Simulation of dc spacecraft power systems using state variable techniques

The modeling and simulation of large dc spacecraft power systems necessitates considering the spacecraft power system as an interconnection of modular components. This paper presents a state-variable-based approach for dc spacecraft power system modeling and simulation. Each modular component is treated as a two-port network, and a state model is written with the port voltages as the inputs. The state model of a component is solved independently of the other components using its state transition matrix. The state variables of each component are updated assuming that the inputs are constant. Network analysis principles are then utilized to calculate the component inputs.

Nelms, R. M.↗
NASA NTRS1987

Application of several variable-valve-timing concepts to an LHR engine

The paper discusses advantages provided by electronically controlled hydraulically activated valves (ECVs) when applied to low heat rejection (LHR) engines. The ECV concept provides additional engine control flexibility by allowing for a variable valve timing as a function of speed and load, or for a given transient condition. The results of a study carried out to assess the benefits that this flexibility can offer to an LHR engine indicated that, when judged on the benefits to BSFC, volumetric efficiency, and peak firing pressure, ECVs would provide only modest benefits in comparison to conventional valve profiles. It is noted, however, that once installed on the engine, the ECVs would permit a whole range of certain more sophisticated variable valve timing strategies not otherwise possible, such as high compression cranking, engine braking, cylinder cutouts, and volumetric efficiency timing with engine speed.

Morel, T.↗
NASA NTRS1987

A search for RR Lyrae variables in NGC 147

An analysis of repeated deep CCD observations of a field in the Local Group dwarf elliptical galaxy NBC 147 has resulted in the detection of 34 variable stars. The periods and shapes of light curves obtained for 13 of these variables show nine of them to be RR Lyrae stars. An intensity-averaged phase-weighted-mean g magnitude of 25.15 (+ 0.17 or - 0.15) is found which indicates a distance modulus to NGC 147 of 23.85 (+ 0.22 or - 0.20), or 589 kpc.

Saha, A.↗
NASA NTRS1987

Intrinsically variable stars

The characteristics of intrinsically variable stars are examined, reviewing the results of observations obtained with the IUE satellite since its launch in 1978. Selected data on both medium-spectral-class pulsating stars (Delta Cep stars, W Vir stars, and related groups) and late-type variables (M, S, and C giants and supergiants) are presented in spectra, graphs, and tables and described in detail. Topics addressed include the calibration of the the period-luminosity relation, Cepheid distance determination, checking stellar evolution theory by the giant companions of Cepheids, Cepheid masses, the importance of the hydrogen convection zone in Cepheids, temperature and abundance estimates for Population II pulsating stars, mass loss in Population II Cepheids, SWP and LWP images of cold giants and supergiants, temporal variations in the UV lines of cold stars, C-rich cold stars, and cold stars with highly ionized emission lines.

Bohm-Vitense, Erika↗
NASA NTRS1987

Single step optimization of manipulator maneuvers with variable structure control

One step ahead optimization has been recently proposed for spacecraft attitude maneuvers as well as for robot manipulator maneuvers. Such a technique yields a discrete time control algorithm implementable as a sequence of state-dependent, quadratic programming problems for acceleration optimization. Its sensitivity to model accuracy, for the required inversion of the system dynamics, is shown in this paper to be alleviated by a fast variable structure control correction, acting between the sampling intervals of the slow one step ahead discrete time acceleration command generation algorithm. The slow and fast looping concept chosen follows that recently proposed for optimal aiming strategies with variable structure control. Accelerations required by the VSC correction are reserved during the slow one step ahead command generation so that the ability to overshoot the sliding surface is guaranteed.

Chen, N.↗
NASA NTRS1988

CCD time-resolved photometry of faint cataclysmic variables. I

CCD time-resolved V and B differential light curves are presented for the dwarf novae AR And, FS Aur, TT Boo, UZ Boo, AF Cam, AL Com, AW Gem, X Leo, RZ Leo, CW Mon, SW UMa, and TW Vir. The time-series observations ranged from 2 to 6 hours and have accuracies of 0.025 mag or better for the majority of the runs. Except for AR And, X Leo, CW Mon, and TW Vir, the periods are below the cataclysmic-variable period gap (about 2 hours), and the systems are potential SU UMa stars. Photometric periods for five of the stars are the first such determinations, while those for the other seven generally confirm previous spectroscopic or photometric observations. In several cases, the photometric modulations are large amplitude (up to 0.5 mag). The results on AL Com and SW UMa indicate they may be magnetic variables.

Howell, Steve↗
NASA NTRS1987

Variable structure control of globally feedback-decoupled deformable vehicle maneuvers

The use of Cayley-Rodrigues attitude parameters as kinematic variables is shown to yield a globally linearized and decoupled model of the equations of motion of a deformable body, where the structural deformation state appears only in the coefficients of the inverse transformation. It is shown how commanded multiaxial attitude maneuvers can be encoded as switching surfaces for a variable-structure control implementation of the corresponding computed slew torques, automatically modulated in respnse only to detected angular rate error signs, for accurate tracking in the presence of separately damped or even uncontrolled (but stable) structural deformations.

Dwyer, T. A. W., III↗
NASA NTRS1988

Variability of metric emission from the sun

Full-disk observations of the sun obtained with the VLA at 317 MHz have been analyzed, with special attention being given to the short period variability of the meter-wavelength sources. A reduction in brightness is noted over coronal holes at 90 cm. Thermal emission models for the slowly varying component are investigated, along with a nonthermal emission mechanism for storm radiation at meter wavelengths. The results indicate that the thermal model encounters difficulties in accommodating short period changes over a few tens of minutes, and that the nonthermal mechanism encounters difficulty in accounting for the variability of low brightness temperatures of less than 10 to the 6th K. A gyrotron synchrotron emission mechanism has been ruled out.

Shevgaonkar, R. K.↗
NASA NTRS1988

Eight years of ultraviolet spectra of the variable BL Lacertae object PKS 2155-304

IUE data obtained on the variable BL Lacertae object PKS 2155-304 over an eight-year period are presented and analyzed in a fashion that minimizes the SNR of individual spectra. In general, the ultraviolet continuum can be fitted by a power law model with an average spectral index of roughly 0.89 + or - 0.12(sigma). The observations show that PKS 2155-304 has been variable on a time scale as short as 10 days and by an overall factor of two. The spectral index has changed very little and is uncorrelated with intensity in an absolute sense.

Urry, C. M.↗
NASA NTRS1989

Variable-Metric Algorithm For Constrained Optimization

Variable Metric Algorithm for Constrained Optimization (VMACO) is nonlinear computer program developed to calculate least value of function of n variables subject to general constraints, both equality and inequality. First set of constraints equality and remaining constraints inequalities. Program utilizes iterative method in seeking optimal solution. Written in ANSI Standard FORTRAN 77.

Frick, James D.↗
NASA NTRS1988

Observations of ultraviolet variability in RV Tauri stars

An IUE program to monitor the ultraviolet variability in RV Tauri stars was initiated. The Mg II region was investigated as a potential probe of atmospheric shocks, which are believed to be associated with the pulsational variability of this class of objects. Observations, a description of the spectra, and findings for V Vul and AC Her are presented. The Mg II emission does vary significantly during the cycle; major changes in the emission line strength occur on a time scale much less than 0.2 in phase; and as the UV (and optical) continuum flux increases, the Mg II lines decrease and increased emission may be seen at 2823, 2844, and 2900 A.

Brugel, Edward W.↗