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At least 1,207 records · Page 67

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NASA NTRS1990

Theoretical characterization of the potential energy surface for H + N2 yields HN2. II - Computed points to define a global potential

A previous calculation for H + N2 (Walch et al., 1989) focused on the minimum energy path (MEP) region of the potential energy surface and on estimates of the lifetime of the HN2 species. In this paper, energies computed at geometries selected to permit a global representation of the potential energy surface (PES) are reported. As in the previous work, the calculations were performed using the complete active space self-consistent field/externally contracted configuration interaction method. The surface was characterized using the same basis set as in the previous paper except that an improved contraction of the H s-basis is used. Calculations with a larger basis set were carried out along an approximate MEP obtained with the smaller basis set. The new PES exhibits a sharp curvature, which was not present in the previous calculations, and has a slightly narrower and smaller barrier to dissociation. Saddle points for H atom exchange via collinear and T shaped HN2 complexes are also reported.

Walch, Stephen P.↗

NASA NTRS1990

Robust H(infinity) control design for the Space Station with structured parameter uncertainty

A robust H(infinity) control design methodology and its application to a Space Station attitude and momentum control problem are presented. This new approach incorporates nonlinear multiparameter variations in the state-space formulation of H(infinity) control theory. An application of this robust control synthesis technique tothe Space Station control problem yields a remarkable result in stability robustness with respect to the moments-of-inertia variation of about 73 percent in one of the structured uncertainty directions. The performance and stability of this new robust H(infinity) controller for the Space Station are compared to those of other controllers designed using a standard linear-quadratic-regulator synthesis technique.

Byun, Kuk-Whan↗

NASA NTRS1990

H-infinity based integrated flight/propulsion control design for a STOVL aircraft in transition flight

This paper presents results from an application of H(infinity) control design methodology to a centralized integrated flight/propulsion control (IFPC) system design for a supersonic STOVL fighter aircraft in transition flight. The overall design methodology consists of a centralized IFPC design with controller partitioning. Design and evaluation vehicle models are summarized, and insight is provided into formulating the H(infinity) control problem such that it reflects the IFPC design objective. The H(infinity) controller is shown to provide decoupled command tracking for the design model. The controller order could be significantly reduced by modal residualization of the fast controller modes without any deterioration in performance.

Garg, Sanjay↗

NASA NTRS1989

Reaction dynamics of H + O2 at 1.6 eV collision energy

The hot hydrogen atom reaction, H + O2 yields OH + O, has been studied at a center of mass collision energy of 1.6 eV. H atoms were generated by 266 nm photolysis of HI in a mixture of HI and O2 at 293 K. The OH product was probed by laser induced fluorescence and the nascent OH vibrational, rotational, and fine structure distributions were determined. The OH(v=0)/OH(v=1) vibrational branching ratio was measured to be 1.72 + or - 0.09. The data suggest that the H + O2 reaction at this collision energy proceeds via two competing mechanisms: reaction involving a long-lived complex and direct reaction.

Bronikowski, Michael J.↗

NASA NTRS1989

H I and dust in the high latitude dark cloud L1642

The high latitude dark cloud L1642 in the 21 cm H I region was mapped using a 100 m radio telescope. A remarkable H I line broadening from 2.5 to 2.9 km/s is observed over a small area on the bright side of L1642, i.e., the side facing the galactic plane. Results are presented concerning the effects of the asymmetrical UV radiation field of OB stars on the H I gas and the very small dust grains associated with L1642.

Liljestroem, Tarja↗

NASA NTRS1990

H I in the giant elliptical galaxy NGC 4406

The Arecibo 305-m radio telescope was used to make 21-cm measurements of NGC 4406, an elliptical galaxy containing 10 to the 10th solar mass of hot diffuse gas. The H I decreases by a factor of 3 one beamwidth from the center and by an order of magnitude two beam widths from the center. Excess H I emission is seen in the X-ray tail, but not in dust-rich regions. The H I measurements do not support the suggestion that the X-ray tail is hot gas being stripped from the galaxy.

Bregman, Joel N.↗

NASA NTRS1991

A large supershell H II region complex in the Large Magellanic Cloud and the interstellar environment of SN 1987A

Results from narrow-bandpass optical imagery of the LMC, acquired prior to the supernova event are discussed, with special emphasis on the region about SN 1987A. The observations reveal the presence of an extremely large H II region complex, mostly embedded in an even larger dust ring. The H II region is roughly 1 kpc in diameter. This region is ionized by over 700 luminous OB stars that comprise the 'star clouds' dispersed throughout the region. The 1.5 x 2 kpc dimension of the large dust ring or supershell enshrouding this H II region is traced out as enhanced extinction in narrow-bandpass imagery of the continuum at 4215 A. The results also indicate that SN 1987A occurred in the cavity carved out by the combined stellar winds and the previous supernova explosions produced by the large number of massive stars in the region.

Bruhweiler, Frederick C.↗

NASA NTRS1991

Warm neutral halos around molecular clouds. II - H I and CO (J = 1-0) observations

Observational data are presented relating to the boundary regions between dense, molecular clouds and the more diffuse atomic gas. The data set consists of H I 21 cm and CO (J = 1-0) strip maps in 14 molecular clouds which traverse 62 known CO cloud boundaries. The angular lengths of the strips range from 0.5 deg to several degrees of arc, which is generally adequate to eliminate confusion with background (and foreground) H I. Most of the crossed boundaries (49 of 62) show clear evidence for the presence of H I emission maxima, or halos, lying outside the molecular clouds' gas. There is also general evidence for limb brightening in the CO emission.

Wannier, Peter G.↗

NASA NTRS1991

A complete Arecibo map of H I 1225 + 01

Complete H I observations of a large low-density cloud, H I 1225 + 01, carried out at Arecibo with an angular resolution of 3.3 arcmin, are reported. The velocity field of the H I distribution is analyzed, and tentative models of the overall mass distribution are proposed. The implications of the observations for the structure and evolution of the system are discussed.

Giovanelli, Riccardo↗

NASA NTRS1990

High frequency, high temperature specific core loss and dynamic B-H hysteresis loop characteristics of soft magnetic alloys

Limited experimental data exists for the specific core loss and dynamic B-H loop for soft magnetic materials for the combined conditions of high frequency and high temperature. This experimental study investigates the specific core loss and dynamic B-H loop characteristics of Supermalloy and Metglas 2605SC over the frequency range of 1 to 50 kHz and temperature range of 23 to 300 C under sinusoidal voltage excitation. The experimental setup used to conduct the investigation is described. The effects of the maximum magnetic flux density, frequency, and temperature on the specific core loss and on the size and shape of the B-H loops are examined.

Wieserman, W. R.↗

NASA NTRS1991

Triton's atmosphere - A source of N and H for Neptune's magnetosphere

Mass loading of the Neptunian magnetosphere occurs primarily by thermal escape of H, H2, and N from Triton's upper atmosphere. The global escape rate of hydrogen is about 7 x 10 exp 25/s, determined by the global average methane photolysis rate, whereas the escape rate of nitrogen for the present preferred model is about 3.4 x 10 exp 25/s, and is controlled by the global and orbital average energy deposition rate due to precipitating magnetospheric electrons. The escape rate of H(+) and N(+) is less than 4 percent of the neutral escape rate and implies that mass loading of the Neptunian magnetosphere is not localized to Triton's corona. The ratio of hydrogen to nitrogen escape rates for the present preferred model is about 2:1, comparable to the H(+)/N(+) abundance ratio inferred for Neptune's magnetosphere.

Summers, Michael E.↗

NASA NTRS1992

The molecular interstellar medium in two giant H II regions in M33 - NGC 604 and NGC 595

This study reports single-dish and interferometric CO observations for the two largest H II regions in M33, NGC 604, and NGC 595. Six individual molecular clouds are identified in the H II regions. Molecular and virial masses for three resolved clouds in NGC 604 agree very well, from which it is inferred that the CO-to-H2 conversion factor in this region is the same as it is in the inner disk of M33 and in the Galaxy, despite a metallicity in NGC 604 that is three times below solar. The total gas mass of the molecular complex associated with NGC 604 estimated from single-dish data is dissimilar to those of molecular complexes in the inner disk of M33, while the molecular gas mass of NGC 505 is roughly an order or magnitude smaller. Both regions contain a large mass fraction of atomic hydrogen, which suggests that photodissociation of molecular hydrogen is an important process in these energetic H II regions.

Wilson, Christine D.↗

NASA NTRS1992

The interstellar medium of M31. III - Narrow-band imagery in H alpha and (SII)

Deep CCD imagery in H alpha and (SII) is presented of the major spiral arms of M31 with particular attention given to the data reduction and the analysis of the (SII)/H alpha flux ratios. A diffuse ionized gas noted in the images is analyzed which shows higher (SII)/H alpha ratios, and 967 discrete nebulae are listed with gray-scale images, finding charts, and absolute fluxes. The differential H-alpha luminosity function is found to have a slope of -0.95 for brighter objects and flattens out below a critical level. The curve is shown to correspond to the point at which single-star ionization accounts for the H alpha luminosities and is consistent with previous observations. The catalog of objects and fluxes is the largest existing sample of this type, and the unresolved objects in the sample are considered to be planetary nebulae.

Walterbos, R. A. M.↗

NASA NTRS1992

H-infinity synthesis using a bilinear pole shifting transform

An H-inifinity control law design is presented for a benchmark problem consisting of an undamped pair of spring-coupled masses with a sensor and actuator that are not collocated. This simple mechanical system captures many of the salient features of more complex aircraft and space structure vibration control problems. The H-infinity problem formulation enables the issue of stability robustness in the face of large mass and spring constant variation to be directly addressed. Constraints on closed-loop dominant pole locations and settling time are accommodated via a simple s-plane bilinear transform. The transform parameter can give direct control of closed-loop disturbance settling time and controller open-loop pole-zero locations. A four-state H-infinity minimum phase controller was found to meet the given specifications of each design. Detailed analysis on the tradeoffs of sensor noise vs control energy are also presented.

Chiang, R. Y.↗

NASA NTRS1992

An analytic formula for H-infinity norm sensitivity with applications to control system design

An analytic formula for the sensitivity of singular value peak variation with respect to parameter variation is derived. As a corollary, the derivative of the H-infinity norm of a stable transfer function with respect to a parameter is presented. It depends on some of the first two derivatives of the transfer function with respect to frequency and the parameter. For cases when the transfer function has a linear system realization whose matrices depend on the parameter, analytic formulas for these first two derivatives are derived, and an efficient algorithm for calculating them is discussed. Examples are given which provide numerical verification of the H-infinity norm sensitivity formula and which demonstrate its utility in designing control systems satisfying H-infinity norm constraints. In the appendix, derivative formulas for singular values are paraphrased.

Giesy, Daniel P.↗

NASA NTRS1992

Ultraviolet Imaging Telescope ultraviolet images - Large-scale structure, H II regions, and extinction in M81

The study employs UV images of M81 obtained by the Ultraviolet Imaging Telescope (UIT) during the December 1990 Astro-1 spacelab mission to determine 2490- and 1520-A fluxes from 46 H II regions and global surface brightness profiles. Comparison photometry in the V band is obtained from a ground-based CCD image. UV radial profiles show bulge and exponential disk components, with a local decrease in disk surface brightness inside the inner Lindblad Resonance about 4 arcmin from the nucleus. The V profile shows typical bulge plus exponential disk structure, with no local maximum in the disk. There is little change of UV color across the disk, although there is a strong gradient in the bulge. Observed m152-V colors of the H II regions are consistent with model spectra for young clusters, after dereddening using Av determined from m249-V and the Galactic extinction curve. The value of Av, so determined, is 0.4 mag greater on the average than Av derived from radio continuum and H-alpha fluxes.

Hill, Jesse K.↗

NASA NTRS1991

H(infinity) and H2 optimal controllers for periodic and multi-rate systems

The solutions to the optimal l2 to l2 disturbance rejection problem H(infinity) as well as to the LQG (linear quadratic Gaussian) H2 problem in periodic systems using the lifting technique are presented. Both problems involve a causality condition on the optimal LTI (linear time invariant) compensator when viewed in the lifted domain. The H(infinity) problem is solved using the Nehari's theorem, whereas in the H2 problem the solution is obtained using the projection theorem in Hilbert spaces. The authors demonstrate that exactly the same method of solution to the H(infinity) and H2 problems in periodic systems applies when considering the same problems in multirate sampled data systems.

Voulgaris, Petros G.↗

NASA NTRS1991

Comparison of high frequency, high temperature core loss and B-H loop characteristics of an 80 Ni-Fe crystalline alloy and two iron-based amorphous alloys

Limited experimental data exists for the specific core loss and dynamic B-H loops for soft magnetic materials for the combined conditions of high frequency and high temperature. This experimental study investigates the specific core loss and dynamic B-H characteristics of a nickel-iron crystalline magnetic alloy (Supermalloy) and two iron-based amorphous magnetic materials (Metglas 2605S-3A and Metglas 2605SC) over the frequency range of 1-50 kHz and temperature range of 23-300 C under sinusoidal voltage excitation. The effects of maximum magnetic flux density, frequency, and temperature on the specific core loss and on the size and shape of the B-H loops are examined. The Supermalloy and Metglass 2605S-3A and 2605SC data are used to compare the core loss of transformers with identical kVA and voltage ratings.

Wieserman, William R.↗