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NASA NTRS1986

Mass spectrometric studies of the electrical breakdown of thin polymer films

The composition of the neutral particles released during the electrical breakdown of 50-micron and 75-micron insulating films of the type used on spacecraft exteriors investigated experimentally using a time-of-flight mass spectrometer triggered by the breakdown event. The experimental apparatus is described in detail, and the results are presented in photographs. It is found that the particle flux from Teflon FEP and PFA films comprise mainly fluorocarbon fragments, some with mass 350 amu or greater, but the flux from Kapton oxygen-ion-beam treated Kapton, Tefzel, and Mylar comprises mainly molecules of mass 44 amu or less.

Kendall, B. R. F.↗

NASA NTRS1987

Active Control of Transition and Turbulence

Two active means of manipulating boundary-layer flow developed, one controlling laminar-to-turbulent transition, other controlling amplitude of turbulent fluctuation. Purpose to control skin-friction drag over surfaces inside inlets and ducts. Resulting turbulence downstream has lower skin-friction drag than equivalent flow developing over same surfaces in absence of intervention. Heating strips trigger turbulence while transition amplitude and bandwidth controlled by acoustic signal.

Maestrello, Lucio↗

NASA NTRS1984

On the utilization of engineering knowledge in design optimization

Some current research work conducted at the University of Michigan is described to illustrate efforts for incorporating knowledge in optimization in a nontraditional way. The incorporation of available knowledge in a logic structure is examined in two circumstances. The first examines the possibility of introducing global design information in a local active set strategy implemented during the iterations of projection-type algorithms for nonlinearly constrained problems. The technique used algorithms for nonlinearly constrained problems. The technique used combines global and local monotinicity analysis of the objective and constraint functions. The second examines a knowledge-based program which aids the user to create condigurations that are most desirable from the manufacturing assembly viewpoint. The data bank used is the classification scheme suggested by Boothroyd. The important aspect of this program is that it is an aid for synthesis intended for use in the design concept phase in a way similar to the so-called idea-triggers in creativity-enhancement techniques like brain-storming. The idea generation, however, is not random but it is driven by the goal of achieving the best acceptable configuration.

Papalambros, P.↗

NASA NTRS1986

Detection of faults and software reliability analysis

Multiversion or N-version programming was proposed as a method of providing fault tolerance in software. The approach requires the separate, independent preparation of multiple versions of a piece of software for some application. Specific topics addressed are: failure probabilities in N-version systems, consistent comparison in N-version systems, descriptions of the faults found in the Knight and Leveson experiment, analytic models of comparison testing, characteristics of the input regions that trigger faults, fault tolerance through data diversity, and the relationship between failures caused by automatically seeded faults.

Knight, J. C.↗

NASA NTRS1986

Simultaneous Solar Maximum Mission (SMM) and very large array observations of solar active regions

The research deals mainly with Very Large Array and Solar Maximum Mission observations of the ubiquitous coronal loops that dominate the structure of the low corona. As illustrated, the observations of thermal cyclotron lines at microwave wavelengths provide a powerful new method of accurately specifying the coronal magnetic field strength. Processes are delineated that trigger solar eruptions from coronal loops, including preburst heating and the magnetic interaction of coronal loops. Evidence for coherent burst mechanisms is provided for both the Sun and nearby stars, while other observations suggest the presence of currents that may amplify the coronal magnetic field to unexpectedly high levels. The existence is reported of a new class of compact, variable moving sources in regions of apparently weak photospheric field.

Lang, K. R.↗

NASA NTRS1986

Filamentary structure in the Orion molecular cloud

A large scale 13CO map (containing 33,000 spectra) of the giant molecular cloud located in the southern part of Orion is presented which contains the Orion Nebula, NGC1977, and the LI641 dark cloud complex. The overall structure of the cloud is filamentary, with individual features having a length up to 40 times their width. This morphology may result from the effects of star formation in the region or embedded magnetic fields in the cloud. We suggest a simple picture for the evolution of the Orion-A cloud and the formation of the major filament. A rotating proto-cloud (counter rotating with respect to the galaxy) contians a b-field aligned with the galaxtic plane. The northern protion of this cloud collapsed first, perhaps triggered by the pressure of the Ori I OB association. The magnetic field combined with the anisotropic pressure produced by the OB-association breaks the symmetry of the pancake instability, a filament rather than a disc is produced. The growth of instabilities in the filament formed sub-condensations which are recent sites of star formation.

Bally, J.↗

NASA NTRS1986

How are quasars fueled? Simulating interstellar gas in tidally disturbed galaxies

Whether gravitational tides from companions trigger global instabilities in spiral galaxy disks and thus rapid flows of gas into the nucleus to fuel activity is investigated. An n-body computer program is used to simulate the disk of the spiral galaxy within a much more stable, high-velocity dispersion spherical halo. Under sufficient perturbation, the disk undergoes violent distortions due to the disturber and its self-gravitation. The tidal action of companions was simulated and the tidal strengths at which the instabilities appear to match those of the observed companions of Seyferts and quasars was shown. With the additional modifications planned, the gas flow will be more realistically simulated to compare with observations (e.g., colors, velocity fields) of active galaxies.

Byrd, Gene G.↗

NASA NTRS1986

Dependence of snow melting and surface-atmosphere interactions on the forest structure

The surface albedo and the surface roughness for forested areas with snow on the ground are expressed in terms of the tree silhouette parameter, the projection on the vertical plane of trees per unit area. The absorption of insolation (direct solar beam) is quantitatively described for a horizontal snow surface with vertical tree trunks, stressing the role of the bark at snow level as triggering the snowmelt.

Ottermann, J.↗

NASA NTRS1986

Coronal manifestations of preflare activity

A variety of coronal manifestations of precursors or preheating for flares are discussed. Researchers found that almost everyone with a telescope sees something before flares. Whether an all-encompassing scenario will ever be developed is not at all clear at present. The clearest example of preflare activity appears to be activated filaments and their manifestations, which presumably are signatures of a changing magnetic field. But researchers have seen two similar eruptions, one without any evidence of emerging flux (Kundu et al., 1985) and the other with colliding poles (Simon et al., 1984). While the reconnection of flux is generally agreed to be required to energize a flare, the emergence of flux from below (at least on short timescales and in compact regions) does not appear to be a necessary condition. In some cases the cancelling of magnetic flux (Martin, 1984) by horizontal motions instead may provide the trigger (Priest, 1985) Researchers found similarities and some differences between these and previous observations. The similarities, besides the frequent involvement of filaments, include compact, multiple precursors which can occur both at and near (not at) the flare site, and the association between coronal sources and activity lower in the atmosphere (i.e., transition zone and chromosphere).

Schmahl, E. J.↗

NASA NTRS1987

Turbulence control on an airborne laser platform

An active flow control device to generate large-scale, periodic structures in a turbulent shear flow is developed. Together with adaptive optics, the device may be used on airborne laser platforms to reduce or eliminate optical distortion caused by the turbulence in the aircraft's boundary layer. A cyclic jet issuing from a spanwise slot is used to collect the turbulent boundary layer for a finite time and then release all of the flow instantaneously in one large eddy that convects downstream. Flow visualization and hot-film probe measurements are used together with pattern recognition algorithms to demonstrate the viability of the flow control method. A flat plate towed in a water channel is used as a test bed. The instantaneous velocity signal is used to compute important statistical quantities of the random velocity field, such as the mean, the root-mean-square, the spectral distribution, and the probability density function. When optimized for a given boundary layer, it is shown that the cyclic jet will produce periodic structures that are similar to the random, naturally occurring ones. These structures seem to trigger the onset of bursting events near the wall of the plate. Thus, the present device generates periodic structures in both the outer and inner regions of a turbulent boundary layer.

Gad-El-hak, Mohamed↗

NASA NTRS1986

Can prominences form in current sheets

Two-dimensional numerical simulations of the formation of cold condensations in a vertical current sheet have been performed using the radiative, resistive MHD equations with line-tied boundary conditions at one end of the sheet. Prominence-like condensations are observed to appear above and below an X-line produced by the onset of the tearing-mode instability. Cooling in the sheet is initiated by Ohmic decay, with the densest condensations occurring in the region downstream of a fast-mode shock. This shock, which is due to the line-tied boundary conditions, terminates one of the two supermagnetosonic reconnection jets that develop when the tearing is fully developed. The condensation properties of shock waves, which may trigger or considerably enhance the conditions for thermal condensation are emphasized.

Malherbe, J. M.↗

NASA NTRS1987

Shuttle get-away special experiments

This presentation describes two shuttle Get-Away-Special (GAS) experiments built by McDonnell Douglas to investigate low-g propellant acquisition and gaging. The first experiment was flown on shuttle mission 41-G in October 1984. The second experiment has been qualified for flight and is waiting for a flight assignment. The tests performed to qualify these experiments for flight are described, and the lessons learned which can be applied to future GAS experiments are discussed. Finally, survey results from 134 GAS experiments flown to date are presented. On the basis of these results it is recommended that future GAS experiments be qualified to shuttle thermal and dynamic environments through a rigorous series of mission operating tests. Furthermore, should automatic activation of the experiment be required during the boost phase of the mission, NASA-supplied redundant barometric switches should be employed to trigger the activation.

Orton, George↗

NASA NTRS1986

Persistent time intervals between features in solar flare hard X-ray emission

Several solar hard X-ray events (greater than 100 keV) were observed simultaneously with identical instruments on the Venera 11, 12, 13, 14, and Prognoz spacecraft. High time resolution (= 2 ms) data were stored in memory when a trigger occurred. The observations of modulation are presented with a period of 1.6 s for the event on December 3, 1978. Evidence is also presented for fast time fluctuations from an event on November 6, 1979, observed from Venera 12 and another on September 6, 1981, observed from the Solar Maximum Mission. Power spectrum analysis, epoch folding, and Monte Carlo simulation were used to evaluate the statistical significance of persistent time delays between features. The results are discussed in light of the MHD model proposed by Zaitsev and Stepanov.

Desai, Upendra D.↗

NASA NTRS1987

Formation mechanisms of laboratory double layers

The evolution processes of double layers were studied in a series of laboratory experiments. It was found that the existence of virtual cathode-type potential wells at the electron injection boundary was the dominant triggering mechanism. The rapid growth of the potential wells led to collisionless ion trapping and the establishment of the necessary trapped ion population. For double layers with small potential drops, collisionless ion trapping actually induced ion-ion streaming instabilities and the formation of ion phase-space vortices. In this regime, the system often exhibited relaxation-type oscillations which correspond to the disruption and the recovery of the double layers.

Chan, Chung↗

NASA NTRS1987

Neutral hydrogen and star formation in irregular galaxies

The Very Large Array and WSTR H I synthesis observations of seven irregular galaxies are presented. The total H I images of four Local Group dwarf irregular galaxies and three larger more distant irregular galaxies are constructed at the identical resolution of 500 pc. When compared to H II region distributions derived from H alpha images, all galaxies studied show an excellent correlation between the H I surface density and the presence of H II regions. This correlation is most easily interpreted in terms of a requisite threshold H I surface density for massive star formation. This threshold is 1 x 10 to the 21st power H I atoms/sq cm for a resolution of 500 pc. Giant extragalactic H II regions are only found near H I surface densities of a factor of 3 to 5 times this threshold level. The observed threshold implies a Jeans length of 150 pc, which is the same as the size scale at which the structure in the H I complexes correlates well with the H II region distribution. This, combined with the fact that in none of the galaxies observed is there H I above the threshold level with concomitant H II regions, implies an exclusively gravitational origin for the star formation events. That is, there is no need to involve a trigger as in the SSPSF theory (Seiden 1983) or feedback as in Dopita (1985).

Skillman, Evan D.↗

NASA NTRS1987

The history of gas in sprial galaxies

The general association of luminous young stars with spiral arms in galaxies has led to widespread acceptance of the idea that the formation of massive stars, at least, is somehow triggered by the interaction of interstellar gas clouds with a spiral density wave. A very simple model for the gas in a spiral galaxy, with a specified initial surface density and angular velocity is examined. Typical results from this simple model, with parameters appropriate to NGC 6946, are shown. The accuracy of the presumption that the molecular gas distributions in galaxies is based upon observations of CO J = 1-0 emission, is discussed.

Maloney, Philip↗

NASA NTRS1987

Induced star formation in interacting galaxies

Measurements of H alpha emission line fluxes and FIR fluxes in approx. 100 interacting spirals were used to investigate the effects of close tidal interactions on the disk and nuclear star formation rates in galaxies. Two samples of interacting spirals were studied, a complete sample of close pairs, and a set of strongly perturbed systems from the Arp atlas. Both the integrated H alpha luminosities and FIR luminosities are enhanced in the interacting galaxies, indicating that the encounters indeed trigger massive star formation in many cases. The response of individual galaxies is highly variable, however. A majority of the interacting spirals exhibit normal star formation rates, while a small fraction are undergoing bursts with luminosities which are rarely, if ever, observed in noninteracting systems. Virtually all of the latter are in the Arp sample, indicating that the Arp atlas is heavily biased to the most active star forming systems.

Kennicutt, R. C.↗

NASA NTRS1987

Cloud fluid models of gas dynamics and star formation in galaxies

The large dynamic range of star formation in galaxies, and the apparently complex environmental influences involved in triggering or suppressing star formation, challenges the understanding. The key to this understanding may be the detailed study of simple physical models for the dominant nonlinear interactions in interstellar cloud systems. One such model is described, a generalized Oort model cloud fluid, and two simple applications of it are explored. The first of these is the relaxation of an isolated volume of cloud fluid following a disturbance. Though very idealized, this closed box study suggests a physical mechanism for starbursts, which is based on the approximate commensurability of massive cloud lifetimes and cloud collisional growth times. The second application is to the modeling of colliding ring galaxies. In this case, the driving processes operating on a dynamical timescale interact with the local cloud processes operating on the above timescale. The results is a variety of interesting nonequilibrium behaviors, including spatial variations of star formation that do not depend monotonically on gas density.

Struck-Marcell, Curtis↗