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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Turbulence structure in clear and cloudy regions of the 7 July 1987 Electra mission

The 7 July mission of the 1987 FIRE Marine Stratocumulus Intensive Field Observations was chosen for analysis because of a well-defined transition from stratocumulus to clear conditions sampled by the aircraft on this day. It is hoped that by studying this case something can be learned about the processes responsible for the maintenance and breakup of stratocumulus layers, a primary objective of FIRE. The preliminary analysis is based on data from the Electra flight of this day. The properties of turbulence elements, i.e., updrafts and downdrafts, are examined to gain information on the nature of the turbulent exchanges through the boundary layer and across the inversion. Since such exchanges in large measure determine the stability and structure of cloud layers, a study of draft properties should be informative. The results will also be useful in the development of boundary layer models that are based on draft circulations (e.g., Randall, 1988; Hanson, 1988).

Khalsa, Siri Jodha Singh↗
NASA NTRS1990

A cloudiness transition in a marine boundary layer

Boundary layer cloudiness plays several important roles in the energy budget of the earth. Low level stratocumulus are highly reflective clouds which reduce the net incoming shortwave radiation at the earth's surface. Climatically, the transition to a small area fraction of scattered cumulus clouds occurs as the air flows over warmer water. Although these clouds reflect less sunlight, they still play an important role in the boundary layer equilibrium by transporting water vapor upwards, and enhancing the surface evaporation. The First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) included a marine stratocumulus experiment off the southern California coast from June 29 to July 19, 1987. The objectives of this experiment were to study the controls on fractional cloudiness, and to assess the role of cloud-top entrainment instability (CTEI) and mesoscale structure in determining cloud type. The focus is one research day, July 7, 1987, when coordinated aircraft missions were flown by four research aircraft, centered on a LANDSAT scene at 1830 UTC. The remarkable feature of this LANDSAT scene is the transition from a clear sky in the west through broken cumulus to solid stratocumulus in the east. The dynamic and thermodynamic structure of this transition in cloudiness is analyzed using data from the NCAR Electra. By averaging the aircraft data, the internal structure of the different cloud regimes is documented, and it is shown that the transition between broken cumulus and stratocumulus is associated with a change in structure with respect to the CTEI condition. However, this results not from sea surface temperature changes, but mostly from a transition in the air above the inversion, and the breakup appears to be at a structure on the unstable side of the wet virtual adiabat.

Betts, Alan K.↗
NASA NTRS1990

Laboratory experiments on stability and entrainment of oceanic stratocumulus. Part 2: Entrainment experiment

A stratified interface is stable to the buoyancy reversal instability for surprisingly large values of D (buoyancy reversal parameter). A new instability mechanism is proposed, which considers the mixing process at the interface. For the type of density curves studied here, under strong perturbations, the mixed parcel must have a buoyancy reversal comparable to the initial stratification before the interface is unstable. This is in accord with a simple model of the interface mixing process, as well as aircraft observations of long-live marine stratocumulus clouds. These clouds' remarkable longevity in the face of finite D indicates that they can be stable (Hanson, 1984; Albrecht et al., 1985; Siems et al., 1989). It is suggested that buoyancy reversal as well as the disturbance must be large for Cloudtop Entrainment Stability. The effect of buoyancy reversal (evaporative cooling) does not always enhance the entrainment rate over that in the inert case, but it may be negligible if Ri (Richardson number) is large (Ri is larger than 50) and D is small (D is smaller than 0.5). This work may shed some light on the fundamental mechanism of the breakup process of the subtropical stratocumulus clouds into tradewind cumulus. These results may also be related to the instability in the Weddell Sea off of Antarctica.

Shy, Shenqyang S.↗
NASA NTRS1989

The Atlas/Centaur lightning strike incident

Consideration is given to an incident on March 26, 1987, in which an Atlas/Centaur 67 vehicle carrying a naval communication satellite was struck by a triggered, cloud-to-ground lightning flash about 48 s into the flight. The lightning current coupled a transient signal into the wiring of the Centaur digital computer unit, ultimately causing the breakup of the vehicle. The meteorological conditions at the time of the launch and the lightning flash triggered by the Atlas/Centaur 67 vehicle are analyzed on the basis of studies of the incident by the NASA Investigation Board.

Christian, H. J.↗
NASA NTRS1989

Protostellar formation in rotating interstellar clouds. VIII - Inner core formation

The results are presented of a variety of spherically symmetric one-dimensional (1D) calculations intended to determine the robustness of the dynamical hiccup phenomenon in protostellar cores. The 1D models show that the phenomenon is relatively insensitive to changes in the equations of state, numerical resolution, initial density and temperature, and the radiative transfer approximation. In 1D, the hiccup results in an explosive destruction of the entire inner protostellar core. Inner core formation is studied with a sequence of three-dimensional models which show that rapid inner core rotation stabilizes the hiccup instability. Instead, the inner core becomes quite flat and undergoes a cycle of binary fragmentation, binary decay into a single object surrounded by a bar, breakup of the bar into a binary, etc. When lesser amounts of rotation are involved, the inner core does hiccup somewhat, but mass is ejected in only a few directions, leading to several broad streams of ejecta.

Boss, Alan P.↗
NASA NTRS1989

Shuttle seated extraction feasibility study

Following the Space Shuttle Challenger accident, serious attention has turned to in-flight escape. Prior to the resumption of flight, a manual bailout system was qualified and installed. For the long term, a seated extraction system to expand the escape envelope is being investigated. This paper describes a 1987 study, conducted jointly by NASA/Johnson Space Center and Langley Research Center, to determine the feasibility of modifying the Space Shuttle Orbiters to incorporate the seated extraction system. Results of the study are positive, indicating retrofit opportunity and high probability of escape for early ascent, late entry, and even for uncontrolled flight such as the Challenger breakup. The system, as envisioned, can extract seven crewmembers within two seconds.

Onagel, Steven R.↗
NASA NTRS1989

Potential SPOT-1 R/B-Cosmos 1680 R/B collision

Detailed NORAD data have revealed updated orbital elements for the Ariane third-stage rocket body that underwent breakup on November 13, 1986, as well as for the Cosmos 1680 rocket body. Applying the maximum expected error due to the extrapolation of orbital elements to the date of the possible collision between the two bodies shows the smallest possible distance between bodies to have been 380 km, thereby precluding collision.

Henize, Karl G.↗
NASA NTRS1989

Current operational practice for the Delta second stage

At the end of operation for the Delta launch vehicle's second-stage propellant tankage, 250 lb of propellants remain. Analyses of on-orbit breakups that may have been caused by fuel tank overpressurization have led to the adoption of a restart operational policy in which the engine is fired for as long as necessary to ensure that as much oxidizer as possible is vented from the tank; tank pressures are thereby reduced to about 75 psi. None of the 12 experimentally restarted stages have broken up since 1981.

Gray, Almon A., Jr.↗
NASA NTRS1989

The Shallowater aubrite - Evidence for origin by planetesimal impacts

This paper presents the results of petrographic, mineralogic, and bulk trace element analyses of Shallowater aubrite samples, and interprets them in terms of the meteorite's origin. On the basis of these analyses, it is argued that Shallowater did not form on the aubrite parent body but represents a fourth enstatite meteorite body (in addition to the H and L enstatite chondrite and aubrite parent bodies). It is shown that its peculiar composition, texture, and cooling history can best be explained by an origin involving a low-velocity collision and breakup of a molten or a partly molten asteroid, consisting largely of a melt of MgSiO3 composition, with a solid, enstatite meteoritelike object.

Keil, Klaus↗
NASA NTRS1990

Unsteady aerodynamic characteristics of a fighter model undergoing large-amplitude pitching motions at high angles of attack

The effects of harmonic or constant-rate-ramp pitching motions (giving angles of attack from 0 to 75 deg) on the aerodynamic performance of a fighter-aircraft model with highly swept leading-edge extensions are investigated experimentally in the NASA Langley 12-ft low-speed wind tunnel. The model configuration and experimental setup are described, and the results of force and moment measurements and flow visualizations are presented graphically and discussed in detail. Large force overshoots and hysteresis are observed and attributed to lags in vortical-flow development and breakup. The motion variables have a strong influence on the persistence of dynamic effects, which are found to affect pitch-rate capability more than flight-path turning performance.

Brandon, Jay M.↗
NASA NTRS1990

Stratospheric temperatures during the 88-89 Northern Hemisphere winter

The Airborne Arctic Stratospheric Expedition (AASE) was conducted during January and February 1989. The polar stratosphere during this period was characterized by cold conditions from January to mid-February. A mid-February wave 2 major warming considerably warmed the polar stratosphere, but did not immediately lead to a lower stratospheric vortex breakup. As inferred from temperature data, January temperatures were sufficient for polar stratospheric cloud (PSC) formation. PSC regional extent was greater than the long-term average, but not significantly larger than in previous years.

Newman, Paul A.↗
NASA NTRS1990

Global impact of the Antarctic ozone hole - Chemical propagation

A model is presented for the chemical mixing of stratospheric air, that combines photochemistry, molecular diffusion, and strain (i.e., the stretching of air parcels due to wind shear). The model is applied to the case in which chemically perturbed air parcels from the Antarctic stratosphere are transported to mid-latidudes and strained into thin ribbon-like filaments until they are diffusively mixed with the ambient stratosphere. Results show that, following the breakup of the polar vortex, Antarctic air with substantially depleted ozone will not contribute any additional ozone loss at mid-latitudes as it is mixed with ambient air, supporting the results of simulations of the Antarctic ozone hole by Prather et al. (1990). Nevertheless, air processed by polar stratospheric clouds but transported to mid-latitudes before substantive ozone depletion, can lead to additional loss after mixing with ambient air.

Prather, Michael↗
NASA NTRS1990

A review of orbital debris environment modeling at NASA/JSC

Orbital debris modeling in three areas is presented in this paper. These are: (1) debris environment evolution, as characterized by a numerical processor using detailed mission model projections, (2) debris cloud evolution for the time immediately after breakup, and (3) two-dimensional flux density modeling to characterize both flux levels and directionality for a debris environment.

Reynolds, Robert C.↗
NASA NTRS1990

Investigation of several passive and active methods for turbulent flow separation control

Relative performance of several passive and active methods for controlling two-dimensional turbulent separated flow associated with a curved backward-facing ramp were investigated at low speeds. Surface static pressure measurement and oil flow visualization results indicate that submerged vortex generators, vortex generator jets, elongated arches at +-alpha, and large-eddy breakup devices at +-alpha placed near the baseline separation location reduce flow separation and increase pressure recovery. Spanwise cylinders reduce flow separation but decrease pressure recovery downstream. Arches with alpha = 0 deg, Helmholtz resonators, and Viets' fluidic flappers examined so far have no significant effect in reducing separation. Wall cooling computation indicates that separation delay on a partially cooled ramp is nearly the same as on a fully-cooled ramp while minimizing the frictional drag increase associated with the wall cooling process.

Lin, J. C.↗
NASA NTRS1990

A cloudiness transition in a marine boundary layer

In situ aircraft data and lidar data are used to analyze a transition in the boundary layer thermodynamic structure from a clear boundary layer through small cumulus and broken stratocumulus to a deck of solid stratocumulus. The data was collected in conjunction with a Landsat overpass on July 7, 1987 off the coast of southern California. A steady progression in mixing line stability is seen associated with the change in cloudiness. The (empirically based) stability threshold for the breakup of this stratocumulus is that the slope of the mixing line is 0.66 + or - 0.04 of the slope of the wet virtual adiabat. A simple linear parameterization is proposed for cloud fraction in terms of mixing line stability. Surface flux measurements are consistent with bulk aerodynamic estimates.

Boers, Reinout↗
NASA NTRS1990

Observations of formaldehyde in Comet Machholtz (1988j)

Comet Machholtz's 1(11)-1(10) formaldehyde emission was detected with the VLA in September, 1988 at 6-cm wavelength; the small blueshift of -0.76 + or - 0.40 km/sec noted by these observations is consistent with an anisotropic outgassing of the cometary nucleus in the solar direction. The derived formaldehyde production rate for Comet Halley, which exhibits a similar emission line blueshift, was 1.5 x 10 to the 28th mol/sec, while for Comet Machholtz the production rate is about an order of magnitude higher, at 2 x 10 to the 29th mol/sec. The fading of Comet Machholtz after these observations is ascribed to its breakup.

Snyder, Lewis E.↗
NASA NTRS1990

Gas-phase flowrate effect on disintegrating cryogenic liquid-jets

Two phase liquid and gaseous nitrogen flow in a pneumatic two fluid atomizer was investigated. Characteristic dropsize for cryogenic sprays were measured with a scattered light scanning instrument developed at NASA-Lewis. Tests were conducted primarily in the aerodynamic stripping regime of liquid jet atomization. At a sampling distance of bar-x =1.3 cm, the Sauter mean, D(sub 32), and volume median, D(sub v.5), drop diameters were measured and correlated with nitrogen gas flowrate, W(sub n), to give the following expressions: D sub 32 to the -1=210W(sub n) to the -1.33 and D(sub v.5) to the -1=150 W(sub n to the -1.33, where reciprocal diameters and gas flowrate are in cm (-1) and g/sec, respectively. The exponent 1.33 for nitrogen gas flowrate, W(sub n), is the same as that predicted by atomization theory for liquid-jet breakup in high velocity gasflow. When the spray was sampled at axial distances of bar-x=2.5 and 4.5 cm downstream of the atomizer, the exponent decreased to 1.2 and 0.9, respectively. This was attributed to the loss of small droplets due to their rapid vaporization.

Ingebo, Robert D.↗
NASA NTRS1990

Dropsize correlation for cryogenic liquid-jet atomization

Momentum transfer from high velocity nitrogen gas flow to liquid-nitrogen jets was investigated. A correlation of aerodynamic and liquid-surface forces with characteristic drop diameter was obtained for cryogenic liquid-jet breakup in Mach 1 gas flow. Nitrogen gas mass-flux was varied by using three differently sized two-fluid fuel atomizers with different nozzle diameters.

Ingebo, Robert D.↗