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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

High resolutions spectroscopy of P/Halley

High resolution spectra of the coma of P/Halley were obtained on 25 April, 2 May, and 3 May 1986. The self-scanned Digicon detector of the 2.7 m telescope coude spectrograph was used to obtain spectra at a resolution of 0.6A. Results of the observation of the NH (A 3 Pi i yields X 3 Sigma (-)) band near 3360 A are presented. The summed NH spectrum represents the results of 190 min of integration centered on the nucleus of the comet. Eight rotational lines from the P and R branches of the band are detected. The lines from the Q branch are especially weak, and are not obviously resolved in the spectrum. The data are compared with the theoretical emission spectrum of Litvak and Kuiper (1982) and substantial discrepancies are found. These discrepancies are explained through a consideration of the relative timescales for collisions, resonance fluorescence, and radiative decays of the NH molecules in the coma.

Cochran, W. D.↗
NASA NTRS1976

The frequency dependence of the viscous component of the magnetic susceptibility of lunar rock and soil samples

The susceptibility of two lunar samples (a soil and a low metamorphic grade breccia) has been measured in a weak field - 0.001 Oe - and as a function of frequency from 0.032 to 1.0 Hz. The measurements were made using a superconducting magnetometer. The results show that the susceptibility decreases linearly with the log of frequency. This observation is in agreement with a theoretical model for viscous decay based on the Neel theory of single-domain and superparamagnetic grains. The relation derived agrees with a model in which there is a uniform distribution of relaxation times.

Hanneken, J. W.↗
NASA NTRS1997

An Experimental Investigation of Steady and Unsteady Flow Field in an Axial Flow Turbine

Measurements were made in a large scale single stage turbine facility. Within the nozzle passage measurements were made using a five hole probe, a two-component Laser Doppler Velocimeter (LDV), and a single sensor hot wire probe. These measurements showed weak secondary flows at midchord, and two secondary flow loss cores at the nozzle exit. The casing vortex loss core was the larger of the two. At the exit radial inward flow was found over the entire passage, and was more pronounced in the wake. Nozzle wake decay was found to be more rapid than for an isolated vane row due to the rotor's presence. The midspan rotor flow field was measured using a two-component LDV. Measurements were made from upstream of the rotor to a chord behind the rotor. The distortion of the nozzle wake as it passed through the rotor blade row was determined. The unsteadiness in the rotor flow field was determined. The decay of the rotor wake was also characterized.

Zaccaria, M.↗
NASA NTRS1993

Backscatter from ice growing on shallow tundra lakes near Barrow, Alaska, winter 1991-1992

The timing of freeze-up and break-up of Arctic lake ice is a potentially useful environmental indicator that could be monitored using SAR. In order to do this, it is important to understand how the properties and structure of the ice during its growth and decay affect radar backscatter and thus lake ice SAR signatures. The availability of radiometrically and geometrically calibrated digital SAR data time series from the Alaska SAR Facility has made it possible for the first time to quantify lake ice backscatter intensity (sigma(sup o)) variations. This has been done for ice growing on shallow tundra lakes near Barrow, NW Alaska, from initial growth in September 1991 until thawing and decay in June 1992. Field and laboratory observations and measurements of the lake ice were made in late April 1992. The field investigations of the coastal lakes near Barrow confirmed previous findings that, (1) ice frozen to the lake bottom had a dark signature in SAR images, indicating weak backscatter, while, (2) ice that was floating had a bright signature, indicating strong backscatter. At all sites, regardless of whether the ice was grounded or floating, there was a layer of clear, inclusion-free ice overlaying a layer of ice with dense concentrations of vertically oriented tubular bubbles. At some sites, there was a third layer of porous, snow-ice overlaying the clear ice.

Jeffries, M. O.↗
NASA NTRSDate not supplied

Simulation and Modeling of Hypersonic Turbulent Boundary Layers Subject to Favorable Pressure Gradients due to Streamline Curvature

Direct numerical simulations (DNS) of favorable-pressure-gradient turbulent boundary layers are presented for a nominal freestream Mach number of 5, with the objective of assessing the limitations of the currently available Reynolds-averaged Navier-Stokes (RANS) models. The favorable pressure gradient is induced by the streamwise curvature of the two-dimensional, planar, convex measurement surface used during experiments at the Texas A&M University. The DNS data shows good comparison with the measured velocity profiles, strain rates, and some, but not all, of the Reynolds-stress components. The discrepancies between the predicted and the measured wall-normal as well as shear stress components are primarily attributed to the lower than actual values inferred from typical PIV measurements of turbulent boundary layers. The DNS data shows a zero or slightly negative Reynolds shear stress in the outer part of the boundary-layer, which is indicative of the decaying turbulent motion under a strong favorable pressure gradient. The DNS data is also compared with the results of RANS computations based on commonly used zero, one, and two equation eddy-viscosity models. The RANS models yield reasonable comparisons with the DNS-based skin friction under zero and weak pressure gradients, but significant discrepancies under a strong pressure gradient. The k-w SST model provided the best overall predictions of skin friction, except in the region where the flow transitions from a favorable to an adverse pressure gradient. While the RANS models examined herein also give good predictions of the Reynolds shear stress under a sufficiently weak pressure gradient, none of those models are able to appropriately capture the reduction in the Reynolds stresses when the flow was subjected to a strong pressure gradient. An a priori assessment of the turbulent heat-flux prediction based on the assumption of a constant turbulent Prandtl number with the DNS data shows that while the constant turbulent Prandtl number model is effective in predicting the wall-normal component of turbulent heat flux, it does not capture the turbulent heat transfer in the streamwise direction for all the pressure gradient cases. The failure of the constant turbulent Prandtl number model highlights a requirement for more advanced models of the turbulent heat flux.

turbulent boundary layers↗
NASA NTRS1981

Surface magnetic fields during the solar activity cycle

The behavior of the magnetic field of the sun as measured in the Fe I 5250 A line is summarized. A latitudinal distribution of the fields observed over 13.5 yr is presented, and episodic formation of the polar fields (about 6 gauss) from a poleward drift originating at the sunspots is noted. Weak magnetic fields (-2 to +2) reach a maximum two years before the maximum in the average field. The total flux remains constant from minimum to maximum, with strong magnetic flux exhibiting an equatorward drift; both strong and new flux appear mostly around sunspots. The appearance of new flux implies the decay of flux elsewhere on the sun, because of the measured constancy of total flux; total replacement of the surface flux can occur within ten days. Field annihilation is concentrated in the active latitudes, where strong opposite polarity fields occur close together.

Howard, R.↗
NASA NTRS1997

ASCA Observations of the Barnard 209 Dark Cloud and an Intense X-Ray Flare on V773 Tauri

ASCA (Advanced Satellite for Cosmology and Astrophysics) detected an intense X-ray flare on the weak-lined T Tauri star V773 Tau (=HD 283447) during a 30 ks observation of the Barnard 209 dark cloud in 1995 September. This star is a spectroscopic binary and shows signs of strong magnetic surface activity including a spot-modulated optical light curve. The flare was seen only during its decay phase but is still one of the strongest ever recorded from a T Tauri star with a peak luminosity L(sub x) = 10(exp 32.4) ergs/s (0.5-10 keV), a maximum temperature of at least 42 million K, and energy release of greater than 10(exp 37) ergs. A shorter ASCA observation taken five months later showed V773 Tau in a quiescent state (L(sub x)= 10(exp 31.0) ergs/s) and detected variable emission from the infrared binary IRAS 04113+2758. The differential emission measure (DEM) distribution during the V773 Tau flare shows a bimodal temperature structure that is almost totally dominated by hot plasma at an average temperature of approx. 37 million K. Using information from time-resolved spectra, we examine the flare decay in terms of solar flare models (cooling loops and two-ribbon flares) and also consider possible nonsolar behavior (interbinary flares, star-disk flares, and rotational X-ray modulation). Solar models are unable to reproduce the unusual convex-shaped X-ray light curve, which decays slowly over a timespan of at least 1 day. However, the light curve decay is accurately modeled as a sinusoid with an inferred X-ray period of 2.97 days, which is nearly identical to the optical rotation period(s) of the two K-type components. This provides tantalizing evidence that the flaring region was undergoing rotational occultation, but periodic X-ray variability is not yet proven since our ASCA observation spans only one-third of a rotation cycle.

Skinner, Stephen L.↗
NASA NTRS1993

Large eddy simulation of shock turbulence interaction

A nonconservative formulation of the energy equation (solving for internal energy) was used to perform large eddy simulations of compressible turbulence in Moin et al. due to its simplicity in implementing SGS models compared to the conservative formulation (solving for total energy). In problems with shocks in the domain, however, the total energy formulation is preferred due to its conservative nature. A conservative set of equations for the LES were derived from the nonconservative equations derived by Moin et al. Performance of the conservative formulation was compared with the experiment on decaying grid-generated turbulence as well as with the filtered DNS field. Various shock-capturing schemes were tested, and an ENO shock-capturing scheme of Shu and Osher was chosen for the simulation of shock/turbulence interaction. The scheme was tested and validated against the data base generated by DNS of weak shock waves. The results obtained with the essentially nonoscillatory (ENO) scheme were within 5 percent from the DNS results, and used less than 25 percent of the CPU time used in the DNS.

Lee, Sangsan↗
NASA NTRS2022

Radiative Losses in the Chromosphere During A C-Class Flare

Context. Solar flares release an enormous amount of energy (~10(exp 32) erg) into the corona. A substantial fraction of this energy is transported to the lower atmosphere, which results in chromospheric heating. The mechanisms that transport energy to the lower solar atmosphere during a flare are still not fully understood. Aims. We aim to estimate the temporal evolution of the radiative losses in the chromosphere at the footpoints of a C-class flare, in order to set observational constraints on the electron beam parameters of a RADYN flare simulation. Methods. We estimated the radiative losses from hydrogen, and singly ionized Ca and Mg using semiempirical model atmospheres, which were inferred from a multiline inversion of observed Stokes profiles obtained with the CRISP and CHROMIS instruments on the Swedish 1-m Solar Telescope. The radiative losses were computed taking into account the effect of partial redistribution and non-local thermodynamic equilibrium. To estimate the integrated radiative losses in the chromosphere, the net cooling rates were integrated between the temperature minimum and the height where the temperature reaches 10 kK. We also compared our time series of radiative losses with those from the RADYN flare simulations. Results. We obtained a high spatial-resolution map of integrated radiative losses around the flare peak time. The stratification of the net cooling rate suggests that the Ca IR triplet lines are responsible for most of the radiative losses in the flaring atmosphere. During the flare peak time, the contribution from Ca II H and K and Mgii h and k lines are strong and comparable to the Ca IR triplet (~32kW m(exp -2)). Since our flare is a relatively weak event, the chromosphere is not heated above 11 kK, which in turn yields a subdued Lyα contribution (~7kW m(exp -2)) in the selected limits of the chromosphere. The temporal evolution of total integrated radiative losses exhibits sharply rising losses (0.4kW m(exp -2) (s(exp -1)) and a relatively slow decay (0.23kW m(exp -2) s(exp -1)). The maximum value of total radiative losses is reached around the flare peak time and can go up to 175kWm􀀀2 for a single pixel located at footpoint. After a small parameter study, we find the best model-data consistency in terms of the amplitude of radiative losses and the overall atmospheric structure with a RADYN flare simulation in the injected energy flux of 5 × 10(exp 10) erg s(exp -1) cm(exp -2).

Chromosphere↗
NASA NTRS1983

H0547-14 - X-ray flux from a weak gamma-ray burst?

Data concerning a fast X ray transient observed by the HEAO 1 on March 17, 1978 are discussed. The object, labeled H0547-14, was located in Lepus, with galactic coordinates 219.37 deg longitude, b = -20.12 deg in the anticenter direction. A 10 sec scan of the transient was obtained with proportional counter detectors. An X ray luminosity of more than 5.1 x 10 to the 31st ergs/sec was measured as a lower limit, with the decay taking 17 min. A lower limit to the flare was set at 30 sec. The flux intensity resided in the range 4.4/1 billion-4/100 million erg/sq cm per sec, with a rise time of a factor of 10 in about 5 sec. Possible characterizations of the object, as either a gamma ray burst or as an M dwarf star, are discussed.

Ambruster, C.↗
NASA NTRS1982

Radiation and attenuation of waves in a random medium

The physical mechanisms of excess attenuation are analyzed on the basis of a one-dimensional time-independent model of propagation in a random medium. Attenuation is regarded as the rate of decrease in the mean intensity and the mean energy flux within a propagation range. A source function is assumed to be determinate, appropriate statistical properties are chosen for the sound speed, and specified statistical properties are found for the wave functions, i.e., the mean intensity and the mean energy flux. The medium is considered to be weakly homogeneous, and expansions are developed for the intensity and mean energy flux, along with an attenuation coefficient in two parts, the second of which defines the excess attenuation. The mean radiated power is defined, and backscattering by the random inhomogeneities in the medium is modeled as redistributing the mean intensity and energy flux, with a resultant decay which occurs more quickly than with randomness.

Wenzel, A. R.↗
NASA NTRS1983

Barotropic instability of weakly non-parallel zonal flows

Integrations of the linear stability problem, in the present numerical investigation of weakly nonparallel zonal flows barotropic instability having localized intense shear regions, reveal the existence of unstable localized wave packets. The spatial structure and eigenfrequencies of these packets depend on two parameters measuring the degree of supercriticality and the zonal length scale of the shear region. It is found that instability structure is defined by conditions ensuring the decay of the wave packet at infinity, and the transition from long to short waves across a turning point region that is controlled by nonparallel effects.

Merkine, L.-O.↗
NASA NTRS2000

Tornadic Supercells on May 3, 1999 Viewed from Space During an Overpass of the NASA TRMM Observatory

The NASA Tropical Rainfall Measuring Mission (TRMM) Observatory made an overpass during the Central Oklahoma tornado outbreak at approximately 04:00 LTC on 4 May, 1999 (23:00 CDT on 3 May). At the time of the TRMM snapshot there were 25 individual storms in Oklahoma that could be identified and cross-correlated between the TRMM measurements and the Twin Lakes (KTLX) NEXRAD radar. Of these, six were significant supercells that could be trended for 30-min prior and subsequent to (1 hr total) the TRMM overpass. Thus, we are able to provide a context for the TRMM snapshot with respect to each individual supercell's recent growth, decay, and severe weather producing history. The most vigorous and electrically active storm at the time of the overpass is the Stroud, OK supercell whose F3 tornado ended only about 10 minutes earlier. Another F3 tornado north of Crescent, OK (- 150 km west of Stroud) is on the ground and a supercell with a developing, yet still weak mesocyclone is just south of the Stroud storm. This latter storm produces an F1 tornado an hour later near Sapulpa, OK. The objective of this study is to provide additional insight into the characteristics of the supercell storms through the unique capabilities and vantagepoint of the TRMM science instruments.

Goodman, Steven J.↗
NASA NTRS1987

Third and higher harmonic plasma emission due to Raman scattering

The theory for third and higher harmonic plasma emission by the weak turbulence (or random phase) process L + T prime to T (where L denotes a Langmuir wave, and T and T prime denote transverse waves) is developed. Kinematic constraints on the characteristics and growth lengths of waves participating in the wave processes are identified. The cases of L waves produced either directly by a streaming instability, or by the decay L to L prime + S (S is an ion sound wave) of L waves generated by a streaming instability, are considered. Limits on the brightness temperature of the radiation are determined, and expressions for the growth rate and path-integrated wave temperatures are derived.

Cairns, Iver H.↗
NASA NTRS1995

Kinetic structure of intermediate shocks: Implications for the magnetopause

A general study of the structure and stability of intermediate shocks (IS) in an isotropic plasma is presented using a hybrid as well as a resistive Hall MHD code. Special emphasis is put on the question of whether the rotational layers observed at the magnetopause can be intermediate shocks. The shocks are formed dynamically by the interaction between a flowing plasma and a stationary piston. Coplanar ISs (both strong and weak) are found to be stable in a collisionless plasma. The existence of slow shocks in a high beta plasma is also established for the first time. Noncoplanar ISs are found to be time-dependent, evolving toward a rotational discontinuity (RD) after some characteristic time tau which can be quite long (1000 Omega(exp -1), where Omega is the ion gyrofrequency). The value tau is larger the closer the rotation angle is to 180 deg. Rotations larger than 180 deg are found to be unstable, decaying into a state of minimum shear (i.e., rotation angle less than 180 deg). There are various length scales associated with an IS in the kinetic regime. The shortest scale is found to be the length scale over which rotation of the transverse component of the magnetic field takes place. This scale can have a half width as small as one ion inertial length (c/omega(sub p)) for electron sense rotations and 3c/omega(sub p) for ion sense rotations, for an upstream ion beta of unity. Both of these scales are consistent with the observed thickness at the magnetopause and identical to the corresponding RD scales. A detailed study of the mode conversion of the Alfven ion cyclotron waves (A/IC) waves across both slow and intermediate shocks and the resulting downstream wave spectrum are presented. The possibility that the large number of relfected ions observed at the magnetopause may be due to the presence of strong ISs is considered. The identification of strong ISs and their distinction from RDs should be possible in observations due to significant differences that exist between jump conditions and overall structure of the two discontinuities. The jumps in the plasma parameters across a weak IS are typically small. This together with the fact that the weak ISs and RDs have very similar thickness and other overall properties makes the distinction between weak ISs and RDs in the observations largely inconsequential. However, at large noncoplanarity angles the weak IS approaches the RD limit in a relatively short time (approximately less than 100 Omega (exp -1)). Thus, magnetopause rotations with large noncoplanarity angles are most likely either RDs or strong ISs. Finally, direct comparisons between fluid (resistive Hall MHD) and kinetic simulations show that fluid theory is not applicable to study of ISs in a collisionless plasma.

Karimabadi, H.↗
NASA NTRS1985

Ephemeral regions versus pseudo ephemeral regions

New studies of the quiet Sun reveal that ephemeral active regions constitute minority rather than a majority of all the short lived, small scale bipolar features on the Sun. In contrast to the recognized patterns of growth and decay of ephemeral regions, various examples of the creation of other temporary bipoles nicknamed pseudo ephemeral regions are illustrated. The pseudo ephemeral regions are the consequence of combinations of small scale dynamic processes of the quiet Sun including: (1) fragmentation of network magnetic fields, (2) the separation of opposite polarity halves of ephemeral regions as they grow and evolve, and (3) the coalescence of weak network or intranetwork magnetic fields. New observations offer the possibility of resolving the discrepancies that have arisen in the association of ephemeral regions with X-ray bright points. Many X-ray bright points may be related to those pseudo ephemeral regions which have begun to exhibit magnetic flux loss.

Martin, S. F.↗
NASA NTRS1996

Active Sonic Boom Control

A theory and simulation code are developed to study nonsteady sources as means to control sonic booms of supersonic aircraft. A key result is that the source of sonic boom pressure is not confined to the length of the aircraft but occupies an extensive segment of flight path. An aircraft in nonsteady flight functions as a synthetic aperture antenna, generating complex acoustic waves with no simple relation to instantaneous volume or lift distributions. The theory applies linear acoustics to slender nonsteady sources but requires no far field approximation. The solution for pressure contains a term not seen in Whitham's theory for sonic booms of distant supersonic aircraft. The term describes a pressure field that decays algebraically behind the Mach cone and, in the case of steady flight, integrates to a ground load equal to the weight of the aircraft. The algebraic term is separate from those that describe the sonic boom. Two nonsteady source phenomena are evaluated: periodic velocity changes (surge), and periodic longitudinal lift redistribution (slosh). Surge can attenuate a sonic boom and convert it into prolonged weak reverberation, but accelerations needed to produce the phenomenon seem too large for practical use. Slosh may be practical and can alter sonic booms but does not, on the average, result in boom attenuation. The conclusion is that active sonic boom abatement is possible in theory but maybe not practical

Crow, Steven C.↗
NASA NTRS2007

On the Propagation and Interaction of Spherical Blast Waves

The characteristics and the scaling laws of isolated spherical blast waves have been briefly reviewed. Both self-similar solutions and numerical solutions of isolated blast waves are discussed. Blast profiles in the near-field (strong shock region) and the far-field (weak shock region) are examined. Particular attention is directed at the blast overpressure and shock propagating speed. Consideration is also given to the interaction of spherical blast waves. Test data for the propagation and interaction of spherical blast waves emanating from explosives placed in the vicinity of a solid propellant stack are presented. These data are discussed with regard to the scaling laws concerning the decay of blast overpressure.

Kandula, Max↗