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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

The solar wind interaction with Mars revisited

No consensus has yet been reached on the nature of the solar wind interaction with Mars, especially with respect to the existence of a small, intrinsic field magnetosphere at Mars, as opposed to a Venus-like ionospheric interaction. An attempt is made to resolve this question through an improved determination of effective obstacle altitude and shape having as its basis the Spreiter and Stahara (1980) catalog of gasdynamic flow solutions. It is also shown that the Martian ionosphere cannot support a Venus-like ionopause at the obstacle altitudes inferred through bow wave observation modelling, even when maximal induced ionospheric magnetic fields and solar maximum EUV levels are assumed. Finally, a search of Mars and Mariner radio occultation measurements has produced no evidence for the existence of an ionopause. It is concluded that Mars probably possesses a small, intrinsic field magnetosphere.

Slavin, J. A.↗
NASA NTRS1984

Space and Time Variations and Turbopause Dynamical Structure

Time spectra of the critical frequency f sub o E sub s and blanketing frequency f sub b E sub s for the records of a spaced chain of ionosphere vertical sounding stations were studied by means of a maximum entropy method, while coherence spectra were analyzed with the Blackman and Tukey method. It was found out that the frequency parameters spectrum of E sub s within the range of 1-10 cycles/h are discrete with one or two maxima. The first, a low frequency maximum with T=40 min, is of larger amplitude, stable and exists permanently; the second one with T = 10 min is less stable, occurs irregularly and more often within f sub o E sub s. Further, the first maximum corresponds to a higher level of coherence which decreases as the distance to the stations increases. It is concluded that the low-frequency maximum is induced by cellular eddies, usually interpreted as wind shears with horizontal dimensions of not more than 300 km. Irregularity, small amplitude, and low coherence in the range of the second maximum are indicative of the fact that turbulence must be its only source.

Ovezgeldiyev, O. G.↗
NASA NTRS1986

Optimal flight trajectories in the presence of windshear, 1984-86

Optimal flight trajectories were determined in the presence of windshear and guidance schemes were developed for near optimum flight in a windshear. This is a wind characterized by sharp change in intensity and direction over a relatively small region of space. This problem is important in the takeoff and landing of both civilian airplanes and military airplanes and is key to aircraft saftey. The topics covered in reference to takeoff problems are: equations of motion, problem formulation, algorithms, optimal flight trajectories, advanced guidance schemes, simplified guidance schemes, and piloting strategies.

Miele, A.↗
NASA NTRS1989

Wind-tunnel investigations of wings with serrated sharp trailing edges

Exploratory wind-tunnel force measurements are presented for two wing geometries with small-scale planar and nonplanar serrated trailing-edge devices (chord-Reynolds numbers ranged from 1.0 to 3.7 million). The planar serrated trailing-edge extensions reduced the drag at conditions when trailing-edge separation occurred at low angles of attack. The introduction of serrations reduced or eliminated the drag penalty, due to the small (1-2 percent of the chord length) nonplanar trailing-edge flaps, while maintaining the effects of increase in camber. The presence of streamwise vortices immediately downstream of the serrated trailing edges is believed to have favorably affected the boundary-layer flow approaching the trailing edge and the near-wake development, resulting in reduced pressure (form) drag.

Vijgen, P. M. H. W.↗
NASA NTRS1989

Wind-tunnel investigations of wings with serrated sharp trailing edges

Exploratory wind-tunnel force measurements are presented for two wing geometries with small-scale planar and nonplanar serrated trailing-edge devices (chord-Reynolds numbers ranged from 1.0 - 3.7 x 10 to the 6th). The planar serrated trailing-edge extensions reduced the drag at conditions when trailing-edge separation occurred at low angles of attack. The introduction of serrations reduced or eliminated the drag penalty due to the small (1-2 percent of the chord length) nonplanar trailing-edge flaps, while maintaining the effects of increase in camber. The presence of streamwise vortices immediately downstream of the serrated trailing edges is believed to have favorably affected the boundary-layer flow approaching the trailing edge and the near-wake development, resulting in reduced pressure drag.

Vijgen, P. M. H. W.↗
NASA NTRS1992

Velocity shear generation of solar wind turbulence

A two-dimensional incompressible MHD spectral code is used to show that shear-driven turbulence is a possible means for producing many observed properties of the evolution of the magnetic and velocity fluctuations in the solar wind and, in particular, the evolution of the cross helicity ('Alfvenicity') at small scales. It is shown that large-scale shear can nonlinearly produce a cascade to smaller scale fluctuations even when the linear Kelvin-Helmholtz mode is stable, and that a roughly power law inertial range is established by this process. The evolution found is similar to that seen in some other simulations of MHD turbulence.

Roberts, D. A.↗
NASA NTRS2001

Stellar Evolution in Starburst Galaxies

The main thrust of the program was to obtain UV spectroscopy of a number of massive and hot luminous (OB type) stars in the nearby galaxy called the Small Magellanic Cloud (SMC). The objective was to analyze their atmospheres and winds so as to determine the effect of the lower abundance of the SIVIC on these parameters. Furthermore, the differences in evolution could be investigated. Additionally, the UV spectra themselves would be suitably weighted and systematically combined to provide a template for comparison to very distant galaxies formed in the early history of the Universe which also have a low abundance of elements. The spectra have been obtained and the analysis is proceeding, primarily by the groups in Munich and at STScl who are the leads for this project. Given the important role of the nearby SMC galaxy as a template of low metal abundance, I have begun to investigate the YOUNGEST phases of massive star birth, before the most massive and hottest stars become optically visible. Typically these stars form in clusters, in some cases having tens to hundreds of OB type stars. In this phase, each star is still buried in its natal cloud and visible only in the infrared (IR) from its self-heated dust and/or from radio free-free emission of the surrounding hydrogen (HII) region. Efforts to find and identify these buried clusters were conducted using a large radio telescope. A number of these were found and further analysis of the data is underway. These clusters are not visible optically, but ought to be seen in the IR, and are a likely topic for HST photometry on NICMOS. A proposal to do this will be made next semester. These objects are the precursors of the optically visible clusters that contain massive and hot luminous stars.

Conti, Peter↗
NASA NTRS2004

Simultaneous Chandra X-ray, HST Ultraviolet, and Ulysses Radio Observations of Jupiter's Aurora

Observations of Jupiter carried out by the Chandra ACIS-S instrument over 24-26 February, 2003, show that the auroral X-ray spectrum consists of line emission consistent with high-charge states of precipitating ions, and not a continuum as might be expected from bremsstrahlung. The part of the spectrum due to oxygen peaks around 650 eV, which indicates a high fraction of fully-stripped oxygen in the precipitating ion flux. A combination of the OVIII emission lines at 653 eV and 774 eV, as well as the OVII emission lines at 561 eV and 666 eV, are evident in the measure auroral spectrum. There is also line emission at lower energies in the spectral region extending from 250 to 350 eV, which could be from sulfur and/or carbon. The Jovian auroral X- ray spectra are significantly different from the X-ray spectra of comets. The charge state distribution of the oxygen ions implied by the measured auroral X-ray spectra strongly suggests that, independent of the source of the energetic ions - magnetospheric or solar wind - the ions have undergone additional acceleration. This spectral evidence for ion acceleration is also consistent with the relatively high intensities of the X-rays compared to the available phase space density of the (unaccelerated) source populations of solar wind or magnetospheric ions at Jupiter, which are orders of magnitude too small to explain the observed emissions. The Chandra X-ray observations were executed simultaneously with observations at ultraviolet wavelengths by the Hubble Space Telescope and at radio wavelengths by the Ulysses spacecraft. These additional data sets suggest that the source of the X-rays is magnetospheric in origin, and that the precipitating particles are accelerated by strong field-aligned electric fields, which simultaneously create both the several-MeV energetic ion population and the relativistic electrons observed in situ by Ulysses that are correlated with approximately 40 minute quasi-periodic radio outbursts.

Elsner, R. F.↗
NASA NTRS2005

The QBO as Potential Amplifier and Conduit to Lower Altitudes of Solar Cycle Influence

In several papers, the solar cycle (SC) effect in the lower atmosphere has been linked to the Quasi-biennial Oscillation (QBO), which is generated primarily by small-scale gravity waves. Salby and Callaghan (2000) analyzed the observed zonal winds of the QBO over more than 40 years and found that it contains a relatively large SC signature at 20 km. Following up on an earlier 2D study with our global-scale Numerical Spectral Model (NSM), we discuss here a 3D study with the QBO under the influence of the SC. For a SC period of 10 years, the amplitude of the relative variations of radiative forcing is taken to vary for simplicity from 0.2% at the surface to 2% at 50 km to 20% at 100 km and above. Covering a limited time span of 40 years, this model produces in the lower stratosphere a relatively large modulation of the QBO, which appears to be related to the SC and is in qualitative agreement with the observations. Some of the energy in the QBO, confined to low latitudes primarily, is redistributed globally by the meridional circulation and planetary waves presumably, so that a measurable SC modulation is generated in the tropospheric temperatures of the polar regions. Further studies are needed, (1) to determine whether the effect is real and prevails in more extensive simulations and whether the results are robust when shorter integration steps are employed, and (2) to explore the mechanism(s) that may ample the apparent SC influence of the UV radiation extending into the lower atmosphere. Quasi-decadal oscillations, generated internally by the QBO interacting with the seasonal cycles, may interfere with or aid the SC effect.

Mayr, Hans G.↗
NASA NTRS2005

Simultaneous Chandra X ray, Hubble Space Telescope Ultraviolet, and Ulysses Radio Observations of Jupiter's Aurora

Observations of Jupiter carried out by the Chandra Advanced CCD Imaging Spectrometer (ACIS-S) instrument over 24-26 February 2003 show that the auroral X-ray spectrum consists of line emission consistent with high-charge states of precipitating ions, and not a continuum as might be expected from bremsstrahlung. The part of the spectrum due to oxygen peaks around 650 eV, which indicates a high fraction of fully stripped oxygen in the precipitating ion flux. A combination of the OVIII emission lines at 653 eV and 774 eV, as well as the OVII emission lines at 561 eV and 666 eV, are evident in the measure auroral spectrum. There is also line emission at lower energies in the spectral region extending from 250 to 350 eV, which could be from sulfur and/or carbon. The Jovian auroral X-ray spectra are significantly different from the X-ray spectra of comets. The charge state distribution of the oxygen ions implied by the measured auroral X-ray spectra strongly suggests that independent of the source of the energetic ions, magnetospheric or solar wind, the ions have undergone additional acceleration. This spectral evidence for ion acceleration is also consistent with the relatively high intensities of the X rays compared with the available phase space density of the (unaccelerated) source populations of solar wind or magnetospheric ions at Jupiter, which are orders of magnitude too small to explain the observed emissions. The Chandra X-ray observations were executed simultaneously with observations at ultraviolet wavelengths by the Hubble Space Telescope and at radio wavelengths by the Ulysses spacecraft. These additional data sets suggest that the source of the X rays is magnetospheric in origin and that the precipitating particles are accelerated by strong field-aligned electric fields, which simultaneously create both the several-MeV energetic ion population and the relativistic electrons observed in situ by Ulysses that are correlated with approx.40 min quasi-periodic radio outbursts.

Elsner, R. F.↗
NASA NTRS2010

When a Standard Candle Flickers

The Crab is the only bright steady source in the X-ray sky. The Crab consists of a pulsar wind nebula, a synchrotron nebula, and a cloud of expanding ejecta. On small scales, the Crab is extremely complex and turbulent. X-ray astronomers have often used the Crab as a standard candle to calibrate instruments, assuming its spectrum and overall flux remains constant over time. Four instruments (Fermi/GBM, RXTE/PCA, Swift/BAT, INTEGRAL/ISGRI) show a approx.5% (50 m Crab) decline in the Crab from 2008-2010. This decline appears to be larger with increasing energy and is not present in the pulsed flux, implying changes in the shock acceleration, electron population or magnetic field in the nebula. The Crab is known to be dynamic on small scales, so it is not too surprising that its total flux varies as well. Caution should be taken when using the Crab for in-orbit calibrations.

Wilson-Hodge, Colleen A.↗
NASA NTRS2011

Axisymmetric Calculations of a Low-Boom Inlet in a Supersonic Wind Tunnel

This paper describes axisymmetric CFD predictions made of a supersonic low-boom inlet with a facility diffuser, cold pipe, and mass flow plug within wind tunnel walls, and compares the CFD calculations with the experimental data. The inlet was designed for use on a small supersonic aircraft that would cruise at Mach 1.6, with a Mach number over the wing of 1.7. The inlet was tested in the 8-ft by 6-ft Supersonic Wind Tunnel at NASA Glenn Research Center in the fall of 2010 to demonstrate the performance and stability of a practical flight design that included a novel bypass duct. The inlet design is discussed here briefly. Prior to the test, CFD calculations were made to predict the performance of the inlet and its associated wind tunnel hardware, and to estimate flow areas needed to throttle the inlet. The calculations were done with the Wind-US CFD code and are described in detail. After the test, comparisons were made between computed and measured shock patterns, total pressure recoveries, and centerline pressures. The results showed that the dual-stream inlet had excellent performance, with capture ratios near one, a peak core total pressure recovery of 96 percent, and a large stable operating range. Predicted core recovery agreed well with the experiment but predicted bypass recovery and maximum capture ratio were high. Calculations of offdesign performance of the inlet along a flight profile agreed well with measurements and previous calculations.

Chima, Rodrick V.↗
NASA NTRS1987

Modeling the interactions between the ocean and the environment for microwave radar sensing

An extensive set of X-band microwave backscatter measurements has been analyzed to determine its dependence on winds near the surface, atmospheric stability, and long-wave slopes. These radar measurements, made from a tower in the Gulf of Mexico, were performed in conjunction with an extensive set of simultaneous environmental measurements. The CW microwave system operated at an incidence angle of 45 deg, with antennas directed into winds and waves. Model functions for the radar cross section (RCS) and the modulation transfer function (MTF) are developed that depend on the geophysical variables, including wind stress. Statistical analysis shows that these models functions yield a significant smaller error when fit to the RCS data than a simple wind-speed function displays. A Taylor-series expansion of the returned power from a small area provides a unified function that demonstrates the complementary roles of wave slope and atmospheric fluctuations on both the RCS, the MTF, and the coherence. The issue of linearity of the MTF is addressed with this data set, yielding evidence that this is a valid assumption. These results have direct application to the remote sensing of the mean and fluctuating winds, and the wave spectrum with coherent and incoherent active radars.

Weissman, David E.↗
NASA NTRS1969

Remote sensing of winds and atmospheric turbulence by cross correlation of passive optical signals

A new method is described for the remote measurement of winds and atmospheric turbulence by the cross-correlation of passive optical signals. If small local variations in atmospheric density, temperature or other parameters cause fluctuations in scattered or thermal radiation detected by a radiometer on the ground, then the cross-correlation of fluctuations detected by two radiometers with crossed fields of view can yield turbulence information pertaining to the region about this intersection point. When the fields of view are not quite crossed turbulent eddies will be convected through the fields of view sequentially, and the transit times of the eddies identified by the correlation procedure will yield wind information. The successful application of this technique, detecting fluctuations in scattered sunlight, has demonstrated both the potential, and the present limitations of the method, which are discussed. Results for the power spectrum of the fluctuations and for winds at an altitude of 61 meters are shown.

Montgomery, A. J.↗
NASA NTRS1975

The enhancement of solar wind fluctuations at the proton thermal gyroradius

Average power spectra of solar wind fluctuations at frequencies up to 0.87 Hz are calculated from Ogo 5 measurements of positive ion flux. Although the general spectral trend follows the power law spectrum observed at lower frequencies, a small but statistically significant power enhancement is observed at the frequency v/2(pi)R, where v is the solar wind velocity and R is the gyroradius of proton thermal motions in the solar wind. The measured power spectrum is in rough agreement with that deduced from radio scintillation observations.

Neugebauer, M.↗
NASA NTRS1979

A numerical model of the Martian polar cap winds

An investigation of the Martian polar cap winds and their response to a variety of factors is carried out by numerical experiments based on a zonally symmetric primitive equation model. The seasonal thermal forcing, mass exchange between polar caps and atmosphere, large-scale topography, and polar cap size are discussed, noting that topography has a small effect, but the circulation intensity increases with cap size. The model results show that surface winds near the edge of a retreating polar cap are enhanced, and that the surficial wind indicators near the south pole are formed during spring and those near the north pole during winter. It is suggested that the high-latitude dune fields in the northern hemisphere are formed when the terrain is covered by frost, and that the saltating particles are 'snowflakes' which formed by the mechanism proposed by Pollack (1976). The model results for the winter simulation compare favorably with general circulation model (GCM) calculations.

Haberle, R. M.↗
NASA NTRS1944

Wind Tunnel Investigation of the Effect of Jet-Motor Operation Stability

The effects of jet-motor operation on the stability and control characteristics of two fighter-type airplanes as determined by wind-tunnel tests of 1/5-scale models are presented. It is shown that the action of the jets is to cause a small loss in stick-fixed stability which is predictable from known theories.

WIND TUNNEL TESTS↗