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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Control of Vortex Breakdown in Critical Swirl Regime Using Azimuthal Forcing

We finally go back to the four swirl cases and see how the flow responds to either forcing m = -1 or m = -2. On the left we see the flow forced at m = -1 We see that the PVC locks onto the applied forcing also for lower swirl number causing this high TKE at the jet center. The amplification of this instability causes VB to occur at a lower swirl number. The opposite can be seen when forcing the flow at m=-2 which is basically growing in the outer shear layer causing VB to move downstream . There is no energy at the center of the vortex showing that the precessing has been damped. The mean flow is most altered at the swirl numbers were VB is unstable.

Oberleithner, Kilian↗
NASA NTRS2015

The Structure of Pre-Transitional Protoplanetary Disks. II Azimuthal Asymmetries, Different Radial Distributions of Large and Small Dust Grains in PDS 70

The formation scenario of a gapped disk, i.e., transitional disk, and its asymmetry is still under debate. Proposed scenarios such as disk-planet interaction, photoevaporation, grain growth, anticyclonic vortex, eccentricity, and their combinations would result in different radial distributions of the gas and the small (sub-micron size) and large (millimeter size) dust grains as well as asymmetric structures in a disk. Optical/near-infrared (NIR) imaging observations and (sub-)millimeter interferometry can trace small and large dust grains, respectively; therefore multi-wavelength observations could help elucidate the origin of complicated structures of a disk. Here we report Submillimeter Array observations of the dust continuum at 1.3 mm and CO-12 J = 2 yields 1 line emission of the pre-transitional protoplanetary disk around the solar-mass star PDS 70. PDS 70, a weak-lined T Tauri star, exhibits a gap in the scattered light from its disk with a radius of approx. 65 AU at NIR wavelengths. However, we found a larger gap in the disk with a radius of approx. 80 AU at 1.3 mm. Emission from all three disk components (the gas and the small and large dust grains) in images exhibits a deficit in brightness in the central region of the disk, in particular, the dust disk in small and large dust grains has asymmetric brightness. The contrast ratio of the flux density in the dust continuum between the peak position to the opposite side of the disk reaches 1.4. We suggest the asymmetries and different gap radii of the disk around PDS 70 are potentially formed by several (unseen) accreting planets inducing dust filtration.

Submillimeter Array observations↗
NASA NTRS2024

Azimuthal Size Scales of Solar Wind Periodic Density Structures

Periodic Density Structures (PDSs) are quasi-periodic variations of solar wind density ranging from a few minutes to a few hours. PDSs advect with the solar wind and have radial length scales (Lx) of tens to several thousand Mm, thus belonging to the class of “mesoscale structures”. Current interplanetary multi-spacecraft observations are not at spatial separations capable of directly measuring the 3D size scale of PDSs or other mesoscale structures. Instead, previous investigations estimated characteristic spatial scales in solar wind parameters using cross-correlation and/or coherence analysis applied to multi-spacecraft observations. For solar wind density and interplanetary magnetic field (IMF) intensity, the reported size scales perpendicular to the Sun-Earth line (Ly) ranged between ≈30 and ≈200 Earth Radii (RE). Here, we implemented a similar approach on the same parameters but focused on high density slow solar wind intervals with PDSs observed by the Wind and ARTEMIS-P1 spacecraft. Additionally, this is the first statistical study of the IMF intensity periodicities in relation to PDSs. We identified intervals in which the two spacecraft observed the same periodicity obtaining two PDS groups based on their radial length scale: Lx1 ≈86RE and Lx2 ≈35RE. Then, we classified the events based on the periodic variations’ coherence level. Reproducing the results with simulations of the PDSs’ transit, we inferred the Ly order of magnitudes for the two PDS groups: Ly1 ≈340RE and Ly2 ≈187RE. Knowing the PDSs’ size scales is fundamental to constrain models aimed at reproducing these structures and is critical to better understand the PDS-magnetosphere coupling.

Interplanetary physics↗
NASA NTRS1987

Optical alignment of Centaur's inertial guidance system

During Centaur launch operations the launch azimuth of the inertial platform's U-accelerometer input axis must be accurately established and maintained. This is accomplished by using an optically closed loop system with a long-range autotheodolite whose line of sight was established by a first-order survey. A collimated light beam from the autotheodolite intercepts a reflecting Porro prism mounted on the platform azimuth gimbal. Thus, any deviation of the Porro prism from its predetermined heading is optically detected by the autotheodolite. The error signal produced is used to torque the azimuth gimbal back to its required launch azimuth. The heading of the U-accelerometer input axis is therefore maintained automatically. Previously, the autotheodolite system could not distinguish between vehicle sway and rotational motion of the inertial platform unless at least three prisms were used. One prism was mounted on the inertial platform to maintain azimuth alignment, and two prisms were mounted externally on the vehicle to track sway. For example, the automatic azimuth-laying theodolite (AALT-SV-M2) on the Saturn vehilce used three prisms. The results of testing and modifying the AALT-SV-M2 autotheodolite to simultaneously monitor and maintain alignment of the inertial platform and track the sway of the vehicle from a single Porro prism.

Gordan, Andrew L.↗
NASA NTRS1981

Equatorial disc and dawn-dusk currents in the frontside magnetosphere of Jupiter - Pioneer 10 and 11

Observations by Pioneer 10 and 11 show that the strongest azimuthal fields are observed near the dawn meridian (Pioneer 10) while the weakest occur near the noon meridian (Pioneer 11), suggesting a strong local time dependence for the corresponding radial current system. Modeling studies of the radial component of the field observed by both spacecraft suggest that the corresponding azimuthal current system must also be a strong function of local time. Both the azimuthal and the radial field component signatures exhibit sharp dips and reversals, requiring thin radial and azimuthal current systems. There is also a suggestion that these two current systems either are interacting or are due, at least in part, to the same current. It is suggested that a plausible current model consists of the superposition of a thin, local-time-independent azimuthal current system plus the equatorial portion of a tail-like current system that extends into the dayside magnetosphere.

Jones, D. E.↗
NASA NTRS1986

Virtual space and 2-dimensional effects in perspective displays

When interpreting three dimensional spatial relationships presented on a two dimensional display surface, the viewer is required to mentally reconstruct the original information. This reconstruction is influenced by both the perspective geometry of the displayed image and the viewer's eye position relative to the display. In a study which manipulated these variables, subjects judged the azimuth direction of a target object relative to a reference object fixed in the center of a perspective display. The results support a previously developed model which predicted that the azimuth judgement error would be a sinusoidal function of stimulus azimuth. The amplitude of this function was correctly predicted to be systematically modulated by both the perspective geometry of the image and the viewer's eye position relative to the screen. Interaction of the two components of the model, the virtual space effect and the 3D-to-2D projection effect, predicted the relative amplitudes of the sinusoidal azimuth error functions for the various conditions of the experiment. Mean azimuth judgements in some directions differed by as much as 25 degrees as a result of different combinations of eye position and image geometry. The results illustrate the need to consider the effects of perspective geometry when designing spatial information instruments, and show the model to be a reliable predictor of average performance.

Mcgreevy, M. W.↗
NASA NTRS1992

Seawifs Technical Report Series. Volume 2: Analysis of Orbit Selection for Seawifs: Ascending Versus Descending Node

Due to range safety considerations, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color instrument may be required to be launched into a near-noon descending node, as opposed to the ascending node used by the predecessor sensor, the Coastal Zone Color Scanner (CZCS). The relative importance of ascending versus descending near-noon orbits was assessed here to determine if descending node will meet the scientific requirements of SeaWiFS. Analyses focused on ground coverage, local times of coverage, solar and viewing geometries (zenith and azimuth angles), and sun glint. Differences were found in the areas covered by individual orbits, but were not important when taken over a 16 day repeat time. Local time of coverage was also different: for ascending node orbits the Northern Hemisphere was observed in the morning and the Southern Hemisphere in the afternoon, while for descending node orbits the Northern Hemisphere was observed in the afternoon and the Southern in the morning. There were substantial differences in solar azimuth and spacecraft azimuth angles both at equinox and at the Northern Hemisphere summer solstice. Negligible differences in solar and spacecraft zenith angles, relative azimuth angles, and sun glint were obtained at the equinox. However, large differences were found in solar zenith angles, relative azimuths, and sun glint for the solstice. These differences appeared to compensate across the scan, however, an increase in sun glint in descending node over that in ascending node on the western part of the scan was compensated by a decrease on the eastern part of the scan. Thus, no advantage or disadvantage could be conferred upon either ascending node or descending node for noon orbits. Analyses were also performed for ascending and descending node orbits that deviated from a noon equator crossing time. For ascending node, afternoon orbits produced the lowest mean solar zenith angles in the Northern Hemisphere, and morning orbits produced the lowest angles for the Southern Hemisphere. For descending node, morning orbits produced the lowest mean solar zenith angles for the Northern Hemisphere; afternoon orbits produced the lowest angles for the Southern Hemisphere.

Hooker, Stanford B.↗
NASA NTRS1992

High resolution observations of the magnetic and velocity field of simple sunspots

We have observed the disk passage of relatively simple round sunspots using a narrowband filter and a large format CCD detector and have created magnetograms, Dopplergrams, and continuum images nearly simultaneously. In addition the spectral resolution of the filter allows the construction of 'spectra' for all points in the field of view. The mean inclination of the magnetic field increases from 45-50 deg to 70-75 deg across the penumbra and there is a fluctuation of the inclination angle about the mean of about 4 +/- 18 deg. The variation in inclination is large enough that substantial amounts of magnetic field are parallel to the solar surface from the mid to outer penumbra. The Evershed flow tends to occur in the regions where the magnetic field is horizontal. This suggests that the Evershed flow is confined to the regions of horizontal fields. We show that a simple sunspot model with azimuthal variations in inclination but no azimuthal variations of field strength is free from azimuthal Lorentz forces. The meridional component of the currents which arise from the azimuthal variation in inclination are parallel to the field lines. This suggests that a cylindrically symmetric magnetostatic sunspot model can be perturbed into one with azimuthal variations in inclination with some adjustment in the meridional force balance.

Title, Alan M.↗
NASA NTRS1999

Development of a Three-Dimensional PSE Code for Compressible Flows: Stability of Three-Dimensional Compressible Boundary Layers

A program is developed to investigate the linear stability of three-dimensional compressible boundary layer flows over bodies of revolutions. The problem is formulated as a two dimensional (2D) eigenvalue problem incorporating the meanflow variations in the normal and azimuthal directions. Normal mode solutions are sought in the whole plane rather than in a line normal to the wall as is done in the classical one dimensional (1D) stability theory. The stability characteristics of a supersonic boundary layer over a sharp cone with 50 half-angle at 2 degrees angle of attack is investigated. The 1D eigenvalue computations showed that the most amplified disturbances occur around x(sub 2) = 90 degrees and the azimuthal mode number for the most amplified disturbances range between m = -30 to -40. The frequencies of the most amplified waves are smaller in the middle region where the crossflow dominates the instability than the most amplified frequencies near the windward and leeward planes. The 2D eigenvalue computations showed that due to the variations in the azimuthal direction, the eigenmodes are clustered into isolated confined regions. For some eigenvalues, the eigenfunctions are clustered in two regions. Due to the nonparallel effect in the azimuthal direction, the eigenmodes are clustered into isolated confined regions. For some eigenvalues, the eigenfunctions are clustered in two regions. Due to the nonparallel effect in the azimuthal direction, the most amplified disturbances are shifted to 120 degrees compared to 90 degrees for the parallel theory. It is also observed that the nonparallel amplification rates are smaller than that is obtained from the parallel theory.

Balakumar, P.↗
NASA NTRS2011

Improved Systematic Pointing Error Model for the DSN Antennas

New pointing models have been developed for large reflector antennas whose construction is founded on elevation over azimuth mount. At JPL, the new models were applied to the Deep Space Network (DSN) 34-meter antenna s subnet for corrections of their systematic pointing errors; it achieved significant improvement in performance at Ka-band (32-GHz) and X-band (8.4-GHz). The new models provide pointing improvements relative to the traditional models by a factor of two to three, which translate to approximately 3-dB performance improvement at Ka-band. For radio science experiments where blind pointing performance is critical, the new innovation provides a new enabling technology. The model extends the traditional physical models with higher-order mathematical terms, thereby increasing the resolution of the model for a better fit to the underlying systematic imperfections that are the cause of antenna pointing errors. The philosophy of the traditional model was that all mathematical terms in the model must be traced to a physical phenomenon causing antenna pointing errors. The traditional physical terms are: antenna axis tilts, gravitational flexure, azimuth collimation, azimuth encoder fixed offset, azimuth and elevation skew, elevation encoder fixed offset, residual refraction, azimuth encoder scale error, and antenna pointing de-rotation terms for beam waveguide (BWG) antennas. Besides the addition of spherical harmonics terms, the new models differ from the traditional ones in that the coefficients for the cross-elevation and elevation corrections are completely independent and may be different, while in the traditional model, some of the terms are identical. In addition, the new software allows for all-sky or mission-specific model development, and can utilize the previously used model as an a priori estimate for the development of the updated models.

Rochblatt, David J.↗
NASA NTRS2011

Diagonal-Axes Stage for Pointing an Optical Communications Transceiver

Traditional azimuth-elevation ("az-el") stages are used to point a variety of devices ranging from large optical telescopes to tank guns. Such a stage typically has an elevation stage having a horizontal axis mounted on an azimuth stage with a vertical axis. Both stages are often motorized. Optical communications transceivers often require two-axis motorized control, as when the communications link is between a ground station and an aircraft or satellite. In such applications, the traditional azimuth-elevation stage has two important drawbacks: a gimbal lock exclusion zone at zenith and susceptibility to pointing errors caused by backlash. Az-el stages in which the azimuth stage cannot rotate a full 360deg have the additional drawback of an azimuth exclusion zone. The diagonal-axes stage described here mitigates or eliminates all of these problems. Instead of one vertical axis and one horizontal axis, a diagonal-axes stage has two horizontal axes, both oriented at 45 to the trajectory of the target. For example, a ground station located on the equator tracking a satellite with an equatorial orbit would have one axis parallel to northeast and southwest, and the other axis parallel to northwest and southeast. The diagonal-axes stage is considerably less vulnerable to backlash. If it is correctly oriented, its axes rotate in only one direction during an overhead pass by a satellite. As a result, the effects of backlash may be inherently eliminated. If the gravity-induced torque on either axis changes during the pass, then backlash may become important during the part of the pass where the gravity torque, instead of opposing the motion of the stage, pushes the stage in the direction of motion. This can result in the loss of gear-to-gear contact in one or more stages of the gear reduction mechanism. In this case, a preload spring used to eliminate backlash need only be sufficiently strong to overcome the gravity torque, i.e. it need not overcome friction in the gear train. The diagonal-axes stage is not backlash-free for arbitrary target trajectories such as an aircraft might execute. If properly oriented for any particular satellite, however, it is backlash-free for all passes of that satellite, which will trace out parallel paths on the sky, and for all passes of any other satellite that are perpendicular to the first. It will also be backlash-free for some fraction of other satellite trajectories.

Regehr, Martin W.↗
NASA NTRS2014

A Design of Experiments Investigation of Offset Streams for Supersonic Jet Noise Reduction

An experimental investigation into the noise characteristics of a dual-stream jet with four airfoils inserted in the fan nozzle was conducted. The intent of the airfoils was to deflect the fan stream relative to the core stream and, therefore, impact the development of the secondary potential core and noise radiated in the peak jet-noise direction. The experiments used a full-factorial Design of Experiments (DoE) approach to identify parameters and parameter interactions impacting noise radiation at two azimuthal microphone array locations, one of which represented a sideline viewing angle. The parameters studied included airfoil angle-of-attack, airfoil azimuthal location within the fan nozzle, and airfoil axial location relative to the fan-nozzle trailing edge. Jet conditions included subsonic and supersonic fan-stream Mach numbers. Heated jets conditions were simulated with a mixture of helium and air to replicate the exhaust velocity and density of the hot jets. The introduction of the airfoils was shown to impact noise radiated at polar angles in peak-jet noise direction and to have no impact on noise radiated at small and broadside polar angles and to have no impact on broadband-shock-associated noise. The DoE analysis showed the main effects impacting noise radiation at sideline-azimuthal-viewing angles included airfoil azimuthal angle for the airfoils on the lower side of the jet near the sideline array and airfoil trailing edge distance (with airfoils located at the nozzle trailing edge produced the lowest sound pressure levels). For an array located directly beneath the jet (and on the side of the jet from which the fan stream was deflected), the main effects impacting noise radiation included airfoil angle-of-attack and airfoil azimuthal angle for the airfoils located on the observation side of the jet as well and trailing edge distance. Interaction terms between multiple configuration parameters were shown to have significant impact on the radiated noise. The models were shown to adequately describe the sound-pressure levels obtained for a configuration in the center of the design space indicating the models can be used to navigate the design space.

Jet Noise↗