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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Experimental investigation of generic three-dimensional sidewall-compression scramjet inlets at Mach 6 in tetrafluoromethane

Three-dimensional sidewall-compression scramjet inlets with leading-edge sweeps of 30 deg and 70 deg were tested in the Langley Hypersonic CF4 Tunnel at Mach 6 and with a ratio of specific heats of 1.2. The parametric effects of leading-edge sweep, cowl position, contraction ratio, and Reynolds number were investigated. The models were instrumented with 42 static pressure orifices that were distributed on the sidewalls, base plate, and cowl. Schlieren movies were made of each test for flow visualization of the effects of the internal flow spillage on the external flow field. To obtain an approximate characterization of the flow field, a modification to two-dimensional, inviscid, oblique shock theory was derived to accommodate the three-dimensional effects of leading-edge sweep. This theory qualitatively predicted the reflected shock structure (i.e., sidewall impingement locations) and the observed increase in spillage with increasing leading-edge sweep. The primary effect of moving the cowl forward was capturing the flow that would have otherwise spilled out ahead of the cowl. Increasing the contraction ratio increases the number of internal shock reflections and hence incrementally increases the sidewall pressure distribution. Significant Reynolds number effects were noted over a small range of Reynolds number.

Holland, Scott D.↗
NASA NTRS2011

Planck Early Results. XV. Spectral Energy Distributions and Radio Continuum Spectra of Northern Extragalactic Radio Sources

Spectral energy distributions (SEDs) and radio continuum spectra are presented for a northern sample of 104 extragalactic radio sources. based on the Planck Early Release Compact Source Catalogue (ERCSC) and simultaneous multi frequency data. The nine Planck frequencies, from 30 to 857 GHz, are complemented by a set of simultaneous observations ranging from radio to gamma-rays. This is the first extensive frequency coverage in the radio and millimetre domains for an essentially complete sample of extragalactic radio sources, and it shows how the individual shocks, each in their own phase of development, shape the radio spectra as they move in the relativistic jet. The SEDs presented in this paper were fitted with second and third degree polynomials to estimate the frequencies of the synchrotron and inverse Compton (IC) peaks, and the spectral indices of low and high frequency radio data, including the Planck ERCSC data, were calculated. SED modelling methods are discussed, with an emphasis on proper. physical modelling of the synchrotron bump using multiple components. Planck ERCSC data also suggest that the original accelerated electron energy spectrum could be much harder than commonly thought, with power-law index around 1.5 instead of the canonical 2.5. The implications of this are discussed for the acceleration mechanisms effective in blazar shock. Furthermore in many cases the Planck data indicate that gamma-ray emission must originate in the same shocks that produce the radio emission.

Aatrokoski, J.↗
NASA NTRS1999

An Adaptive Unstructured Grid Method by Grid Subdivision, Local Remeshing, and Grid Movement

An unstructured grid adaptation technique has been developed and successfully applied to several three dimensional inviscid flow test cases. The approach is based on a combination of grid subdivision, local remeshing, and grid movement. For solution adaptive grids, the surface triangulation is locally refined by grid subdivision, and the tetrahedral grid in the field is partially remeshed at locations of dominant flow features. A grid redistribution strategy is employed for geometric adaptation of volume grids to moving or deforming surfaces. The method is automatic and fast and is designed for modular coupling with different solvers. Several steady state test cases with different inviscid flow features were tested for grid/solution adaptation. In all cases, the dominant flow features, such as shocks and vortices, were accurately and efficiently predicted with the present approach. A new and robust method of moving tetrahedral "viscous" grids is also presented and demonstrated on a three-dimensional example.

Pirzadeh, Shahyar Z.↗
NASA NTRS1998

Low sonic boom shock control/alleviation surface

A sonic control device that reduces the effects of shock waves generated by an aircraft traveling at supersonic speeds. The control device includes a control surface located at or near the nose section of the aircraft. The position of the control surface can be moved between a retracted position and an extended position. When in a deflected position, the control surface increases the air pressure at the nose section. The increase in air pressure at the nose section decreases both the pressure amplitude and the slope of the overall shock wave as the wave travels toward the ground. Additionally, the deflection of the control surface may induce a downward directed pressure increase which creates less of a drag penalty than a truly blunt nose. When shock control is not desired, the control surface is moved back to the retracted position to reduce the drag on the plane. The moving control device allows a supersonic aircraft to efficiently travel above both land and water.

Morgenstern, John M↗
NASA NTRS1991

Broadband shock-associated noise from supersonic jets in flight

The stochastic model theory of broadband shock-associated noise from supersonic jets, developed earlier (Tam, 1987, 1990), is extended to jets in flight. Forward flight affects broadband shock-associated noise generation through noise source modification. It also affects the intensity and characteristics of the noise radiated in a given direction through the mean flow convection effect. Noise source modification arises when the external flow associated with forward flight alters the mean velocity profile and shock cell structure of the jet. The mean flow convection effect arises because the radiated sound waves now have to propagate through a moving medium to reach the observation point. The present theory suggests that both the noise source modification effect and the mean flow convection effect would produce narrower broadband shock-associated noise spectral peaks with lower peak frequencies. The theory is further extended to the case of a supersonic jet embedded in an open wind tunnel bounded by a vortex sheet. Detalied comparisons between the calculated results and the measurements of Norum and Shearin (1984, 1986) are reported here. Very good agreements are found over a broad range of jet Mach numbers and wind tunnel Mach numbers.

Tam, C. K. W.↗
NASA NTRS1992

Impact-generated winds on Venus: Causes and effects

The pressure of the dense atmosphere of Venus significantly changes the appearance of ejecta deposits relative to craters on the Moon and Mercury. Conversely, specific styles and sequences of ejecta emplacement can be inferred to represent different intensities of atmospheric response winds acting over different timescales. Three characteristic timescales can be inferred from the geologic record: surface scouring and impactor-controlled (angle and direction) initiation of the long fluidized run-out flows; nonballistic emplacement of inner, radar-bright ejecta facies and radar-dark outer facies; and very late reworking of surface materials. These three timescales roughly correspond to processes observed in laboratory experiments that can be scaled to conditions on Venus (with appropriate assumptions): coupling between the atmosphere and earlytime vapor/melt (target and impactor) that produces an intense shock that subsequently evolves into blast/response winds; less energetic dynamic response of the atmosphere to the outward-moving ballistic ejecta curtain that generates nonthermal turbulent eddies; and late recovery of the atmosphere to impact-generated thermal and pressure gradients expressed as low-energy but long-lived winds. These different timescales and processes can be viewed as the atmosphere equivalent of shock melting, material motion, and far-field seismic response in the target. The three processes (early Processes, Atmospheric Processes, and Late Recovery Winds) are discussed at length.

Schultz, Pater H.↗
NASA NTRS1972

Movement of the Surveyor 3 spacecraft

It was concluded that the Surveyor 3 spacecraft moved, probably as a result of a sudden failure of the leg 3 shock absorber, between May 1967 and November 1969. The time at which the movement occurred can be estimated only from comparison of the shielded and unshielded portions of footpad 3 and a knowledge of the mechanism and rate of the process that tans the painted surface.

Scott, R. F.↗
NASA NTRS1981

Magnetospheric substorm models - Comparison with neutral sheet magnetic field observations

Four models for geomagnetic substorms, a quiet tail model, and models incorporating structural effects of the tail are examined for consistency with magnetic-field data during satellite crossings of the tail neutral sheet/plasma sheet. For this data the tearing mode instability model is always consistent, and inward moving distant neutral line is sometimes consistent, quasi-steady reconnection with slow shock and intermediate wave structure and locally quiet tail rarely consistent, and an outward propagating rarefaction wave is never consistent with the magnetic observations. In several cases structural effects of the tail are consistent with key features of the magnetic signatures.

Speiser, T. W.↗
NASA NTRS1990

Line emission from clumpy photodissociation regions

A theoretical parameter study of dense photodissociation regions is presented. It is found that when the gas density is sufficiently high relative to the FUV flux, self-shielding of the molecules can move the C(+)/CO and H/H2 atomic-molecular transitions close to the surfaces of the molecular cloud, where they can feel the full effect of heating by the FUV radiation field. For n of 100,000/cu cm, collisional deexcitation of the FUV-pumped H2 can move the lower levels toward the LTE, producing line ratio resembling those of shocked regions for these low-v levels, while the high-v level line ratios retain a 'fluorescent' value. Appreciable emission in high-J transitions of CO originates in this warm molecular gas. Comparison with observations suggests that a small volume filling factor of high density clumps embedded within a moderate density interclump medium are a common phenomenon in photodissociation regions.

Burton, Michael G.↗
NASA NTRS1992

Off-center nuclei in galaxies

The nucleus of a galaxy orbits around the mass centroid. Orbital motions appear overstable in numerical experiments started with a galaxy's nucleus at rest atop its mass centroid. The amplitude doubles in 6-10 orbital periods. Orbits precess, nutate, and change their amplitudes, but they keep fairly constant periods. Orbital periods are in resonance with local particle motions, and amplitudes reach a core radius. This resonance suggests that center motions are a local, rather than a global, phenomenon. The overstability implies that a galaxy cannot be formed in nature with its nucleus at rest atop its mass centroid, and that nuclei orbit the mass centroid in real galaxies. These center motions should show up observationally as a shift of the nucleus away from the center defined by nearby isophotes. Off-center nuclei have been reported in many galaxies (e.g., M33, M101, NGC 3379, NGC 3384). Other kinds of observations confirmed the picture of nonsteady galactic centers as well. Gas trapped in moving nuclear regions of a galaxy should show strange flow patterns with possible shocks. The nuclear regions of galaxies including Milky Way and of globular clusters are not likely to be in a static steady state.

Miller, R. H.↗
NASA NTRS1992

A brief description of a new numerical framework for solving conservation laws: The method of space-time conservation element and solution element

A new numerical method for solving conservation laws is being developed. It differs substantially from the well established methods, i.e., finite difference, finite volume, finite element, and spectral methods, in both concept and methodology. It is much simpler than a typical high resolution method. No flux limiter or any technique related to characteristics is involved. No artificial viscosity or smoothing is introduced, and no moving mesh is used. Yet this method is capable of generating highly accurate shock tube solutions. The slight numerical overshoot and/or oscillations generated can be removed if a simple averaging formula initially used is replaced by a weighted formula. This modification has little effect on other parts of the solution. Because of its simplicity, generalization of this new method for multi-dimensional problems is straightforward.

Chang, Sin-Chung↗
NASA NTRS1999

Comparisons of Shock Noise Predictions with Flight Data

A flight test was performed at NASA Dryden Research Center in November 1991 utilizing both F18 and Fl6 aircraft. These flights were designed to provide (1) acoustic data that could be extrapolated to that of an HSCT at various points of its climb-to-cruise operation and (2) a data base for noise from a supersonic jet exhausting from an aircraft moving at high subsonic speeds. This presentation utilizes data obtained from these flyovers to evaluate predictions of broadband shock noise from supersonic jets in flight. The Fl8 is particularly suitable for flyovers of shock noise since it can be flown with one engine at flight idle. The second engine can then be operated at a pressure high enough to produce a supersonic nozzle exhaust and still maintain an unaccelerated, level flyover.

Norum, T. D.↗
NASA NTRS1992

Dynamics of Kepler's supernova remnant

Observations of Kepler's SNR have revealed a strong interaction with the ambient medium, far in excess of that expected at a distance of about 600 pc away from the Galactic plane where Kepler's SNR is located. This has been interpreted as a result of the interaction of supernova ejecta with the dense circumstellar medium (CSM). Based on the bow-shock model of Bandiera (1985), we study the dynamics of this interaction. The CSM distribution consists of an undisturbed stellar wind of a moving supernova progenitor and a dense shell formed in its interaction with a tenuous interstellar medium. Supernova ejecta drive a blast wave through the stellar wind which splits into the transmitted and reflected shocks upon hitting this bow-shock shell. We identify the transmitted shock with the nonradiative, Balmer-dominated shocks found recently in Kepler's SNR. The transmitted shock most probably penetrated the shell in the vicinity of the stagnation point.

Borkowski, Kazimierz J.↗
NASA NTRS1975

Standing waves at low Mach number laminar bow shocks

Explorer 43 data were used to study 34 bow shock crossings observed from 5 to 16 earth radii upstream of the average bow shock location. Waves with periods of 6 to 130 s having amplitudes up to delta-B/B = 1 were detected. Wave polarization for the low-frequency waves is right-handed in relation to the average field direction when the observer moves from the upstream to downstream direction but is left-handed when the observer moves in the opposite sense. This fact identified the waves as standing whistler waves in the coordinate system of the shock. The waves are in agreement with collisionless low Mach number laminar shock theory. When the measured parameters were used to calculate theoretical wavelengths, the observed wave frequencies could be used to calculate velocities for the shock-wave coordinate system past the spacecraft; such velocities are mostly between 10 and 30 km/s. It is suggested that the higher-frequency propagating whistler waves may evolve from the standing whistler waves through a decay instability.

Fairfield, D. H.↗
NASA NTRS1980

Resonant Doppler velocimeter

A technique is presented for visualizing and quantitatively measuring velocity, temperature, and pressure by shining a single frequency laser beam into a gaseous flow which is seeded with an atomic species. The laser is tuned through the absorption frequencies of the seeded species and the absorption profile is detected by observing fluorescence as the atoms relax back to the ground state. The flow velocity is determined by observing the Doppler shift in the absorption frequency. Spectroscopic absorption line broadening mechanisms furnish information regarding the static temperature and pressure of the moving gas. Results of experiments conducted in the free stream and in the bow shock of a conical model mounted in a hypersonic wind tunnel indicate that the experimental uncertainties in the measurement of average values for the velocity, temperature and pressure of the flow are 0.1, 5 and 10 percent respectively.

Zimmermann, M.↗
NASA NTRS1990

A numerical study on the use of sulfur hexafluoride as a test gas for wind tunnels

A numerical study is presented which investigates effects of using sulfur hexafluoride, S F6, as a substitute for air. Inviscid results for airfoils indicate that for transonic cases the shock location calculated for S F6 is vastly different from that in air and moves progressively forward on the airfoil as the freestream pressure is increased and real gas effects become more pronounced. Application of a simple Mach number scaling procedure results in good correlation between S F6 abnd air even for pressures at which nonideal gas effects are significant. Computations for subsonic turbulent flows over a NACA 0012 airfoil show that the maximum angle of attack at which steady lift can be obtained is different between air and S F6. In addition, for S F6, this angle of attack depends greatly on the freestream conditions. Close agreement with air can be achieved by altering the freestream Mach number according to the inviscid scaling procedure.

Anderson, W. Kyle↗
NASA NTRS1982

Measurements on a shock wave generated by a solar flare

Shock waves generated by intense solar flares may be driven by a large amount of ejected mass, about 5 x 10 to the 16th g, and the total energy involved may be of the order of 10 to the 32nd erg. The shocks may have initial velocities of the order of 2,000 km/s and, in their exodus through the corona, may be accompanied by fast-moving optical transients, the emission of highly characteristic radio signatures and the acceleration of particles to quasi-relativistic velocities. Here, a review is presented of data on a high-velocity shock generated by a flare on 18 August 1979, 1400 UT, and comments are provided on some previously deduced velocities for the shock. Attention is given to a model, based on current computer programs to account for the overall characteristics of the shock as it propagated through the corona and the interplanetary plasma.

Maxwell, A.↗
NASA NTRS1990

Pure-oxygen radiative shocks with electron thermal conduction

Steady state radiative shock models in gas composed entirely of oxygen are calculated with the purpose of explaining observations of fast-moving knots in Cas A and other oxygen-rich SNRs. Models with electron thermal conduction differ significantly from models in which conduction is neglected. Conduction reduces postshock electron temperatures by a factor of 7-10 and flattens temperature gradients. The O III ion, whose forbidden emission usually dominates the observed spectra, is present over a wide range of shock velocities, from 100 to 170 km/s. The electron temperature in the O III forbidden line formation region is 30,000 K, in agreement with the 20,000 K derived from observations. All models with conduction have extensive warm (T above 4000 K) photoionization zones, which provides better agreement with observed optical O I line strengths.

Borkowski, Kazimierz J.↗