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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

A natural low frequency oscillation in the wake of an airfoil near stalling conditions

An unusually low frequency oscillation in the flow over an airfoil was explored experimentally. Wind tunnel measurements were carried out with a two dimensional airfoil model at a chord Reynolds number of 100,000. During deep stall the usual bluff-body shedding occurred at a Strouhal number. But at the onset of stall a low frequency periodic oscillation occurred, the corresponding Strouhal number being an order of magnitude lower. The phenomenon occurred in relatively unclean flow when the freestream turbulence was raised to 0.4 percent, but did not in the cleaner flow with turbulence intensity of 0.1 percent. It could also be produced by certain high frequency acoustic excitation. Details of the flow field are compared between a case of low frequency oscillation at alpha = 15 deg and a case of bluff-body shedding at alpha = 22.5 deg. The origin of the low frequency oscillation traces to the upper surface of the airfoil and is seemingly associated with the periodic formation and breakdown of a large separation bubble. The intense flow fluctuations impart significant unsteady forces to the airfoil but diminish rapidly within a distance of one chord from the trailing edge.

Zaman, K. B. M. Q.↗
NASA NTRS1993

Wave generation by contaminant ions

Investigations dealing with the generation of waves by contaminant ions are reported. The studies included the properties of the velocity distribution function of such ions. It was found that it takes about one ion-cyclotron period for the distribution function to transform from a beam to a ring distribution. A linear instability analysis was performed to examine the possibility of wave excitation by an ion beam streaming perpendicular to the ambient magnetic field in an ionospheric type of plasma. A simulation code was developed to study the nonlinear behavior of the waves excited by beam and ring types of ion distributions. The code treats both electrons and ions as particles kinetically to also factor in the possible involvement of lower-hybrid waves.

Singh, N.↗
NASA NTRS1984

Turbulent solutions of the equations of fluid motion

Some turbulent solutions of the unaveraged Navier-Stokes equations (equations of fluid motion) are reviewed. Those equations are solved numerically in order to study the nonlinear physics of incompressible turbulent flow. Initial three-dimensional cosine velocity fluctuations and periodic boundary conditions are used in most of the work considered. The three components of the mean-square velocity fluctuations are initially equal for the conditions chosen. The resulting solutions show characteristics of turbulence such as the linear and nonlinear excitation of small-scale fluctuations. For the stronger fluctuations, the initially nonrandom flow develops into an apparently random turbulence. Thus randomness or turbulence can arise as a consequence of the structure of the Navier-Stokes equations. The cases considered include turbulence which is statistically homogeneous or inhomogeneous and isotropic or anisotropic. A mean shear is present in some cases. A statistically steady-state turbulence is obtained by using a spatially periodic body force. Various turbulence processes, including the transfer of energy between eddy sizes and between directional components, and the production, dissipation, and spatial diffusion of turbulence, are considered. It is concluded that the physical processes occurring in turbulence can be profitably studied numerically.

Deissler, R. G.↗
NASA NTRS1987

Variations of the semi-diurnal tidal wind in the meteor region with periods of about 27 and 13.5 days

Daily values of sunspot number and solar radio emission at 10.7 cm wavelength show a well known strong modulation with a period of 27 days, the Sun's rotation period. Recent satellite measurements revealed the same modulation of the ultraviolet irradiance at wavelength below 300 nm. These UV variations can influence the thermal heating of the ozone layer by altering the chemical composition. Therefore, one can also expect a corresponding variation in the middle atmosphere of the semi-diurnal tide, which is thermally excited essentially by absorption of UV between 200 and 370 nm in the upper ozone layer. Results of radar meteor wind measurements were used to detect a presumable very weak 27 day modulation within the natural and artificial noise of the daily values of the semi-diurnal wind amplitude. The summer period was chosen because of its relatively steady conditions and the maximum amplitudes during the year. The results of this investigation are given and discussed.

Greisiger, K. M.↗
NASA NTRS1981

Morphology of Saturn's aurora

Aurorally-excited emissions of atomic and molecular hydrogen come from a narrow circumpolar band near 80 deg north and south latitude on Saturn. The aurorae, which lie near the edge of the polar cap region, are continuously excited in both the north and south. If the strong variations observed in the auroral intensity are temporal, rather than longitudinal, they may be related to the periodic structure in the Saturn kilometric radiation.

Sandel, B. R.↗
NASA NTRS1986

Studies of radiative transfer in planetary atmospheres

The research emphasis during the period of this report has been on radio observations of comets, related to the International Halley Watch. Observations of the 18 cm lambda-doublet of OH have been carried out monthly. Both of the 1667 and 1665 MHz lines have been repeatedly detected for Comets Halley, Giacobini-Zinner, Thiele, and Hartley-Good. The first astronomical detection of the weak satellite line at 1720 MHz was made for P/Halley. These data promise to supply the highest signal-to-noise-ratio data over an extended period ever obtained for cometary radio observations. Analysis will provide gas production rates as a function of heliocentric distance and allow for detailed comparisons with the OH excitation model involving ultraviolet pumping. In the case of Halley, the lines appear quite symmetric, usually being centered within 0.1 km/s with respect to the nominal nuclear velocity. Line widths have been typically within 10% of 2 km/s. There is thus little evidence for asymmetric emission in the data obtained to date. It seems likely that non-steady-state models of the coma will be required to interpret the data. Evidence for significant departures from the LTE hyperfine ratios was found for Comet Giacobini-Zinner. Smaller, but nonetheless significant, deviations have been found for Comet Halley.

Irvine, W. M.↗
NASA NTRS1988

Theoretical investigation of the oceanic inverted barometer response

Dynamic tidal theory is generalized in order to predict the oceanic response to atmospheric pressure variations. The oceans are found to exhibit distinct dynamic behavior when forced at periods less than one week; depending on the harmonic type of forcing, the amplitude of the response can differ by about 20 percent or more from the static response. Even at roughly two months the traditional 'inverted barometer' may not be sufficiently accurate for applications in which detection of long-term sea level trends or vertical crustal motion is the goal. For forcing at periods of order one year the oceanic response differs by at most a few percent from the static (not the inverted barometer) response; thus calculations of meteorological excitation of the annual and Chandler wobbles can employ the static approximation without reservation.

Dickman, S. R.↗
NASA NTRS2003

Application of Satellite Altimeter Data to Studies of Ocean Surface Heat Flux and Upper Ocean Thermal Processes

This is a one-year cost extension of previous grant but carrying a new award number for the administrative purpose. Supported by this one-year extension, the following research has continued and obtained significant results. 20 papers have been published (9) or submitted (11) to scientific journals in this one-year period. A brief summary of scientific results on: 1. A new method for estimation of the sensible heat flux using satellite vector winds, 2. Pacific warm pool excitation, earth rotation and El Nino Southern Oscillations, 3. A new study of the Mediterranean outflow and Meddies at 400-meter isopycnal surface using multi-sensor data, 4. Response of the coastal ocean to extremely high wind, and 5. Role of wind on the estimation of heat flux using satellite data, are provided below as examples of our many research results conducted in the last year,

Yan, Xiao-Hal↗
NASA NTRS2003

The Nonlinear Coupling of Electromagnetic Ion Cyclotron and Lower Hybrid Waves in the Ring Current Region: The Magnetic Storm May 1-7 1998

The excitation of lower hybrid waves (LHWs) is a widely discussed mechanism of interaction between plasma species in space, and is one of the unresolved questions of magnetospheric multi-ion plasmas. In this paper we present the morphology, dynamics, and level of LHW activity generated by electromagnetic ion cyclotron (EMIC) waves during the May 2-7, 1998 storm period on the global scale. The LHWs were calculated based on our newly developed self-consistent model that couples the system of two kinetic equations: one equation describes the ring current (RC) ion dynamic, and another equation describes the evolution of EMIC waves. It is found that the LHWs are excited by helium ions due to their mass dependent drift in the electric field of EMIC waves. The level of LHW activity is calculated assuming that the induced scattering process is the main saturation mechanism for these waves. The calculated LHWs electric fields are consistent with the observational data.

Khazanov, G. V.↗
NASA NTRS1976

Theoretical models of Beta Cephei stars constructed with new radiative opacities

Equilibrium models for the main-sequence and early post-main-sequence phases of evolution are constructed for stars of 10.9 and 15 solar masses with two different initial hydrogen and metals abundances. Carson's (1976) radiative opacities are used in the calculations. It is found that the models which lie off the zero-age main sequence have cooler effective temperatures than corresponding models based on other opacities and cross the observed Beta Cephei strip in the H-R diagram only during the main phase of core hydrogen burning. The stability of radial and nonradial pulsations is studied by applying linear nonadiabatic perturbations to the models. The results show that all the modes examined are stable and that the margin of stability decreases with advancing evolution. It is suggested that the closeness of the periods of the two lowest nonradial quadrupole p-modes may be responsible for the 'beat' phenomenon observed in many Beta Cephei stars, if these modes are somehow excited. Possible instability mechanisms are evaluated, particularly the ultimate ionization of CNO elements.

Stothers, R.↗
NASA NTRS1982

Observations and analysis of the R Aquarii jet

Ultraviolet, optical and radio observations of the symbiotic star R Aquarii are discussed in the light of the discovery of a bright radio and optical jet from this star. The star is probably a binary with a period of 44 years. The VLA maps of the jet reveal a protruding structure extending approximately 10 arc sec from the central radio source with a position angle virtually identical to that of the optical jet observed at Lick. The observations of R Aqr are interpreted as indicating the existence of an accretion disk around an unseen companion. The hot subdwarf has effective temperature approximately 65,000 K. It is suggested that the Mira primary and the hot secondary are in orbit around each other with a high eccentricity. At periastron the hot subdwarf accretes at super critical rates and a jet forms. It is difficult to understand how an accretion disk would have eclipsed the Mira in 1928-1935 and 1974-1980. The suppression of maximum light in these two periods is interpreted as due to a distortion of the Mira envelope at periastron by the tidal interaction with the secondary. The jet may help to explain the excitation of the R Aqr nebula. It is possible that R Aqr flared up as a nova approximately 1000 years ago forming the nebula.

Kafatos, M.↗
NASA NTRS1991

Oscillation of high-altitude balloons

The Vaisala-Brunt frequencies of atmospheric waves lie sufficiently near He-filled balloon buoyancy oscillation frequencies to cause a near-resonance condition, especially in the 11.6-24.4 km altitude range. This excitation will hold for such balloons irrespective of size and payload variations due to the independence of balloon buoyant frequencies from size. Vertical perturbations of approximately 5-min period should accordingly be anticipated by balloon flight-control system designers.

Anderson, William J.↗
NASA NTRS1993

Video luminescent barometry - The induction period

Video monitoring of oxygen quenching of the photoluminescence of platinum octaethylporphyrin (PtOEP) in silicone polymer resin may be used to measure pressure distribution over an airfoil. A continuous increase of the luminescence intensity of PtOEP on exposure to the exciting light is known as the induction effect. The effect of several factors on PtOEP photoluminescence and the induction effect was investigated. The experimental apparatus is described and results are presented. It was observed that the relative induction amplitude and induction time increase at higher oxygen pressure and with thicker films. These observations may be explained if the singlet oxygen produced by oxygen quenching is consumed by the polymer and is therefore unavailable for further quenching. Researchers using this method for measuring pressure distribution on airfoil surfaces should be aware of the induction effect and its implications.

Uibel, Rory H.↗
NASA NTRS2004

The Nonlinear Coupling of Electromagnetic Ion Cyclotron and Lower Hybrid Waves in the Ring Current Region

The excitation of lower hybrid waves (LHWs) is a widely discussed mechanism of interaction between plasma species in space, and is one of the unresolved questions of magnetospheric multi-ion plasmas. In this paper we present the morphology, dynamics, and level of LHW activity generated by electromagnetic ion cyclotron (EMIC) waves during the May 2-7, 1998 storm period on the global scale. The LHWs were calculated based on a newly developed self-consistent model (Khazanov et. al., 2002, 2003) that couples the system of two kinetic equations: one equation describes the ring current (RC) ion dynamic, and another equation describes the evolution of EMIC waves. It is found that the LHWs are excited by helium ions due to their mass dependent drift in the electric field of EMIC waves. The level of LHW activity is calculated assuming that the induced scattering process is the main saturation mechanism for these waves. The calculated LHWs electric fields are consistent with the observational data.

Khazanov, G. V.↗
NASA NTRS2012

Slow Magnetosonic Waves and Fast Flows in Active Region Loops

Recent extreme ultraviolet spectroscopic observations indicate that slow magnetosonic waves are present in active region (AR) loops. Some of the spectral data were also interpreted as evidence of fast (approx 100-300 km/s) quasiperiodic flows. We have performed three-dimensional magnetohydrodynamic (3D MHD) modeling of a bipolar AR that contains impulsively generated waves and flows in coronal loops. The model AR is initiated with a dipole magnetic field and gravitationally stratified density, with an upflow-driven steadily or periodically in localized regions at the footpoints of magnetic loops. The resulting flows along the magnetic field lines of the AR produce higher density loops compared to the surrounding plasma by injection of material into the flux tubes and the establishment of siphon flow.We find that the impulsive onset of flows with subsonic speeds result in the excitation of damped slow magnetosonic waves that propagate along the loops and coupled nonlinearly driven fast-mode waves. The phase speed of the slow magnetosonic waves is close to the coronal sound speed. When the amplitude of the driving pulses is increased we find that slow shock-like wave trains are produced. When the upflows are driven periodically, undamped oscillations are produced with periods determined by the periodicity of the upflows. Based on the results of the 3D MHD model we suggest that the observed slow magnetosonic waves and persistent upflows may be produced by the same impulsive events at the bases of ARs.

Magnetohydrodynamics-Sun Activity↗
NASA NTRS1973

Studies of hydrodynamic events in stellar evolution. 3: Ejection of planetary nebulae

The dynamic behavior of the H-rich envelope (0.101 solar mass) of an evolved star (1.1 solar mass) as the luminosity rises to 19000 solar luminosity during the second ascent of the red giant branch. For luminosities in the range 3100 L 19000 solar luminosity the H-rich envelope pulsates like a long-period variable (LPV) with periods of the order of a year. As L reaches 19000 solar luminosity, the entire H-rich envelope is ejected as a shell with speeds of a few 10 km/s. The ejection occurs on a timescale of a few LPV pulsation periods. This ejection is associated with the formation of a planetary nebula. The computations are based on an implicit hydrodynamic computer code. T- and RHO-dependent opacities and excitation and ionization energies are included. As the H-rich envelope is accelerated off the stellar core, the gap between envelope and core is approximated by a vacuum, filled with radiation. Across the vacuum, the luminosity is conserved and the anisotropy of the radiation is considered as well as the solid angle subtended by the remnant star at the inner surface of the H-rich envelope. Spherical symmetry and the diffusion approximation are assumed.

Sparks, W. M.↗
NASA NTRS1979

The optical identification of 2A 0311-227 with a new AM Herculis-type object

The X-ray source 2A 0311-227 has been precisely positioned by using the HEAO 1 scanning modulation collimator, resulting in an optical candidate with a B magnitude of about 15. Medium-resolution optical spectra of this star show remarkable similarity to the spectrum of AM Her, a short-period binary containing a magnetic white dwarf. The Balmer decrement in 2A 0311-227 is also similar to that in AM Her and may likewise be attributed to collisional excitation in a plasma with an electron density of the order of 10 to the 13th per cu cm.

Griffiths, R. E.↗
NASA NTRS1982

The injection of energy into the Io plasma torus

Voyager EUV observations of the Io plasma torus indicate the presence of an intensity modulation in the corotating reference frame of significant magnitude on a 10-hour time scale. The phenomenon has a persistence which suggests it is a permanent feature, and the magnitude is such that a substantial amount of energy is injected into the torus at a 10-hour periodic rate. An investigation is conducted regarding the nature of the energy enjection process as determined by the plasma parameters and the observed characteristics. Attention is given to excitation-relaxation relations, aspects of ion-electron relaxation, and electron-electron relaxation and energy transfer rates. The radial distribution of temperature at the eastern and western elongations is found to indicate that the energy input to the electrons is distributed over the width of the torus. The dominant mechanism of energy transport to the plasma appears to be electron-electron heating.

Shemansky, D. E.↗