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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 145 records · Page 8
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NASA NTRSDate not supplied

Global Magnetic Reconnection with Weakly Shocked Solar Wind During Geomagnetic Storms

The solar wind and magnetic field emanating from the sun typically reaches Earth moving at a super-Alfvénic velocity (multiple times the local Alfvén wave speed). This leads to the formation of a bow shock upstream of the planet where the plasma is slowed and heated and the interplanetary magnetic field is compressed before it interacts with Earth's magnetic field. However, a number of recent geomagnetic storm events have been associated with sustained intervals of nearly or indeed fully sub-Alfvénic flows. A weakly- or un-shocked magnetosheath changes the conditions for magnetic reconnection at the magnetopause and subsequently the storm-time evolution of the magnetosphere. We perform Multiscale Atmosphere-Geospace Environment (MAGE) simulations of three different geomagnetic storms each containing a period of nearly or fully sub-Alfvénic solar wind driving and examine the properties of global reconnection. The structure of the night-side magnetosphere and polar cusps are also examined. The dynamical evolution of the global magnetosphere during strong driving represents a key science target for the NASA Heliophysics System Observatory, particularly Magnetospheric Multiscale (MMS) targeting magnetotail/magnetopause reconnection and Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) observing ion dispersion in the cusps.

Brandon Burkholder↗
NASA NTRS1987

MHD wave breaking in the outer plasmasphere

Empirical models of the average magnetospheric magnetic field, plasma density, and temperature distributions are used to construct a model of the distribution of MHD wave mode speeds within the magnetosphere. A persistent feature of the derived optical structure is a pronounced minimum of the wave speeds in the outer plasmasphere, i.e., a magnetospheric 'shoal'. This feature does not map along magnetic field lines, but is confined to the equatorial region, leading to a positive radial gradient of wave speeds near synchronous orbit. The breaking of earthward propagating disturbances in this region may play an essential role in the formation of the substorm injection boundary and in the creation of equatorially trapped warm ion distributions.

Moore, T. E.↗
NASA NTRS2022

Density and Velocity Fluctuations of Alpha Particles in Magnetic Switchbacks

Magnetic switchbacks, or sudden reversals in the magnetic field’s radial direction, are one of the more striking observations of the Parker Solar Probe (PSP) in its mission thus far. While their precise production mechanisms are still unknown, the two main theories are via interchange reconnection events and in situ generation. In this work, density and abundance variations of alpha particles are studied inside and outside individual switchbacks. We find no consistent compositional differences in the alpha particle abundance ratio, nαp, inside versus outside switchbacks, nor do we observe any signature when separating the switchbacks according to Vαp/Vpw, the ratio of the alpha–proton differential speed to the wave phase speed (the speed at which the switchback is traveling). We argue that these measurements cannot be used to rule in favor of one production mechanism over the other, due to the distance between PSP and the postulated interchange reconnection events. In addition, we examine the 3D velocity fluctuations of protons and alpha particles within individual switchbacks. While switchbacks are always associated with increases in proton velocity, alpha velocities may be enhanced, unchanged, or decrease. This is due to the interplay between Vpw and Vαp, with the Alfvénic motion of the alpha particles vanishing as the difference |Vpw – Vαp| decreases. We show how the Alfvénic motion of both the alphas and the protons through switchbacks can be understood as an approximately rigid arm rotation about the location of the wave frame, and illustrate that the wave frame can therefore be estimated using particle measurements alone, via sphere fitting.

Michael D. McManus↗
NASA NTRS1985

The speed of wave-wave interactions in the atmosphere

Resonant wave-wave interactions are considered. Studies of the interaction coefficient show that rapid transfer of wave action can take place in the disjoined parts of the spectrum for three processes, namely: elastic scattering, parametric subharmonic instability and induced diffusion. Of the three processes, the vertical shear plays a role in two. The vertical shear of a moderate scale interacts through elastic scattering to make the spectrum vertically symmetric. On the other hand, the vertical shear of a large scale interacts through induced diffusion and is responsible for diffusion in k sub z space. When interacting with a vertical shear, it is known that the vertical shear acts as a catalyst and is not involved in energy transfer. Consequently, in both elastic scattering and induced diffusion, the vertical shear does not gain or lose energy. Through parametric subharmonic instability the more energetic large-scale waves are feeding energy into moderate and small scale waves of an elevation angle of 60 deg or larger.

Yeh, K. C.↗
NASA NTRS1987

Extension of Kirchhoff's formula to radiation from moving surfaces

Kirchhoff's formula for radiation from a closed surface has been used recently for prediction of the noise of high speed rotors and propellers. Because the closed surface on which the boundary data are prescribed in these cases is in motion, an extension of Kirchhoff's formula to this condition is required. In this paper such a formula, obtained originally by Morgans for the interior problem, is derived for regions exterior to surfaces moving at speeds below the wave propagation speed by making use of some results of generalized function theory. It is shown that the usual Kirchhoff formula is a special case of the main result of the paper. The general result applies to a deformable surface. However, the special form it assumes for a rigid surface in motion is also noted. In addition, Morgans' result is further extended by showing that edge line integrals appear in the formula when applied to a surface that is piecewise smooth. Some possible areas of application of the formula to problems of current interest in aeroacoustics are discussed.

Farassat, F.↗
NASA NTRS1988

Extension of Kirchhoff's formula to radiation from moving surfaces

Kirchhoff's formula for radiation from a closed surface has been used recently for prediction of the noise of high speed rotors and propellers. Because the closed surface on which the boundary data are prescribed in these cases is in motion, an extension of Kirchhoff's formula to this condition is required. In this paper such a formula, obtained originally by Morgans for the interior problem, is derived for regions exterior to surfaces moving at speeds below the wave propagation speed by making use of some results of generalized function theory. It is shown that the usual Kirchhoff formula is a special case of the main result of the paper. The general result applies to a deformable surface. However, the special form it assumes for a rigid surface in motion is also noted. In addition, Morgans' result is further extended by showing that edge line integrals appear in the formula when applied to a surface that is piecewise smooth. Some possible areas of application of the formula to problems of current interest in aeroacoustics are discussed.

Farassat, F.↗
NASA NTRS1989

On the three families of instability waves of high-speed jets

An analytical and computational study of the normal-mode small-amplitude waves of high-speed jets is presented. Three families of instability waves have been identified: (1) the familiar Kelvin-Helmholtz instability waves; (2) supersonic instability waves; and (3) subsonic waves. It is demonstrated that the computed wave patterns and propagation characteristics of these three wave types are consistent with the findings of Oetel (1979, 1980, 1982). The subsonic waves are shown to be unstable only for jets with mixing layers of finite thickness.

Tam, Christopher K. W.↗
NASA NTRSDate not supplied

Rayleigh-Scattering-Based Measurement of 'trapped waves' in High-speed Jets

Recently reported research by others identified a system of waves in the near-exit region of high-speed jets that are different from the well-known Kelvin-Helmholtz waves. In an experimental study, Rayleigh-scattering-based measurement of density fluctuations associated with these waves was used. Simultaneously, a microphone placed near the exit of the jet measured pressure fluctuations associated with the ‘trapped waves’. The pressure fluctuations were observed as a series of peaks in the spectra. Measurements were acquired in high subsonic and under-expanded screeching supersonic flows from a 1-inch diameter convergent nozzle in a free jet facility. The Rayleigh measurement volume was translated throughout the flow field while the microphone was held fixed just outside the flow near the nozzle exit. Even though the signal-to-noise ratio for the Rayleigh data was poor, cross-correlation of density fluctuations with the microphone signal provided some insight into the propagation characteristics of the trapped waves as well as the screech component.

Rayleigh scattering↗
NASA NTRS2003

Sound Transmission Through a Curved Honeycomb Composite Panel

Composite structures are often used in aircraft because of the advantages offered by a high strength to weight ratio. However, the acoustical properties of these light and stiff structures can often be less than desirable resulting in high aircraft interior noise levels. In this paper, measurements and predictions of the transmission loss of a curved honeycomb composite panel are presented. The transmission loss predictions are validated by comparisons to measurements. An assessment of the behavior of the panel is made from the dispersion characteristics of transverse waves propagating in the panel. The speed of transverse waves propagating in the panel is found to be sonic or supersonic over the frequency range from 100 to 5000 Hz. The acoustical benefit of reducing the wave speed for transverse vibration is demonstrated.

Klos, Jacob↗
NASA NTRS2010

Improving NOAA's NWLON Through Enhanced Data Inputs from NASA's Ocean Surface Topography

This report assesses the benefit of incorporating NASA's OSTM (Ocean Surface Topography Mission) altimeter data (C- and Ku-band) into NOAA's (National Oceanic and Atmospheric Administration) NWLON (National Water Level Observation Network) DSS (Decision Support System). This data will enhance the NWLON DSS by providing additional inforrnation because not all stations collect all meteorological parameters (sea-surface height, ocean tides, wave height, and wind speed over waves). OSTM will also provide data where NWLON stations are not present. OSTM will provide data on seasurface heights for determining sea-level rise and ocean circulation. Researchers and operational users currently use satellite altimeter data products with the GSFCOO NASA data model to obtain sea-surface height and ocean circulation inforrnation. Accurate and tirnely inforrnation concerning sea-level height, tide, and ocean currents is needed to irnprove coastal tidal predictions, tsunarni and storm surge warnings, and wetland restoration.

Guest, DeNeice C.↗
NASA NTRS1955

Arrangement of fusiform bodies to reduce the wave drag at supersonic speeds

By means of linearized-body theory and reverse-flow theorems, the wave drag of a system of fusiform bodies at zero angle of attack and supersonic speeds is studied to determine the effect of varying the relative location of the component parts. The investigation is limited to two-body and three-body arrangements of Sears-Haack minimum-drag bodies. It is found that in certain arrangements the interference effects are beneficial, and may even result in the two or three-body system having no more wave drag than that of the principal body alone. The most favorable location appears to be one in which the maximum cross-section of the auxiliary body is slightly forward of the Mach cone from the tail of the main body. The least favorable is the region between the Mach cone from the nose and the forecone from the tail of the main body. (author)

Friedman, Morris D↗
NASA NTRS1977

Nonlinear Alfven waves in high-speed solar wind streams

A nonlinear proton distribution function that is an exact stationary solution of the nonlinear Vlasov equation and Maxwell's equations and which supports a single nonlinear transverse Alfven (ion cyclotron) wave that is circularly polarized and nondispersive is proposed for most of the observations during high-speed solar wind streams. This nonlinear distribution removes the strong Alfven wave instability, inconsistent with the persistence of the observed proton distribution functions in high-speed streams, found by the linear stability analysis. Model temperature anisotropies and drift velocities of the two spatially inhomogeneous bi-Maxwellian components are consistent with typical proton velocity distributions measured in high-speed streams at 1 AU. Two derived relations for each of the wave number and the phase velocity of the wave are obeyed within experimental uncertainties by two typical proton measurements. Our model also predicts that the alpha particle bulk flow velocity exceeds the proton particle bulk flow velocity, as is observed.

Abraham-Shrauner, B.↗
NASA NTRS1989

Experimental investigation of the dependence of radar backscattering on wind speed, wind stress and wave height

During summer 1988, radar measurements were performed in conjunction with detailed environmental observations on Lake Washington at the University of Washington Sand Point field station. Radar data were collected at 5.3 and 10 GHz for incidence angles between 30 and 60 deg with VV-polarization. The environmental measurements included wind speed and direction, large-wave heights, the high-frequency portion of the wave spectrum, humidity, and air and water temperatures. The small-scale wave spectrum was measured using a resistance wire gauge. The results show that backscatter increased with wind speed as expected. However, little difference was observed in the scattering coefficient for upwind and crosswind directions. The results also indicated an increase in the amplitude of small waves with friction velocity.

Gogineni, S. P.↗
NASA NTRS1995

Evolution of Photon and Particle Spectra in Compact, Luminous Objects

Physical conditions in the radiating plasma in the cores of radio-strong quasars and active galactic nuclei cannot be derived from observations until the effects of relativistic aberration are understood. This requires determining both the bulk flow speeds and any wave or signal speed in the parsec-scale nuclear jets. In this project we studied several aspects of such waves. We considered constraints on jet deceleration by mass pickup, and found that bolometric luminosities of the active nuclei cannot constrain core jet speeds usefully. We also simulated observations of ballistic, helical trajectories and helical waves moving directly outwards along the jet. We found that ballistic trajectories are not allowed by the data; the helical features seen are very likely to be helical waves. We believe these are waves propagating in the jet plasma. To this end, we studied waves propagating in relativistic pair plasma jets. In particular, we undertook a program whose goal was to determine the nature of waves which can propagate in relativistic pair plasmas, and how such waves propagating in streaming jet plasma would be observed by an external observer. We developed the possibility of using pulsars as test cases for our models; this takes advantage of new technology in pulsar observations, and the similarity of the physical conditions in the pulsar magnetosphere to the dense, relativistic pair plasmas which exist in radio-strong quasars.

Eilek, Jean A.↗
NASA NTRS1990

Some comparisons of linear stability theory with experiment at supersonic and hypersonic speed

Measurements of waves initiated by wind-tunnel sounds are obtained for both supersonic and hypersonic speed. Flat-plate experiments carried out at Mach numbers of 4.5, 3.7, and 2.4 under the condition of laminar tunnel-wall boundary layers whereby a low background fluctuation level prevailed are outlined, as well as flat-plate experiments with sound-induced waves at supersonic speeds. Experimental studies of 'natural' wave growth on sharp cones at zero angle of attack and Mach numbers near 8 are discussed. Attention is focused on experimental research needed on boundary-layer receptivity to eddy Mach-wave radiation, hypersonic speed, and the stability-transition relationship in zero-pressure-gradient layers.

Kendall, James M., Jr.↗