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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Formation of a compound stream between 0.85 AU and 6.2 AU and its effects on solar energetic particles and galactic cosmic rays

This paper describes the formation of a compound stream as a result of the interaction and coalescence of a series of five streams (a slow transient stream, two corotating streams, and two exceptionally fast transient streams) observed by the Helios B (HB) satellite near 0.85 AU. The compound stream and two merged interaction regions were also observed by the Voyager 1 satellite near 6.2 AU when it was nearly radially aligned with the HB. Closely associated with this compound stream was one of the largest solar energetic particle events observed beyond 5 AU. The relationship of this compound stream and its magnetic fields to the intensity profiles of solar energetic particles and galactic cosmic rays is discussed.

Burlaga, L. F.↗
NASA NTRS1987

Cosmic ray variations and magnetic field fluctuations in the outer heliosphere

It is formally confirmed that Galactic-cosmic-ray intensity variations measured by Voyager 2 during recovery from solar maximum are caused by traveling compressions and rarefactions in the mean interplanetary magnetic field. Voyager magnetic-field data are used as input to a time-independent, spherically symmetric, cosmic-ray-transport equation in the force-field approximation. The solutions closely followed the count rate of cosmic rays greater than 75 MeV/nucleon over 4 years, during the recovery phase of the 11-year solar-driven cosmic-ray cycle. This strongly supports prior theoretical assertions that turbulent interaction regions traveling with the solar wind are the major cause of the solar-cycle variation of Galactic cosmic rays in the ecliptic region.

Perko, J. S.↗
NASA NTRS1987

Numerical simulation of solar wind and magnetospheric phenomena

The nature of the nonlinear evolution of Kelvin-Helmholtz instability in the presence of sheared magnetic fields was investigated via numerical simulation. Models of the magnetosheath-magnetopause boundary in earth's tail and stream interaction regions in the inner heliosphere were studied. The development of the instability is influenced strongly by the orientation and magnitude of the magnetic field. Large vortical structures that resemble observations in the earth's tail can form while other cases generate turbulent spectra that provide insight into the generation of Alfven turbulence in the solar wind.

Goldstein, Melvyn L.↗
NASA NTRS1988

Experimental and numerical investigation of the effect of distributed suction on oblique shock wave/turbulent boundary layer interaction

A combined experimental and numerical study of the interaction of an incident oblique shock wave with a turbulent boundary layer on a rough plate and on a porous plate with suction is presented. The experimental phase involved the acquisition of mean data upstream of, within, and downstream of the interaction region at Mach numbers 2.5 and 3.0. Data were taken at unit Reynolds numbers of 1.66 E7 and 1.85 E7 m respectively, and for flow deflection angles of 0, 4, 6 and 8 degs. Measured data include wall static pressure, pitot pressure profiles, and local bleed distributions on the porous plate. On the rough plate, with no suction, the boundary layer profiles were modified near the wall, but not separated for the 4 deg flow deflection angle. For the higher deflection angles of 6 and 8 degs, the boundary layer was separated. Suction increases the strength of the incident shock required to separate the turbulent boundary layer; for all shock strengths tested, separation is completely eliminated. The pitot pressure profiles are affected throughout the whole boundary layer; they are fuller than the ones obtained on the rough plate. It is also found that the combination of suction and roughness introduces spatial perturbations.

Benhachmi, Driss↗
NASA NTRS1987

Submillimeter backward wave oscillators

A NASA program for the development of backward wave oscillators (BWOs) for the 500-2000 GHz frequency range is discussed. The BWO operation is based on a periodic etched slow wave structure which supports the induced traveling electromagnetic wave. The present submillimeter wave BWOs will employ a variation of the folded wave guide, the interdigital line. The use of a noncompressed electron beam results in less thermal loading of the circuit, smaller magnetic focusing fields, and higher current density of the circuit interaction region. The diamond etching procedure is described. Aluminum will be used for the metallization of the diamonds.

Dayton, J. A.↗
NASA NTRS1987

Low-frequency magnetic field fluctuations in comet P/Halley's magnetosheath - Giotto observations

The interaction region between comets and the solar wind is characterized by large amplitude, low frequency magnetic field fluctuations, both within the upstream region as well as in the magnetosheath. Magnetosheath observations of the magnetic field experiment onboard Giotto indicate values of delta B/B equal to about O(1). Power spectral peaks appear at frequencies of 10mHz with the spectrum exhibiting a power law dependence with an exponent of the order 2. Radial variation of the fluctuation level does not clearly increase with decreasing distance from the cometary nucleus as observed by the magnetometer onboard Vega-1 and as expected from quasi-linear theory. The entrance into the cometary bow shock is furthermore characterized by an order of magnitude increase of the fluctuation level, both on the in- and outbound pass of Giotto.

Glassmeier, K. H.↗
NASA NTRS1987

Cometary MHD and chemistry

An MHD and chemical comet-coma model was developed, applying the computer program of Huebner (1985) for the detailed chemical evolution of a spherically expanding coma and the program of Schmidt and Wegman (1982) and Wegman (1987) for the MHD flow of plasma and magnetic field in a comet to the Giotto-mission data on the ion abundances measured by the HIS ion mass spectrometer. The physics and chemistry of the coma are modeled in great detail, including photoprocesses, gas-phase chemical kinetics, energy balance with a separate electron temperature, multifluid hydrodynamics with a transition to free molecular flow, fast-streaming atomic and molecular hydrogen, counter and cross streaming of the ionized species relative to the neutral species in the coma-solar wind interaction region with momentum exchange by elastic collisions, mass-loading through ion pick-up, and Lorentz forces of the advected magnetic field. The results, both inside and outside of the contact surface, are discussed and compared with the relevant HIS ion mass spectra.

Wegmann, R.↗
NASA NTRS1988

Excitation of instability waves in free shear layers. II - Experiments

The acoustical excitation of shear layers is investigated experimentally. Acoustical excitation causes, for example, the so-called 'orderly structures' in shear layers and jets. The deviations in the spreading rate between different turbulent-shear-layer experiments are due to the same excitation mechanism. The present investigations focus on measurements in the linear interaction region close to the edge from which the shear layer is shed. Two sets of experiments (Houston, 1981, and Berlin, 1983 and 1984) are reported on. The measurements have been carried out with laminar shear layers in air using hot-wire anemometers and microphones. The agreement between these measurements and the theory is good. Details of the fluctuating flow field are found to correspond to theoretical predictions, such as the local occurrence of negative phase speeds.

Bechert, D. W.↗
NASA NTRS1988

Period doubling in the outer heliosphere

From approximately July 28 to November 26, 1984, IMP 8 at 1 AU observed quasi-periodic interaction regions in the solar wind characterized by a peak every 13.4 days in the magnetic field strength, plasma density, and temperature, corresponding to an inertial period of 12.5 days. When the same solar wind reached Voyager 2 (which moved from 15.2 to 16.1 AU during the corresponding time interval, September 27, 1984, to January 27, 1985), the enhancements in the magnetic field strength and the plasma density and temperature recurred with a period of approximately 25 days. Thus the period of the large-scale fluctuations in B, N, and T doubled between 1 AU and 15.2 AU. The magnetic field strength increased linearly with the density and the temperature in the Voyager 2 data. The tails of the distributions of B, N, and T in the Voyager 2 data were approximately exponential.

Burlaga, L. F.↗
NASA NTRS1988

Origin of interplanetary southward magnetic fields responsible for major magnetic storms near solar maximum (1978-1979)

Simultaneous ISEE-3 field and plasma data were used to examine interplanetary phenomena associated with 10 major magnetic storms detected from August 16, 1978, to December 28, 1979, in a study of Gonzalez and Tsurutani (1987), and, in particular to determine the origins of the southward magnetic fields which caused the storms. In nine of the 10 cases, the responsible interplanetary events were found, as expected, to be associated with the high magnetic fields in the stream-stream interaction regions (sheaths) or driver gases, with the events following the interplanetary shocks. The tenth event was found to be associated not with a high-speed stream, but with a noncompressional density-enhancement event. The results of this study indicate the equal importance of both the sheath fields or draped fields and the driver gas fields for the generation of major geomagnetic storms.

Tsurutani, Bruce T.↗
NASA NTRS1989

An LDA (Laser-Doppler Anemometry) investigation of three-dimensional normal shock wave boundary-layer interactions

Nonintrusive measurements were made of a normal shock wave/boundary layer interaction. Two dimensional measurements were made throughout the interaction region while 3-D measurements were made in the vicinity of the shock wave. The measurements were made in the corner of the test section of a continuous supersonic wind tunnel in which a normal shock wave had been stabilized. Laser Doppler Anemometry, surface pressure measurement and flow visualization techniques were employed for two freestream Mach number test cases: 1.6 and 1.3. The former contained separated flow regions and a system of shock waves. The latter was found to be far less complicated. The results define the flow field structure in detail for each case.

Chriss, R. M.↗
NASA NTRS1988

Meridional plasma flow in the outer heliosphere

Voyager 2 observations made in the outer heliosphere near 25 AU and within 2 deg of the heliographic equatorial plane show periodic variations in the meridional (North/South) flow velocities that are much more prominent than the East/West variations. An autocorrelation analysis shows that the flow variation has a period of about 25.5 days in the latter half of 1986, in approximate agreement with the solar rotation period. The results suggest that increased pressure in interaction regions remains the best candidate for the driver of the nonradial flows.

Lazarus, A. J.↗
NASA NTRS1989

Rocket observations of the precipitation of electrons by ground VLF transmitters

Recent results obtained with electric and magnetic receivers aboard a NASA sounding rocket launched on July 31, 1987 are presented which relate multiple electron spectral peaks observed in the bounce loss cone fluxes to the resonant interaction of electrons with VLF waves from ground transmitters. The correlation of transmitter signals passing through the ionosphere with the precipitated electrons was investigated. The analysis of these in situ wave and particle data addresses the propagation of waves through the ionosphere, and, through an application of the resonant theory, enables an estimation of the cold plasma density in the interaction region.

Arnoldy, Roger L.↗
NASA NTRS1989

Remote sensing of the termination of the solar wind via in situ plasma measurements

Since 1983 the Plasma Wave experiments on the two Voyager spacecraft have detected low frequency radio noise in the outer heliosphere which has been postulated to emanate from the terminal shock of the solar wind or, possibly, from the heliopause itself. The solar wind data from the Plasma Science experiment on the Voyager spacecraft are studied to search for correlations with these radio emissions. It is found that two anomalous high speed streams passed Voyager 2 in late 1982 and early 1983 and it is suggested that the interaction of the streams with the heliospheric terminal shock is responsible for the generation of the most intense radio noise observed later in the same year. If the stream speeds did not decrease in traveling to the interaction region, that region is about 135 AU from the sun. This is consistent with previous estimates of the distance to the inner heliospheric shock.

Mcnutt, R. L., Jr.↗
NASA NTRS1990

Unsteady three-dimensional marginal separation, including breakdown

A situation involving a three-dimensional marginal separation is considered, where a (steady) boundary layer flow is on the verge of separating at a point (located along a line of symmetry/centerline). At this point, a triple-deck is included, thereby permitting a small amount of interaction to occur. Unsteadiness is included within this interaction region through some external means. It is shown that the problem reduces to the solution of a nonlinear, unsteady, partial-integro system, which is solved numerically by means of time-marching together with a pseudo-spectral method spatially. A number of solutions to this system are presented which strongly suggest a breakdown of this system may occur, at a finite spatial position, at a finite time. The structure and details of this breakdown are then described.

Duck, Peter W.↗
NASA NTRS1990

On the instability of hypersonic flow past a flat plate

The instability of hypersonic boundary-layer flows over flat plates is considered. The viscosity of the fluid is taken to be governed by Sutherland's law, which gives a much more accurate representation of the temperature dependence of fluid viscosity at hypersonic speeds than Chapman's approximate linear law; although at lower speeds the temperature variation of the mean state is less pronounced so that the Chapman law can be used with some confidence. Attention is focussed on the so-called (vorticity) mode of instability of the viscous hypersonic boundary layer. This is thought to be the fastest growing inviscid disturbance at hypersonic speeds; it is also believed to have an asymptotically larger growth rate than any viscous or centrifugal instability. As a starting point the instability of the hypersonic boundary layer which exists far downstream from the leading edge of the plate is investigated. In this regime the shock that is attached to the leading edge of the plate plays no role, so that the basic boundary layer is non-interactive. It is shown that the vorticity mode of instability of this flow operates on a significantly different lengthscale than that obtained if a Chapman viscosity law is assumed. In particular, it is found that the growth rate predicted by a linear viscosity law overestimates the size of the growth rate by O(M(exp 2). Next, the development of the vorticity mode as the wavenumber decreases is described, and it is shown that acoustic modes emerge when the wavenumber has decreased from it's O(1) initial value to O(M (exp -3/2). Finally, the inviscid instability of the boundary layer near the leading edge in the interaction zone is discussed and particular attention is focussed on the strong interaction region which occurs sufficiently close to the leading edge. It is found that the vorticity mode in this regime is again unstable, and that it is concentrated in the transition layer at the edge of the boundary layer where the temperature adjusts from its large, O(M(exp 2), value in the viscous boundary layer, to its O(1) free stream value. The existence of the shock indirectly, but significantly, influences the instability problem by modifying the basic flow structure in this layer.

Blackaby, Nicholas↗
NASA NTRS1990

Experimental studies of shock wave/wall jet interaction in hypersonic flow

The interaction between a planar shock wave and a wall jet produced by slot cooling in turbulent hypersonic flow was experimentally studied. Detailed distributions of heat transfer and pressure are obtained in the incident shock/wall jet interaction region for a series of shock strengths and impingement positions for two nozzle heights. The major result is that the cooling film could be readily dispersed by relatively weak incident shocks such that the peak heating in the recompression region was not significantly reduced by even the largest levels of film cooling. Regions of boundary layer separation were induced in the film cooling layer, the size of which first increased and then decreased with increasing film cooling. The size of the separated regions and magnitude of the recompression heating were not strongly influenced by the thickness of the cooling film or point of shock impingement relative to the exit plane of the nozzles.

Holden, Michael S.↗
NASA NTRS1990

A time-dependent wave packet approach to atom-diatom reactive collision probabilities - Theory and application to the H + H2(J = 0) system

This paper describes a new approach to the study of atom-diatom reactive collisions in three dimensions employing wave packets and the time-dependent Schroedinger equation. The method uses a projection operator approach to couple the inelastic and reactive portions of the total wave function and optical potentials to circumvent the necessity of using product arrangement coordinates. Reactive transition probabilities are calculated from the state resolved flux of the wave packet as it leaves the interaction region in the direction of the reactive arrangement channel. The present approach is used to obtain such vibrationally resolved probabilities for the three-dimensional H + H2 (J = 0) hydrogen exchange reaction, using a body-fixed system of coordinates.

Neuhauser, Daniel↗