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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↗

NASA NTRS1989

Gamma-ray evidence for a stellar-mass black hole near the Galactic center

An analysis of the time variability of the observed 511-keV line emission from the direction of the Galactic center and the correlation of its variations in the continuum emission above 511 keV from the same direction suggest the existence of a compact object at or near the Galactic center. A possible mechanism of the observed positron annihilation is consistent with a compact interaction region of the order of 10 to the 8th cm. A black hole of several hundred solar masses is favored as a candidate for this compact object; arguments in support of this suggestion are presented.

Ramaty, Reuven↗

NASA NTRS1990

A plasma radiation model for the prompt radio emission of supernova 1987 A

It is proposed that the prompt radio emission of supernova 1987a a few days after the explosion is the result of a plasma interaction between the ejecta and the presupernova circumstellar matter. The emission frequency then is determined by the electron density in the front of the ejecta, and the radio flux density mainly depends on the velocity of the ejecta and the circumstellar electron density. Free-free absorptions in the interaction region and by the circumstellar medium limit the emission on the high, respectively low, frequency side and inhibit observable emission in the first two days.

Benz, A. O.↗

NASA NTRS1990

An LDA investigation of the normal shock wave boundary layer interaction

Nonintrusive measurements have been made of two normal shock wave-boundary layer interactions. Two-dimensional measurements were made throughout the interaction region while three-dimensional measurements were made in the vicinity of the shock wave. The measurements were made in the corner of the test section of a continuous flow supersonic wind tunnel in which a normal shock wave had been stabilized. LDA, 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 reported results define the flowfield structure in detail for each case.

Chriss, R. M.↗

NASA NTRS1986

Mars Aeronomy Observer: Report of the Science Working Team

The Mars Aeronomy Observer (MAO) is a candidate follow-on mission to Mars Observer (MO) in the Planetary Observer Program. The four Mariner and two Viking spacecraft sent to Mars between 1965 and 1976 have provided a wealth of information concerning Martian planetology. The Mars Observer, to be launched in 1990, will build on their results by further examining the elemental and mineralogical composition of the surface, the strength and multipolar composition of the planetary magnetic field, the gravitational field and topography, and the circulation of the lower atmosphere. The Mars Aeronomy Observer is intended to address the last major aspects of Martian environment which have yet to be investigated: the upper atmosphere, the ionsphere, and the solar wind interaction region.

Hunten, Donald M.↗

NASA NTRS1991

A laser velocimeter investigation of the normal shock-wave boundary layer interaction

Nonintrusive 3-D measurements were made of a normal shock wave/turbulent boundary layer interaction. The measurements were made through a quadrant of a square test section of a continuous flow supersonic wind tunnel in which a normal shock wave had been stabilized. Two dimensional measurements were made throughout the interaction region while 3-D measurements were made near the corner in the vicinity of the shock where 3-D flow effects were expected to be significant. Laser Doppler velocimetry, surface static pressure measurements, and flow visualization techniques were used for two freestream nominal Mach number test cases: 1.6 and 1.3. No turbulence information was obtained. The mean velocity measurements were converted to Mach number by recording the wind tunnel total temperature. Some shock oscillation was present during both of the test cases. After startup of the wind tunnel, the shock oscillated with an amplitude of approx. + or - 1 cm, however, after reaching steady condition, the shock oscillation amplitude was greatly reduced, as evidenced by the laser velocimeter results. The Mach 1.3 test case resulted in a nearly uniform flow without secondary shock waves and with no or very isolated corner separation.

Chriss, Randall M.↗

NASA NTRS1990

Simulation of period doubling of recurrent solar wind structures

Based on satellite observations of a recurrent solar wind structure conducted in 1974, an MHD simulation model, and input functions generated from plasma and magnetic field data, the continuing evolution of the solar wind structure outside 5 AU is studied. The model uses the Rankine-Hugoniot relations to describe the jumps in flow properties across the shocks, and it treats shocks as surfaces of discontinuity with zero thickness. Two interaction processes (the collision and the merging of shocks) play important roles in restructuring the solar wind in the outer heliosphere. The simulation result shows that period doubling occurs between 5 and 10 AU. The recurrent solar wind appears to be a persistent new structure between 10 and 20 AU, and it consists of one merged interaction region per solar rotation.

Whang, Y. C.↗