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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Modeling of an interplanetary disturbance event tracked by the interplanetary scintillation method

The interplanetary disturbance event of August 25-29, 1978 was modeled in an attempt to reproduce the corresponding interplanetary scintillation observations and simultaneous ISEE-3 satellite data. A shock wave generated from the region of a disappearing filament on August 23 is shown to account for the observed shock wave structure, but not the broad high-speed stream behind the shock wave. A shock wave generated by the sudden activation of the coronal hole on the same day is able to account for the high-speed stream.

Akasofu, S.-I.↗
NASA NTRS1973

Steepening of nonlinear waves in the solar wind.

Two shock-like structures (i.e., abrupt step-like increases in number density, bulk speed, and proton temperature) were found in the data from Mariner 5 at 0.98 AU on June 26 and at 0.85 AU on Aug. 29, 1967. The thickness of these 'structures' determined from the magnetic field data was more than 1000 proton Larmor radii. They were also observed by Explorer 33, 34, and 35 at 1 AU, where the thicknesses were much smaller than in the Mariner 5 data. It is suggested that these two structures were nonlinear magnetoacoustic waves that were in the processes of steepening. It is further suggested that shocks would be formed just beyond 1 AU. These structures were not associated with corotating streams, but they could be associated with impressive solar events in which a flare was followed by type 2 and type 4 radio emission and a rare chromospheric wave.

Chao, J. K.↗
NASA NTRS1993

Swept shock/boundary-layer interactions: Scaling laws, flowfield structure, and experimental methods

A general review is given of several decades of research on the scaling laws and flowfield structures of swept shock wave/turbulent boundary layer interactions. Attention is further restricted to the experimental study and physical understanding of the steady-state aspects of these flows. The interaction produced by a sharp, upright fin mounted on a flat plate is taken as an archetype. An overall framework of quasiconical symmetry describing such interactions is first developed. Boundary-layer separation, the interaction footprint, Mach number scaling, and Reynolds number scaling are then considered, followed by a discussion of the quasiconical similarity of interactions produced by geometrically-dissimilar shock generators. The detailed structure of these interaction flowfields is next reviewed, and is illustrated by both qualitative visualizations and quantitative flow images in the quasiconical framework. Finally, the experimental techniques used to investigate such flows are reviewed, with emphasis on modern non-intrusive optical flow diagnostics.

Settles, Gary S.↗
NASA NTRS1994

Sonic boom interaction with turbulence

A recently developed transonic small-disturbance model is used to analyze the interactions of random disturbances with a weak shock. The model equation has an extended form of the classic small-disturbance equation for unsteady transonic aerodynamics. It shows that diffraction effects, nonlinear steepening effects, focusing and caustic effects and random induced vorticity fluctuations interact simultaneously to determine the development of the shock wave in space and time and the pressure field behind it. A finite-difference algorithm to solve the mixed-type elliptic hyperbolic flows around the shock wave is presented. Numerical calculations of shock wave interactions with various deterministic vorticity and temperature disturbances result in complicate shock wave structures and describe peaked as well as rounded pressure signatures behind the shock front, as were recorded in experiments of sonic booms running through atmospheric turbulence.

Zvi Rusak↗
NASA NTRS1986

The development of an environmental disturbance model for large space structures after the onset of thermal shock

Expressions for the temperature response across a thin large space structure during and immediately following the onset of thermal shock are obtained as a function of the properties of the material and the solar incidence angle. The thermal gradients induced due to solar radiation heating result in thermal deformation. From the temperature gradient information, expressions are developed for the time history of the thermally induced deflections based on the thermoelastic relationships. The effect of the variation of the solar incidence angle and the variation of the emissivity of the surface is considered. From the deflection time history, a model of the disturbance moments is developed and evaluated for different solar incidence angles.

Hamsath, N.↗
NASA NTRS1989

A new rotational relaxation model for use in hypersonic computational fluid dynamics

The theoretical basis for the Landau-Teller equation commonly used to model rotational nonequilibrium is reviewed. Several assumptions underlying this model are indicated to be unrealistic for rarefied hypersonic flow. A new rotational nonequilibrium model based on recent measurements up to 2000K of quantum state transition rates is presented. The new model is applied to the continuum study of normal shock wave structure using both the Burnett equations and a simplified nonlinear constitutive relation. Comparisons are made between shock wave temperature profiles generated from the new model and those generated using the Landau-Teller model. Comparisons of shock reciprocal thicknesses between experimental data for nitrogen and continuum solutions using both rotational models are made. The new rotational model agrees well with experiment up to Mach 6, and under predicts shock thickness at higher Mach numbers. The Landau-Teller model agrees well with experimental shock thickness up to Mach 3, and over predicts the thickness at higher Mach numbers. A modification to the rotational collision number in the Landau-Teller model is found to give results which agree with experimental shock thicknesses at all Mach numbers up to 11.

Chapman, Dean E.↗
NASA NTRS1990

Enstrophy budget in decaying compressible turbulence

The interaction of vorticity and shock waves in decaying compressible turbulence is investigated by direct numerical simulation of the Navier-Stokes equation using a spectral method with 64-cubed collocation points. The Taylor microscale Reynolds number decreases from 36 to 17 and the rms Mach number from 0.8 to 0.4 during the decay. It is observed that vorticity is created inside curved shocks through baroclinic interaction and intensified by the compression of fluid elements. Vortex stretching is, on the other hand, dominant outside shocks and is responsible for enhancing the strength of vorticity. A dipole of vorticity is typically created at the vertex of curved shocks, the structure of which is explained in terms of jump relations of velocity derivatives across a curved shock.

Kida, Shigeo↗
NASA NTRS1990

Formation, structure, and stability of MHD intermediate shocks

It was recently shown by Wu (1987) that intermediate shocks are admissible and can be formed through nonlinear wave steepening from continuous waves. In this paper, the formation, structure, and stability of intermediate shocks in dissipative MHD are considered in detail. The differences between the conventional theory and the present one are pointed out and clarified. It is shown that all four types of intermediate shocks can be formed from smooth waves. It is also shown that there are free parameters in the structure of the intermediate shocks, and that these parameters are related to the shock stability. In addition, the paper shows that a rotational discontinuity can not exist with finite width, indicates how this is related to the existence of time-dependent intermediate shocks, and shows why the conventional theory is not a good approximation to dissipative MHD solutions whenever there is rotation in magnetic field.

Wu, C. C.↗
NASA NTRS2004

Lunar and Planetary Science XXXV: Effects of Impacts: Shock and Awe

This document discusses the following topics: Zircon as a Shock Indicator in Impactites of Drill Core Yaxcopoil-1, Chicxulub Impact Structure, Mexico; Experimental Investigation of Shock Effects in a Metapelitic Granulite; Experimental Reproduction of Shock Veins in Single-Crystal Minerals; Post-Shock Crystal-Plastic Processes in Quartz from Crystalline Target Rocks of the Charlevoix Impact Structure; Shock Reequilibration of Fluid Inclusions; How Does Tektite Glass Lose Its Water?; Assessing the Role of Anhydrite in the KT Mass Extinction: Hints from Shock-loading Experiments; A Mineralogical and Geochemical Study of the Nonmarine Permian/Triassic Boundary in the Southern Karoo Basin, South Africa; Extraterrestrial Chromium in the Permian-Triassic Boundary at Graphite Peak, Antarctica; Magnetic Fe,Si,Al-rich Impact Spherules from the P-T Boundary Layer at Graphite Peak, Antarctica; A Newly Recognized Late Archean Impact Spherule Layer in the Reivilo Formation, Griqualand West Basin, South Africa; Initial Cr-Isotopic and Iridium Measurements of Concentrates from Late Eocene Cpx-Spherule Deposits; An Ordinary Chondrite Impactor Composition for the Bosumtwi Impact Structure, Ghana, West Africa: Discussion of Siderophile Element Contents and Os and Cr Isotope Data.

Kyte, F. T.↗
NASA NTRS1992

Observations of short large-amplitude magnetic structures at a quasi-parallel shock

Results of a detailed analysis of short large-amplitude magnetic structures (SLAMS) observed at an encounter of the quasi-parallel blow shock by the AMPTE UKS and IRM satellites are presented. Isolated SLAMS, surrounded by solar wind conditions, and embedded SLAMS, which lie within or form the boundary with regions of significant heating and deceleration, are identified. The duration, polarization, and other characteristics of SLAMS are all consistent with their growth directly out of the ULF wave field, including the common occurrence of an attached whistler as found in ULF shocklets. The plasma rest frame propagation speeds and two-spacecraft time delays for all cases show that the SLAMS attempt to propagate upstream against the oncoming flow, but are convected back downstream. The speeds and delays vary systematically with SLAMS amplitude in the way anticipated from nonlinear wave theory, as do their polarization features. Inter-SLAMS regions and boundary regions with solar wind contain hot deflected ions of lesser density than within the SLAMS.

Schwartz, Steven J.↗
NASA NTRS1985

Evolution and interaction of large interplanetary streams

A computer simulation for the evolution and interaction of large interplanetary streams based on multi-spacecraft observations and an unsteady, one-dimensional MHD model is presented. Two events, each observed by two or more spacecraft separated by a distance of the order of 10 AU, were studied. The first simulation is based on the plasma and magnetic field observations made by two radially-aligned spacecraft. The second simulation is based on an event observed first by Helios-1 in May 1980 near 0.6 AU and later by Voyager-1 in June 1980 at 8.1 AU. These examples show that the dynamical evolution of large-scale solar wind structures is dominated by the shock process, including the formation, collision, and merging of shocks. The interaction of shocks with stream structures also causes a drastic decrease in the amplitude of the solar wind speed variation with increasing heliocentric distance, and as a result of interactions there is a large variation of shock-strengths and shock-speeds. The simulation results shed light on the interpretation for the interaction and evolution of large interplanetary streams. Observations were made along a few limited trajectories, but simulation results can supplement these by providing the detailed evolution process for large-scale solar wind structures in the vast region not directly observed. The use of a quantitative nonlinear simulation model including shock merging process is crucial in the interpretation of data obtained in the outer heliosphere.

Whang, Y. C.↗
NASA NTRS1992

Bow shock models for the velocity structure of ultracompact H II regions

The velocity structure of ultracompact H II regions is modeled assuming that O stars moving supersonically through molecular clouds sweep up bow shocks to produce the observed objects. The expected radio recombination line emission is calculated for the case of an optically thin continuum and the strong effect of changing the viewing angle is shown. The kinematic information removes the degeneracy with ram pressure of a previous model, allowing measurement of stellar velocity vectors and cloud densities. A detailed model for G29.96-0.02 shows good agreement with observations by Wood and Churchwell, supporting the bow shock hypothesis. It is found that the exciting star of G29 is moving at 20 km/s relative to the gas, suggesting that O stars acquire a large velocity dispersion early in their lives.

Van Buren, Dave↗
NASA NTRS1994

Relativistic particle acceleration in plerions

We discuss recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is either pure electrons and positrons or primarily electrons and positrons with an admixture of heavy ions. Particle-in-cell simulation techniques as well as analytic theory have been used to show that such shocks in pure pair plasmas are fully thermalized -- the downstream particle spectra are relativistic Maxwellians at the temperature expected from the jump conditions. On the other hand, shocks containing heavy ions which are a minority constituent by number but which carry most of the energy density in the upstream medium do put approximately 20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) varies as E(exp -2), where N(E)dE is the number of particles with energy between E and E+dE. The mechanism of thermalization and particle acceleration is found to be synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front. The synchrotron maser modes radiated by the heavy ions are absorbed by the pairs at their (relativistic) cyclotron frequencies, allowing the maximum energy achievable by the pairs to be gamma(sub +/-)m(sub +/-)c squared = m(sub i)c squared gamma(sub 1)/Z(sub i), where gamma(sub 1) is the Lorentz factor of the upstream flow and Z(sub i) is the atomic number of the ions. The shock's spatial structure is shown to contain a series of 'overshoots' in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value. This shock model is applied to an interpretation of the structure of the inner regions of the Crab Nebula, in particular to the 'wisps,' surface brightness enhancements near the pulsar. We argue that these surface brightness enhancements are the regions of magnetic overshoot, which appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar's wind in the Crab Nebula is spatially resolved, and allows one to measure gamma(sub 1) and a number of other properties of the pulsar's wind. We also discuss applications of the shock theory to the termination shocks of the winds from rotation-powered pulsars embedded in compact binaries. We show that this model adequately accounts for (and indeed predicted) the recently discovered X-ray flux from PSR 1957+20, and we discuss several other applications to other examples of these systems.

Arons, Jonathan↗
NASA NTRS1972

Propagation of weak shock waves through turbulence.

The effect of turbulence on the structure of weak shock waves is investigated. The equilibrium structure is shown to be governed by a balance between nonlinear steepening and the turbulent scattering of acoustic energy out of the main wave direction. The scattered energy appears as perturbations behind the shock front. For conditions typical of sonic booms in atmospheric turbulence the wave structure is governed by a Burgers equation similar to that describing viscous shocks, except that parameters related to the turbulence appear instead of the viscosity coefficient. The magnitude of the perturbations following a shock is estimated from first-order scattering applied to a thickened shock. Predictions of shock thicknesses and perturbations compare favourably with available experimental data.

Plotkin, K. J.↗
NASA NTRS1987

The structure of the time-dependent interstellar shocks and grain destruction in the interstellar medium

A new theoretical analysis of the structure of interstellar shocks and the grain dynamics of these shocks is presented. The basic hydrodynamic equations for J-shocks in interstellar gas are given in which shock-driving pressure is allowed to be weakly time-dependent and the grains are treated as a separate two-dimensional fluid. Specific equations for the grain dynamics in the cooling postshock gas are derived. An analytic theory for the propagation of a shock driven into an interstellar cloud by the blast wave of a supernova remnant which sweeps over the cloud is developed. An improved calculation of the grain charge in postshock gas is described, giving a simple analytic approximation for the results. The consequences of including these processes in the numerical code of Seab and Shull (1983) are addressed, including the effect of realistic magnetic fields in low-density gas.

Mckee, Christopher F.↗