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NASA NTRS2006

Shock Structure Analysis and Aerodynamics in a Weakly Ionized Gas Flow

The structure of a shock wave propagating through a weakly ionized gas is analyzed using an electrofluid dynamics model composed of classical conservation laws and Gauss Law. A viscosity model is included to correctly model the spatial scale of the shock structure, and quasi-neutrality is not assumed. A detailed analysis of the structure of a shock wave propagating in a weakly ionized gas is presented, together with a discussion of the physics underlying the key features of the shock structure. A model for the flow behind a shock wave propagating through a weakly ionized gas is developed and used to analyze the effect of the ionization on the aerodynamics and performance of a two-dimensional hypersonic lifting body.

Saeks, R.↗
NASA NTRS2007

Simulation of Boundary-Layer Cumulus and Stratocumulus Clouds using a Cloud-Resolving Model With Low- and Third-Order Turbulence Closures

The effects of subgrid-scale condensation and transport become more important as the grid spacings increase from those typically used in large-eddy simulation (LES) to those typically used in cloud-resolving models (CRMs). Incorporation of these effects can be achieved by a joint probability density function approach that utilizes higher-order moments of thermodynamic and dynamic variables. This study examines how well shallow cumulus and stratocumulus clouds are simulated by two versions of a CRM that is implemented with low-order and third-order turbulence closures (LOC and TOC) when a typical CRM horizontal resolution is used and what roles the subgrid-scale and resolved-scale processes play as the horizontal grid spacing of the CRM becomes finer. Cumulus clouds were mostly produced through subgrid-scale transport processes while stratocumulus clouds were produced through both subgrid-scale and resolved-scale processes in the TOC version of the CRM when a typical CRM grid spacing is used. The LOC version of the CRM relied upon resolved-scale circulations to produce both cumulus and stratocumulus clouds, due to small subgrid-scale transports. The mean profiles of thermodynamic variables, cloud fraction and liquid water content exhibit significant differences between the two versions of the CRM, with the TOC results agreeing better with the LES than the LOC results. The characteristics, temporal evolution and mean profiles of shallow cumulus and stratocumulus clouds are weakly dependent upon the horizontal grid spacing used in the TOC CRM. However, the ratio of the subgrid-scale to resolved-scale fluxes becomes smaller as the horizontal grid spacing decreases. The subcloud-layer fluxes are mostly due to the resolved scales when a grid spacing less than or equal to 1 km is used. The overall results of the TOC simulations suggest that a 1-km grid spacing is a good choice for CRM simulation of shallow cumulus and stratocumulus.

Xu, Kuan-Man↗
NASA NTRS1980

Criteria for fragmentation in a collapsing rotating cloud

The isothermal collapse of a rotating protostar with pressure and self-gravity is calculated by a hydrodynamic computer code that treats three space dimensions. A number of initial conditions are tested to determine the conditions under which a cloud is unstable to fragmentation. It is shown that fragmentation does not proceed to a significant extent during the first free-fall time, but that in most cases fragmentation has occurred by 1.5 to 2 times the initial free-fall time. The extent to which the density in the fragments is enhanced over that in their surroundings on this time scale depends on the initial ratio of thermal to gravitational energy, but is only weakly dependent on the type and amplitude of the perturbation and on the initial rotational energy.

Bodenheimer, P.↗
NASA NTRS1984

Photometric Studies of Heat Flow at the Photosphere

Continuum photometry is carried out and the results of comparing these observations with models of photospheric heat flow are described. The main results are: (1) a possible detection of weak bright rings around some spot penumbrae; (2) no evidence is found for large scale photospheric brightness inhomogeneities exceeding 2-3 K which places tighter constraints on models of global scale convection; (3) supergranular scale continuum structures observed across the photosphere appear mainly due to random clumping of granules; (4) the one case observed of a sunspot emergence shows no thermal shadow exceeding 1.5 K rms one day prior to umbra appearance; (5) network and faculae are found to show a small excess brightness even at mu = 1, so detection of faculae at mu = 1 by differential photometry indicates a gentler temperature gradient near tau = 1 in the facular (relative to cell) atmosphere; (6) the limb darkening study shows no significant global variations to within 0.1% rms.

Foukal, P.↗
NASA NTRS1987

Numerical simulations of turbulent trapping in the weak beam-plasma instability

Numerical simulations of the weak beam-plasma instability were done in the turbulent regime where small-scale trapping is a dominant feature of the instability, a regime with behavior not predicted by quasi-linear theory. The results of the simulations were compared with those of a specific model of the turbulence, the so-called 'turbulent trapping' model, which gives precise formulas for the particle correlation functions, and predicts a growth rate well enhanced over the quasi-linear value. It was found that the model gives accurate predictions for the correlation functions. On the other hand, while growth rates were enhanced over the quasi-linear values, the enhancements observed are smaller than expected from the quantitative predictions of the model.

Theilhaber, K.↗
NASA NTRS1991

Strong plasma turbulence in the earth's electron foreshock

A quantitative model is developed to account for the distribution in magnitude and location of the intense plasma waves observed in the earth's electron foreshock given the observed rms levels of waves. In this model, nonlinear strong-turbulence effects cause solitonlike coherent wave packets to form and decouple from incoherent background beam-excited weak turbulence, after which they convect downstream with the solar wind while collapsing to scales as short as 100 m and fields as high as 2 V/m. The existence of waves with energy densities above the strong-turbulence wave-collapse threshold is inferred from observations from IMP 6 and ISEE 1 and quantitative agreement is found between the predicted distribution of fields in an ensemble of such wave packets and the actual field distribution observed in situ by IMP 6. Predictions for the polarization of plasma waves and the bandwidth of ion-sound waves are also consistent with the observations. It is shown that strong-turbulence effects must be incorporated in any comprehensive theory of the propagation and evolution of electron beams in the foreshock. Previous arguments against the existence of strong turbulence in the foreshock are refuted.

Robinson, P. A.↗
NASA NTRS1992

Wave diffraction in weak cosmic-ray-modified shocks

Weakly multidirectional, long-wavelength cosmic-ray-modified shocks are studied via multiple scale perturbation techniques. The effects of diffraction are discussed in terms of Green's function solutions of the linearized 1 + 3D Burgers and 1 + 3D KdVB equations, and also in terms of solutions with singular Dirac delta initial distributions. The solutions show a monotonic decrease of the wave-front curvature with increasing time owing to the effects of wave diffraction. The shape of the wave surface is discussed in terms of solutions S to the wave eikonal equation corresponding to singular initial conditions. For the fast magnetosonic wave propagating in the positive x-direction, the wave phase surface S = 0 has elliptic cross sections with the planes x = constant and has a convex paraboloidal shape. Plane-wave solutions of the 1 + 3D KdVB equation are discussed.

Webb, G. M.↗
NASA NTRS1992

Weakly inhomogeneous MHD turbulence and transport of solar wind fluctuations

An evaluation is conducted of recent theories of small-scale MHD turbulence transport in an inhomogeneous background that are pertinent to the evolution of solar wind turbulence. Attention is given to the WKB formalism that has been used in many solar wind-related physics applications, with a view to its shortcomings. Also discussed are the structure of two-scale transport theories, and their relationship to WKB theory in light of multiple-scales analysis.

Matthaeus, W. H.↗
NASA NTRS1993

Mixing in a stratified shear flow: Energetics and sampling

Direct numerical simulations of the time evolution of homogeneous stably stratified shear flows have been performed for Richardson numbers from 0 to 1 and for Prandtl numbers between 0.1 and 2. The results indicate that mixing efficiency R(sub f) varies with turbulent Froude number in a manner consistent with laboratory experiments performed with Prandtl numbers of 0.7 and 700. However, unlike the laboratory results, for a particular Froude number, the simulations do not show a clear dependence on the magnitude of R(sub f) on Pr. The observed maximum value of R(sub f) is 0.25. When averaged over vertical length scales of an order of magnitude greater than either the overturning or Ozmidov scales of the flow, the simulations indicate that the dissipation rate epsilon is only weakly lognormally distributed with an intermittency of about 0.01 whereas estimated values in the ocean are 3 to 7.

Ivey, G. N.↗
NASA NTRS1994

Coupling between middle atmosphere trend estimates and solar effects in ozone vertical structure

Studies are performed on both the Stratospheric Aerosol and Gas Experiment (SAGE) II (1985-1989) and Solar Backscatter Ultraviolet Spectrometer (SBUV)-Version 6 (1979-1986) global ozone vertical structure satellite data sets to determine the long-term trends in ozone as a function of altitude (pressure) and latitude. SAGE II data are only available during the period of increasing solar activity and show increases in ozone with time in the upper stratosphere which are attributed largely to rising solar activity. Looking at this data set independently, the solar effects and trends are highly coupled and cannot be clearly separated. However, a study of combined SBUV and SAGE II data over the 11-year solar cycle shows a clear response of ozone to 11-year solar variations and allows a decoupling of solar effects, quasibiennial oscillations (QBO), and trends. The detailed pattern of long-term ozone trends become clear using this approach. In the upper stratosphere, ozone depletion increases sharply with latitude. Global trends are fairly symmetric about the equator but are somewhat stronger in the Southern Hemisphere. Near the equator, some layers of ozone are decreasing with time while others appear to be increasing. Near 30 mb, there is evidence of intrusion to mid latitudes of high latitude negative trends. Near 15 mb, trends appear to be very weak. Near the tropopause there appears to be strong ozone depletion on a global scale. Two regions of unexpectedly strong ozone response to 11-year solar variations were detected: the first near 2 mb and the second near 30 mb at low latitudes and near 15 mb at mid latitudes.

Keating, G. M.↗
NASA NTRS1998

X-Ray Emission from the Unidentified Gamma-Ray Transient GRO J1838-0415

The gamma-ray transient GRO J1838-04 is one of the most enigmatic sources in the sky. Despite its closeness to the Galactic plane, it showed a strong gamma-ray flare in 1995 typical of blazars. However, no blazar is detected in its approx. 1 deg. error box, and a Galactic source is suspected. Other time variable gamma-ray sources are known in the Galactic disk, and GRO J1838-04 may belong to a new class of sources (isolated young pulsars, the only proven Galactic gamma-ray sources, do not show variability). The Advanced Satellite for Cosmology and Astrophysics (ASCA) observed the centroid of the Energetic Gamma Ray Experiment Telescope (EGRET) error box of GRO J1838-04 in April 1997. Five weak X-ray sources are detected, one of which appears to be diffuse (3 arcmins). No known radio source is coincident with the X-ray sources. A preliminary report of our results appeared and a more systematic report is being written . The fact that no prominent X-ray source appears in the field, excludes an X-ray transient source with strong persistent emission is a possible counterpart of GRO JI838- 04 (such as superluminal transients). To uncover the nature of GRO J1838-04 requires more study of the weak X-ray sources in its error box, and a long-time scale monitoring of their variability.

Tavani, Marco↗
NASA NTRS1979

Spiral clouds on Mars - A new atmospheric phenomenon

Viking Orbiter images obtained during the 1978 northern summer on Mars show unusual spiral cloud patterns at high northern latitudes. Well-developed spirals occurred only during early summer at a time when the sublimation of the annual CO2 frost cap in the north had ceased. The systems ranged in size from 200 to 500 km and all spirals were wound in a counter-clockwise sense. An explanation is proposed, based on the radiatively driven instability described by Gierasch, Ingersoll, and Williams. The model predicts disturbances with the correct scale and lifetime. However, the successful development of the spirals depends on the existence of weak mean winds, suggesting that these instabilities can only develop at special null points in the general circulation. It is argued that such special circumstances are most likely to occur during early northern summer and are less likely to obtain at a comparable season in the Southern Hemisphere.

Gierasch, P.↗
NASA NTRS1984

Galaxy formation

Implications of the isotropy of the cosmic microwave background on large and small angular scales for galaxy formation are reviewed. In primeval adiabatic fluctuations, a universe dominated by cold, weakly interacting nonbaryonic matter, e.g., the massive photino is postulated. A possible signature of photino annihilation in our galactic halo involves production of cosmic ray antiprotons. If the density is near its closure value, it is necessary to invoke a biasing mechanism for suppressing galaxy formation throughout most of the universe in order to reconcile the dark matter density with the lower astronomical determinations of the mean cosmological density. A mechanism utilizing the onset of primordial massive star formation to strip gaseous protogalaxies is described. Only the densest, early collapsing systems form luminous galaxies.

Silk, J.↗
NASA NTRS1985

Seasonal changes in the structure of the stratospheric circulation

The seasonal evolution of the stratospheric circulation in both hemispheres is studied using a coordinate independent diagnostic tied to the vortex, wherever it lies, rather than to latitude circles. Zonally averaged quantities are highly variable in the winter stratosphere, as the vortex moves on and off the pole, masking systematic changes in the structure of the vortex. The quantities shown here are the areas A(Q), where Ertel's potential vorticity is greater than a specified value Q, for various Q. In these calculations, Q is evaluated on the 850K isentropic surface which lies near 10 mb in the middle of the stratosphere. The advantage of using contours of Q (rather than contours of geopotential height, say) is that an area enclosed by such contours is approximately constant for quasi nondivergent, adiabatic, inviscid motion. Changes in area will occur if any of the above restrictions on the motion are invalid, and also if scales of motions are generated below those that can be adequately resolved. The evolution of area shows the systematic change in the average structure of the vortex. At a given time, the rate of change of Q with A(Q) gives a measure of the average gradient of Q. The gradient of Q is relevant for the propagation of large-scale disturbances into the stratosphere. Experience has also shown that winds tend to be strong (weak) when the gradient of Q is strong (weak).

Oneill, A.↗
NASA NTRS2014

Improved Composites Using Crosslinked, Surface-Modified Carbon Nanotube Materials

Individual carbon nanotubes (CNTs) exhibit exceptional tensile strength and stiffness; however, these properties have not translated well to the macroscopic scale. Premature failure of bulk CNT materials under tensile loading occurs due to the relatively weak frictional forces between adjacent CNTs, leading to poor load transfer through the material. When used in polymer matrix composites (PMCs), the weak nanotube-matrix interaction leads to the CNTs providing less than optimal reinforcement.Our group is examining the use of covalent crosslinking and surface modification as a means to improve the tensile properties of PMCs containing carbon nanotubes. Sheet material comprised of unaligned multi-walled carbon nanotubes (MWCNT) was used as a drop-in replacement for carbon fiber in the composites. A variety of post-processing methods have been examined for covalently crosslinking the CNTs to overcome the weak inter-nanotube shear interactions, resulting in improved tensile strength and modulus for the bulk sheet material. Residual functional groups from the crosslinking chemistry may have the added benefit of improving the nanotube-matrix interaction. Composites prepared using these crosslinked, surface-modified nanotube sheet materials exhibit superior tensile properties to composites using the as received CNT sheet material.

polymer matrix composite↗
NASA NTRS1981

The nature of the solar wind interaction with CO2/CO-dominated comets

The nature of the interaction of the solar wind with the ionospheres (field-free ion cavities) of comets with CO or CO2 as their dominant volatile species is considered and compared with the solar wind interaction with H2O-diominated comets. Calculations of the heliocentric variation of scale lengths defined as the nuclear distance at which the mean free path for ion-neutral collisions becomes equal to the nuclear distance, and the ion and neutral stand-off distances are presented for different modes of energy addition, nuclear radii and ionization time scales. Results indicate that whereas little activity can be expected from H2O-dominated comets at distances beyond 3 AU, COand CO2-dominated comets can be expected to be quite prolific at such distances. In contrast to H2O-dominated comets characterized by weak shocks, CO/CO2-dominated comet solar wind interactions are found to produce strong bow shocks within a specific heliocentric distance for the isothermal case of a CO-dominated comet, and a weak shock outside of this distance. In the general case, two types of cometary ionosphere transitions are found: a transition from strong to weak bow shock and from a compressible (soft) to an incompressible (hard) ionosphere, and a transition from a soft to a hard ionosphere with the bow shock remaining weak at a heliocentric distance dependant on atmospheric optical depth, bolometric albedo, nuclear radius, latent heat of sublimation of nuclear material, ionization time scale and type of heat addition. It is pointed out that the behavior of Comet Humason can be explained by solar wind interactions with a CO- or CO2-dominated comet.

Houpis, H. L. F.↗
NASA NTRS1991

Suppression of turbulent transport by a weak magnetic field

Two-dimensional numerical simulations with high spatial resolution are used to study the effects of a large-scale magnetic field on its turbulent transport. It is commonly believed that the nonlinear back-reaction of the magnetic field on the turbulence becomes important when the field strength is close to equipartition. However, turbulent diffusion is effectively reduced even when the large-scale field is much weaker than equipartition.

Cattaneo, Fausto↗