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At least 433 records · Page 24

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NASA NTRS1981

The rigid shell component for superrotation in planetary atmospheres: Angular momentum budget, mechanical analog and simulation of the spin up process

An analysis of superrotation in the atmosphere of planets, with rotation axis perpendicular to the orbital plane is presented. As the atmosphere expands, Hadley cells develop producing a redistribution of mass and angular momentum. A three dimensional thermally driven zonally symmetric spectral model and Laplace transformation simulate the time evolution of a fluid leading from corotation under globally uniform heating to superrotation under globally nonuniform heating. For high viscosities the rigid shell component of atmospheric superrotation can be understood in analogy with a pirouette. During spin up angular momentum is transferred to the planet. For low iscosities, the process is reversed. A tendency toward geostrophy, combined with increase of surface pressure toward the poles (due to meridional mass transport), induces the atmosphere to subrotate temporarily at lower altitudes. Resultant viscous shear near the surface permits angular momentum to flow from the planet into the atmosphere propagating upwards to produce high altitude superrotation rates.

Mayr, H. G.↗

NASA NTRS1982

A Numerical simulation of transition in plane channel flow

A numerical simulation of the final stages of transition to turbulence in plane channel flow at a Reynolds number of 7500 is described. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time evolution of two and three dimensional finite amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32 by 33 by 32 grid. Solutions indicate the existence of structures similar to those observed in the laboratory and which are characteristic of various stages of transition that lead to final breakdown. Details of the resulting flow field after breakdown indicate the evolution of streak-like formations found in turbulent flows. Although the flow field does approach a steady state (turbulent channel flow), implementation of subgrid-scale terms are necessary to obtain proper turbulent statistics.

Goglia, G.↗

NASA NTRS1982

Electromagnetic deep-probing (100-1000 KMS) of the Earth's interior from artificial satellites: Constraints on the regional emplacement of crustal resources

The two stages of analysis of MAGSAT magnetic data which are designed to evaluate electromagnetic induction effects are described. The first stage consists of comparison of data from contiguous orbit passes over large scale geologic boundaries, such as ocean-land interfaces, at several levels of magnetic disturbance. The purpose of these comparisons is to separate induction effects from effects of lithospheric magnetization. The procdure for reducing the data includes: (1) identifying and subtracting quiet time effects; (2) modelling and subtracting first order ring current effects; and (3) projecting an orbit track onto a map as a nearly straight line so it can serve as an axis on which to plot the corresponding orbit pass data in the context of geography. The second stage consists of comparison of MAGSAT data with standard hourly observatory data. The purpose is to constrain the time evolution of ionospheric and magnetospheric current systems. Qualitative features of the ground based dataset are discussed. Methods for reducing the ground based data are described.

Hermance, J. F.↗

NASA NTRS1981

Transport, charge exchange and loss of energetic heavy ions in the earth's radiation belts - Applicability and limitations of theory

Computer simulations of processes which control the relative abundances of ions in the trapping regions of geospace are compared with observations from discriminating ion detectors. Energy losses due to Coulomb collisions between ions and exospheric neutrals are considered, along with charge exchange losses and internal charge exchanges. The time evolution of energetic ion fluxes of equatorially mirroring ions under radial diffusion is modelled to include geomagnetic and geoelectric fluctutations. Limits to the validity of diffusion transport theory are discussed, and the simulation is noted to contain provisions for six ionic charge states and the source effect on the radiation belt oxygen ion distributions. Comparisons are made with ion flux data gathered on Explorer 45 and ISEE-1 spacecraft and results indicate that internal charge exchanges cause the radiation belt ion charge state to be independent of source charge rate characteristics, and relative charge state distribution is independent of the radially diffusive transport rate below the charge state redistribution zone.

Spjeldvik, W. N.↗

NASA NTRS1981

Simulation of space plasma phenomena

The importance of computer modeling in investigating physical phenomena is discussed, noting that modern computers can follow the time evolution of systems containing many millions of degrees of freedom, all of which are simultaneously interacting with each other. The two types of models used in simulating plasma, that is, particle and fluid, are described. Attention is given to particle models and some recent results obtained concerning the diffuse aurora. The simulations are shown to give clear evidence that a type of nonlinear cyclotron resonance process for the cold particles is responsible for the saturation observed in these calculations. It is also responsible for the cold electron heating and it is found that at saturation the rate of heating of the cold electrons essentially balances the rate at which energy is fed into unstable waves.

Dawson, J. M.↗

NASA NTRS1982

Extended adiabatic blast waves and a model of the soft X-ray background

The suggestion has been made that much of the soft X-ray background observed in X-ray astronomy might arise from being inside a very large supernova blast wave propagating in the hot, low-density component of the interstellar (ISM) medium. An investigation is conducted to study this possibility. An analytic approximation is presented for the nonsimilar time evolution of the dynamic structure of an adiabatic blast wave generated by a point explosion in a homogeneous ambient medium. A scheme is provided for evaluating the electron-temperature distribution for the evolving structure, and a procedure is presented for following the state of a given fluid element through the evolving dynamical and thermal structures. The results of the investigation show that, if the solar system were located within a blast wave, the Wisconsin soft X-ray rocket payload would measure the B and C band count rates that it does measure, provided conditions correspond to the values calculated in the investigation.

Cox, D. P.↗

NASA NTRS1982

Global pictures of the ozone field from high altitudes from DE-I

Detailed synoptic views of the column ozone field can be obtained by the Spin-Scan Ozone Imager (SOI) (Keating et al., 1981) aboard the Dynamics Explorer I satellite. The eccentric polar orbit with an apogee altitude of 23,000 km allows high resolution global-scale images to be obtained within 12 minutes, and allows regions to be viewed for long periods of time. At perigee, a pixel size of nadir measurements of 3 km is possible, and measurements are determined using the backscattered ultraviolet technique. A wavelength measurement of 317.5 nm is used as there are limitations in filter locations and it allows comparison with Nimbus 7 SBUV/TOMS data. Consideration of the reflectivities of this data aids in checking the SOI data reduction algorithm. SOI data show short-term (less than one day) variations in the observed ozone field, and a negative correlation (greater than 0.9) between ozone and tropopause heights. It is expected, due to this correlation, that SOI data will aid in understanding the time evolution of dynamics near the tropopause.

Keating, G. M.↗

NASA NTRS1983

Numerical simulation of a disk-shaped electron accelerating electrostatic probe

This paper presents a method utilizing the numerical plasma simulation particle-in-cell technique for studying electron accelerating probe characteristics. A probe configuration other than the commonly used plane, cylindrical, or spherical case will be studied; the probe configuration used is that of a disk. Cylindrical geometry is used in the simulation model. One of the advantages of a numerical simulation is the ease with which the details of the system can be examined. Therefore, in addition to the current-voltage characteristics of the static probe, other results are presented such as the time evolution of the plasma properties and studies of the sheath region surrounding the probe.

Nonnast, J. H.↗

NASA NTRS1983

A numerical simulation of transition in plane channel flow

This paper involves a numerical simulation of the final stages of transition to turbulence in plane channel flow at a Reynolds number of 7500. Three-dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time-evolution of two- and three-dimensional finite-amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32 x 33 x 32 grid. Solutions indicate the existence of structures similar to those observed in the laboratory and which are characteristic of various stages of transition that lead to final breakdown. Details of the resulting flow field after breakdown indicate the evolution of streak-like formations found in turbulent flows. Although the flow field does approach a steady-state (turbulent channel flow), implementation of subgrid-scale terms are necesary to obtain proper turbulent statistics.

Biringen, S.↗

NASA NTRS1982

A numerical study of the two-dimensional Navier-Stokes equations in vorticity-velocity variables

The application of solution methods for compact finite-difference schemes to a vorticity-velocity form of the two-dimensional unsteady Navier-Stokes equations is described. An account is also given of numerical experiments for stagnation point and driven cavity flows. One experiment treats the flow impinging on a flat plate, and the numerical results can be compared with the analytical steady state solution (Batchelor, 1967). The other deals with the driven cavity problem, the primary purpose being to describe the time evolution of this classical problem (Donovan, 1970).

Gatski, T. B.↗

NASA NTRS1983

Time-resolved study of the laser optogalvanic effect in I2

A time-resolved study of the optogalvanic effect in a pure iodine discharge is reported for pulsed dye laser irradiation at 585 nm. By varying both the spatial location and pulse energy of the laser probe, several contributions to the time evolution of the optogalvanic signal are identified and their origin discussed.

Haner, D. A.↗

NASA NTRS1983

Transition to turbulence in plane channel flow

A numerical simulation of the final stages of transition to turbulence in plane channel flow is reported. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time-evolution of two and three dimensional finite amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32x51x32 grid. Results are presented for no-slip boundary conditions at the solid walls as well as for periodic suction-blowing to simulate active control of transition by mass transfer. Solutions indicate that the method is capable of simulating the complex character of vorticity dynamics during the various stages of transition and final breakdown. In particular, evidence points to the formation of a lambda-shape vortex and the subsequent system of horseshoe vortices inclined to the main flow direction as the main elements of transition. Calculations involving suction-blowing indicate that interference with a wave of suitable phase and amplitude reduces the disturbance growth rates.

Biringen, S.↗

NASA NTRS1984

Transition to turbulence in plane channel flows

Results obtained from a numerical simulation of the final stages of transition to turbulence in plane channel flow are described. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time evolution of two and three dimensional finite amplitude disturbances. Computations are performed on CYBER-203 vector processor for a 32x51x32 grid. Results are presented for no-slip boundary conditions at the solid walls as well as for periodic suction blowing to simulate active control of transition by mass transfer. Solutions indicate that the method is capable of simulating the complex character of vorticity dynamics during the various stages of transition and final breakdown. In particular, evidence points to the formation of a lambda-shape vortex and the subsequent system of horseshoe vortices inclined to the main flow direction as the main elements of transition. Calculations involving periodic suction-blowing indicate that interference with a wave of suitable phase and amplitude reduces the disturbance growth rates.

Biringen, S.↗

NASA NTRS1983

Spectral characteristics of the MST radar returns

The salient features of the spectra of atmospheric returns due to random refractivity fluctuations in the Mesosphere, Stratosphere, Troposphere MST region are reviewed. The nonhomogeneous layered structure of turbulence is often evident as multiple peaks in the spectra. The time evolution of the spectra observed with a fine Doppler resolution provides evidence for thin regions of turbulence associated with gravity waves and shear instabilities. Embedded in these regions are horizontally extended refractivity structures that produce enhanced returns due to specular reflections. It is conceivable that some enhanced returns arise due to anisotropy of small scale refractivity structures. Observed correlations of the strength of the returns with their Doppler spread, wind shears, and winds provide insights into the physical mechanisms that produce turbulence.

Rastogi, P. K.↗

NASA NTRS1983

A mathematical model for crop spectral-temporal trajectories based on a plant growth model

The Kubelka-Munk radiative transfer model is combined with an approximation of Kauth-Thomas greeness and brightness transforms to derive approximate closed form expressions for crop greeness and brightness surrogates in terms of canopy biomass. The greeness relation derived resembles an existing empirical relation between leaf area index and greeness. A simple growth model based on interception and utilization of photosynthetically active radiation is developed and used to describe the time evolution of greeness and brightness. The model developed does not yet yield definitive profile calculations but suggests a conceptual framework which may be found useful for further profile analysis.

Woolford, T. L.↗

NASA NTRS1984

Spiral structure and star formation. I - Formation mechanisms and mean free paths

The wide variety of optical appearances observed in spiral galaxies has encouraged the growth of two theoretical approaches to explain the spiral patterns exhibited by such young objects as OB associations, giant H II regions, and dark dust lanes. These approaches are related to the density wave theory of spiral structure and 'stochastic, self-propagating star formation'. Levinson and Roberts (1981) tried to reconcile these two theoretical approaches, and considered a disk filled with discrete gas clouds. The present investigation is concerned with refinements and extensions of the model of Levinson and Roberts. Attention is given to gravitational forces and dynamical propagation, cloud-cloud collisions, supernova interactions, computational models, a theory concerning the interactions of the gas cloud and stellar association systems, the time evolution of the gas cloud-stellar association systems, and aspects of collisionally triggered star formation.

Roberts, W. W., Jr.↗

NASA NTRS1983

Dust particles in the anomalous tail of Comet Kohoutek (1973 XII)

Intermediate results from theoretical models of the anomalous spikelike dust distribution observed in the tail of Comet Kohoutek using the Skylab white-light coronograph on December 28.996, 1973, are reported. The time evolution is studied using the model of Finson and Probstein (1968) and assuming power-law variation of the dust-production rate and the particle-size distribution. Brightness profiles are calculated and presented graphically for comparison with those derived from the observation photograph: the model predicts a longer duration of the spike phenomenon than was observed. It is shown qualitatively that this discrepancy can be removed by introducing a finite dust-emission velocity (e.g., 1 m/s) into the model.

Keller, H. U.↗

NASA NTRS1985

Uniformly high-order accurate non-oscillatory schemes, 1

The construction and the analysis of nonoscillatory shock capturing methods for the approximation of hyperbolic conservation laws was begun. These schemes share many desirable properties with total variation diminishing schemes (TVD), but TVD schemes have at most first order accuracy, in the sense of truncation error, at extreme of the solution. A uniformly second order approximation was constucted, which is nonoscillatory in the sense that the number of extrema of the discrete solution is not increasing in time. This is achieved via a nonoscillatory piecewise linear reconstruction of the solution from its cell averages, time evolution through an approximate solution of the resulting initial value problem, and averaging of this approximate solution over each cell.

Harten, A.↗