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At least 667 records · Page 37
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NASA NTRS1980

Climate sensitivity from fluctuation dissipation - Some simple model tests

Leith has suggested that climatic response to change in external forcing parameters of the climate system may be estimated via the fluctuation-dissipation theorem (FDT). The method, which uses the natural fluctuations of the atmosphere to probe its dynamics, is tested here using a twenty-variable truncation model of the barotropic vorticity equation. Dissipative terms are added to the equations, so that the model is pushed away from the region where it is expected to satisfy the FDT. It is found that, even though the FDT is no longer satisfied in every detail, the FDT continues to provide an excellent estimate of the climatic sensitivity of the model.

Bell, T. L.↗
NASA NTRS1981

Scattering of electromagnetic waves from random media with strong permittivity fluctuations

By taking into account the singularity of the dyadic Green's function in the renormalization method, a theory is derived for vector electromagnetic wave propagation in a random medium with large permittivity fluctuations and with anisotropic correlation function. The strong fluctuation theory is then applied to a discrete scatterer problem in which the permittivity can assume only two values. The results are found to be consistent with those derived from discrete scatterer theory for all values of dielectric constants of the scatterers.

Tsang, L.↗
NASA NTRS1981

On a theory of temporal fluctuations in the electrostatic potential structures associated with auroral arcs

A possible mechanism is presented for the generation of large-amplitude temporal fluctuations in the structure of the electron energization region associated with auroral arcs. The mechanism is based on the observation that the auroral arc system resembles a laboratory circuit consisting of the series connection of battery, resistance and a forward biased diode containing collisionless plasma in which large-amplitude relaxation oscillations are sometimes observed to be superimposed on the steady-state current. It is shown that in both the laboratory and auroral systems, in which a localized auroral arc dynamo, the ionosphere and the electron energization region are involved, the oscillations are controlled by the times for ions and electrons to traverse the acceleration region, which also characterize the low- and high-frequency structure of the fluctuating waveform. It is demonstrated that a plausible one-dimensional double-layer model of the auroral arc acceleration region exhibits the dynamic negative resistance necessary for the generation of oscillations by the present mechanism. Finally, consideration is given to two kinds of auroral phenomena which might be associated with the mechanism: the 10-Hz quasi-periodic flickering aurora and 10-Hz modulations in the intensity of electrostatic hydrogen cyclotron waves.

Silevitch, M. B.↗
NASA NTRS1981

Crossflow effects on steady and fluctuating pressures on an ogive-cylinder cone-frustum model in supersonic separated flow

Wind-tunnel tests were conducted on an ogive-cylinder model with two axisymmetric protuberances having cone frustum angles of cone = 23 deg and 45 deg that were used to generate detached shock waves and the resulting separated flow areas downstream of the shock. The tests were conducted in a 9 by 7 foot supersonic wind tunnel at a free-stream Mach number of 2.0 and at Reynolds numbers of 1.5 x 1 million and 3.9 x 1 million, based on body diameter. The model had an afterbody fineness ratio of 8.3, and the ogive nose had a fineness ratio of 3.0. Two characteristics of the fluctuating pressures in surface vortex flows that result from the crossflow component, (velocity along the tunnel longitudinal axis free stream angle of attack), in combination with changes in the longitudinal pressure gradient were measured: (1) the broadband, rms-pressure coefficients and (2) the power spectral densities. Measurements are presented for various flow regions on the model such as the attached turbulent boundary layer, the detached frustum shock wave, and separated flow areas. The results indicate that the pressure fluctuations around or in the neighborhood of the foci of the vortex flows had broadband intensities and power spectral densities nearly identical to the levels previously measured in separated-flow regions at angles of attack of 0 deg.

Dods, J. B., Jr.↗
NASA NTRS1982

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows

A principal element to be derived from modeling turbulent reacting flows is an expression for the reaction rates of the various species involved in any particular combustion process under consideration. A temperature-derived most-likely probability density function (pdf) was used to describe the effects of temperature fluctuations on the Arrhenius reaction rate constant. A most-likely bivariate pdf described the effects of temperature and species concentrations fluctuations on the reaction rate. A criterion is developed for the use of an "appropriate" temperature pdf. The formulation of models to calculate the mean turbulent Arrhenius reaction rate constant and the mean turbulent reaction rate is considered and the results of calculations using these models are presented.

Foy, E.↗
NASA NTRS1981

Statistical properties of MHD fluctuations associated with high speed streams from HELIOS 2 observations

Helios 2 magnetic data were used to obtain several statistical properties of MHD fluctuations associated with the trailing edge of a given stream served in different solar rotations. Eigenvalues and eigenvectors of the variance matrix, total power and degree of compressibility of the fluctuations were derived and discussed both as a function of distance from the Sun and as a function of the frequency range included in the sample. The results obtained add new information to the picture of MHD turbulence in the solar wind. In particular, a dependence from frequency range of the radial gradients of various statistical quantities is obtained.

Bavassano, B.↗
NASA NTRS1982

Abundance fluctuations in the interstellar medium

The determination of abundances within the interstellar medium is reviewed. It appears that interstellar abundances within 1 kpc of the Sun are uniform to within a factor of two or three, but it is not yet possible to determine whether there are real fluctuations at this level except for deuterium for which the factor of two variations appear to be real. Establishing the level of local fluctuations in the abundances is of considerable importance for understanding the history of nucleosynthesis in the solar neighborhood, the evolution of the interstellar medium and the formation of stars.

Jura, M.↗
NASA NTRS1982

Simulation of the fluctuating field of a forced jet

The fluctuating field of a jet excited by transient mass injection is simulated numerically. The model is developed by expanding the state vector as a mean state plus a fluctuating state. Nonlinear terms are not neglected and the effect of nonlinearity is studied. The results show a significant spectral broadening in the flow field due to the nonlinearity. In addition, large scale structures are broken down into smaller scales.

Bayliss, A.↗
NASA NTRS1981

Wave-particle transport by weak electrostatic flow shear fluctuations

A description is presented of the first consistent theoretical treatment of transport due to weak electrostatic fluctuations from microinstabilities driven by a shear in plasma flow parallel to a uniform magnetic field. The model used considers electrostatic fluctuations in a Vlasov plasma with sheared bulk velocity parallel to a uniform magnetic field. The linear stability theory for the model has been studied by Gary and Schwartz (1980). In the current investigation, a calculation is performed of the wave-particle transport associated with the electrostatic flow shear instability.

Gary, S. P.↗
NASA NTRS1982

Thermal fluctuations and the diffuse electrostatic emissions

The suggestion that the weak-banded electrostatic emissions observed in the magnetosphere at frequencies between the electron gyrofrequency and upper hybrid frequency are thermal fluctuations is quantitatively explored, by means of a hot-cold model for dayside electron distributions with density and temperature ratios much greater than unity. It is determined that, for typical dayside conditions, the observed waves are weakly damper Bernstein-Harris modes whose spectral density and polarization, perpendicular to the ambient magnetic field, can be accounted for by the theory. It is suggested that the formalism developed may be applicable to weak electron cyclotron emissions at Jupiter and Saturn, and can be extended to lower-frequency ion cyclotron fluctuations by modifying the theory to include ion contributions.

Sentman, D. D.↗
NASA NTRS1982

Cloud fluctuation statistics

A space-time statistical analysis of total outgoing infrared radiation is used to determine the gross features of day-to-day cloudiness fluctuations over the Pacific Ocean in summer and winter. IR fluctuations arise from the passage of cloudiness systems through a grid box as well as the creation and destruction of cloudiness in the box. Which process dominates depends upon the size of the box relative to the size, speed and persistence time of a typical cloudiness system. In most regions the statistical analysis yields advection speeds characteristic of 700 mb mean flow with spatial dependence resembling the 300 mb mean flow. Spatial scales less than 2000 km predominate, smaller scales having less persistence. Characteristic time scales are on the order of one or two days, even for a grid box spanning the entire North Pacific storm track. This result is remarkable in view of the much longer time scales commonly associated with atmospheric disturbances. Apparently many cloudiness systems are created and destroyed during the lifetime of a single disturbance.

Cahalan, R. F.↗
NASA NTRS1982

Induced vorticity fluctuations in counterflowing He II

It is shown that vortex-line-density fluctuations in counterflowing He II can be greatly enhanced by modulating the heater with a noise generator. This technique is used to test Vinen's equation for vortex-line turbulence. Vinen's equation, linearized to describe fluctuations, appears to be correct except near the lambda transition.

Barenghi, C. F.↗
NASA NTRS1982

Current carrying properties of double layers and low frequency auroral fluctuations

A summary is given of a systematic parametric study in which the ion-transit time effects on double-layer fluctuations are investigated in a series of numerical simulations involving several lengths of finite extent plasmas. Typical values of ion drift velocities both smaller and larger than the ion-thermal speed on the high potential end of the simulation plasma are given. It is shown that the link between the electron-current fluctuations and ion-transit time is the Langmuir criterion for the existence of double layers. The time period of the recurring phenomena depends on the ion dynamics; for ions entering the simulation plasma in such a way that the ion drift velocity is less than approximately the ion thermal speed, the time period is governed by the ion transit time from the location of ion injection to the location of the double layer.

Singh, N.↗
NASA NTRS1982

Interaction of small-amplitude fluctuations with a strong magnetohydrodynamic shock

The interaction of small fluctuations with a strong magnetohydrodynamic shock is considered theoretically in the most general situation using an ideal model. An analytical treatment of the cutoff conditions for penetration of the upstream fluctuation through the bow shock is presented. The situation in which there exist unique solutions of the interaction is investigated quantitatively. Numerical examples of the interaction of waves in the solar wind with the terrestrial bow shock are given to illustrate the refracted waves to be found in the earth's magnetosheath.

Zhuang, H.-C.↗
NASA NTRS1982

Analysis of the decay of temperature fluctuations in isotropic turbulence

The Lagrangian dispersion theory of Durbin (1980) is used to analyze experiments by Warhaft and Lumley (1978) and by Sreenivasan et al. (1980) on temperature fluctuations in grid-generated turbulence. Both theory and experiment show that the decay exponent m depends on the ratio of the initial length scales of velocity and temperature, although when this ratio is greater than 2.5 such dependence is negligible. The theory shows that m is not truly constant, but within the range covered by the experiments it is nearly so. The agreement between theory and experiment lends credence to the idea that the decay of fluctuations is controlled largely by turbulent relative dispersion.

Durbin, P. A.↗
NASA NTRS1982

DE-2 cusp observations - Role of plasma instabilities in topside ionospheric heating and density fluctuations

Observations by the low altitude Dynamics Explorer satellite (DE-2) in the polar cusp show the ionospheric plasma electron temperature and the ratio of electron to ion temperature to be increased, and the electron density fluctuations to be enhanced. Also, downward fluxes of energetic eletrons and ions increase in the cusp, and the magnetic field structure are consistent with the existence of a field-aligned current. Simultaneously, there is characteristic broadband electrostatic noise (BEN) with amplitudes of 1-10 mV/m, peaking in the cusp but extending into the polar cap. These emissions range from far below the local 0(+) gyrofrequency F sub O(+) to the vicinity of the proton gyrofrequency, but below the oxygen lower hybrid frequency. The BEN observations are compared to the predictions of several theories. The amplitude of these waves is shown to be far too small to contribute significantly to the observed ionospheric heating or density fluctuations by local wave-particle interactions. Rather, the observed spatial variations are attributed to nonlocal field aligned heating processes and reflect the nonuniformity of the magnetosheath plasma's penetration into the ionosphere.

Curtis, S. A.↗
NASA NTRS1983

Preliminary measurements of velocity, density and total temperature fluctuations in compressible subsonic flow

The heat transfer characteristics of a three-wire hot-wire probe operated with a constant temperature anemometer were investigated in the subsonic compressible flow regime. The sensitivity coefficients, with respect to velocity, density and total temperature, were measured and the results were used to calculate the velocity, density, and total temperature fluctuations in the test section of the Langley 0.3-m Transonic Cryogenic Tunnel (TCT). These results were extended to give estimates for fluctuations due to vorticity, sound, and entropy. In addition, attempts were made to determine the major source of disturbances in the 0.3-m TCT.

Stainback, P. C.↗
NASA NTRS1983

Magnetic field fluctuations in the Venus magnetosheath

Using a model for the convection pattern of the shocked solar wind flow around the Venus obstacle, Pioneer Venus observations of ultra-low-frequency (about 10-40 s period) magnetic field fluctuations in the magnetosheath have been traced along streamlines to the regions of the quasi-parallel bow shock. The periods and polarizations of the sinusoidal fluctuations are similar to those observed upstream of the quasi-parallel bow shock, where streaming superthermal particles are believed to produce MHD waves by a beam-plasma instability. The results suggest that both disturbances at the ionopause at Venus and the earth's magnetopause may be caused by convection of turbulent magnetic fields from the subsolar bow shock when the interplanetary field direction produces a quasi-parallel shock there.

Luhmann, J. G.↗