SEARCH · Engineering Papers

Results for “FLUCTUATIONS”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 253 records · Page 14

All sources NASA NTRS DOE OSTI

NASA NTRS1983

Backscattering cascade of beam modes off ambient density fluctuations

The effects of a given nonthermal low-frequency density-fluctuation spectrum on the amplification of Langmuir waves by a 'bump-on-tail' beam of electrons are studied. The density-fluctuation spectrum is assumed to contain a uniform distribution of wavelengths ranging from much shorter than the beam mode wavelength to of the same order. This permits multiple large-angle (back) scattering to occur. One-dimensional numerical solutions of the kinetic equations are found which yield criteria for linear saturation of the beam instability of a cascade of backscattering to high wavenumber. The relevant time scales and spectral shapes are also determined in both the stable and unstable regions. Linear damping and Cerenkov emission by a possible nonthermal tail of electrons is taken into account. An application is made to the beam modes observed simultaneously with density fluctuations off the Jovian bow shock. It is shown that the observed level of density fluctuations is sufficient to saturate the unstable Langmuir waves, although nonthermal Landau damping may prevent a cascade to very high wavenumbers.

Russell, D. A.↗

NASA NTRS1986

Large amplitude, low frequency plasma fluctuations at Comet Giacobini-Zinner

Very large amplitude fluctuations in electron density, temperature, and flow velocity were a prominent aspect of the solar wind interaction with Comet Giacobini-Zinner during the ICE encounter in September 1985. These fluctuations were detected at a distance of at least 900,000 km and grew in amplitude as ICE approached the comet, peaking in amplitude 60,000 km from closest approach. A typical period associated with the fluctuations was about 2 min, which corresponds to a scale length in the solar wind frame of about 50,000 km. For the most part these fluctuations appear to be a product of one or more plasma instabilities associated with the solar wind pick up of cometary ions.

Gosling, J. T.↗

NASA NTRS1987

Surface pressure fluctuations in a separating turbulent boundary layer

A novel, microphone-employing technique for the measurement of pressure fluctuations through pinhole apertures in the flow surface is used in the present study of separating boundary layer pressure fluctuation spectra and wavespeeds. The rms of the surface pressure fluctuation increases monotonically through the adverse-pressure gradient attached flow region and the detached flow zone. A correlation of the detached flow data is given which is derived from velocity and length scales of the separated flow. The celerity of the surface pressure fluctuations for the attached flow increases with frequency to a maximum, and the streamwise coherence decreases drastically after the begining of flow reversal.

Simpson, Roger L.↗

NASA NTRS1988

On the theory of the conversion of metallic mixed phases. 5: Fluctuations and nucleation in sub-cooled phases

A comparison is made between fluctuation-theoretical calculations and empirical results. Concentration fluctuations and the resulting separation, and the distribution function of the reversible fluctuations are approximately calculated. The relation of this distribution to the concentration-dependence of the free energy is discussed. Possible effects of fluctuations are discussed, and other attempts to explain the sub-cooling are described.

Borelius, G.↗

NASA NTRS1990

Plasma fluctuations in the magnetosheath downstream from Uranus

The Voyager plasma experiment observed large-amplitude plasma fluctuations in the Uranian magnetosheat downstream from the planet. This is a region that has not been well sampled in the earth's magnetosphere. These waves have periods of tens of minutes, are characterized by an anticorrelation between the plasma density and temperature, and are associated with deflections in the flow angle of the plasma. These fluctuations are observed only in regions where the magnetic field is rapidly varying. These waves have time and distance scales placing them in the MHD regime, but their characteristics are not compatible with any known solution of the MHD equations. It is suggested that these fluctuations are produced by the solar wind interaction with the magnetosphere at the bow shock, but the physics governing the production and propagation of these fluctuations is not understood.

Richardson, John D.↗

NASA NTRS1991

Small-scale fluctuations of the X-ray background

Fluctuations of the diffuse X-ray background are investigated using the Einstein Observatory Deep Survey data. Small angular-scale variations of the numbers of counts in the energy range 0.8-3.5 keV in the central 32 x 32 sq arcmin of the IPC are analyzed by means of the two-point correlation function. It is shown that detector modulation contributes substantially to the observed fluctuations. Instrumental effects are estimated and subtracted from the raw observations. The residual fluctuations are used to estimate the log N-log S relation of the X-ray sources below the Einstein sensitivity limit. Taking into account numerous uncertainties involved in this analysis, it is shown that the amplitude of fluctuations observed in the Deep Survey data is consistent with a relatively small number of sources producing the X-ray background.

Soltan, A. M.↗

NASA NTRS1991

Fluctuation diagrams for hot-wire anemometry in subsonic compressible flows

The concept of using 'fluctuation diagrams' for describing basic fluctuations in compressible flows was reported by Kovasznay in the 1950's. The application of this technique, for the most part, was restricted to supersonic flows. Recently, Zinovev and Lebiga published reports where they considered the fluctuation diagrams in subsonic compressible flows. For the above studies, the velocity and density sensitivities of the heated wires were equal. However, there are considerable data, much taken in the 1950's, which indicate that under some conditions the velocity and density sensitivities are not equal in subsonic compressible flows. Therefore, possible fluctuation diagrams are described for the cases where the velocity and density sensitivities are equal and the more general cases where they are unequal.

Stainback, P. C.↗

NASA NTRS1991

The structure and annual variation of antisymmetric fluctuations of tropical convection and their association with Rossby-gravity waves

The present study explores the signature of 4-5-day-period Rossby-gravity waves in the tropical convection field across the Indian and Pacific oceans. The convergence/divergence field of these waves in the lower troposphere is anticipated to produce an antisymmetric fluctuation in tropical convection. Antisymmetric fluctuations of tropical convection are shown to exhibit a pronounced spectral peak at a 4-5-day period only during boreal fall and only within about 30 deg longitude of the date line. The peak amplitude occurs around 7.5 deg latitude. These fluctuations propagate westward at 15-20 m/s with zonal wavelength of about 7000-9000 km. The fluctuations of convections are coherent and out of phase with the equatorial meridional wind, which also exhibits a pronounced spectral peak at a 4-5-day period in the lower troposphere near the date line. The horizontal distributions of the 4-5-day power and coherence of the winds and convection are consistent with that produced by a convectively coupled Rossby-gravity wave that is confined near the date line.

Hendon, Harry H.↗

NASA NTRS1993

Cluster evolution as a probe of primordial density fluctuations

Although COBE's detection of large angle microwave background anisotropies fixes the amplitude of density fluctuations on length scales k exp -1 approximately = (300-6000) h(exp -1)Mpc, what is crucial for the level of large scale clustering is the amplitude of density fluctuations on scales (5-50) h(exp -1)Mpc. The level of dynamical clustering is usually parameterized by the size of the mass fluctuations in 8 h exp -1 Mpc spheres, sigma sub 8. For the cold dark matter model, COBE gives sigma sub 8 approximately = 1, while models with extra large scale power give sigma sub 8 approximately = 1/2. The most massive clusters of galaxies (greater than or approximately = 10 exp 15 solar mass) form from rare 'peak patches' found in the initial mass density distribution. Their abundance as a function of redshift is a sensitive probe of the wave number band k(exp -1) approx. (3-8) h(exp -1)Mpc, hence of sigma sub 8, and so cluster evolution can discriminate among models allowed by the COBE results. We use our Hierarchical Peaks Method, which accurately reproduces the results of P3M N-body simulations, to calculate the evolution of cluster x-ray flux counts, luminosity, and temperature functions as a function of sigma sub 8 for CDM models and those with more large scale power. We find that the EMSS and Edge et al. cluster samples support sigma sub 8 in the range from approx. 0.6-0.9, and that models with more large scale power (and hence flatter fluctuation spectra in the cluster regime) fit the x-ray bright end better.

Bond, J. Richard↗

NASA NTRS1993

Damping of surface pressure fluctuations in hypersonic turbulent flow past expansion corners

Surface pressure fluctuations of Mach 8 turbulent flow past a 2.5- and a 4.25-deg expansion corner maintained a Gaussian distribution but were severely attenuated by the expansion process. The pressure fluctuations did not recover to those of an equilibrium turbulent flow even though the mean pressures reached downstream inviscid values in four to six boundary-layer thicknesses. The fluctuations were convected with a velocity comparable to that on a flat plate, and they maintained their identities longer for the stronger expansion. The damping of pressure fluctuations at hypersonic Mach numbers, even by small corner angles, may be exploited in fatigue design.

Chung, Kung-Ming↗

NASA NTRS1994

Wall pressure fluctuations in the reattachment region of a supersonic free shear layer

The primary aim of this research program was to investigate the mechanisms which cause the unsteady wall-pressure fluctuations in shock wave turbulent shear layer interactions. The secondary aim was to find means to reduce the magnitude of the fluctuating pressure loads by controlling the unsteady shock motion. The particular flow under study is the unsteady shock wave interaction formed in the reattachment zone of a separated supersonic flow. Similar flows are encountered in many practical situations, and they are associated with high levels of fluctuating wall pressure. The free shear layer is formed by the flow over a backward facing step, using an existing model, with the base pressure on the step adjusted so that there is no pressure discontinuity at the lip. The shear layer therefore develops in a zero pressure gradient. The primary advantage of this flow configuration is that the reattachment process can be studied in the absence of a separation shock. The mean flow data, and some preliminary hot-wire measurements of the mass-flux fluctuations were made by Baca and Settles, Baca, Williams and Bogdonoff, who showed that the shear layer became self-similar at about 17 delta(sub 0) downstream of the lip, and that it grew at a rate typical of the observed Mach number difference (about 1/3rd the incompressible growth rate). The turbulence measurements were later extended by Hayakawa, Smits and Bogdonoff under NASA Headquarters support.

Smits, Alexander J.↗

NASA NTRS1993

Thermal magnetic fluctuations of whistlers in a Maxwellian plasma

Thermal fluctuations were measured with a magnetic-loop antenna inside a large afterglow plasma in the whistler-wave regime. The magnetic fluctuations exhibit a 1/f-like spectrum for whistlers, no resonant enhancement at the electron cyclotron frequency, and a flat spectrum in the evanescent regime. The observed fluctuations are therefore described neither by blackbody radiation laws nor by cyclotron emission, but resemble the decaying Alfvenic fluctuations spectrum calculated by Cable and Tajima (1992).

Golubyatnikov, G.↗

NASA NTRS1994

Thermonuclear runaways in nova outbursts. 2: Effect of strong, instantaneous, local fluctuations

In an attempt to understand the manner in which nova outbursts are initiated on the surface of a white dwarf, we investigate the effects fluctuations have on the evolution of a thermonuclear runaway. Fluctuations in temperature density, or the composition of material in the burning shell may arise due to the chaotic flow field generated by convection when it occurs, or by the accretion process itself. With the aid of two-dimensional reactive flow calculations, we consider cases where a strong fluctutation in temperature arises during the early, quiescent accretion phase or during the later, more dynamic, explosion phase. In all cases we find that an instantaneous, local temperature fluctuation causes the affected material to become Rayleigh-Taylor unstable. The rapid rise and subsequent expansion of matter immediately cools the hot blob, which prevents the lateral propagation of burning. This suggests that local temperature fluctuations do not play a significant role in directly initiating the runaway, especially during the early stages. However, they may provide an efficient mechanism of mixing core material into the envelope (thereby pre-enriching the fuel for subsequent episodes of explosive hydrogen burning) and of mixing substantial amounts of the radioactive nucleus N-13 into the surface layers, making novae potential gamma-ray sources. This suggests that it is the global not the local, evolution of the core-envelope interface to high temperatures which dominates the development of the runaway. We also present a possible new scenario for the initiation of nova outbursts based on our results.

Shankar, Anurag↗

NASA NTRS1995

Variation of fractional electron density fluctuations inside 40 R(sub 0) observed by Ulysses ranging measurements

The first measurements of fractional electron density fluctuations delta-n(sub e)/n(sub e), where delta-n(sub e) is rms electron density fluctuation and n(sub e) is the mean electron density, have been carried out inside 40 R(sub 0) using 1991 Ulysses dual-frequency S- and X-band (13 and 3.6 cm) ranging (time delay) measurements. In the frequency band of approximately 6 x 10(exp -5) - 8 x 10(exp -4) Hz (periods of 20 min to 5 hr), delta-n(sub e)/n(sub e) varies from a high near 20% in the slow wind close to the neutral line to a low of 1% in the fast wind far from the neutral line. For spatial wavenumber K approximately = 1.4 x 10(exp -6)/km (period of 5 hr at 250 km/s), delta-n(sub e)/n(sub e) is essentially independent of heliocentric distance over 0.03-1.0 AU in the slow wind; it is a factor of 30 lower in the fast wind than in the slow wind inside 0.1 AU, but exhibits dramatic growth with heliocentric distance inside 0.3 AU. This latter result reinforces current views of the evolution of MHD turbulence and the association of Alfven waves with high speed streams based on in situ fields and particles measurements beyond 0.3 AU. That regions of enhanced density fluctuations near or above the neutral line coincide with regions of enhanced density confirms previous conclusions that they are the interplanetary manifestation of the heliospheric current sheet and extensions of coronal streamers. While the regions of enhanced density fluctuations lie within those of enhanced density, they have boundaries that are distinctly more abrupt, suggesting the separation of plasma of different nature and origin.

Woo, Richard↗

NASA NTRS1995

Nonequilibrium, large-amplitude MHD fluctuations in the solar wind

Compressible MHD simulations in one dimension with three-dimensional vectors are used to investigate a number of processes relevant to problems in interplanetary physics. The simulations indicate that a large-amplitude nonequilibrium (e.g., linearly polarized) Alfvenic wave, which always starts with small relative fluctuations in the magnitude B of the magnetic field, typically evolves to flatten the magnetic profile in most regions. Under a wide variety of conditions B and the density rho become anticorrelated on average. If the mean magnetic field is allowed to decrease in time, the point where the transverse magnetic fluctuation amplitude delta B(sub T) is greater than the mean field B(sub 0) is not special, and large values of delta B(sub T)/B(sub 0) do not cause the compressive thermal energy to increase remarkably or the wave energy to dissipate at an unusually high rate. Nor does the 'backscatter' of the waves that occurs when the sound speed is less than the Alfven speed result, in itself, in substantial energy dissipation, but rather primarily in a phase change between the magnetic and velocity fields. For isolated wave packets the backscatter does not occur for any of the parameters examined; an initial radiation of acoustic waves away from the packet establishes a stable traveling structure. Thus these simulations, although greatly idealized compared to reality, suggest a picture in which the interplanetary fluctuations should have small deltaB and increasingly quasi-pressure balanced compressive fluctuations, as observed, and in which the dissipation and 'saturation' at delta B(sub T)/B(sub 0) approximately = 1 required by some theories of wave acceleration of the solar wind do not occur. The simulations also provide simple ways to understand the processes of nonlinear steepening and backscattering of Alfven waves and demonstrate the existence of previously unreported types of quasi-steady MHD states.

Roberts, D. Aaron↗

NASA NTRS1995

Fluctuations of thermal conductivity and morphological stability

Compositional fluctuations of the binary alloy result in the corresponding fluctuations of the thermal conductivity of the material. During crystal growth, these fluctuations can significantly modify the local temperature fields at the liquid-solid interface. This, in turn, will affect the morphological stability of the growing interface. In this work, the temperature dependence of the thermal conductivity of the solid phase has been included into the Mullins-Sekerka formalism. A significant effect on the onset of the instability of planar interface has been predicted. It has been found, in particular, that for binary systems with the segregation coefficient above unity a flat interface is always unstable. The shape of the interface fluctuation should have a single harmonic character with a well defined wavelength.

Mazuruk, K.↗

NASA NTRS1994

Measurements of the Low Frequency Gain Fluctuations of a 30 GHz High-Electron-Mobility-Transistor Cryogenic Amplifier

Low frequency gain fluctuations of a 30 GHz cryogenic HEMT amplifier have been measured with the input of the amplifier connected to a 15 K load. Effects of fluctuations of other components of the test set-up were eliminated by use of a power-power correlation technique. Strong correlation between output power fluctuations of the amplifier and drain current fluctuations of the transistors comprising the amplifier are observed. The existence of these correlations introduces the possibility of regressing some of the excess noise from the HEMT amplifier's output using the measured drain currents.

Jarosik, Norman↗

NASA NTRS1995

Alfvenic fluctuations in the solar wind observed by Ulysses

One of the striking results of the Sun's south polar pass by Ulysses was the discovery of large amplitude, long period Alfvenic fluctuations that were continuously present in the solar wind flow from the polar coronal hole. The fluctuations dominate the variances and power spectra at periods greater than or equal to 1 hour and are evident as correlated fluctuations in the magnetic field and solar wind velocity components. Various properties of the fluctuations in the magnetic field, in the velocity, and in the electric field have been established. The waves appear to have important implications for galactic cosmic rays and for the solar wind, topics which have continued to be investigated. Their origin is also under study, specifically whether or not they represent motions of the ends of the field lines at the Sun. The resolution of these issues has benefited from the more recent observations as the spacecraft traveled northward toward the ecliptic and passed into the northern solar hemisphere. All these observations will be presented and their implications will be discussed.

Smith, E. J.↗