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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.

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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Toward a theory of interstellar turbulence. 2: Strong alfvenic turbulence

We continue to investigate the possibility that interstellar turbulence is caused by nonlinear interactions among shear Alfven waves. Here, we restrict attention to the symmetric case where the oppositely directed waves carry equal energy fluxes. This precludes application to the solar wind in which the outward flux significantly exceeds the ingoing one. All our detailed calculations are carried out for an incompressible magnetized fluid. In incompressible magnetohydrodynamics (MHD), nonlinear interactions only occur between oppositely direct waves. We present a theory for the strong turbulence of shear Alfven waves. It has the following main characteristics. (1) The inertial-stage energy spectrum exhibits a critical balance between linear wave periods and nonlinear turnover timescales. (2) The 'eddies' are elongated in the direction of the field on small spatial scales; the parallel and perpendicular components of the wave vector, k(sub z) and k(perpendicular) are related by k(sub z) approximately equals k(sub perpendicular to)(exp 2/3) L(exp -1/3), where L is the outer scale of the turbulence. (3) The 'one-dimensional' energy spectrum is proportional to k(sub perpendicular)(exp -5/3)-an anisotropic Kolmogorov energy spectrum. Shear Alfvenic turbulence mixes specific entropy as a passive contaminant. This gives rise to an electron density power spectrum whose form mimics the energy spectrum of the turbulence. Radio wave scattering by these electron density fluctuations produces anisotropic scatter-broadened images. Damping by ion-neutral collisions restricts Alfvenic turbulence to highly ionized regions of the interstellar medium.

Goldreich, P.↗
DOE OSTI2022

Measurements of the imploding plasma sheath in triple-nozzle gas-puff z pinches

Gas-puff z-pinch implosions are characterized by the formation of a dense annular plasma shell, the sheath, that is driven to the axis by magnetic forces and therefore subject to the magneto-Rayleigh–Taylor instability. Here, the conditions within these sheaths are measured on the 1-MA COBRA generator at Cornell University for various gas species and initial fill densities. The gas-puff loads are initialized by a 7 cm diameter triple-nozzle gas valve assembly with concentric outer and inner annular nozzles and a central gas jet. Thomson scattering and laser interferometry provide spatially resolved flow, temperature, and electron density profiles midway through the implosion, while extreme ultraviolet pinhole cameras record the evolution of the plasma column and photoconducting diodes measure x-ray emission. Analysis of the scattering spectra includes a means of discriminating between thermal and non-thermal broadening to test for the presence of hydrodynamic turbulence. Two types of sheath profiles are observed, those with sharp discontinuities at the leading edge and those with smooth gradients. In both cases, non-thermal broadening is generally peaked at the front of the sheath and exhibits a characteristic decay length that roughly scales with the sheath ion mean free path. Here, we demonstrate that this non-thermal broadening term is inconsistent with laminar velocity gradients and is more consistent with dissipative turbulence driven by unstable plasma waves in a collisionless shock. The resulting differences in sheath profile are then set by the sheath ion collisionality in a manner consistent with recent 1D kinetic simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2004

Gravity Wave Forcing of the Mesosphere and Lower Thermosphere: Mountain and Convective Waves Ascending Vertically (MaCWAVE)

The specific objectives of this research effort included the following: 1) Quantification of gravity wave propagation throughout the lower and middle atmosphere in order to define the roles of topographic and convective sources and filtering by mean and low-frequency winds in defining the wave field and wave fluxes at greater altitudes; 2) The influences of wave instability processes in constraining wave amplitudes and fluxes and generating turbulence and transport; 3) Gravity wave forcing of the mean circulation and thermal structure in the presence of variable motion fields and wave-wave interactions, since the mean forcing may be a small residual when wave interactions, anisotropy, and momentum and heat fluxes are large; 4) The statistical forcing and variability imposed on the thermosphere at greater altitudes by the strong wave forcing and interactions occurring in the MLTI.

Fritts, David C.↗
NASA NTRS1990

Turbulence modeling for sharp-fin-induced shock wave/turbulent boundary-layer interactions

Solutions of the Reynolds averaged Navier-Stokes equations are presented and compared with a family of experimental results for the 3-D interaction of a sharp fin induced shock wave with a turbulent boundary layer. Several algebraic and two equation eddy viscosity turbulence models are employed. The computed results are compared with experimental surface pressure, skin friction, and yaw angle data as well as the overall size of the interaction. Although the major feature of the flow fields are correctly predicted, several discrepancies are noted. Namely, the maximum skin friction values are significantly underpredicted for the strongest interaction cases. These and other deficiencies are discussed.

Horstman, C. C.↗
NASA NTRS1983

On the large-scale dynamics of rapidly rotating convection zones

The fact that the values of the eight basic waves present in turbulent flows in the presence of rotation prohibit a tilt of eddy towards the axis of rotation is incorporated into a formalism for rapidly rotating convection zones. Equations for turbulent velocities are defined in a rotating coordinate system, assuming that gravity and grad delta T act in a radial direction. An expression is derived for the lifetime of a basic wave and then for the average velocity vector. A real convective eddy is formulated and the wave vectors are calculated. The velocity amplitude and the stress tensor amplitude are integrated over the eddy domain. Applied to the solar convective zone, it is found that the convective cells are aligned along the axis of rotation at the poles and at the equator, a model that conflicts with nonrotating mixng length theory predictions.

Durney, B. R.↗
NASA NTRS1987

MHD energy fluxes for late type dwarfs

The efficiency of MHD wave generation by turbulent motions in stratified stellar atmospheres with embedded uniform magnetic fields is calculated. In contradiction with previous results, it is shown that there is no significant increase in the efficiency of wave generation because of the presence of magnetic fields, at least within the theory's limits of applicability. It is shown that MHD energy fluxes for late-type stars are less than those obtained for acoustic waves in a magnetic-field-free atmosphere, and do not vary enough for a given spectral type in order to explain observed UV and X-ray fluxes. Thus, the results show that MHD energy fluxes obtained if stellar surface magnetic fields are uniform cannot explain the observed stellar coronal emissions.

Rosner, R.↗
NASA NTRS1983

Existence of a persistent background of turbulence

A plausible scenario for the existence of a persistent back-ground of turbulence in the free atmosphere is described. The MST radar technique is the only existing technique that can be used to describe the morphology of occurrence of turbulence as a function of altitude, wind speed, shear, weather conditions, geographical location, etc. This technique was used also to assess the degree of universality of shape and amplitude of the buoyancy wave spectrum and the relation between the buoyancy wave spectrum and turbulence.

Vanzandt, T. E.↗
NASA NTRS1986

Strong hydromagnetic fluctuations in the Comet P/Halley magnetosphere observed by the Giotto magnetic field experiment

Strong hydromagnetic fluctuations were observed in the induced magnetosphere of comet Halley. Several regions were identified showing either strong turbulence, characterized by the absence of strong fluctuations, or showing isolated, quasi-coherent wave trains above a turbulent background. Visual inspection of the wave trains shows no preferred sense of polarization, i.e., they may be of transverse, compressional, or mixed polarization. Spectral analysis shows peak power at various frequencies, corresponding, for example, to water group or hydrogen ion cyclotron resonances. Analysis of a peculiar type of oscillation with 25 sec period suggests strong contributions from off-angle propagating waves.

Glassmeier, K. H.↗
NASA NTRS1978

Ion acoustic waves in the solar wind

Plasma wave measurements on the Helios 1 and 2 spacecraft have revealed the occurrence of electric field turbulence in the solar wind at frequencies between the electron and ion plasma frequencies. Wavelength measurements with the Imp 6 spacecraft now provide strong evidence that these waves are shortwavelength ion acoustic waves which are Doppler-shifted upward in frequency by the motion of the solar wind. Comparison of the Helios results with measurements from the earth-orbiting Imp 6 and 8 spacecraft shows that the ion acoustic wave turbulence detected in interplanetary space has characteristics essentially identical to those of bursts of electrostatic turbulence generated by protons streaming into the solar wind from the earth's bow shock. In a few cases, enhanced ion acoustic wave intensities have been observed in direct association with abrupt increases in the anisotropy of the solar wind electron distribution. This relationship strongly suggests that the ion acoustic waves detected by Helios far from the earth are produced by an electron heat flux instability, as was suggested by Forslund. Possible related mechanisms which could explain the generation of ion acoustic waves by protons streaming into the solar wind from the earth's bow shock are also considered.

Gurnett, D. A.↗
NASA NTRS1991

Speckles in interstellar radio-wave scattering

Observations of speckles in the scattering disk of the Vela pulsar are presented and speckle techniques for studying and circumventing scattering of radio waves by the turbulent interstellar plasma are discussed. The speckle pattern contains, in a hologrammatic fashion, complete information on the structure of the radio source as well as the distribution of the scattering material. Speckle observations of interstellar scattering of radio waves are difficult because of their characteristically short timescales and narrow bandwidths. Here, first observations are presented, taken at 13 cm wavelength with elements of the SHEVE VLBI network, of speckles in interstellar scattering.

Desai, K. M.↗
NASA NTRS2011

Multiple Spacecraft Study of the Impact of Turbulence on Reconnection Rates

Magnetic turbulence and secondary island formation have reemerged as possible explanations for fast reconnection. Recent three-dimensional simulations reveal the formation of secondary islands that serve to shorten the current sheet and increase the accelerating electric field, while both simulations and observations witness electron holes whose collapse energizes electrons. However, few data studies have explicitly investigated the effect of turbulence and islands on the reconnection rate. We present a more comprehensive analysis of the effect of turbulence and islands on reconnection rates observed in space. Our approach takes advantage of multiple spacecraft to find the location of the spacecraft relative to the inflow and the outflow, to estimate the reconnection electric field, to indicate the presence and size of islands, and to determine wave vectors indicating turbulence. A superposed epoch analysis provides independent estimates of spatial scales and a reconnection electric field. We apply k-filtering and a new method adopted from seismological analyses to identify the wavevectors. From several case studies of reconnection events, we obtain preliminary estimates of the spectral scaling law, identify wave modes, and present a method for finding the reconnection electric field associated with the wave modes.

Wendel, Deirdre↗
NASA NTRS1991

Heat transfer, velocity-temperature correlation, and turbulent shear stress from Navier-Stokes computations of shock wave/turbulent boundary layer interaction flows

The properties of 2-D shock wave/turbulent boundary layer interaction flows were calculated by using a compressible turbulent Navier-Stokes numerical computational code. Interaction flows caused by oblique shock wave impingement on the turbulent boundary layer flow were considered. The oblique shock waves were induced with shock generators at angles of attack less than 10 degs in supersonic flows. The surface temperatures were kept at near-adiabatic (ratio of wall static temperature to free stream total temperature) and cold wall (ratio of wall static temperature to free stream total temperature) conditions. The computational results were studied for the surface heat transfer, velocity temperature correlation, and turbulent shear stress in the interaction flow fields. Comparisons of the computational results with existing measurements indicated that (1) the surface heat transfer rates and surface pressures could be correlated with Holden's relationship, (2) the mean flow streamwise velocity components and static temperatures could be correlated with Crocco's relationship if flow separation did not occur, and (3) the Baldwin-Lomax turbulence model should be modified for turbulent shear stress computations in the interaction flows.

Wang, C. R.↗
DOE OSTI2025

Multi-modal effects on indirect noise induced by turbulent entropy fields

Planar entropy waves are commonly assumed for predicting indirect combustion noise. However, the non-planar and turbulent nature of flows found in most practical combustors challenges this assumption. In the present paper, we examine the indirect noise generated by non-planar and turbulent entropy fields through subsonic nozzles. Firstly, we introduce a new transfer function framework that accounts for the contribution of non-planar Fourier modes of the entropy field to the indirect noise spectra. When applied to a turbulent flow field, this method demonstrates a significant improvement in spectral predictions compared with a conventional approach that only considers the planar mode. Secondly, simulations show that non-planar Fourier modes become significant above a threshold frequency ƒ thresh , found in the mid- to high-frequency range. This contribution of non-planar modes is explained by two-dimensional shear effects that distort the entropy waves. A scaling relation that uses residence times along streamlines is developed for ƒ thresh , showing good agreement with simulation results. Finally, we show that the indirect noise from non-planar entropy modes found in aviation combustors can be significant at frequencies below 1 kHz, which might be relevant in situations of thermo-acoustic instabilities coupled to indirect noise.

42 ENGINEERING↗
NASA NTRS2020

Non-Turbulent Liquid-Bearing Polar Clouds: Observed Frequency of Occurrence and Simulated Sensitivity to Gravity Waves

A common feature of polar liquid-bearing clouds (LBCs) is radiatively-driven turbulence, which may variously alter cloud lifecycle via vertical mixing, droplet activation, and subsequent feedbacks. However, polar LBCs are commonly initiated under stable, non-turbulent conditions. Using long-term data from the North Slope of Alaska and McMurdo, Antarctica, we show that non-turbulent conditions prevail in ~25% of detected LBCs, surmised to be preferentially early in their lifecycle. We conclude that non-turbulent LBCs are likely common over the polar regions owing primarily to atmospheric temperature and stability. Such stable environments are known to support gravity wave activity. Using large-eddy simulations we find that short to intermediate period gravity waves may catalyze turbulence formation when aerosol particles available for activation are sufficiently small. We posit that the frequent occurrence of non-turbulent LBCs over the polar regions has implications for polar aerosol-cloud interactions and their parameterization in large-scale models.

Clouds↗
NASA NTRS1974

Application of wave mechanics theory to fluid dynamics problems: Boundary layer on a circular cylinder including turbulence

The application of the elements of quantum (wave) mechanics to some special problems in the field of macroscopic fluid dynamics is discussed. Emphasis is placed on the flow of a viscous, incompressible fluid around a circular cylinder. The following subjects are considered: (1) the flow of a nonviscous fluid around a circular cylinder, (2) the restrictions imposed the stream function by the number of dimensions of space, and (3) the flow past three dimensional bodies in a viscous fluid, particularly past a circular cylinder in the symmetrical case.

Krzywoblocki, M. Z. V.↗
NASA NTRS1995

Studies of radar backscatter as a function of wave properties and the winds in the turbulent marine atmosphere

The research on model functions for ADEOS and ERS-1 are summarized and an analysis of the differences between the three kinds of models is provided in this final report. The success of the AMI on ERS-1 obtained at GSFC and NMC is highlighted. The problem of wind stress description is reviewed within and the scatterometer model being developed for high winds monitoring for the AMI on ERS-1 and ERS-2 is described.

Pierson, Willard J., Jr.↗
NASA NTRS1992

Evolution of the shock front and turbulence structures in the shock/turbulence interaction

The interaction of a weak shock front with isotropic turbulence has been investigated using Direct Numerical Simulation (DNS). Two problems were considered: the ability of the field equation (the equation for a propagating surface) to model the shock; and a quantitative study of the evolution of turbulence structure using the database generated by Lee et al. Field equation model predictions for front shape have been compared with DNS results; good agreement is found for shock wave interaction with 2D turbulence and for a single steady vorticity wave. In the interaction of 3D isotropic turbulence with a normal shock, strong alignment of vorticity with the intermediate eigenvector of the rate of strain tensor (S(sup *)(sub ij) = S(sub ij) - (1/3)(delta(sub ij))(S(sub kk))) is seen to develop upstream of the shock and to be further amplified on passage through the shock. Vorticity tends to align at 90 deg to the largest eigenvector, but there is no preferred alignment with the smallest eigenvector. Upstream of the shock, the alignments continue to develop even after the velocity derivative skewness saturates. There is a significant tendency, which increases with time throughout the computational domain, for velocity to align with vorticity. The alignment between velocity and vorticity is strongest in eddy regions and weakest in convergence regions.

Kevlahan, N.↗