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All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Propagation of electromagnetic waves in a turbulent medium

Theoretical modeling of the wealth of experimental data on propagation of electromagnetic radiation through turbulent media has centered on the use of the Heisenberg-Kolmogorov (HK) model, which is, however, valid only for medium to small sized eddies. Ad hoc modifications of the HK model to encompass the large-scale region of the eddy spectrum have been widely used, but a sound physical basis has been lacking. A model for large-scale turbulence that was recently proposed is applied to the above problem. The spectral density of the temperature field is derived and used to calculate the structure function of the index of refraction N. The result is compared with available data, yielding a reasonably good fit. The variance of N is also in accord with the data. The model is also applied to propagation effects. The phase structure function, covariance of the log amplitude, and variance of the log intensity are calculated. The calculated phase structure function is in excellent agreement with available data.

Canuto, V. M.↗
DOE OSTI2020

Recent advances in computational mathematics and applications

We are honored to bring you this special issue dedicated to recent advances in computational mathematics and applications in science and engineering. The papers in this special issue were presented in ”Conference on Computational Mathematics and Applications (CCMA)” held at the University of Nevada Las Vegas (UNLV) during October 25–27, 2019. The conference has attracted about 90 attendees from six countries. The 15 papers in the special issue comprise a diverse collection of theoretical numerical analysis as well as applications in various subjects. Topics cover optimal control of PDEs, discontinuous Galerkin methods, fluid flow in porous media, turbulence flow, static and moving interface problems, complex fluids composed by the mixture of Newtonian fluid and nematic (liquid crystal) flows, modeling of ocean–atmosphere system, image segmentation algorithm, uncertainty quantification problems for turbulence model and Maxwell’s equations, iterative solver for systems resulting from mixed methods, and novel methods for solving systems of nonlinear equations. As the Guest Editors we would like to thank Dr. Leland Jameson at NSF (National Science Foundation) for kindly supporting our conference proposal which made this conference possible. We would also like to thank Darren Sugrue from Elsevier who provided partial support for our conference, and Thennarasu Gunasekaran and his production team from Elsevier who produced this nice special issue. We are grateful to many people (Dr. Zhijian Wu, Lori Ornelas, Elsa Juarez, and many Ph.D. students) at the Department of Mathematical Sciences of UNLV who provided tremendous support for the conference. Finally, we very much appreciate all authors who contributed their precious results to this special issue.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS1973

Nonlinear analysis of phased-locked loops with rapidly varying phase

The performance of command and telemetry systems, useful in deep-space communications, is frequently affected by the radio-frequency phase error which is introduced at the point of reception by means of the carrier tracking loop. In low data rate communications, this phase error may vary rapidly over the duration of the signaling interval. In this paper such phase variation is characterized by a sinusoidal input phase, K sin (omega sub o t+, pi/6), which models a typical phase variation in communication over turbulent media. Conditions for synchronization stability and the acquisition behavior are examined by detailed computer study of the phase-plane trajectories for the second and third-order loops with perfect integrator. It is demonstrated that for the phase variation considered the third-order loop has no real advantage over the second-order loop. Finally, it is shown that nonzero initial conditions may result in large steady-state phase error.

Chen, C.↗
NASA NTRS1976

Heating of the solar chromosphere and corona. I - Generalized inhomogeneous wave equation for magnetoacoustic motions

The generalized inhomogeneous wave equation that governs magnetoacoustic, vortical, and thermal motions in compressible fluids and that is applicable to the problem of heating of the solar chromosphere and corona is obtained. The effects of kinematic and bulk viscosity, heat conduction, Joule dissipation, and magnetic diffusivity are included. Under the usual assumptions, the generalized wave equation reduces to the well-known equations of Lighthill, Kulsrud, Phillips, and others. The major problems encountered in applying Lighthill's (1952) mechanism to sound generation in turbulent media are reviewed for both the subsonic and supersonic cases.

Anand, S. P. S.↗
NASA NTRS1988

Studying Turbulence Using Numerical Simulation Databases, 2. Proceedings of the 1988 Summer Program

The focus of the program was on the use of direct numerical simulations of turbulent flow for study of turbulence physics and modeling. A special interest was placed on turbulent mixing layers. The required data for these investigations were generated from four newly developed codes for simulation of time and spatially developing incompressible and compressible mixing layers. Also of interest were the structure of wall bounded turbulent and transitional flows, evaluation of diagnostic techniques for detection of organized motions, energy transfer in isotropic turbulence, optical propagation through turbulent media, and detailed analysis of the interaction of vortical structures.

Source record↗
NASA NTRS2000

Size Scales for Thermal Inhomogeneities in Mars' Atmosphere Surface Layer: Mars Pathfinder

Atmospheric temperature measurement at three heights with thin wire thermocouples on the 1.1 m Mars Pathfinder meteorology must allow estimates of the integral scale of the atmospheric thermal turbulence during an 83 sol period that begins in the summer. The integral scale is a measure for regions of perturbations. In turbulent media that roughly characterizes locations where the perturbations are correlated. Excluding some to intervals with violent excursions of the mean temperatures, integral scale values are found that increase relatively rapidly from a few tenths meters or less near down to several meters by mid-morning. During mid-morning, the diurnal and shorter time scale wind direction variations often place the meteorology mast in the thermal wake of the Lander.

Mihalov, John D.↗
NASA NTRS1988

Turbulent transport of He II in active and passive phase separators using slit devices and porous media

The turbulent transport mode of vapor liquid phase separators (VLPS) for He II has been investigated comparing passive porous plug separators with active phase separators (APS) using slits of variable flow paths within a common frame of reference. It is concluded that the basic transport regimes in both devices are identical. An integrated Gorter-Mellink (1949) equation, found previously to predict VLPS results of porous plugs, is employed to analyze APS data published in the literature. It is found that the Gorter-Mellink flow rate parameter for 9-micron and 14-micron APS slit widths are relatively independent of the slit width, having a rate constant of about 9 + or - 10 percent. This agrees with the early heat flow results for He II entropy transport at zero net mass flow in wide capillaries and slits.

Yuan, S. W. K.↗
DOE OSTI2021

flash -light on the ring : hydrodynamic simulations of expanding supernova shells near supermassive black holes

ABSTRACT The way supermassive black holes (SMBHs) in Galactic Centres (GCs) accumulate their mass is not completely determined. At large scales, it is governed by galactic encounters, mass inflows connected to spirals arms and bars, or due to expanding shells from supernova (SN) explosions in the central parts of galaxies. The investigation of the latter process requires an extensive set of gas dynamical simulations to explore the multidimensional parameter space needed to frame the phenomenon. The aims of this paper are to extend our investigation of the importance of SNe for inducing accretion on to an SMBH and carry out a comparison between the fully hydrodynamic code flash and the much less computationally intensive code ring, which uses the thin shell approximation. We simulate 3D expanding shells in a gravitational potential similar to that of the GC with a variety of homogeneous and turbulent environments. In homogeneous media, we find convincing agreement between flash and ring in the shapes of shells and their equivalent radii throughout their whole evolution until they become subsonic. In highly inhomogeneous, turbulent media, there is also a good agreement of shapes and sizes of shells, and of the times of their first contact with the central 1-pc sphere, where we assume that they join the accretion flow. The comparison supports the proposition that an SN occurring at a galactocentric distance of 5 pc typically drives 1–3 M⊙ into the central 1 pc around the GC.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Molecular-gas-dynamics simulations of turbulent Couette flow over a mean-free-path-scale permeable substrate

Here we report flow statistics and visualizations from molecular-gas-dynamics simulations using the direct simulation Monte Carlo (DSMC) method for turbulent Couette flow in a minimal domain where the lower wall is replaced by an idealized permeable fibrous substrate representative of thermal-protection-system materials for which the Knudsen number is O(10 -1 ). Comparisons are made with smooth-wall DSMC simulations and smooth-wall direct numerical simulations (DNS) of the Navier-Stokes equations for the same conditions. Roughness, permeability, and noncontinuum effects are assessed. In the range of Reynolds numbers considered herein, the scalings of the skin friction on the permeable substrate and of the mean flow within the substrate suggest that they are dominated by viscous effects. While the regenerative cycle characteristic of smooth-wall turbulence remains intact for all cases considered, we observe that the near-wall velocity fluctuations are modulated by the permeable substrate with a wavelength equal to the pore spacing. Additionally, the flow within the substrate shows significant rarefaction effects, resulting in an apparent permeability that is 13% larger than the intrinsic permeability. In contrast, the smooth-wall DSMC and DNS simulations exhibit remarkably good agreement for the statistics examined, despite the Knudsen number based on the viscous length scale being as large as O(10 -1 ). This latter result is at variance with classical estimates for the breakdown of the continuum assumption and calls for further investigations into the interaction of noncontinuum effects and turbulence.

42 ENGINEERING↗
DOE OSTI2022

Clustering of inertial particles in turbulent flow through a porous unit cell

We report direct numerical simulation is used to investigate effects of turbulent flow in the confined geometry of a face-centred cubic porous unit cell on the transport, clustering and deposition of fine particles at different Stokes numbers (St = 0.01, 0.1, 0.5, 1, 2) and at a pore Reynolds number of 500. Particles are advanced using one-way coupling and the collision of particles with pore walls is modelled as perfectly elastic with specular reflection. Tools for studying inertial particle dynamics and clustering developed for homogeneous flows are adapted to take into account the embedded, curved geometry of the pore walls. The pattern and dynamics of clustering are investigated using the volume change of Voronoi tesselation in time to analyse the divergence and convergence of the particles. Similar to the case of homogeneous, isotropic turbulence, the cluster formation is present at large volumes, while cluster destruction is prominent at small volumes and these effects are amplified with the Stokes number. However, unlike homogeneous, isotropic turbulence, the formation of a large number of very small volumes was observed at all Stokes numbers and attributed to the collision of particles with the pore wall. Multiscale wavelet analysis of the particle number density indicates that the peak of the energy density spectrum, representative of enhanced particle clustering, shifts towards larger scales with an increase in the Stokes number. Scale-dependent skewness and flatness quantify the intermittent void and cluster distribution, with cluster formation observed at small scales for all Stokes numbers, and void regions at large scales for large Stokes numbers.

42 ENGINEERING↗
DOE OSTI2022

Magnetic Field Strength from Turbulence Theory. I. Using Differential Measure Approach

The mean plane-of-sky magnetic field strength is traditionally obtained from the combination of polarization and spectroscopic data using the Davis–Chandrasekhar–Fermi (DCF) technique. However, we identify the major problem of the DCF technique to be its disregard of the anisotropic character of MHD turbulence. On the basis of the modern MHD turbulence theory we introduce a new way of obtaining magnetic field strength from observations. Unlike the DCF technique, the new technique uses not the dispersion of the polarization angle and line-of-sight velocities, but increments of these quantities given by the structure functions. To address the variety of astrophysical conditions for which our technique can be applied, we consider turbulence in both media with magnetic pressure higher than the gas pressure, corresponding, e.g., to molecular clouds, and media with gas pressure higher than the magnetic pressure, corresponding to the warm neutral medium. We provide general expressions for arbitrary admixtures of Alfvén, slow, and fast modes in these media and consider in detail particular cases relevant to diffuse media and molecular clouds. We successfully test our results using synthetic observations obtained from MHD turbulence simulations. We demonstrate that our differential measure approach, unlike the DCF technique, can be used to measure the distribution of magnetic field strengths, can provide magnetic field measurements with limited data, and is much more stable in the presence of induced large-scale variations of nonturbulent nature. Furthermore, our study uncovers the deficiencies of earlier DCF research.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1987

VLA and low-frequency VLBI observations of the radio source 0503 + 467 - Austere constraints on interstellar scattering in two media

The radio source 0503 + 467 lies near the Galactic plane (l = 161.0 deg, b = 3.7 deg) and at the edge of the supernova remnant (SNR) HB 9. The VLA observations show that it has a spectrum typical of a compact extragalactic radio source. The resultant small angular size of the source makes it an excellent probe of turbulence in two media: the diffuse, or 'type A', component of interstellar turbulence and a hypothesized region of hydromagnetic turbulence upstream of the supernova remnant. An eight-station VLBI experiment at 326 MHz indicates that the source is less than about 20 milliarcseconds (mas) in angular diameter. A value of 16 mas is most appropriate as an upper limit to the interstellar scattering contribution to the measured angular size. The implications of this upper limit are twofold. First, the galactocentric radial scale to the type-A turbulence is probably less than or equal to about 6 kpc. Second, no evidence is seen for shock-associated turbulence upstream of HB 9. The measurements make it possible to constrain a parameter which is a function of the rms density fluctuation in the upstream region, the outer scale to the density turbulence, and the thickness of SNR foreshock region.

Spangler, Steven R.↗
NASA NTRS1981

The optical and probe measurement of NO - A comparative study

Measurements were made of NO formation in both subsonic turbulent and nonturbulent media generated from gaseous and liquid fuels. Quartz and metallic, cooled and uncooled probes were employed, and optical measurements were made in the UV with narrow-line and continuum lamps. The extent of systematic error bias in these methods was studied over a wide range of conditions. High resolution measurements were made only on flat flames using a high pressure Xe lamp, while low resolution measurements were done with a hollow cathode lamp on flat flame, swirl burner and jet combustor exhausts. Optical data were reduced using a first principles spectral model and multiple zone treatment of the optical path. No major discrepancies related to processes occurring in probes were identified. Good agreement can be expected in measurements made in subsonic exhaust streams up to 2000 K.

Zabielski, M. F.↗
NASA NTRS1989

Group-kinetic theory and modeling of atmospheric turbulence

A group kinetic method is developed for analyzing eddy transport properties and relaxation to equilibrium. The purpose is to derive the spectral structure of turbulence in incompressible and compressible media. Of particular interest are: direct and inverse cascade, boundary layer turbulence, Rossby wave turbulence, two phase turbulence; compressible turbulence, and soliton turbulence. Soliton turbulence can be found in large scale turbulence, turbulence connected with surface gravity waves and nonlinear propagation of acoustical and optical waves. By letting the pressure gradient represent the elementary interaction among fluid elements and by raising the Navier-Stokes equation to higher dimensionality, the master equation was obtained for the description of the microdynamical state of turbulence.

Tchen, C. M.↗
DOE OSTI2024

Nonlinear propagation of chirped laser pulses through a dispersive and turbulent atmosphere

The evolution of ultrashort laser pulses in dispersive, turbulent, nonlinear, and dissipative media is discussed in connection with nonlinear self-focusing collapse and the onset of laser filamentation. In quiescent air, a laser pulse propagating with a peak power greater than a critical power for self-focusing will undergo a catastrophic, transverse collapse until the intensity is large enough for photoionization. At this point, self-focusing is arrested and balanced by plasma refraction, forming a laser filament. By applying an appropriate chirp, the dispersive properties of the medium can be used to enhance this process and control its onset, and to counter dissipative effects such as molecular absorption and atmospheric scattering. This paper presents an analysis of the effect of atmospheric turbulence on the propagation of nonlinear pulses with dispersion compensation (chirp). Furthermore, the analytical results are compared with wave optics simulations and found to be in reasonable agreement as long as the pulse maintains a near-Gaussian spatiotemporal profile.

47 OTHER INSTRUMENTATION↗
NASA NTRS1983

Visualization studies of turbulent transition flows in a porous medium

Results are reported for flow-visualization studies of the flow regimes of water passing through a porous medium consisting of cylindrical glass and plexiglas rods arranged in a complex and fixed three-dimensional geometry. The Reynolds number (Re) varied from 50 to 700; the flow was visualized by injecting a 5% potassium permanganate dye solution into the pores and photographing the resulting dye streaklines with both a still camera and a movie camera. The results indicate that four distinct flow regimes exist in the porous medium: (1) Darcy or creeping flow up to Re = 3; (2) steady inertia-dominated laminar flow for Re = 3-150; (3) unsteady transitional laminar flow for Re = 150-250; and (4) fully turbulent flow for Re greater than 250. It is concluded that a laminar wake instability mechanism typical of the external flow about bluff bodies may be responsible for the overall transition from laminar to turbulent flow in porous media.

Bilardo, V. J.↗