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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 361 records · Page 20
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NASA NTRS1979

Large eddy simulation of turbulent channel flow: ILLIAC 4 calculation

The three-dimensional time dependent equations of motion were numerically integrated for fully-developed turbulent channel flow. A large scale flow field was obtained directly from the solution of these equations, and small scale field motions were simulated through an eddy viscosity model. The calculations were carried out on the ILLIAC 4 computer. The computed flow patterns show that the wall layer consists of coherent structures of low speed and high speed streaks alternating in the spanwise direction. These structures were absent in the regions away from the wall. Hot spots, small localized regions of very large turbulent shear stress, were frequently observed. The profiles of the pressure velocity-gradient correlations show a significant transfer of energy from the normal to the spanwise component of turbulent kinetic energy in the immediate neighborhood of the wall ('the splatting effect').

Kim, J.↗
NASA NTRS1978

Eddy diffusivities within Jupiter

The Flasar and Gierasch (1977) model for small-scale thermally driven turbulent convection in rapidly rotating systems is used to estimate eddy diffusivities within Jupiter's atmosphere and interior. It is assumed that heat transport is governed by a mixing-length law, that Jupiter's interior heat flow derives from thermal cooling, and that the entropy loss per nucleon is uniform throughout. A characteristic scalar diffusivity is obtained as a function of pressure level and latitude. The results show that the latitude variation in the diffusivity might be as large as a factor of 100 at 4600 bars. The consequences of such variations are considered.

Flasar, F. M.↗
NASA NTRS1980

Dependence of red edge on eddy viscosity model parameters

The dependence of the red edge location on the two fundamental free parameters in the eddy viscosity treatment was extensively studied. It is found that the convective flux is rather insensitive to any reasonable or allowed value of the two free parameters of the treatment. This must be due in part to the fact that the convective flux is determined more by the properties of the hydrogen ionization region than by differences in convective structure. The changes in the effective temperature of the red edge of the RR Lyrae gap resulting from these parameter variations is quite small (approximately 150 K). This is true both because the parameter variation causes only small changes and because large changes in the convective flux are required to produce any significant change in red edge location. The possible changes found are substantially less than the approximately 600 K required to change the predicted helium abundance mass fraction from 0.3 to 0.2.

Deupree, R. G.↗
NASA NTRS1980

Large eddy simulation of turbulent channel flow: ILLIAC 5 calculation

The capabilities of large eddy simulation in the prediction and analyses of wall-bounded turbulent shear flows are demonstrated. The dynamical equations for large scale field motions are derived. The computational grid network is described and its relation to the observed physical length scales in the flow are discussed. Some aspects of the mechanics and structure of the flow are examined both in the vicinity of the wall and in regions away from the wall. An attempt is made to correlate numerical results with laboratory observations. Other significant observations and conclusions are presented.

Kim, J.↗
NASA NTRS1980

Improved subgrid-scale models for large-eddy simulation

The paper analyzes models for subgrid-scale turbulence. The analysis indicates that there is sufficient information in the resolved scales to determine some characteristics of the complete flow field. The kinetic energy of the small-scale motions can be decomposed into two parts: one results from the large scales and is correlated with them, and the other part is uncorrelated which leads to a two-component eddy-viscosity model. The 'production equals dissipation' argument does not apply to the small scales in the decay of turbulence because it does not account for the uncorrelated component. The exchange between the large and small scales takes place mainly between the smallest scales of the former and the largest scales of the latter; this argument is the basis of a new model shown to be superior to the Smagorinsky model (1963).

Bardina, J.↗
NASA NTRS1980

Eddy diffusion coefficients and the variance of the atmosphere 30-60 km

The results of numerical models or of new observational programs are checked by comparing them with past observations. In view of the differing analysis techniques or differing data samples, the eddy diffusivities presented here agree remarkably well with past estimates. However, in the application of K-values to two-dimensional models, the actual magnitude of the diffusivities is no more important than their spatial patterns, i.e., their gradients with height and latitude. It should thus be noted that the present patterns are often much different from those of past results.

Nastrom, G. D.↗
NASA NTRS1979

Large Eddy Simulation of turbulent shear flows

The conceptual foundation underlying Large Eddy Simulation (LES) is summarized, and the numerical methods developed for simulation of the time-developing turbulent mixing layer and turbulent plane Poiseuille flow are discussed. Computational results show that the average Reynolds stress profile nearly attains the equilibrium shape which balances the downstream mean pressure gradient in the regions away from the walls. In the vicinity of the walls, viscous stresses are shown to be significant; together with the Reynolds stresses, these stresses balance the mean pressure gradient. It is stressed that the subgrid scale contribution to the total Reynolds stress is significant only in the vicinity of the walls. The continued development of LES is urged.

Moin, P.↗
NASA NTRS1981

Large-eddy simulation of the shear-free turbulent boundary layer

The shear-free turbulent boundary layer is calculated by the large-eddy simulation technique. The filtered Navier-Stokes equations are used; the method of integration employs Fourier expansions in the homogeneous directions and finite differences in the cross-stream direction. Results indicate that the simulation is capable of predicting the primary Reynolds-number effects.

Biringen, S.↗
NASA NTRS1982

Laminar flow transition: A large-eddy simulation approach

A vectorized, semi-implicit code was developed for the solution of the time-dependent, three dimensional equations of motion in plane Poiseuille flow by the large-eddy simulation technique. The code is tested by comparing results with those obtained from the solutions of the Orr-Sommerfeld equation. Comparisons indicate that finite-differences employed along the cross-stream direction act as an implicit filter. This removes the necessity of explicit filtering along this direction (where a nonhomogeneous mesh is used) for the simulation of laminar flow transition into turbulence in which small scale turbulence will be accounted for by a subgrid scale turbulence model.

Biringen, S.↗
NASA NTRS1981

Interaction of eddies and mean zonal flow on Jupiter as inferred from Voyager 1 and 2 images

Voyagers 1 and 2 narrow angle frames are used to obtain displacements of features at resolutions of 130 km over time intervals of 1 Jovian rotation. It is shown that the mean zonal velocity profile does not change by a measurable amount between Voyagers 1 and 2, which is consistent with previous observations. It is also shown that the curvature of the velocity profile vanes varies with latitudes in the range from -3 beta to +2 beta. The barotropic stability criterion is violated at 10 latitudes between + and - 60 deg, and the rate of conversion of eddy kinetic energy into zonal mean kinetic energy is in the range from 1.5 to 3.0 per sq Wm for a layer 2.5 bar deep. The rate of energy conversion is more than 10% of the total infrared heat flux for Jupiter, as compared to the earth where it is only 0.1% of the infrared, which suggests that the two planets possess fundamentally different thermomechanical energy cycles.

Ingersoll, A. P.↗
NASA NTRS1981

Oceanic eddy variability measured by GEOS 3 altimeter crossover differences

Eddy statistics of sea height variations associated with dynamic ocean currents, gathered by GEOS 3 altimetry (25 cm), are discussed. Time variable flow is noted to be measureable using altimeter height and orbit information by means of three techniques: collinear tracks, where rms variability can be determined for mesoscale phenomena measured along parallel tracks; mean surfaces, using a 3-dimensional mapping of the sea surface for an accurate representation of the surface undulations; and crossover differences, such as between data tracks from GEOS and Seasat which have reduced ephemeris error over scales of a few thousand km. Mesoscale energetics have been found suitable for tracing meandering ocean currents by the crossover method. Possible sources of error are considered, including altimeter noise, orbit error, short wavelength continental shelf tides, etc., and it is determined that observations of mesoscale sea height appear to be limited only by uncertainties in the altimetry, which for Seasat was 5 cm over 60% of the world's oceans.

Cheney, R. E.↗
NASA NTRS1981

Eddy current probe response to open and closed surface flaws

A general analysis of eddy current response to certain types of open and closed surface flaws is presented for both standard low-frequency and ferromagnetic-resonance (FMR) probes. It is shown analytically that for two-dimensional and three-dimensional surface flaws interrogated by a uniform probe field, the crack opening sensitivity increases with the operating frequency of the probe, this behavior being due to the Faraday induction effect. Experiments with low-frequency probes operating at or below 1 MHz and with the FMR probe operating at approximately 1000 MHz confirm this increase of the crack mouth opening displacement for practical situations where the probe field is not uniform in the vicinity of the flaw.

Auld, B. A.↗
NASA NTRS1982

An experimental search for near-wall boundary conditions for large eddy simulation

Instantaneous wall shear stress and streamwise velocities have been measured simultaneously in a flat plate, turbulent boundary layer at moderate Reynolds number in an effort to provide experimental support for large eddy simulations. Data were obtained by using a buried-wire wall shear gage and a hot-wire rake positioned in the log region of the flow. All data processing was accomplished with digital data analysis techniques on a minicomputer. Fluctuations of the instantaneous U plus versus Y plus profiles about a mean law of the wall are shown to be significant and complex. Peak cross-correlation values between wall shear stress and the velocities are high and reflect the passage of a large structure inclined at a small angle to the wall. Estimates of this angle are consistent with those made by other investigators. Conditional sampling techniques were used to detect the passage of various sizes and types of flow disturbances (events) and to estimate their mean frequency of occurrence. Events characterized by large and sudden streamwise accelerations were found to be highly coherent throughout the log region and were strongly correlated with large fluctuations in wall shear-stress. Phase randomness between the near-wall quantities and the outer velocities was small. The results suggest that the flow events detected by conditional sampling applied to velocities in the log region may be related to the bursting process.

Robinson, S. K.↗
NASA NTRS1983

Eddy-Current Damage Test for Carbon Composites

Nondestructive test method detects cracks in materials of low conductivity. Test setup includes eddycurrent unit with frequency of 2 to 20 MHz/ and storage oscilloscope. High frequency required to assure depth of penetration less than substrate thickness, to prevent thickness from influencing reading. Eddy-current analysis confirmed by visual inspection as damaged area was sectioned and evaluated.

Hamilton, A. B.↗
NASA NTRS1984

Eddy-Current Inspection of Narrow Metal Tubes

Inspection technique for narrow-bore metal tubing involves use of small internal eddy-current probe. Probe consists of thin copper wire wrapped on bobbin. Probe small enough to pass through bends in tube being measured. Technique useful for strain measurements where operating conditions or inaccessibility prevent use of such conventional methods as X-ray diffraction, electrical-resistance measurements, strain gages, or holography.

Ambrose, H. H.↗
NASA NTRS1982

Vertical eddy diffusion coefficient from the LANDSAT imagery

Analysis of five stable cases of the smoke plumes that originated in eastern Cabo Frio (22 deg 59'S; 42 deg 02'W), Brazil using LANDSAT imagery is presented for different months and years. From these images the lateral standard deviation (sigma sub y) and the lateral eddy diffusion coefficient (K sub y) are obtained from the formula based on Taylor's theory of diffusion by continuous moment. The rate of kinetic energy dissipation (e) is evaluated from the diffusion parameters sigma sub y and K sub y. Then, the vertical diffusion coefficient (K sub z) is estimated using Weinstock's formulation. These results agree well with the previous experimental values obtained over water surfaces by various workers. Values of e and K sub z show the weaker mixing processes in the marine stable boundary layer. The data sample is apparently to small to include representative active turbulent regions because such regions are so intermittent in time and in space. These results form a data base for use in the development and validation of mesoscale atmospheric diffusion models.

Viswanadham, Y.↗
NASA NTRS1983

Eddy current X-Y scanner system

The Nondestructive Evaluation Branch of the Materials and Processes Laboratory became aware of a need for a miniature, portable X-Y scanner capable of performing eddy current or other nondestructive testing scanning operations such as ultrasonic, or small areas of flat plate. The technical description and operational theory of the X-Y scanner system designed and built to fulfill this need are covered. The scanner was given limited testing and performs according to its design intent, which is to scan flat plate areas of approximately 412 sq cm (64 sq in) during each complete cycle of scanning.

Kurtz, G. W.↗
NASA NTRS1983

Eddy intrusion of hot plasma into the polar cap and formation of polar-cap arcs

Plasma and electric field signatures of polar cap arcs in their formation region above 5000 km are examined. Signatures for active and for quiet magnetic conditions are presented, both indicating a virtual equivalence of plasma and electric field signatures between the polar and evening discrete arcs, although their optical configurations can be very different. It is proposed that the two types of arcs are indeed the same at active times except that the polar cap arcs occur at multiple convection reversal boundaries of plasmasheet plasmas which intrude into the normal polar cap in the form of multiple cellular eddies. A far-reaching consequence of this hypothesis is the unification of arc formation theory clarifying the distinct roles of arc formation parallel electric fields and of arc configuration and location. Optical and particle data which support this hypothesis are discussed.

Chiu, Y. T.↗