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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 937 records · Page 52
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NASA NTRS1987

Shear-layer structures in near-wall turbulence

The structure of internal shear layer observed in the near-wall region of turbulent flows is investigated by analyzing flow fields obtained from numerical simulations of channel and boundary-layer flows. It is found that the shear layer is an important contributor to the turbulence production. The conditionally averaged production at the center of the structure was almost twice as large as the long-time mean value. The shear-layer structure is also found to retain its coherence over streamwise distances on the order of a thousand viscous length units, and propagates with a constant velocity of about 10.6 u sub rho throughout the near wall region.

Johansson, A. V.↗
NASA NTRS1987

Active layer model for wall-bounded turbulence

The active-layer model for wall-bounded turbulence hypothesizes that the nonlinear terms are large only in a thin layer near the wall, and hence the turbulence in the region outside the active inner layer can be modeled as a linear fluctuating flow driven by the active layer. This hypothesis is tested using data obtained from a direct simulation of turbulent channel flow. It is found that the nonlinear effects are the strongest near the wall with a maximum at around y sup + = 20 and, outside the near-wall region, these involve primarily the cascading mechanism leading to dissipation.

Landahl, M. T.↗
NASA NTRS1988

On governing equations for crack layer propagation

Results of analysis on damage distribution of a crack layer, in a model material, supported the self-similarity hypothesis of damage evolution which has been adopted by the crack layer theory. On the basis of measurements of discontinuity density and the double layer potential technique, a solution to the crack damage interaction problem has been developed. Evaluation of the stress intensity factor illustrated the methodology. Analysis of experimental results showed that Arrhenius type constitutive relationship described very well the expansion of the active zone of a crack layer.

Chudnovsky, A.↗
NASA NTRS1986

Boundary layer flow visualization for flight testing

Flow visualization is used extensively in flight testing to determine aerodynamic characteristics such as surface flow direction and boundary layer state. Several visualization techniques are available to the aerodynamicist. Two of the most popular are oil flows and sublimating chemicals. Oil is used to visualize boundary layer transition, shock wave location, regions of separated flow, and surface flow direction. Boundary layer transition can also be visualized with sublimating chemicals. A summary of these two techniques is discussed, and the use of sublimating chemicals is examined in some detail. The different modes of boundary layer transition are characterized by different patterns in the sublimating chemical coating. The discussion includes interpretation of these chemical patterns and the temperature and velocity operating limitations of the chemical substances. Information for selection of appropriate chemicals for a desired set of flight conditions is provided.

Obara, Clifford J.↗
NASA NTRS1987

A model of the wall boundary layer for ducted propellers

The objective of the present study is to include a representation of a wall boundary layer in an existing finite element model of the propeller in the wind tunnel environment. The major consideration is that the new formulation should introduce only modest alterations in the numerical model and should still be capable of producing economical predictions of the radiated acoustic field. This is accomplished by using a stepped approximation in which the velocity profile is piecewise constant in layers. In the limit of infinitesimally thin layers, the velocity profile of the stepped approximation coincides with that of the continuous profile. The approach described here could also be useful in modeling the boundary layer in other duct applications, particularly in the computation of the radiated acoustic field for sources contained in a duct.

Eversman, Walter↗
NASA NTRS1987

Roll-up of vorticity in adverse-pressure-gradient boundary layers

It is shown how the unsteady, nonlinear critical-layer equation determines the evolution of instability waves in a weak adverse-pressure-gradient boundary layer. Numerical solutions show that the nonlinearity halts the growth of these inviscidly unstable waves. The stabilizing effect of nonlinearity, in the present case, can be described as a consequence of either the increase (toward zero) of the phase jump across the critical layer or the roll-up of the critical-layer disturbance vorticity.

Goldstein, M. E.↗
NASA NTRS1988

Mixing control in a plane shear layer

An investigation of mixing processes in a plane shear layer by a direct numerical simulation with a high-order numerical technique, the isoparametric spectral element method, is presented. The computational domain includes a region with the splitter plate that allows to start the flow as a laminar (Blasius) boundary layers with different velocities and concentrations, merging in the shear layer and undergoing transition to highly-unsteady structures with intensive mixing. The augmentation of mixing as a function of a downstream location, the intensity and type of vortical structures in the layer is investigated by an introduction of disturbance at the high-velocity inflow. The character of the dynamic mixing is displayed along with time averaged profiles and statistical characteristics.

Korczak, K. Z.↗
NASA NTRS1988

Numerical investigation of three-dimensional flow separation using the boundary layer equations

The steady, incompressible, three-dimensional laminar and turbulent boundary-layer equations are solved in a streamline coordinate system and in a self-adaptive grid system using Matsuno's finite difference method. Techniques are described for calculating laminar and turbulent separation using the boundary-layer equations. Any type (bubble type or free vortex-layer type) of major separation line can be calculated at an angle of attack on ellipsoids of revolution by this boundary layer code. Results are presented for ellipsoids of revolution at angles of attack up to 45 degrees. Agreements with other numerical and experimental results are very good for laminar flows. Turbulent flows are also investigated with algebraic turbulence models proposed by Rotta and Cebeci and Smith. Good agreement with experimental results was obtained at a small angle of attack (10 degrees) but only qualitative agreement was obtained at a high angle of attack (30 degrees) for turbulent flow on a 6:1 ellipsoid of revolution.

Wie, Yong-Sun↗
NASA NTRS1987

The field-aligned flow approximation for electrons within layers possessing a normal mass flux - A corollary to the deHoffmann-Teller theorem

In this paper, an approximate extension of the deHoffmann-Teller (1950) theorem for electrons within layers possessing a normal mass flux is derived. It is shown that the deHoffmann-Teller frame electron bulk velocity is nearly parallel to the magnetic field everywhere through those one-dimensional time-stationary layers which possess a normal mass flux, even though electric field perpendicular to the magnetic field exists throughout such layers. Examples of layers of this class are the fast and slow shocks as well as rotational discontinuities.

Scudder, J. D.↗
NASA NTRS1987

Vertical temperature gradients on Uranus - Implications for layered convection

Analyses of Voyager IR spectroscopy and radio occultation measurements confirm ground-based observations that the deep lapse rate of Uranus exceeds that for an adiabat of H2 in thermodynamic equilibrium. However, in approximately the same region of the atmosphere, Voyager IR and ground-based observations also indicate that the ratio of ortho to para hydrogen is near the equilibrium value. These two sets of observations can be reconciled by postulating the existence of rapid convective overturning within layers which are thin in comparison to a pressure scale height. Two forms of layered convection are examined. In one case, deposition of kinetic energy results in thin, rapidly overturning layers. Possible kinetic energy sources include braking waves and local instabilities. In the second case considered, molecular-weight discontinuities due to CH4 stratification stabilize the interfaces of thin layers. It is argued that both forms may be important in the convective portion of the Uranus atmosphere.

Gierasch, Peter J.↗
NASA NTRS1988

Boundary layer ozone - An airborne survey above the Amazon Basin

Ozone data obtained over the forest canopy of the Amazon Basin during July and August 1985 in the course of NASA's Amazon Boundary Layer Experiment 2A are discussed, and ozone profiles obtained during flights from Belem to Tabatinga, Brazil, are analyzed to determine any cross-basin effects. The analyses of ozone data indicate that the mixed layer of the Amazon Basin, for the conditions of undisturbed meteorology and in the absence of biomass burning, is a significant sink for tropospheric ozone. As the coast is approached, marine influences are noted at about 300 km inland, and a transition from a forest-controlled mixed layer to a marine-controlled mixed layer is noted.

Gregory, Gerald L.↗
NASA NTRS1988

The fully nonlinear development of Goertler vortices in growing boundary layers

A combination of asymptotic and numerical methods is used to study the fully nonlinear development of small-wavelength Goertler vortices in a growing boundary layer. It is shown that the vortices spread out across the boundary layer, effectively driving the boundary layer, and that the mean flow adjusts so as to make the large-amplitude vortices locally neutral. In the region where the vortices exist, the mean flow is found to have a square-root profile, and the vortex velocity field can be written in closed form. The region of vortex activity is shown to ultimately include almost all of the original boundary layer and much of the free stream.

Hall, P.↗
NASA NTRS1987

Boundary-layer instability mechanisms on a swept-leading edge at Mach 3.5

Correlations have been made in NASA Langley's Mach 3.5 Pilot Quiet Tunnel for the transitions occurring from laminar to turbulent flow, in the cases of 45-deg and 60-deg swept cylinders. While freestream noise variations had no effect on boundary layer transition, the addition of boundary layer trips to the leading edges led to transition at lower Re numbers, depending on both trip height and wind tunnel noise level. Also presented are the results of compressible linear stability calculations for the boundary layer of an infinite swept cylinder; Tollmien-Schlichting waves are found to be amplified in the attachment line boundary layer.

Creel, T. R., Jr.↗
NASA NTRS1987

Measurements of time-dependent boundary layer turbulence in an oscillating flow

Changes in the time-dependent boundary-layer turbulence in an oscillating flow were investigated using the oscillating-flow facility described by Cook et al. (1985). Hot wire measurements were made across the boundary layer for frequencies ranging from 4 to 21 Hz at various positions from the boundary layer origin and with different amplitudes of velocity oscillation. The results clearly demonstrate that the turbulence level is influenced by the imposed oscillation. The variation of the turbulence level is not predicted well by the variation in boundary-layer thickness alone, indicating that the interaction between the imposed oscillation and the turbulence structure is more complex. It is suggested that turbulence models developed for steady flow may not be adequate for periodic turbulent flows.

Cook, William J.↗
NASA NTRS1988

Potential vorticity and layer thickness variations in the flow around Jupiter's Great Red Spot and White Oval BC

Using Voyager images, layer thickness variations in the flow around Jupiter's Great Red Spot (GRS) and White Oval BC were investigated by treating potential vorticity as a conserved tracer. Fluid trajectories around the GRS and the White Oval BC were calculated assuming the flow to be frictionless, adiabatic, hydrostatic, and steady in the reference frame of the vortex. The data obtained constitute a useful diagnostic which will help to differentiate between models of Jovian vortices. Implications of the observations were studied in the context of a one-layer quasi-geostrophic model in which a thin upper weather layer, which contains the vortex, is supported hydrostatically by a much deeper lower layer.

Dowling, Timothy E.↗
NASA NTRS1988

Do large structures control their own growth in a mixing layer? - An assessment

Two different two-dimensional free shear layers, the T-layer developing in time from an initial tangential velocity discontinuity separating the two half-spaces, and the S-layer which develops downstream of the origin where two uniform streams of unequal velocity are brought into tangential contact, are compared. Calculations are performed in order to determine to what extent the perturbations induced upstream by large concentrations of vorticity found downstream hasten or retard the subharmonic instability that leads to the formations of these large structures. The results show that the elliptic influence, or the feedback, in a mixing layer is relatively small for small velocity ratios.

Kaul, Upender K.↗
NASA NTRS1988

Numerical models of thermally and mechanically coupled two-layer convection of highly viscous fluids

Thermal convection in the earth mantle is investigated by means of numerical simulations. The mantle models comprise two horizontal layers of viscous incompressible fluid (with identical or differing properties) separated by a fixed horizontal interface; an isothermal, fixed-heat-flux, or insulating lower boundary; and heat supplied either from below or internally (in the lower layer only, equally in both layers, or primarily in the upper layer). The mathematical formulation of the models is explained, and results for linear stability and finite-amplitude convection are presented in extensive tables and graphs and discussed in detail. Particular attention is given to the presence of thermal coupling without interface distortion in many cases, the predominance of long-wavelength cells in the finite-amplitude models, and the large (50 percent) temperature difference across the interface in all cases.

Ellsworth, Kirk↗
NASA NTRS1988

Nonlinear roll-up of externally excited free shear layers

The effects of strong critical-layer nonlinearity on the spatially growing instabilities of a shear layer between two parallel streams are considered. A composite expansion technique is used to obtain a single formula that accounts for both shear-layer spreading and nonlinear critical-layer effects. Nonlinearity causes the instability to saturate well upstream of the linear neutral stability point. It also produces vorticity roll-up that cannot be predicted by linear theory.

Goldstein, M. E.↗