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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Preliminary results on the fracture analysis of multi-site cracking of lap joints in aircraft skins

This paper presents results of a fracture mechanics analysis relevant to fatigue crack growth at rivets in lap joints of aircraft skins are presented. Multi-site damage (MSD) is receiving increased attention within the context of problems of aging aircraft. Fracture analyses previously carried out include small-scale modeling of rivet/skin interactions, larger-scale two-dimensional models of lap joints similar to that developed here, and full scale three-dimensional models of large portions of the aircraft fuselage. Fatigue testing efforts have included flat coupon specimens, two-dimensional lap joint tests, and full scale tests on specimens designed to closely duplicate aircraft sections. Most of this work is documented in the proceedings of previous symposia on the aging aircraft problem. The effect MSD has on the ability of skin stiffeners to arrest the growth of long skin cracks is a particularly important topic that remains to be addressed. One of the most striking features of MSD observed in joints of some test sections and in the joints of some of the older aircraft fuselages is the relative uniformity of the fatigue cracks from rivet to rivet along an extended row of rivets. This regularity suggests that nucleation of the cracks must not be overly difficult. Moreover, it indicates that there is some mechanism which keeps longer cracks from running away from shorter ones, or, equivalently, a mechanism for shorter cracks to catch-up with longer cracks. This basic mechanism has not been identified, and one of the objectives of the work is to see to what extent the mechanism is revealed by a fracture analysis of the MSD cracks. Another related aim is to present accurate stress intensity factor variations with crack length which can be used to estimate fatigue crack growth lifetimes once cracks have been initiated. Results are presented which illustrate the influence of load shedding from rivets with long cracks to neighboring rivets with shorter cracks. Results will also be included for the effect of residual stress due to the riveting process itself. All the results presented in this paper should be regarded as preliminary in the sense that further work by ourselves and others will be required to validate them and to establish their sensitivity to more embellished modelling assumptions.

Aging Aircraft↗
NASA NTRS1979

A similarity rule for compressibility and sidewall-boundary-layer effects in two-dimensional wind tunnels

The effect of the sidewall boundary layer on flow in two-dimensional wind tunnels is determined. The small-disturbance and isentropic approximations are made, and the sidewall-boundary-layer dynamics are modeled with the von Karman momentum-integral equation. The effects of the edge-velocity-gradient term in the sidewall momentum integral, which is usually dominant near the model, and the compressibility term are shown to be similar. It is shown that the effect of sidewall suction around the model is not similar to two-dimensional flow. Comparisons with experiment are made to verify the similarity rule.

Barnwell, R. W.↗
NASA NTRS1978

Tropical sea surface temperature - An interactive one-dimensional atmosphere-ocean model

It is shown that the (cumulus) convective processes in the tropics may be described by a one-dimensional cloud model, while the near-surface ocean may similarly be described by a one-dimensional mixed-layer model. The coupling is achieved through a sea surface flux budget combined with the flux parameterizations implied by Monin-Obukhov similarity theory. The coupled one-dimensional atmosphere-ocean model is applied to the equilibrium situation in which all temperatures reach a steady state. For the ocean, the fluxes must vanish in equilibrium, but the atmosphere maintains a stable lapse rate by balancing cumulonimbus heating against net radiative cooling. All water precipitating from cumulonimbus clouds must have evaporated from the sea. It is shown that this equilibrium system is closed and determinable solely in terms of the solar constant.

Sarachik, E. S.↗
NASA NTRS1985

A three-dimensional turbulent separated flow and related mesurements

The applicability of and the limits on the applicability of 11 near wall similarity laws characterizing three-dimensional turbulent boundary layer flows were determined. A direct force sensing local wall shear stress meter was used in both pressure-driven and shear-driven three-dimensional turbulent boundary layers, together with extensive mean velocity field and wall pressure field data. This resulted in a relatively large number of graphical comparisons of the predictive ability of 10 of these 11 similarity models relative to measured data over a wide range of flow conditions. Documentation of a complex, separated three-dimensional turbulent flow as a standard test case for evaluating the predictive ability of numerical codes solving such flows is presented.

Pierce, F. J.↗
DOE OSTI2021

Plasmons in two-dimensional topological insulators

Here, we analyze collective excitations in models of two-dimensional topological insulators using the random phase approximation. In a two-dimensional extension of the Su-Schrieffer-Heeger model, edge plasmonic excitations with induced charge-density distributions localized at the boundaries of the system are found in the topologically nontrivial phase, dispersing similarly as one-dimensional bulk plasmons in the conventional Su-Schrieffer-Heeger chain. For two-dimensional bulk collective modes, we reveal regimes of enhanced interband wave function correlations, leading to characteristic hardening and softening of inter- and intraband bulk plasmonic branches, respectively. In the two-dimensional Haldane Chern insulator model, chiral, unidirectional edge plasmons in nano-ribbon architectures are observed, which can be characterized by an effective Coulomb interaction cross section. Bulk collective excitations in the two-dimensional Haldane model are shown to be originated by single-particle band structure details in different topological phases.

2-dimensional systems↗
NASA NTRS1996

Improved Models for the Porous Surface with Passive Control

A computational investigation of afterbody flow using a passive control method is conducted. The passive control method consists of a porous surface placed over a plenum. The purpose of the passive control method is to exploit the adverse pressure gradient present in afterbody flow in an attempt to reduce boundary layer separation and afterbody drag. Four different porous wall models are used to model the transpiration velocity in the region of passive control. A three-dimensional, time-dependent, Reynolds-averaged, simplified Navier-Stokes solver, PAB3D, is used to simulate afterbody flow with and without passive control. Three afterbody configurations with boat-tail angles of 10, 20, and 30 deg. are used to obtain two-dimensional solutions with a freestream Mach number of 0.6 and nozzle pressure ratio of 6. The region of passive control was initially placed from 20-60% of the nozzle length. The effect of the porous placement and porous extent is also studied. Baseline (no porosity) two-dimensional solutions are qualitatively similar to experimental data but under-predict the magnitude of the pressure recovery. Results for the subsonic solutions show losses in the pressure recovery for some cases with passive control. Three-dimensional effects are also investigated and seen to be very significant. Three-dimensional baseline solutions, for both sub- and super-critical freestream Mach numbers, compare very favorably with the experimental data in comparison to the two-dimensional solution. Future work is required to examine three-dimensional afterbody flows with passive porosity.

Kraushaar, Sandra L.↗
NASA NTRS1972

Shock wave-turbulent boundary layer interactions in rectangular channels.

Interaction regions created by the impingement of full span, externally generated, shock waves on a nozzle wall boundary layer were investigated. Incident shock strength was varied to produce unseparated, incipient, and fully separated flow fields. Significant departures from two-dimensionality were observed over the entire range of shock strengths tested and were identified with sidewall and corner boundary layer effects. However, comparisons of present centerline results with published two-dimensional data, obtained under similar test conditions and geometrical constraints, showed excellent agreement (e.g., incipient separation pressure levels, wall pressure distributions, free-interaction, and scale of the interaction region). This raises some question concerning the degree of two-dimensionality achieved in these previous investigations.

Reda, D. C.↗
NASA NTRS2001

Differentiation and Genomic Instability in a Human Mammary Cell Model

Harvest of prophylactic mastectomy specimens from an obligate heterozygote for ataxia-telangiectasia provided autologous fibroblasts as well epithelial cells (HMEC). The routine availability of these autologous cells has provided an opportunity to study cell-cell interactions in coculture and monoculture, and in 3-dimensional cultures grown in the NASA rotating bioreactor. HMEC and stromal fibroblasts grown in 2-dimensional monoculture were both observed to produce extracellular matrix. Similar matrix was encountered in 3-dimensional cultures containing HMEC. Metaphases were analyzed. For stromal fibroblasts, genomic aberrations were found in 18% of metaphase spreads. For HMEC, aberrations were greater such that a majority were found to be abnormal. The level of genomic instability determined for these noncancerous cells in 2-dimensional monoculture should be useful for generating a human cell model that can correlate the effects of differentiation in 3-dimensional coculture on the level of genomic instability.

Richmond, R.↗
NASA NTRS1980

On the stabilization of three-dimensional boundary layers by suction and cooling

A significant reduction in the drag of transonic aircraft can be achieved by using an active method of boundary-layer control to maintain laminar flow on the aerodynamic surfaces. The stabilizing influence of suction for both favorable and adverse pressure gradients is demonstrated by means of the self-similar incompressible three-dimensional boundary layers on yawed wedges. Profile instability is measured by the maximum amplification rate of fixed-frequency disturbances computed according to linearized, locally parallel, spatial stability theory. Suction is found to be more effective in controlling Tollmien-Schlichting instability than stationary cross-flow disturbances. The effectiveness of surface cooling as a method of stabilization is compared with suction for the boundary layers on a transonic 35 deg swept wing of infinite span. It is found from compressible stability theory that when the surface is cooled to a uniform temperature such that the maximum cross-flow velocity is reduced by the same amount as with a uniform suction distribution, the stabilizing effect of cooling on cross-flow disturbances is less than with suction.

Mack, L. M.↗
NASA NTRS1992

An extended Lagrangian method

A unique formulation of describing fluid motion is presented. The method, referred to as 'extended Lagrangian method', is interesting from both theoretical and numerical points of view. The formulation offers accuracy in numerical solution by avoiding numerical diffusion resulting from mixing of fluxes in the Eulerian description. Meanwhile, it also avoids the inaccuracy incurred due to geometry and variable interpolations used by the previous Lagrangian methods. Unlike the Lagrangian method previously imposed which is valid only for supersonic flows, the present method is general and capable of treating subsonic flows as well as supersonic flows. The method proposed in this paper is robust and stable. It automatically adapts to flow features without resorting to clustering, thereby maintaining rather uniform grid spacing throughout and large time step. Moreover, the method is shown to resolve multi-dimensional discontinuities with a high level of accuracy, similar to that found in one-dimensional problems.

Liou, Meng-Sing↗
NASA NTRS1987

Differential scanning calorimetric survey of brominated PAN, pitch-based and vapor-grown fibers

A differential scanning calorimetric survey of brominated PAN, pitch-based and vapor-grown fibers revealed that only vapor-grown fibers heat-treated to temperatures of 2250 C or above exhibited the phase change at 100 C associated with the two-dimensional melting of bromine, similar to that observed in HOPG. All the pitch-based fibers tested, as well as vapor-grown fibers annealed to 2250 C, showed a substantial decrease in resistivity upon bromination but gave no indication of a phase change, suggesting that the bromine in these fibers does not have the long-range order needed to undergo two-dimensional melting. The brominated PAN fibers and unannealed vapor-grown fibers showed no phase change and no change in resistivity, indicating that these fibers do not intercalate bromine.

Jaworske, D. A.↗
NASA NTRS1990

Fractal dimension of bioconvection patterns

Shallow cultures of the motile algal strain, Euglena gracilis, were concentrated to 2 x 10 to the 6th organisms per ml and placed in constant temperature water baths at 24 and 38 C. Bioconvective patterns formed an open two-dimensional structure with random branches, similar to clusters encountered in the diffusion-limited aggregation (DLA) model. When averaged over several example cultures, the pattern was found to have no natural length scale, self-similar branching, and a fractal dimension (d about 1.7). These agree well with the two-dimensional DLA.

Noever, David A.↗
NASA NTRS1994

Direct Simulation of a Self-Similar Turbulent Mixing Layer

Three direct numerical simulations of incompressible turbulent plane mixing layers have been performed. All the simulations were initialized with the same two velocity fields obtained from a direct numerical simulation of a turbulent boundary layer with a momentum thickness Reynolds number of 300 computed by Spalart (J. Fluid Mech. 187, 61, 1988). In addition to a baseline case with no additional disturbances, two simulations were begun with two-dimensional disturbances of varying strength in addition to the boundary layer turbulence. After a development stage, the baseline case and the case with weaker additional two-dimensional disturbances evolve self-similarly, reaching visual thickness Reynolds numbers of up to 20 000. This self-similar period is characterized by a lack of large-scale organized pairings, a lack of streamwise vortices in the 'braid' regions, and scalar mixing that is characterized by 'marching' Probability Density Functions (PDFs). The case begun with strong additional two-dimensional disturbances only becomes approximately self-similar, but exhibits sustained organized large-scale pairings, clearly defined braid regions with streamwise vortices that span them, and scalar PDFs that are 'nonmarching.' It is also characterized by much more intense vertical velocity fluctuations than the other two cases. The statistics and structures in several experiments involving turbulent mixing layers are in better agreement with those of the simulations that do not exhibit organized pairings.

Rogers, Michael M.↗
NASA NTRS1993

Initial development of the two-dimensional ejector shear layer - Experimental results

An experimental investigation designed to study the development of shear layers in a two-dimensional single-nozzle ejector has been completed. In this study, combinations of air/air, argon/air, helium/air, and air/helium were used as the supersonic primary and subsonic secondary, respectively. Mixing of the gases occurred in a constant-area tube 39.1 mm high by 25.4 mm wide, where the inlet static pressure was maintained at 35 kPa. The cases studied resulted in convective Mach numbers between 0.058 and 1.64, density ratios between 0.102 and 3.49, and velocity ratios between 0.065 and 0.811. The resulting data shows the differences in the shear-layer development for the various combinations of independent variables utilized in the investigation. The normalized growth-rates in the near-field were found to be similar to two-dimensional mixing layers. These results have enhanced the ability to analyze and design ejector systems as well as providing a better understanding of the physics.

Benjamin, M. A.↗
NASA NTRS1998

Development of Numerical Extended Hydrodynamics for Transition-Regime Non-Equilibrium Flows Encountered in Semiconductor Manufacturing Processes

Six months of funding was received for the proposed three year research program (funding for the period from March 1, 1997 to August 31, 1997). Although the official starting date for the project was March 1, 1997, no funding for the project was received until July 1997. In the funded research period, considerable progress was made on Phase I of the proposed research program. The initial research efforts concentrated on applying the 10-, 20-, and 35-moment Gaussian-based closures to a series of standard two-dimensional non-reacting single species test flow problems, such as the flat plate, couette, channel, and rearward facing step flows, and to some other two-dimensional flows having geometries similar to those encountered in chemical-vapor deposition (CVD) reactors. Eigensystem analyses for these systems for the case of two spatial dimensions was carried out and efficient formulations of approximate Riemann solvers have been formulated using these eigenstructures. Formulations to include rotational non-equilibrium effects into the moment closure models for the treatment of polyatomic gases were explored, as the original formulations of the closure models were developed strictly for gases composed of monatomic molecules. The development of a software library and computer code for solving relaxing hyperbolic systems in two spatial dimensions of the type arising from the closure models was also initiated. The software makes use of high-resolution upwind finite-volumes schemes, multi-stage point implicit time stepping, and automatic adaptive mesh refinement (AMR) to solve the governing conservation equations for the moment closures. The initial phase of the code development was completed and a numerical investigation of the solutions of the 10-moment closure model for the simple two-dimensional test cases mentioned above was initiated. Predictions of the 10-moment model were compared to available theoretical solutions and the results of direct-simulation Monte Carlo (DSMC) calculations. The first results of this study were presented at a meeting last year.

Groth, Clinton P. T.↗
NASA NTRS1987

Local intermodal energy transfer of the secondary instability in a plane channel

A mathematical technique for analyzing local energy-transfer rates among wave-vector triads is developed and applied to the data generated by direct numerical simulations of waves with various types of initial conditions. Starting the simulation with a primary two-dimensional wave and random noise produced structures very similar to those which evolved from three-dimensional center modes. The local transfer rates determined in this case help to explain the eventual deformation of the primary two-dimensional wave which was observed in wind-tunnel experiments. Including weak streamwise vortices in the initial flow field results in large amounts of energy being transferred to the K-type modes early in the simulation. The later development of the waves (and hence the energy-transfer rates) is similar to the previous case.

Singer, Bart A.↗
NASA NTRS1976

Model predictions of latitude-dependent ozone depletion due to aerospace vehicle operations

Results are presented from a two-dimensional model of the stratosphere that simulates the seasonal movement of ozone by both wind and eddy transport, and contains all the chemistry known to be important. The calculated reductions in ozone due to NO2 injection from a fleet of supersonic transports are compared with the zonally averaged results of a three-dimensional model for a similar episode of injection. The agreement is good in the northern hemisphere, but is not as good in the southern hemisphere. Both sets of calculations show a strong corridor effect in that the predicted ozone depletions are largest to the north of the flight corridor for aircraft operating in the northern hemisphere.

Borucki, W. J.↗