SEARCH · Engineering Papers

Results for “Closure”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,171 records · Page 65

All sources NASA NTRS DOE OSTI

NASA NTRS1990

Relic gravitational waves and extended inflation

In extended inflation, a new version of inflation where the transition from an inflationary to a radiation-dominated universe is accomplished by bubble nucleation, bubble collisions supply a potent - and potentially detectable - source of gravitational waves. The energy density in relic gravitons from bubble collisions is expected to be about 0.00005 of closure density. Their characteristic wavelength depends on the reheating temperature. If black holes are produced by bubble collisions, they will evaporate, producing shorter-wavelength gravitons.

Turner, Michael S.↗

NASA NTRS1991

An efficient solution technique for shockwave-boundary layer interactions with flow separation and slot suction effects

An efficient method for computing two-dimensional compressible Navier-Stokes flow fields is presented. The solution algorithm is a fully-implicit approximate factorization technique based on an unsymmetric line Gauss-Seidel splitting of the equation system Jacobian matrix. Convergence characteristics are improved by the addition of acceleration techniques based on Shamanskii's method for nonlinear equations and Broyden's quasi-Newton update. Characteristic-based differencing of the equations is provided by means of Van Leer's flux vector splitting. In this investigation, emphasis is placed on the fast and accurate computation of shock-wave-boundary layer interactions with and without slot suction effects. In the latter context, a set of numerical boundary conditions for simulating the transpiration flow in an open slot is devised. Both laminar and turbulent cases are considered, with turbulent closure provided by a modified Cebeci-Smith algebraic model. Comparisons with computational and experimental data sets are presented for a variety of interactions, and a fully-coupled simulation of a plenum chamber/inlet flowfield with shock interaction and suction is also shown and discussed.

Edwards, Jack R.↗

NASA NTRS1990

On near-wall turbulent flow modelling

The behavior of various terms in the Reynolds-stress transport equations is evaluated in view results of near-wall turbulence characterizations which show that all components of the velocity-pressure gradient correlation vanish at the wall. A methodology for deriving an asymptotically correct model of the velocity-pressure gradient correlation is proposed, and a model capable of approaching the high-Reynolds number model for pressure distribution far from the wall is derived. A parametric study of the model constants introduced by the near-wall closure reveals that one constant in the dissipation-rate equation is Reynolds number-dependent; with this modification, excellent agreement is obtained with measured and simulated near-wall turbulence statistics.

Lai, Y. G.↗

NASA NTRS1990

An evaluation of the pressure proof test concept for thin sheet 2024-T3

The concept of pressure proof testing of fuselage structures with fatigue cracks to insure structural integrity was evaluated from a fracture mechanics viewpoint. A generic analytical and experimental investigation was conducted on uniaxially loaded flat panels with crack configurations and stress levels typical of longitudinal lap-splice joints in commercial transport aircraft fuselage. The results revealed that the remaining fatigue life after a proof test was longer than that without the proof test because of crack growth retardation due to increased crack closure. However, based on a crack length that is slightly less than the critical value at the maximum proof test stress, the minimum assured life or proof test interval must be no more than 550 pressure cycles for a 1.33 proof factor and 1530 pressure cycles for a 1.5 proof factor to prevent in-flight failures.

Dawicke, D. S.↗

NASA NTRS1991

Modeling turbulent compressible flows - The mass fluctuating velocity and squared density

This paper deals with single-point closure theory for compressible turbulent flow, including the effects of compressibility on the turbulence. In particular, the combination of the pressure dilatation and the dilatation dissipation, terms which appear on the turbulent kinetic energy equation, are modeled. Model parameters in these transport equations are determined by comparing predictions with boundary layer measurements. Finally, predictions with a k-epsilon model, including the new formulations, are presented for the compressible shear layer.

Taulbee, D.↗

NASA NTRS1991

A two-equation turbulence model for compressible reacting flows

A two-equation turbulence model (k-epsilon) has been modified to model compressible reacting flows. A compressibility correction model based on modeling the dilatational terms in the Reynolds stress equations has been used. Computations are carried out using the Navier-Stokes solver SPARK which includes a finite rate chemistry model for hydrogen-air combustion. Initially, planar compressible mixing layers are simulated. Comparisons of the predictions with available experimental data and the predictions of a compressible Reynolds stress closure indicate that the model is well suited for the study of such flows. The decrease in the growth rate of the mixing layer with increasing convective Mach number is well predicted. The model is then tested on a three-dimensional supersonic flow problem where solid boundaries are present. An axisymmetric nonreacting mixing layer flow involving dissimilar gases is evaluated next using the model and the predictions are compared with available experimental data. Finally, two reacting turbulent flow configurations have been studied and comparison of predictions and experimental data has been done. The comparisons indicate that the model predicts the overall features of the test cases very well.

Narayan, J. R.↗

NASA NTRS1991

Numerical simulation of transition in wall-bounded shear flows

The current status of numerical simulation techniques for the transition to turbulence in incompressible channel and boundary-layer flows is surveyed, and typical results are presented graphically. The focus is on direct numerical simulations based on the full nonlinear time-dependent Navier-Stokes equations without empirical closure assumptions for prescribed initial and boundary conditions. Topics addressed include the vibrating ribbon problem, space and time discretization, initial and boundary conditions, alternative methods based on the triple-deck approximation, two-dimensional channel and boundary-layer flows, three-dimensional boundary layers, wave packets and turbulent spots, compressible flows, transition control, and transition modeling.

Kleiser, Leonhard↗

NASA NTRS1990

Fatigue crack propagation in aerospace aluminum alloys

This paper reviews fracture mechanics based, damage tolerant characterizations and predictions of fatigue crack growth in aerospace aluminum alloys. The results of laboratory experimentation and modeling are summarized in the areas of: (1) fatigue crack closure, (2) the wide range crack growth rate response of conventional aluminum alloys, (3) the fatigue behavior of advanced monolithic aluminum alloys and metal matrix composites, (4) the short crack problem, (5) environmental fatigue, and (6) variable amplitude loading. Remaining uncertainties and necessary research are identified. This work provides a foundation for the development of fatigue resistant alloys and composites, next generation life prediction codes for new structural designs and extreme environments, and to counter the problem of aging components.

Gangloff, R. P.↗

NASA NTRS1991

A second-order modeling study of confined swirling flow

A numerical study of a confined strong swirling flow is presented. Computations are performed using a differential second-moment closure. The effect of inlet dissipation rate on calculated mean and turbulence fields is investigated. Two pressure-strain models with various model constants are examined to demonstrate their influences on the predicted results. Finally, comparison of the differential second-moment calculation is made with algebraic second-moment results to determine a better suited model for complex swirling flows.

Nikjooy, M.↗

NASA NTRS1991

A frequency based approach to dynamic stress intensity analysis

A hybrid finite element/modal analysis/spectral analysis method is presented for the extraction of frequency dependent strain energy release rates and stress intensity factors. The approach is based on the crack closure technique and is formulated directly in the frequency domain. It is demonstrated for a center cracked panel subject to static in-plane and acoustic loading.

Rizzi, Stephen A.↗

NASA NTRS1991

Interference-free measurements of the subsonic aerodynamics of slanted-base ogive cylinders

Drag, lift, pitching moment, and base-pressure measurements have been made, free of support interference, on a range of slanted-base ogive cylinders, using the NASA Langley Research Center 13-in magnetic suspension and balance system. Test Mach numbers were in the range 0.04-0.2. Two types of wake flow were observed, a quasi-symmetric turbulent closure or a longitudinal vortex flow. Aerodynamic characteristics differ dramatically between the two wake types. Drag measurements are shown to be in agreement with previous tests. A hysteretic behavior of the wake with varying Reynold's number has been discovered for the 45-deg base. An interaction between forebody boundary-layer state and wake flow and base pressures has been detected for higher slant angles.

Britcher, Colin P.↗

NASA NTRS1990

Ignitability test method. II

To determine functional performance of initiating devices, the NASA's Langley Research Center's novel ignitability research on percussion primers has been expanded in 1989 to include measurements of function time, the evaluation of six primer lots (five types), and the determination of the effects of the military cold-temperature requirement of -65 F and primer output closure disks. This test method, a major improvement over the prior primer output test methods, fully met all objectives, while showing a significant amount of ignition variability.

Bement, Laurence J.↗

NASA NTRS1991

Apollo 14 glasses and the origin of lunar soils

Electron microprobe for comparison with soil glass data were used to analyze homogeneous and heterogeneous glass clasts in four Apollo 14 regolith breccias (14042, 14301, 14313, and 14315). Glass types in the Apollo 14 samples were found to be dominated by highland compositions, which include KREEP, LKFM and highland basalt varieties. Only 14042 has a highland glass population similar to those of local Apollo 14 soils. Breccia 14301 stands out in that it is enriched in KREEP glasses with high K2O content, which are similar in composition to Apollo 12 ropy glasses. Only 14042 could be made from local present-day soils. Some of the ancient soils did not undergo breccia formation and closure, and they evolved by meteorite impact processing, by mixing together in various proportions, and by changes made by the addition of lithic fragments and other components. It is suggested that the Apollo 14 soils are made from mixtures of comminuted regolith breccias. A likely age sequence is presented.

Wentworth, S. J.↗

NASA NTRS1991

Laboratory experiments on the electrodynamic behavior of tethers in space

The transient current systems between tethered plasmas in a large magnetoplasma are investigated experimentally for extrapolation to electrodynamic tethers in space. The studies measure the perturbed magnetic fields and the current density associated with pulsed currents to electrodes in three-dimensional space and time. The electrodes excite electron whistlers because they produce fields that dominantly couple to electrons, allowing pulsed currents to propagate and disperse as whistler wave packets. The wave packets evolve into force-free, flux-ropelike field configurations, and a whistler 'wedge' is formed in the plasma due to 'eddy' currents caused by insulated tethers with dc currents. Substantial radiation into the whistler mode happens with moving VLF antennas as well as tethers, and the wave spread within the ray cone is the most significant characteristic event. The wave spread widens the current channel, incites current closure, and is also associated with a 'phantom loop' phenomenon.

Stenzel, Reiner L.↗

NASA NTRS1991

Hypersonic Shock/Boundary-Layer Interaction Database

Turbulence modeling is generally recognized as the major problem obstructing further advances in computational fluid dynamics (CFD). A closed solution of the governing Navier-Stokes equations for turbulent flows of practical consequence is still far beyond grasp. At the same time, the simplified models of turbulence which are used to achieve closure of the Navier-Stokes equations are known to be rigorously incorrect. While these models serve a definite purpose, they are inadequate for the general prediction of hypersonic viscous/inviscid interactions, mixing problems, chemical nonequilibria, and a range of other phenomena which must be predicted in order to design a hypersonic vehicle computationally. Due to the complexity of turbulence, useful new turbulence models are synthesized only when great expertise is brought to bear and considerable intellectual energy is expended. Although this process is fundamentally theoretical, crucial guidance may be gained from carefully-executed basic experiments. Following the birth of a new model, its testing and validation once again demand comparisons with data of unimpeachable quality. This report concerns these issues which arise from the experimental aspects of hypersonic modeling and represents the results of the first phase of an effort to develop compressible turbulence models.

Settles, G. S.↗

NASA NTRS1991

Optical transitions in diamond at ultrahigh pressures

Optical studies on diamond at ultrahigh pressures are described which show that the unique properties of diamonds are affected by such stresses. Spectroscopic measurements show that the optical absorption edge shifts from UV to red with increasing pressures, and Raman scattering measurements how evidence for new structural transitions associated with large macroscopic deformation, beginning at a pressure of about 150 GPa. The changes are reversible and are associated with intense luminescence peaks at 2.0-2.2 eV under 458-514 nm radiation. These results may be related to the onset of bandgap closure in the approach to a new high-pressure phase. These spectral features must also be taken into account when diamond is used as optical windows for ultrahigh-pressure investigations.

Mao, H. K.↗

NASA NTRS1991

Pressure flux-split viscous solutions for subsonic diffusers

A method for predicting flow in diffusers with inlet swirl has been developed. Solutions of the pressure flux-split Reduced Navier Stokes (RNS) equations are obtained for flows in axisymmetric diffusers. Viscous-Inviscid interactions and flow fields with toroidal recirculation regions are efficiently captured. The computational model is verified by comparision with experimental data and other computations. The extreme sensitivity to grid, turbulent closure model and inlet profiles is discussed.

Pordal, H. S.↗

NASA NTRS1989

The fatigue response of the aluminium-lithium alloy, 8090

The fatigue response of an Al-Li-Cu-Mg-Zr (8090) alloy has been studied at room temperature. The initiation and growth of small and long cracks has been examined at R = 0.1 and at a frequency of 100 Hz. Initiation was observed to occur dominantly at sub-grain boundaries. The growth of the small cracks was crystallographic in character and exhibited little evidence of retardation or arrest at the grain boundaries. The long crack data showed the alloy to have a high resistance to fatigue crack growth with underaging providing the optimum heat treatment for fatigue crack growth resistance. In general, this can be attributed to high levels of crack closure which resulted from the presence of extensive microstructurally related asperities.

Birt, M. J.↗