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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Influence of flaw geometries on hole-crack stress intensities

The paper deals with a frozen-stress photoelastic study of a class of flaw shapes where the ratio of the hole diameter to plate thickness is approximately unity. The study was carried out by a technique developed for estimating stress intensity factor variations along the border of flaws in three-dimensional problems. The results show that for deep flaws, the flaw shapes are not simple and the flow growth is not self-similar. The implications of this effect are discussed.

Smith, C. W.↗
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 NTRS1981

Mathematical theory of cylindrical isothermal blast waves in a magnetic field

An analysis is conducted regarding the properties of cylindrically symmetric self-similar blast waves propagating away from a line source into a medium whose density and magnetic field (with components in both the phi and z directions) both vary as r to the -(omega) power (with omega less than 1) ahead of the blast wave. The main results of the analysis can be divided into two classes, related to a zero azimuthal field and a zero longitudinal field. In the case of the zero longitudinal field it is found that there are no physically acceptable solutions with continuous postshock variations of flow speed and gas density.

Lerche, I.↗
NASA NTRS1981

Direct numerical simulations of turbulent shear flows

Numerical simulations of wakes of axisymmetric bodies and of turbulent mixing layers are reported. The flows were assumed to be statistically homogeneous in the mean flow direction, in concert with experimental data and the self-similarity theorem. The nonlinear Navier-Stokes equations were solved by a pseudo-spectral numerical method using a 32 x 32 x 33 point grid and an algorithm for fast Fourier transforms and inverse transforms. Leapfrog time differencing was employed on nonlinear terms and time differencing on viscous terms. Towed wakes and wakes behind a self-propelled body were simulated, showing that the towed wakes exhibited a proper temporal behavior after an initial period of adjustment, including the development of a kurtosis near the wake edge, which is experimentally verifiable. The mixing-layer simulation displayed the laboratory demonstrated presence of large scale features such as vortex cores, while the lateral coherence was weak.

Metcalfe, R. W.↗
NASA NTRS1982

Approximate solution to the Hopf Phi equation for isotropic homogeneous fluid turbulence

Consistent with the observed t to the -n decay laws for isotropic homogeneous turbulence and the form of the longitudinal correlation function f(r, t) for small r, the Hopf Phi equation is shown to be satisfied approximately by an asymptotic power series in t to the -n. This solution features a self-similar universal equilibrium functional which manifests Kolmogoroff-type scaling.

Rosen, G.↗
NASA NTRS1982

On a study of the /Delta T/c and C/asterisk/ integrals for fracture analysis under non-steady creep

Applications of a vector quantity, path-independent integral which has an energy interpretation to the characterization of crack-tip fields in the range from fast to slow crack propagation are examined. The crack tip characterization parameter is defined in terms of a conservation integral for an area around the crack tip in a two-dimensional cracked body. The actual physical interpretation of the parameter is shown to be the difference in crack lengths displayed by two identical bodies which have equal load histories. A steady-state value is obtained for the parameter for cases of steady-state creep and is shown to be related to the standard path-independent integral for macroscopic self-similar crack growth under mode I conditions. A finite element model is developed for viscoplastic material models, using an initial strain approach with steps in a size employed in tangent stiffness methods.

Stonesifer, R. B.↗
NASA NTRS1983

A unifying strain criterion for fracture of fibrous composite laminates

Fibrous composite materials, such as graphite/epoxy, are light, stiff, and strong. They have great potential for reducing weight in aircraft structures. However, for a realization of this potential, designers will have to know the fracture toughness of composite laminates in order to design damage tolerant structures. In connection with the development of an economical testing procedure, there is a great need for a single fracture toughness parameter which can be used to predict the stress-intensity factor (K(Q)) for all laminates of interest to the designer. Poe and Sova (1980) have derived a general fracture toughness parameter (Qc), which is a material constant. It defines the critical level of strains in the principal load-carryng plies. The present investigation is concerned with the calculation of values for the ratio of Qc and the ultimate tensile strain of the fibers. The obtained data indicate that this ratio is reasonably constant for layups which fail largely by self-similar crack extension.

Poe, C. C., Jr.↗
NASA NTRS1983

Flow measurements of an airfoil with deflected spoiler

Spoilers are used as an effective control surface in various stages of flight. The steady and unsteady flow characteristics change with incidence and spoiler angle. An experimental investigation is undertaken to understand the flow phenomena. Typical boundary layer and wake profiles at various incidences and spoiler angles are presented and discussed. A laminar separation is observed at low incidence and high spoiler angle. At incidence, a turbulent boundary layer develops and separation point moves downstream. Generally the wake profiles are asymmetric. When different length scales are used, the profiles become self-similar and symmetric. The importance of laser velocimetry in the reverse flow region is demonstrated by comparing the preliminary LDV measurements with the hot-wire results.

Lee, C. S.↗
NASA NTRS1983

Expansion of a multi-ion plasma into a vacuum

A numerical investigation of the expansion of a plasma with two ion species into a vacuum is presented. A set of Vlasov equations describe the ion behavior and the electrostatic potential is modelled by the Poisson equation. Electrons are assumed to follow Boltzmann's law. A plasma with H(+) and O(+) ions is considered, with the ions forming various combinations. Hydrodynamic calculations are performed for ions and electrons at equal temperatures, and for the presence of hot electrons. Self-similarity is shown to be valid where charge neutrality is dominant. An absence of significant quantities of ion-acoustic oscillations were observed.

Singh, N.↗
NASA NTRS1983

The effect of a coronal shock wave on the solar wind ionization state

In connection with studies of solar wind ionization state freezing, it is shown that, by using a Lagrangian approach of following individual fluid parcels, the techniques used previously for calculating ionization state variations in a steady state case can be extended straightforwardly to time-varying flows. The specific ionization state calculations presented are for a relatively simple picture of time-dependent coronal flow, based on a well-known model of a self-similar shock wave propagating through the corona. Time-dependent ionization effects for the sudden transition between two otherwise steady flows are likely to be limited to a narrow range of gas parcels which, having been shocked within the coronal freezing-in radius, pass a fixed interplanetary observer in an interval of a few tens of minutes. The amplitude of any rise in interplanetary ionization temperature associated with the coronal shock is likely to be considerably smaller than the jump in electron temperature that actually occurs in the corona.

Owocki, S. P.↗
NASA NTRS1983

Optical radiation associated with gamma-ray bursts

Calculations are made of the approximate characteristics of the reprocessed optical radiation resulting from the absorption of a gamma-ray burst by a nearby star. The overall reprocessing time scale, including contributions from gamma-ray transfer and diffusion of the optical radiation, is estimated. It is noted that diffusive cooling occurs by either a 'self-similar' or a 'transparency' wave of postabsorption stellar surface temperatures greater or less than 10,000 K, respectively. Depending on the combination of stellar and gamma-ray burst properties, the reprocessing time scale can vary from the duration of the gamma-ray burst (approximately 1 s) to the maximum calculated cooling time (approximately 1000 s). Calculations for close binary systems are made of the number of optical photons per square centimeter expected at earth as a function of the observed gamma-ray burst fluences and likely distances. It is predicted that, if all gamma-ray bursters are in close binaries, the number of detections per year by an all sky monitor of 200 photons per sq cm sensitivity will be about 100.

London, R. A.↗
NASA NTRS1984

Correlation of thermophoretically-modified small particle diffusional deposition rates in forced convection systems with variable properties, transpiration cooling and/or viscous dissipation

A cooled object (heat exchanger tube or turbine blade) is considered to be immersed in a hot fluid stream containing trace amounts of suspended vapors and/or small particles. Numerical prediction calculations were done for self-similar laminar boundary layers and law-of-the-wall turbulent boundary layers. Correlations are presented for the effect of thermophoresis in the absence of transpiration cooling and viscous dissipation; the effect of real suction and blowing in the absence of thermophoresis; the effect of viscous dissipation on thermophoresis in the absence of transpiration cooling; and the combined effect of viscous dissipation and transpiration cooling on thermophoresis. The final correlation, St/St-sub-zero, is insensitive to particle properties, Euler number, and local mainstream temperature.

Gokoglu, S. A.↗
NASA NTRS1985

Fractal structure of the interplanetary magnetic field

Under some conditions, time series of the interplanetary magnetic field strength and components have the properties of fractal curves. Magnetic field measurements made near 8.5 AU by Voyager 2 from June 5 to August 24, 1981 were self-similar over time scales from approximately 20 sec to approximately 3 x 100,000 sec, and the fractal dimension of the time series of the strength and components of the magnetic field was D = 5/3, corresponding to a power spectrum P(f) approximately f sup -5/3. Since the Kolmogorov spectrum for homogeneous, isotropic, stationary turbulence is also f sup -5/3, the Voyager 2 measurements are consistent with the observation of an inertial range of turbulence extending over approximately four decades in frequency. Interaction regions probably contributed most of the power in this interval. As an example, one interaction region is discussed in which the magnetic field had a fractal dimension D = 5/3.

Burlaga, L. F.↗
NASA NTRS1984

Temperature relaxation in supernova remnants, revisited

Some supernova remnants are expanding into a partially neutral medium. The neutral atoms which are engulfed by the fast blast shock are collisionally ionized to eject low-energy secondary electrons. Calculations are conducted of the temperature relaxation through Coulomb collisions among the secondary electrons, the shocked electrons, and the ions, assuming that the three species have Maxwellian velocity distributions. The results are applied to a self-similar blast wave. If the efficiency of collisionless electron heating at the shock front is high in young remnants such as Tycho, the secondary electrons may be much cooler than both the shocked electrons and the ions. In this case, the emergent X-ray continuum spectrum will have a two-temperature, or a power-law, appearance. This effect may have been observed in the bright rim of the remnant of SN 1006.

Itoh, H.↗
NASA NTRS1985

The destruction of dust grains in collisions of a supernova with a circumstellar medium

Dust grains exist, if not abundantly, in systems composed of a Type II supernova and a circumstellar medium (CSM). The grains, however, may subsequently suffer intense sputtering by X-ray emitting plasmas during the collision of the supernova ejecta with the CSM. An approximate expression is obtained for the amount of grain erosion in both the blast-shocked CSM and the reverse-shocked ejecta, assuming their self-similar interaction. The distortion of the grain size distribution is also calculated. In the remnant of SN 1979c, the population of the grains of radius 0.1 micron, for example, will be diminished substantially in both the ejecta and the CSM, while in the remnant of SN 1980k, it will be affected considerably only in the ejecta. The infrared fluxes expected from the blast-shocked CSM are roughly estimated. The present destruction mechanism may be responsible for the severe under-abundance of dust in the Cassiopeia A supernova remnant.

Itoh, H.↗
NASA NTRS1985

Vortex motion in wall-bounded viscous flow

A factor of general interest in a broad class of wall-bounded flows is the dynamic evolution of vortical structures through the flow. The structures are three-dimensional, and an overall mathematical description of such entities has not yet been formulated. One of the objectives of the present investigation is concerned with the establishment of a framework, based on first principles, which may form a basis for more detailed analytical studies. Another aim is related to the establishment of boundary and initial conditions in numerical experiments. The mathematical framework employed involves the method of matched asymptotic expansions, and an inner solution field is constructed which consists of a two-dimensional vortical structure. The outer solution field is taken to be an otherwise undisturbed laminar two-dimensional parallel or self-similar viscous flowfield.

Gatski, T. B.↗
NASA NTRS1985

Two-dimensional modelling of compressive failure in delaminated laminates

An analytical model is developed to assess the compressive strength criticality of near-surface interlaminar defects in laminated composites. The delaminated region is elliptic in shape, separating a thick isotropic plate from a thin orthotropic layer whose material axes coincide with the ellipse axes. The growth conditions and growth behavior of this defect are studied by breaking the overall problem into an elastic stability problem and a fracture problem. Post-buckling solution for the elliptic section is obtained using the Rayleigh-Ritz method while an energy balance criterion based on a self-similar disbond growth governs the fracture. The parameters controlling the growth or arrest of the delamination damage are identified as the fracture energy, disbond depth and elastic properties of the materials from both sides of the delaminating interface. By varying the degree of material anisotropy relative to the loading axis a range in growth behavior was found including stable or unstable crack growth parallel to or normal to the loading axis.

Chai, H.↗
NASA NTRS1985

A study of plasma expansion phenomena in laboratory generated plasma wakes - Preliminary results

The plasma expansion into the wake of a large rectangular plate immersed in a single-ion, collisionless, streaming plasma has been investigated in the laboratory. Several characteristics of the process involved in 'plasma expansion into vacuum' that have been predicted theoretically were observed, including the creation and motion of a rarefaction wave disturbance; the creation and motion of an expansion front; and the acceleration of ions into the wake at speeds above the ion-acoustic speed. The expansion was limited to early times; i.e., a few ion plasma periods, by the combination of plasma drift speed and vacuum chamber size. This prevented detailed comparison with self-similar theory, but results are in good agreement with numerical simulations and other laboratory experiments for the early time expansion. The conclusion is that the plasma expansion process is the dominant wake filling mechanism in the near wake of a body, whose potential is approximately the plasma space potential.

Wright, K. H., Jr.↗