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At least 919 records · Page 51

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NASA NTRS1984

Transition to turbulence in plane channel flows

Results obtained from a numerical simulation of the final stages of transition to turbulence in plane channel flow are described. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time evolution of two and three dimensional finite amplitude disturbances. Computations are performed on CYBER-203 vector processor for a 32x51x32 grid. Results are presented for no-slip boundary conditions at the solid walls as well as for periodic suction blowing to simulate active control of transition by mass transfer. Solutions indicate that the method is capable of simulating the complex character of vorticity dynamics during the various stages of transition and final breakdown. In particular, evidence points to the formation of a lambda-shape vortex and the subsequent system of horseshoe vortices inclined to the main flow direction as the main elements of transition. Calculations involving periodic suction-blowing indicate that interference with a wave of suitable phase and amplitude reduces the disturbance growth rates.

Biringen, S.↗

NASA NTRS1984

The role of cobalt on the creep of Waspaloy

Cobalt was systematically replaced with nickel in Waspaloy (which normally contains 13% Co) to determine the effects of cobalt on the creep behavior of this alloy. Effects of cobalt were found to be minimal on tensile strengths and microstructure. The creep resistance and the stress rupture resistance determined in the range from 704 to 760 C (1300 to 1400 C) were found to decrease as cobalt was removed from the standard alloy at all stresses and temperatures. Roughly a ten-fold drop in rupture life and a corresponding increase in minimum creep rate were found under all test conditions. Both the apparent creep activation energy and the matrix contribution to creep resistance were found to increase with cobalt. These creep effects are attributed to cobalt lowering the stacking fault energy of the alloy matrix. The creep resistance loss due to the removal of cobalt is shown to be restored by slightly increasing the gamma' volume fraction. Results are compared to a previous study on Udimet 700, a higher strength, higher gamma' volume fraction alloy with similar phase chemistry, in which cobalt did not affect creep resistance. An explanation for this difference in behavior based on interparticle spacing and cross-slip is presented.

Jarrett, R. N.↗

NASA NTRS1984

Mechanical contact induced transformation from the amorphous to the crystalline state in metallic glass

Friction and wear tests were conducted with 3.2- and 6.4-millimeter-diameter aluminum oxide spheres sliding, in reciprocating motion, on a Fe67Co18B14Si1 metallic foil. Crystallites with a size range of 10 to 150 nanometers were produced on the wear surface of the amorphous alloy. A strong interaction between transition metals and metalloids such as silicon and boron results in strong segregation during repeated sliding, provides preferential transition metal-metalloid clustering in the amorphous alloy, and subsequently produces the diffused honeycomb structure formed by dark grey bands and primary crystals, that is, alpha-Fe in the matrix. Large plastic flow occurs on an amorphous alloy surface with sliding and the flow film of the alloy transfers to the aluminum oxide pin surface. Multiple slip bands due to shear deformation are observed on the side of the wear track. Two distinct types of wear debris were observed as a result of sliding: an alloy wear debris, and/or powdery-whiskery oxide debris.

Miyoshi, K.↗

NASA NTRS1984

Particle size distribution of typical ceramic raw materials by laser granulometry

The principles of the method are explained and the working of the CILAS 715 laser granulometer is described. The particle size distributions of milled glazes, quartz, feldspar and china clay were determined by this instrument and by Andreasen sedimentation. The agreement was good for isometric particles, but the china clay appears finer by sedimentation, because the platelets arrange themselves horizontally during sedimentation, while in the laser granulometer preferred orientation is prevented by circulation between the sample holder and the vibrated and stirred reservoir of the slip.

Wojnarovitsne, I. H.↗

NASA NTRS1984

Contact stresses in pin-loaded orthotropic plates

The effects of pin elasticity, friction, and clearance on the stresses near the hole in a pin-loaded orthotropic plate are described. The problem is modeled as a contact elasticity problem using complex variable theory, the pin and the plate being two elastic bodies interacting through contact. This modeling is in contrast to previous works which assumed that the pin is rigid or that it exerts a known cosinusoidal radial traction on the hole boundary. Neither of these approaches explicitly involves a pin. A collocation procedure and iteration were used to obtain numerical results for a variety of plate and pin elastic properties and various levels of friction and clearance. Collocation was used to enforce the boundary and iteration was used to find the contact and no-slip regions on the boundary. Details of the numerical scheme are discussed.

Hyer, M. W.↗

NASA NTRS1983

Solar tests of aperture plate materials for solar thermal dish collectors

In parabolic dish solar collectors, walk-off of the spot of concentrated sunlight is a hazard if a malfunction causes the concentrator to stop following the Sun. Therefore, a test program was carried out to evaluate the behavior of various ceramics, metals, and polymers under solar irradiation of about 7000 kW/sq m. (peak) for 15 minutes. The only materials that did not slump or shatter were two grades of medium-grain extruded graphite. High purity, slip-cast silica might be satisfactory at somewhat lower flux. Oxidation of the graphite appeared acceptable during tests simulating walk-off, acquisition (2000 cycles on/off Sun), and spillage (continuous on-Sun operation).

Jaffe, L. D.↗

NASA NTRS1984

The Corfu Landslide: Analog to Giant Landslides on Mars

In an analog to the great landslides of the Vales Marineris, Mars, a detailed study was made of the Corfu Landslide in south-central Washington. This prehistoric slide is located on the northern flank of the Saddle Mountains, southwest of Othello, Washington. The slide covers a 13 square km area centered on section 11 of T.15N., R.27E., Willamette Meridian, adjacent to the Corfu townsite. Approximately 1 cubic km of material is involved in sliding that was probably initiated by Missoula flooding through the Channeled Scabland. It is concluded that there were four primary factors involved in the initiation of the Corfu landsliding: (1) A slip surface was present at the right orientation; (2) Glacial flooding undercut the slope; (3) Wetter climatic conditions prevailed during that time period; and (4) Some seismic vibrations, known to occur locally, probably acted as a trigger. These factors show that special conditions were required in conjunction to produce landsliding. Studies in progress of the Vales Marieneris suggest that the same factors probably contributed to landsliding there.

Scott W. Lewis↗

NASA NTRS1984

Rheological study of clay-kaolin aqueous suspensions

Rheological characteristics of clay-kaolin aqueous suspensions were studied by a rotational viscometer to correlate their behavior with the properties of ceramic slips for casting containing quartz, feldspars, and other nonplastic materials. In particular, the effects of the different amounts of dry materials and deflocculant (mixture 1:1 of Na2CO3 and Na2SiO3) and of temperatures on the shear-time-dependent properties of suspensions were examined.

Lapasin, R.↗

NASA NTRS1984

Deflocculation of clay suspensions using sodium polyacrylates

Rheological properties of elutriated kaolin suspensions deflocculated by Na polyacrylate (DAC 3 and DAC 4) were studied and compared to those deflocculated by the conventional Na2CO3 water and glass and imported Dispex N40. The deflocculating effect of Na polyacrylate was comparable to that of Dispex N40. The optimum amounts of Na polyacrylate were determined for suspensions based on 5-type kaolin. The Na polyacrylate can be successfully used for decreasing the water content of ceramic slips for casting and spray drying.

Jedlicka, P.↗

NASA NTRS1984

The Influence of Surface Topography on Polymer Friction

Current friction models for polymers are examined to determine their applicability in predicting the effects of roughness on friction. The two component, adhesion deformation model of friction predicts the variation of friction with the change in the sliding direction with respect to the lay direction on anisotropic rough surfaces. The two component model does not predict the change in friction with changes in roughness. A combination of an energy based model and a slip line deformation model may provide a friction model which could better represent measured friction.

Eiss, N. S., Jr.↗

NASA NTRS1984

Solar-array-materials passive LDEF experiment (A0171)

The objective of this experiment is to evaluate the synergistic effects of the space environment on various solar-array materials, including solar cells, cover slips with various antireflectance coatings, adhesive, encapsulants, reflector materials, substrate strength materials, mast and harness materials, structural composites, and thermal control treatments. The experiment is passive and consists of an arrangement of material specimens mounted in a 3-in.-deep peripheral tray. The effects of the space environment on the specimens will be determined by comparison of preflight and postflight measurements of mechanical, electrical, and optical properties.

Whitaker, A. F.↗

NASA NTRS1984

Real Surface Effects in Elastohydrodynamic Lubrication

Following a general discussion of real effects in EHL, consideration is given in detail to the role played by stochastic roughness superimposed on the nominal solid boundaries. Particular attention is given to the full-film EHL regime where incipient asperity contact bears a negligible fraction of the load. Flow, from which an averaged Reynolds equation can be formed, is nonetheless modified by the amplitude and texture of the roughness patterns. In describing these effects by means of certain well-defined flow factors, it is found that only two extra parameters are needed - the rms surface height and the ratio of the correlation lengths in the two principal roughness directions. In a lowest order perturbation expansion of these flow factors in powers of the ratio of rms roughness to nominal film thickness, no other properties of the roughness appear. In most cases of practical interest, the factors describing Poiseuille flow are separable into the sum of two single-surface flow factors which means that a combination of a single equivalent rough surface versus an ideal smooth surface can always be found. For flow entrained by slip velocity the factors separate instead into a difference of single-surface factors and it becomes significant which of the two surfaces carries the equivalent roughness. Results are discussed of some applications of the averaged Reynolds equation based on the flow factor method to the EHL line contact problem. Finally, the partial EHL regime is considered where comparable load fractions are carried by the hydrodynamic film and by incipient mechanical contact. An extension of the method into this regime by combining it with asperity contact models appears most encouraging.

John H Tripp↗

NASA NTRS1984

Effects of NRZ-M Modulation on Convolutional Codes Performance

Non-return-to-zero mark (NRZ-M) modulation is often used to resolve data sense in suppressed carrier telemetry systems because such systems are subject to half cycle slips that result in complementing the encoded data stream. The performance of coded telemetry systems with NRZ-M is sensitive to the order in which the various operations are done. A system that demodulates the NRZ-M waveform and then decodes performs differently from a system that does the decoding first. The performance of the NASA standard (7, 1/2) convolutional codes is determined for several systems using NRZ-M. Several different demodulation schemes for NRZ-M are considered. It is shown that, even for the best soft-decision method examined, there is a 2.7 dB loss at a decoded bit error rate of 0.005 if the NRZ-M demodulation occurs before rather than after Viterbi decoding.

Deutsch, L. J.↗

NASA NTRS1984

Heads up display

A heads up aircraft display which allows the pilot to view the display without diverting his attention from the scene ahead is disclosed. The display is designed for use on propeller driven aircraft comprised of a radially disposed row of lamps embedded in the rear surface of a propeller. Measurements of flight data are made by conventional means and converted into digital signals. These digital signals are applied to graphic generators which control lamp drivers which in turn control lamps through slip rings. The lamps are lit at the appropriate times during each revolution of the propeller to display the flight data in graphic form to the pilot. The combination of graphic generators and radially disposed lamps embedded in an aircraft propeller enables the pilot to view the display without diverting his attention from the scene ahead.

Garner, H. D.↗

NASA NTRS1984

Operating characteristics of a three-piece-inner-ring large-bore roller bearing to speeds of 3 million DN

A 118 mm bore roller bearing with a three piece inner ring ran successfully at 300,000 DN for 20 hr. Provisions were made for lubrication and cooling through the inner ring. In some tests the outer ring was also cooled. Power loss within the bearing increased with both speed and total oil flow rate to the inner ring. Outer ring temperature decreased by as much as 22 K (40 F) when outer ring cooling was employed whereas inner ring temperature remained essentially constant. Cage slip was greatly reduced or even eliminated by using a bearing with a very tight clearance at operating speed. A three piece inner ring bearing had higher inner ring temperatures and less temperature difference between the inner and outer rings than a conventional one piece inner ring bearing.

Schuller, F. T.↗

NASA NTRS1984

Characterization of erosion of metallic materials under cavitation attack in a mineral oil

Cavitation erosion and erosion rates of eight metallic materials representing three crystal structures were studied using a 20-kHz ultrasonic magnetostrictive oscillator in viscous mineral oil. The erosion rates of the metals with an fcc matrix were 10 to 100 times higher than that of an hcp-matrix titanium alloy. The erosion rates of iron and molybdenum, with bcc matrices, were higher than that of the titanium alloy but lower than those of the fcc metals. Scanning electron microscopy indicates that the cavitation pits are initially formed at the grain boundaries and precipitates and that the pits that formed at the triple points grew faster than the others. Transcrystalline craters formed by cavitation attack over the surface of grains and roughened the surfaces by multiple slip and twinning. Surface roughness measurements show that the pits that formed over the grain boundaries deepended faster than other pits. Computer analysis revealed that a geometric expression describes the nondimensional erosion curves during the time period 0.5 t(0) t 2.5 t(0), where t(0) is the incubation period. The fcc metals had very short incubation periods; the titanium alloy had the longest incubation period.

Rao, B. C. S.↗

NASA NTRS1984

Development of the first infrared satellite observatory

A development history is given for the Infrared Astronomical Satelite (IRAS), whose primary mission objective is an unbiased, all-sky survey in the 8-120 micron wavelength range. A point source catalog of more than 200,000 IR sources, to be published later this year, represents the accomplishment of this objective. IRAS has also conducted 10,000 pointed observations of specific objects. Attention is given to the cost increases and schedule slips which resulted from the substantial technical challenges of IRAS hardware and software development, and to the management techniques which had to be employed in this major international project.

Smith, G. M.↗

NASA NTRS1984

Incorporation of wind shear terms into the governing equations of aircraft motion

Conventional analyses of aircraft motion in the atmosphere have neglected wind speed variability on the scales associated with many atmospheric phenomena such as thunderstorms, low-level jets, etc. These phemonena produce wind shears that have been determined as the probable cause in many recent commercial airline accidents. This paper derives the six degrees equations of motion or an aircraft incorporating the variable wind terms. The equations are presented in several coordinate systems (i.e., body coordinates, inertial coordinates, etc.). The wind shear terms, including the temporal and spatial gradients of the wind, appear differently in the various coordinate system. These terms are discussed. Also, the influence of wind shear on inputs to computing the aerodynamic coefficients such as the effects of wind velocity vector rotation on relative angular rates of rotation and on the time rate of change of angles of attack and side slip are addressed.

Frost, W.↗