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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 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

A length-scale model for developing turbulent flow in a rectangular duct

A three-dimensional mixing length model is proposed for modeling local Reynolds stress behavior in rectangular ducts of arbitrary aspect ratio. The model is applicable to both developing and fully-developed flows, and can be applied to other 90-degree corner flows with mild streamwise pressure gradients. Comparisons between theory and experiment show that all components of the Reynolds stress tensor are modeled reasonably well, both in the vicinity of a corner and in two-dimensional regions away from the corner.

Gessner, F. B.↗
NASA NTRS1977

Kinetic theory analysis of rarefied gas flow through finite length slots

A kinetic-theory analysis is made of the flow of a rarefied monatomic gas through a two-dimensional slot connecting two reservoirs. Numerical solutions are obtained by the moment and discrete-ordinate methods. The former method portrays the transition-regime characteristics well but has limitations in the free-molecule regime. The latter method gives accurate results in the free-molecule and slip regimes and bolsters confidence in the accuracy of the transition-regime results. The numerical solution for the mass flux through the slot agrees well with an approximate analytical solution of the moment equations for length-to-width ratios from 6 to 0.5, pressure ratios from 0.8 to 0.1, and Knudsen numbers from 5 to 0.5.

Raghuraman, P.↗
NASA NTRS1977

Surface recombination velocity and diffusion length of minority carriers in heavily doped silicon layers

Quantitative analysis of the electron beam-induced current and the dependence of the effective diffusion length of the minority carriers on the penetration depth of the electron beam were employed for the analysis of the carrier recombination characteristics in heavily doped silicon layers. The analysis is based on the concept of the effective excitation strength of the carriers which takes into consideration all possible recombination sources. Two dimensional mapping of the surface recombination velocity of P-diffused Si layers will be presented together with a three dimensional mapping of minority carrier lifetime in ion implanted Si. Layers heavily doped with As exhibit improved recombination characteristics as compared to those of the layers doped with P.

Gatos, H. C.↗
NASA NTRS1978

Determination of bulk diffusion lengths for angle-lapped semiconductor material via the scanning electron microscope: A theoretical analysis

A standard procedure for the determination of the minority carrier diffusion length by means of a scanning electron microscope (SEM) consists in scanning across an angle-lapped surface of a P-N junction and measuring the resultant short circuit current I sub sc as a function of beam position. A detailed analysis of the I sub sc originating from this configuration is presented. It is found that, for a point source excitation, the I sub sc depends very simply on x, the variable distance between the surface and the junction edge. The expression for the I sub sc of a planar junction device is well known. If d, the constant distance between the plane of the surface of the semiconductor and the junction edge in the expression for the I of a planar junction is merely replaced by x, the variable distance of the corresponding angle-lapped junction, an expression results which is correct to within a small fraction of a percent as long as the angle between the surfaces, 2 theta sub 1, is smaller than 10 deg.

Vonroos, O.↗
NASA NTRS1978

The dynamical halo and the variation of cosmic-ray path length with energy

It is shown that the dynamical halo model offers a natural explanation for the form of the variation of the cosmic-ray path length with energy. The variation above approximately 1 GeV/nucleon can be understood as due to the variation of the diffusion coefficient, and hence the resident time, with energy. The flattening of the curve below 1 GeV/nucleon is seen to mark a transition to a convection dominated regime where coefficient is no longer the determining parameter. A fit to the observations yields a halo outflow velocity of 8 km sec/1. An attempt is made to determine the overall scale of the halo and the diffusion coefficient using recent Be-10 flux measurements but the data do not agree well enough to pin down these variables to within less than four or five orders of magnitude.

Jones, F. C.↗
NASA NTRS1979

Length and bursting of separation bubbles: A physical interpretation

A physical interpretation of the observed form of the pressure distribution beneath a two-dimensional separation bubble which modified the external inviscid pressure distribution only locally is given in terms of boundary layer concepts. A simple method for estimating along a mean streamline entrained into the underside of a growing shear layer through the transition region is discussed, and predictions of bubble bursting conditions and a lower bound on the bubble length are compared with experiment.

Russell, J. M.↗
NASA NTRS1979

On the determination of diffusion lengths by means of angle-lapped p-n junctions

A standard procedure for determining the minority carrier diffusion length by means of SEM consists of scanning an angle-lapped surface of a p-n junction and measuring the resulting short circuit current as a function of beam position. The present paper points out that the usual expression linking the short circuit current induced by the electron beam to the angle between the semiconductor surface and the junction plane is incorrect. The correct expression is discussed and it is noted that, for angles less than 10 deg, the new and the old expression are practically indistinguishable.

Von Roos, O.↗
NASA NTRS1979

The dynamical halo and the variation of cosmic-ray path length with energy

It is shown that the dynamical halo model offers a natural explanation for the form of the variation of the cosmic-ray path length with energy. The variation above about 1 GeV per nucleon can be understood as due to the variation of the diffusion coefficient, and hence the resident time, with energy. The flattening of the curve below 1 GeV per nucleon is seen to mark a transition to a convection-dominated regime where the diffusion coefficient is no longer the determining parameter. A fit to the observations yields a halo outflow velocity of 8 km/s. An attempt is made to determine the overall scale of the halo and the diffusion coefficient using recent Be-10 flux measurements, but the data do not agree well enough to pin down these variables to within less than four or five orders of magnitude.

Jones, F. C.↗
NASA NTRS1980

Optimal periodic binary codes of lengths 28 to 64

Results from computer searches performed to find repeated binary phase coded waveforms with optimal periodic autocorrelation functions are discussed. The best results for lengths 28 to 64 are given. The code features of major concern are where (1) the peak sidelobe in the autocorrelation function is small and (2) the sum of the squares of the sidelobes in the autocorrelation function is small.

Tyler, S.↗
NASA NTRS1981

The effect of elastomer chain length on properties of silicone-modified polyimide adhesives

A series of polyimides containing silicone elastomers was synthesized in order to study the effects of the elastomer chain length on polymer properties. The elastomer with repeat units varying from n=10 to 105 was chemically reacted into the backbone of an addition polyimide oligomer via reactive aromatic amine groups. Glass transition temperatures of the elastomer and polyimide phases were observed by torsional braid analysis. The elastomer-modified polyimides were tested as adhesives for bonding titanium in order to determine their potential for aerospace applications. Adhesive lap shear tests were performed before and after aging bonded specimens at elevated temperatures.

St.clair, A. K.↗
NASA NTRS1980

Accommodation of diverse length scales in general viscous flows

A numerical method developed with the primary aim of relieving computational requirements for high Reynolds number general viscous flows was found to also alleviate the cause of several solution inaccuracies. The approach is tested and calibrated through the solution of a number of flow problems involving appreciable regions of separation. It is concluded on the basis of the analyses and numerical examples presented that the accommodation of diverse length scales in general viscous flows yields superior solution efficiency and accuracy.

Wu, J. C.↗
NASA NTRS1980

Minority carrier diffusion lengths and absorption coefficients in silicon sheet material

Most of the methods which have been developed for the measurement of the minority carrier diffusion length of silicon wafers require that the material have either a Schottky or an ohmic contact. The surface photovoltage (SPV) technique is an exception. The SPV technique could, therefore, become a valuable diagnostic tool in connection with current efforts to develop low-cost processes for the production of solar cells. The technique depends on a knowledge of the optical absorption coefficient. The considered investigation is concerned with a reevaluation of the absorption coefficient as a function of silicon processing. A comparison of absorption coefficient values showed these values to be relatively consistent from sample to sample, and independent of the sample growth method.

Dumas, K. A.↗
NASA NTRS1981

Green's function for cracks of equal length from a circular hole in an infinite sheet

Stress-intensity factors are obtained for twin collinear cracks emanating from a hole in an infinite plate. Point forces that tend to make the crack surfaces separate and slide are applied to one of the cracks. A conformal mapping and the Muskelishvili formulation involving series representation of the Kolosov stress functions are used to derive the solutions. The variation of stress-intensity factor with crack length and with point of application of the loads is presented for both crack tips.

Shivakumar, V.↗
NASA NTRS1983

Three-dimensional turbulent-mixing-length modeling for discrete-hole coolant injection into a crossflow

Three dimensional mixing length models of a flow field immediately downstream of coolant injection through a discrete circular hole at a 30 deg angle into a crossflow were derived from the measurements of turbulence intensity. To verify their effectiveness, the models were used to estimate the anisotropic turbulent effects in a simplified theoretical and numerical analysis to compute the velocity and temperature fields. With small coolant injection mass flow rate and constant surface temperature, numerical results of the local crossflow streamwise velocity component and surface heat transfer rate are consistent with the velocity measurement and the surface film cooling effectiveness distributions reported in previous studies.

Wang, C. R.↗
NASA NTRS1982

Finite-length solutions for rotodynamic coefficients of turbulent annular seals

A finite-length solution procedure is developed for perturbation equations which are based on Hirs' (1973) turbulent lubrication model. The equations apply to small motions about a centered position and include the influence of swirl and changes in Reynolds number due to perturbations in clearances. Numerical results are presented for a range of L/D ratios, with and without swirl. For zero swirl, changes in the L/D ratios show results which are similar to those obtained by Black and Jenssen (1970), but when L/D = 1, differences of about 15 percent appear. The results including swirl give physically insupportable results at small L/D ratios, such as a negative cross-coupled stiffness coefficient at L/D = 0.2. This result demonstrates that the complete Hirs turbulence model is not suitable for short seals with significant swirling flow.

Childs, D. W.↗
NASA NTRS1982

Minority carrier diffusion length measurements - A review and comparison of techniques

A review of the major techniques for measuring minority carrier diffusion lengths in solar cells is given. Limits of applicability are indicated for each method, as relevant to indirect bandgap materials such as silicon, and direct bandgap materials like gallium arsenide. A discussion and bibliography is presented for the following techniques: uniform generation, electron beam induced current, Schottky barrier (and other steady-state) photocurrent, surface photovoltage, transient decay, and luminescence.

Khan, A. A.↗
NASA NTRS1982

The effect of space charge fields due to finite length electron beams in the free-electron laser

The space charge electric field of a finite length electron beam in the free electron laser amplifier with a tapered wiggler is analyzed. In the free drift region between the accelerator and laser, expressions for the increase of energy spread due to the self field are presented. In the FEL interaction region, the general conditions on the importance of the self electric field in the equations of motion is obtained. A numerical example of the FEL experiment at 10.6 microns is given.

Tang, C.-M.↗