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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Dynamics of narrow rings

The ring models described here were developed to account for the dynamical problems posed by the narrow rings of Uranus. Some of these rings are now known to be eccentric, inclined, nonuniform in width, optically thick, and narrow, with very sharp edges. The eccentric rings have common pericenters and large, positive eccentricity gradients. The theory of shepherding satellites successfully accounts for most of these features and can also account for some features of the narrow Saturnian rings, in particular, waves, kinks, and periodic variations in brightness. Outstanding problems include the putative relation between eccentricity and inclination displayed by eight of the nine Uranian rings, and the magnitudes of the tidal torques acting on the shepherding satellites. The horseshoe-orbit model, although viable, probably has more application to the narrow rings from which the Saturnian coorbital satellites formed. The angular momentum flow rate due to particle collisions is a minimum at the Lagrangian equilibrium points L(4) and L(5), and one can expect accretion to be rapid at these points.

Dermott, S. F.↗
NASA NTRS1986

Computer simulation of a wind tunnel test section with discrete finite-length wall slots

A computer simulation of a slotted wind tunnel test section which includes a discrete, finite-length wall slot representation with plenum chamber constraints and accounts for the nonlinear effects of the dynamic pressure of the slot outflow jet and of the low energy of slot inflow air was developed. The simulation features were selected to be those appropriate for the intended subsequent use of the simulation in a wall interference assessment procedure using sparsely located wall pressure measurements. It is demonstrated that accounting for slot discreteness is important in interpreting wall pressure measured between slots, and that accounting for nonlinear slot flow effects produces significant changes in tunnel-induced velocity distributions and, in particular, produces a longitudinal component of tunnel-induced velocity due to model lift. A characteristic mode of tunnel flow interaction with constraints imposed by the plenum chamber and diffuser entrance is apparent in simulation results and is derived analytically through a simplified analysis.

Kemp, W. B., Jr.↗
NASA NTRS1985

A discrete control model of PLANT

A model of the PLANT system using the discrete control modeling techniques developed by Miller is described. Discrete control models attempt to represent in a mathematical form how a human operator might decompose a complex system into simpler parts and how the control actions and system configuration are coordinated so that acceptable overall system performance is achieved. Basic questions include knowledge representation, information flow, and decision making in complex systems. The structure of the model is a general hierarchical/heterarchical scheme which structurally accounts for coordination and dynamic focus of attention. Mathematically, the discrete control model is defined in terms of a network of finite state systems. Specifically, the discrete control model accounts for how specific control actions are selected from information about the controlled system, the environment, and the context of the situation. The objective is to provide a plausible and empirically testable accounting and, if possible, explanation of control behavior.

Mitchell, C. M.↗
NASA NTRS1986

Calculations of cosmogenic nuclide production rates in the Earth's atmosphere and their inventories

The production rates of cosmogenic isotopes in the Earth's atmosphere and their resulting terrestrial abundances have been calculated, taking into account both geomagnetic and solar-modulatory effects. The local interstellar flux was assumed to be that of Garcia-Munoz, et al. Solar modulation was accounted for using the heliocentric potential model and expressed in terms of the Deep River neutron monitor count rates. The geomagnetic field was presented by vertical cutoffs calculated by Shea and Smart and the non-vertical cutoffs calculated using ANGRI. The local interstellar particle flux was first modulated using the heliocentric potential field. The modulated cosmic-ray fluxes reaching the earth's orbit then interacted with the geomagnetic field as though it were a high-pass filter. The interaction of the cosmic radiation with the Earth's atmosphere was calculated utilizing the Bolztmann transport equation. Spallation cross sections for isotope production were calculated using the formalism of Silberberg and Tsao and other cross sections were taken from standard sources. Inventories were calculated by accounting from the variation in solar modulation and geomagnetic field strength with time. Results for many isotope, including C-14, Be-7 and Be-10 are in generally good agreement with existing data. The C-14 inventory, for instance, amounts to 1.75/sq cm(e)/s, in excellent agreement with direct estimates.

Obrien, K.↗
NASA NTRS1984

Characterization of sprays

It is pointed out that most practical power generation and propulsion systems involve the burning of different types of fuel sprays, taking into account aircraft propulsion, industrial furnaces, boilers, gas turbines, and diesel engines. There has been a lack of data which can serve as a basis for spray model development and validation. A major aim of the present investigation is to fill this gap. Experimental apparatus and techniques for studying the characteristics of fuel sprays are discussed, taking into account two-dimensional still photography, cinematography, holography, a laser diffraction particle sizer, and a laser anemometer. The considered instruments were used in a number of experiments, taking into account three different types of fuel spray. Attention is given to liquid fuel sprays, high pressure pulsed diesel sprays, and coal-water slurry sprays.

Chigier, N.↗
NASA NTRS1986

D to H ratio and the origin and evolution of Titan's atmosphere

Potential causative factors for the unusually large ratio of CH3D to CH4 reported in the stratosphere of Titan (6 x 10 to the -4th) compared to that of Saturn (8.7 x 10 to the -5th) or Jupiter (6.7 x 10 to the -5th) are examined. With uncertainty in the observations and data reduction taken into account, an atmospheric enrichment factor of three or more is calculated for Titan relative to the planets. This enrichment factor is seen to be accounted for by condensation to form tropospheric methane clouds, fractionation occurring over a hypothetical CH4-C2H6 ocean and between the ocean and the clathrate crust beneath, fractionation which occurred during the formation of Titan, and fractionation as a result of the evolution of Titan's atmosphere. It is concluded that the greater part of the observed fractionation is probably derived from the last two of these mechanisms. The mechanisms studied do not account for the observed CH3D/CH4 value of 1.7 x 10 to the -3rd obtained by Kim and Caldwell (1982).

Pinto, J. P.↗
NASA NTRS1986

Heat transport in steep temperature gradients. I - Small flaring solar loops

Results on nonlocal heat transport which properly takes into account the presence of fast electrons with mean free paths much longer than the temperature scale height L are reviewed. In terms of the mean free path for the slow bulk electrons, lambda(s), the nonlocal effects are important whenever lambda(s)/L greater than 0.001, with the following consequences. The heat flux in the hot part of the gradient is reduced relative to the Spitzer-Haerm value q(SH) which does not take into account the heat carried away by the fast electrons. The heat flux in the cold part of the gradient is enhanced relative to the value q(SH) which does not take into account the heat deposited by the fast electrons. These quite general results, which should have several applications in astrophysics, are applied to the problem of thermal hard X-ray burst models. It is shown that heat is not bottled up as effectively as in some past models, and temperatures achieved for realistic energy input rates are consequently not as high. As a result such sources can be effective only in the soft part (10-30 keV) of the hard X-ray range for energy input rates up to 6,400 ergs/cu cm s. The analysis is based on a fluid model and does not consider the X-ray signature of fast electrons which escape to distances far beyond the conduction fronts formed. It is shown that such electrons could at most be effective in the soft part of the hard X-ray range.

Smith, D. F.↗
NASA NTRS1987

Wall jet analysis for circulation control aerodynamics. Part 1: Fundamental CFD and turbulence modeling concepts

An overview of parabolic and PNS (Parabolized Navier-Stokes) methodology developed to treat highly curved sub and supersonic wall jets is presented. The fundamental data base to which these models were applied is discussed in detail. The analysis of strong curvature effects was found to require a semi-elliptic extension of the parabolic modeling to account for turbulent contributions to the normal pressure variations, as well as an extension to the turbulence models utilized, to account for the highly enhanced mixing rates observed in situations with large convex curvature. A noniterative, pressure split procedure is shown to extend parabolic models to account for such normal pressure variations in an efficient manner, requiring minimal additional run time over a standard parabolic approach. A new PNS methodology is presented to solve this problem which extends parabolic methodology via the addition of a characteristic base wave solver. Applications of this approach to analyze the interaction of wave and turbulence processes in wall jets is presented.

Dash, S. M.↗
NASA NTRS1987

A Numerical Model of Unsteady, Subsonic Aeroelastic Behavior

A method for predicting unsteady, subsonic aeroelastic responses was developed. The technique accounts for aerodynamic nonlinearities associated with angles of attack, vortex-dominated flow, static deformations, and unsteady behavior. The fluid and the wing together are treated as a single dynamical system, and the equations of motion for the structure and flow field are integrated simultaneously and interactively in the time domain. The method employs an iterative scheme based on a predictor-corrector technique. The aerodynamic loads are computed by the general unsteady vortex-lattice method and are determined simultaneously with the motion of the wing. Because the unsteady vortex-lattice method predicts the wake as part of the solution, the history of the motion is taken into account; hysteresis is predicted. Two models are used to demonstrate the technique: a rigid wing on an elastic support experiencing plunge and pitch about the elastic axis, and an elastic wing rigidly supported at the root chord experiencing spanwise bending and twisting. The method can be readily extended to account for structural nonlinearities and/or substitute aerodynamic load models. The time domain solution coupled with the unsteady vortex-lattice method provides the capability of graphically depicting wing and wake motion.

Strganac, Thomas W.↗
NASA NTRS1987

A substructure technique for dynamics of flexible mechanical systems with contact-impact

A substructure synthesis method is proposed to account for contact-impact effects in flexible components of mechanical systems. Components that may come into contact is divided into substructures, on each of which local deformation modes are defined to described deformation fields of components. Constraint modes and fixed interface normal modes are used to account for elastic deformation within each substructure. A constraint addition-deletion technique is used to account for contact between impacting bodies. Lagrange multipliers associated with the constraints, which represent constraint forces, are used to determine separation of contacting nodes. Use of the method is illustrated on problems of longitudinal and transverse impact of bodies.

Wu, S.-C.↗
NASA NTRS1988

The cometary and asteroidal impactor flux at the earth

The cratering records on the Earth and the lunar maria provide upper limits on the total impactor flux at the Earth's orbit over the past 600 Myr and the past 3.3 Gyr, respectively. These limits can be compared with estimates of the expected cratering rate from observed comets and asteroids in Earth-crossing orbits, corrected for observational selection effects and incompleteness, and including expected temporal variations in the impactor flux. Both estimates can also be used to calculate the probability of large impacts which may result in biological extinction events on the Earth. The estimated cratering rate on the Earth for craters greater than 10 km-diameter, based on counted craters on dated surfaces is 2.2 + or - 1.1 x 10 to the minus 14th power km(-2) yr(-1) (Shoemaker et al., 1979). Using a revised mass distribution for cometary nuclei based on the results of the spacecraft flybys of Comet Halley in 1986, and other refinements in the estimate of the cometary flux in the terrestrial planets zone, it is now estimated that long-period comets account for 11 percent of the cratering on the Earth (scaled to the estimate above), and short-period comets account for 4 pct (Weissman, 1987). However, the greatest contribution is from large but infrequent, random cometary showers, accounting for 22 pct of the terrestrial cratering.

Weissman, Paul R.↗
NASA NTRS1989

Navier-Stokes analysis of cold scramjet-afterbody flows

The progress of two efforts in coding solutions of Navier-Stokes equations is summarized. The first effort concerns a 3-D space marching parabolized Navier-Stokes (PNS) code being modified to compute the supersonic mixing flow through an internal/external expansion nozzle with multicomponent gases. The 3-D PNS equations, coupled with a set of species continuity equations, are solved using an implicit finite difference scheme. The completed work is summarized and includes code modifications for four chemical species, computing the flow upstream of the upper cowl for a theoretical air mixture, developing an initial plane solution for the inner nozzle region, and computing the flow inside the nozzle for both a N2/O2 mixture and a Freon-12/Ar mixture, and plotting density-pressure contours for the inner nozzle region. The second effort concerns a full Navier-Stokes code. The species continuity equations account for the diffusion of multiple gases. This 3-D explicit afterbody code has the ability to use high order numerical integration schemes such as the 4th order MacCormack, and the Gottlieb-MacCormack schemes. Changes to the work are listed and include, but are not limited to: (1) internal/external flow capability; (2) new treatments of the cowl wall boundary conditions and relaxed computations around the cowl region and cowl tip; (3) the entering of the thermodynamic and transport properties of Freon-12, Ar, O, and N; (4) modification to the Baldwin-Lomax turbulence model to account for turbulent eddies generated by cowl walls inside and external to the nozzle; and (5) adopting a relaxation formula to account for the turbulence in the mixing shear layer.

Baysal, Oktay↗
NASA NTRS1988

Yarkovsky thermal drag on LAGEOS

Based on a thermal model including the radiative heat transfer between the retroreflector, its mounting rings, and the aluminum cavity in which it sits, it is estimated that thermal drag accounts for about 70 percent of the observed average drag on the LAGEOS satellite. It is found that neutral particle drag accounts for about 13 percent of the average drag, with charged particle drag accounting for at least 5 percent. It is suggested that the remaining 12 percent is probably due to charged particle drag as well.

Rubincam, David Parry↗
NASA NTRS1989

The infrared emission bands. II - A spatial and spectral study of the Orion bar

The 3-13 micron emission of the Orion Bar is spectroscopically and spatially studied. There are three emission components, one from 'classical' dust that accounts for the bulk of the emission longward of 20 microns, a second one from large amorphous carbon grains or polycyclic aromatic hydrocarbon (PAH) clusters accounting for the broad features, and a third from PAH molecules that accounts for the sharp bands. The 3.3 and 11.3 micron features, which are due to C-H modes, are well correlated spatially, while the 7.7 micron band, due to C=C modes, has a different distribution than the 3.3 and 11.1 micron bands. It is concluded that the sharp emission bands arise in the photodissociation transition region between the H II region and the molecular cloud and are not present in the H II region. The broad continuum feature from 11-13 microns is strong in both regions.

Bregman, J. D.↗
NASA NTRS1989

A computationally efficient modelling of laminar separation bubbles

In order to predict the aerodynamic characteristics of airfoils operating at low Reynolds numbers, it is necessary to accurately account for the effects of laminar (transitional) separation bubbles. Generally, the greatest difficulty comes about when attempting to determine the increase in profile drag that results from the presence of separation bubbles. While a number of empirically based separation bubble models have been introduced in the past, the majority assume that the bubble development is fully predictable from upstream conditions. One way of accounting for laminar separation bubbles in airfoil design is the bubble analog used in the design and analysis program of Eppler and Somers. A locally interactive separation bubble model was developed and incorporated into the Eppler and Somers program. Although unable to account for strong interactions such as the large reduction in suction peak sometimes caused by leading edge bubbles, it is able to predict the increase in drag and the local alteration of the airfoil pressure distribution that is caused by bubbles occurring in the operational range which is of most interest.

Dini, Paolo↗
NASA NTRS1990

Igniter adapter-to-igniter chamber deflection test

Testing was performed to determine the maximum RSRM igniter adapter-to-igniter chamber joint deflection at the crown of the inner joint primary seal. The deflection data was gathered to support igniter inner joint gasket resiliency predictions which led to launch commit criteria temperature determinations. The proximity (deflection) gage holes for the first test (Test No. 1) were incorrectly located; therefore, the test was declared a non-test. Prior to Test No. 2, test article configuration was modified with the correct proximity gage locations. Deflection data were successfully acquired during Test No. 2. However, the proximity gage deflection measurements were adversely affected by temperature increases. Deflections measured after the temperature rise at the proximity gages were considered unreliable. An analysis was performed to predict the maximum deflections based on the reliable data measured before the detectable temperature rise. Deflections to the primary seal crown location were adjusted to correspond to the time of maximum expected operating pressure (2,159 psi) to account for proximity gage bias, and to account for maximum attach and special bolt relaxation. The maximum joint deflection for the igniter inner joint at the crown of the primary seal, accounting for all significant correction factors, was 0.0031 in. (3.1 mil). Since the predicted (0.003 in.) and tested maximum deflection values were sufficiently close, the launch commit criteria was not changed as a result of this test. Data from this test should be used to determine if the igniter inner joint gasket seals are capable of maintaining sealing capability at a joint displacement of (1.4) x (0.0031 in.) = 0.00434 inches. Additional testing should be performed to increase the database on igniter deflections and address launch commit criteria temperatures.

Cook, M.↗
NASA NTRS1990

Drag on the Lageos satellite

The drag data on the Lageos satellite collected between 1976 and 1987 are analyzed, and the cause of fluctuations observed at times when the orbit intersects the earth's shadow is investigated. The average drag on the Lageos consists of three types of drag: the Yarkovsky thermal drag, which accounts for about 70 percent of the observed drag; the neutral particle drug, accounting for about 14 percent of the drag for a hydrogen number density of 5 x 10 to the 9th/cu m; and the charged particle drag, which accounts for 12 percent of average drag at a number density of 3 x 10 to the 9th and a temperature of 5000 K in the plasmasphere for a satellite potential of -1 V. The observed drag agrees with the particle environment at the Lageos altitude and the drag models. However, the fluctuations in Lageos's along-track acceleration when the orbit intersects the earth's shadow are still not well understood.

Rubincam, David Parry↗
NASA NTRS1990

Non-grey radiation in a liquid rocket combustion chamber

The effects of radiation on droplets gasification in liquid rocket combustion chambers has been studied. The modeling includes a Legendre pseudo-spectral collocation approximation to solve the Radiative Transfer Equation (RTE). The band model has been utilized to account for non-grey emitting and absorbing gases present in the comustion chamber. The GEMCHIP II code has been utilized to study the fuel and oxidizer droplets combustion and interaction. The submodels within this code are capable of accounting for group combustion and conjugate effects between many droplets. The radiative model has been coupled with the GEMCHIP II code accounting for radial effects only, to provide the tool for studying the combustion-radiation coupling effects in a bipropellant system. The results indicate that the gasification/combustion process will be enhanced upstream of the chamber causing thicker flame sheet and associated higher combustion efficiency.

Kehtarnavaz, H.↗