Search NASA⌕ Search

SEARCH · Search NASA

Results for “Monte Carlo Methods”

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.

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

At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Thermal radiation model for solid rocket booster plumes

The Monte Carlo method is used to model the thermal radiation field of the plumes for the dual solid rocket boosters astride the Space Shuttle launch configuration. The model accounts for axial and radial variations in radiative properties of the plumes. The plumes are considered to be composed of a dispersion of aluminum oxide (Al2O3) particles immersed in the gaseous products of combustion. The principal emitting gases are taken to be CO, CO2, H2O, and HCl. The thermal model is based on local thermodynamic equilibrium. Scattering of radiant energy by Al2O3 particles may be treated as isotropic or anisotropic. Sample radiant heating rates to the base region of the Space Shuttle are shown. Space Shuttle geometries are simulated as combinations of quadric surfaces.

Watson, G. H.↗
NASA NTRS1977

Metastable states of small rare gas crystallites

Metastable states of rare gas crystallites containing N atoms are investigated for N = 5,6,7, and 8. In particular, the stability, structures, structural transformation, and binding energy versus temperature are determined using a Monte Carlo method. The square pyramid isomer for N = 5 is found to be unstable at any finite temperature. The other metastable isomers are all found to make spontaneous transitions to the ground state if the temperature is greater than about one half that of meltings. Comparisons with previous work are also made

Etters, R. D.↗
NASA NTRS1978

One-dimensional outgassing problem

A predominantly numerical study, using the direct-simulation Monte Carlo method, of the flow from outgassing surfaces with plane, cylindrical, or spherical geometry is presented. The numerical results for the plane flow case are compared with existing theoretical and experimental results and also with a new formulation of the jump conditions across the Knudsen layer between the surface and the continuum outflow. This outflow is sonic for the free expansion case but subsonic in the presence of back-pressure effects. The self-scattered return flux is 19% of the outgassed flux in the sonic outflow case and increases at lower Mach numbers. Thermal accomodation and reflection of this return flux produces an outgassing cooling effect. The cylindrical and spherical geometries introduce an additional Knudsen number defined by the ratio of the mean free path in the effusing gas to the radius. The plane flow results apply when this Knudsen number is less than 0.001 and, when it is of order unity or above, the return flux is inversely proportional to it.

Tran Cong, T.↗
NASA NTRS1978

Remote sensing of optical properties in continuously stratified waters

The radiative transfer equation is solved by Monte Carlo methods for natural waters in which the optical properties are distributed with depth. It is demonstrated that interpreting the reflectance of a continuously stratified ocean in terms of an equivalent homogeneous ocean yields the average of a particular combination of the water's optical properties over the dimensionless penetration depth. Although in general the dimensionless penetration depth cannot be remotely measured, a method is presented for estimating the actual penetration depth from the remote observations if the medium's absorption coefficient is known, independent of depth, and sufficiently large. The application of this to the remote measurement of the vertical distribution of suspended sediments is discussed in detail.

Gordon, H. R.↗
NASA NTRS1978

The effect of finite geometry on the three-dimensional transfer of solar irradiance in clouds

Results are presented for a Monte Carlo model applied to a wide range of cloud widths and heights, and for an analytical model restricted in its application to cuboidally shaped clouds whose length, breadth, and depth may be varied independently; the clouds must be internally homogeneous with respect to their intrinsic radiative properties. Comparative results from the Monte Carlo method and the derived analytical model are presented for a wide range of cloud sizes, with special emphasis on the effects of varying the single scatter albedo, the solar zenith angle, and the scattering phase angle.

Davies, R.↗
NASA NTRS1980

Monte Carlo variance reduction

Computer program incorporates technique that reduces variance of forward Monte Carlo method for given amount of computer time in determining radiation environment in complex organic and inorganic systems exposed to significant amounts of radiation.

Byrn, N. R.↗
NASA NTRS1979

Electron impact on atmospheric gases. III - Spatial yield spectra for N2

Spatial yield spectra have been calculated for electron energy degradation into molecular nitrogen gas using a Monte Carlo method for 0.1- to 5.0-keV incident electrons. The spectra contain the spatial yield information about the electron degradation process and can be employed to calculate a 'yield' for any inelastic state at any position in the medium. Because of the spectrum's useful nature and simple characteristics, the three-variable spatial yield spectrum U(E, z, E sub zero) is analytically represented as well. This analytic form can then be easily applied to atmospheric and laboratory problems dealing with energetic electron degradation.

Jackman, C. H.↗
NASA NTRS1979

Solution of Boltzmann equation for highly nonequilibrium diatomic gases rotational translational energy relaxation

The direct simulation Monte Carlo method is applied to solve the Boltzmann equation for collisions between internally excited diatomic gases in highly nonequilibrium states. The semiclassical transition probability is incorporated in the simulation for energy exchange between rotational and translational energy. The results provide details on the fundamental mechanisms of gas kinetics where analytical methods are impractical. The validity of the local Maxwellian assumption and relaxation time, rotational-translational energy transition, and a velocity analysis of the inelastic collision are discussed in detail.

Yoshikawa, K. K.↗
NASA NTRS1981

Electromagnetic cascades in pulsars

The development of pair photon cascades initiated by high energy electrons above a pulsar polar cap is simulated numerically. The calculation uses the energy of the primary electron, the magnetic field strength, and the period of rotation as parameters and follows the curvature radiation emitted by the primary, the conversion of this radiation e(+) - e(-) pairs in the intense fields, and the quantized synchrotron radiation by the secondary pairs. A recursive technique allows the tracing of an indefinite number of generations using a Monte Carlo method. Gamma ray and pair spectra are calculated for cascades in different parts of the polar cap and with different acceleration models. It is found that synchrotron radiation from secondary pairs makes an important contribution to the gamma ray spectrum above 25 MeV, and that the final gamma ray and pair spectra are insensitive to the height of the accelerating region, as long as the acceleration of the primary electrons is not limited by radiation reaction.

Daugherty, J. K.↗
NASA NTRS1981

Spacecraft outgas ambient flow interaction

An investigation has been made of the extent to which outgassed or emitted molecules return to a spacecraft as a result of intermolecular collisions with ambient freestream molecules. The governing parameters are discussed and heuristic arguments are used to predict the combination of parameters that is most likely to describe the return flux ratio. This is then tested by an extensive set of numerical calculations using the direct simulation Monte Carlo method. Computations have been made for the flow past spheres and past circular cylinders with their axes normal in the stream. The numerical results lead to empirical expressions that can be used for engineering estimates in spacecraft contamination studies.

Bird, G. A.↗
NASA NTRS1981

Spatial aspects of electron energy degradation in atomic oxygen

Spatial (radial and longitudinal) yield spectra for electron energy degradation in atomic oxygen have been obtained using a Monte Carlo method for 25 eV to 10 keV incident electrons. Four-dimensional yield spectra have been analytically represented in terms of a model containing three simple microplumes. We find that the scaled spatial yield spectra for O is approximately the same as for N2. This feature provides a basis for inferring yield spectra for any atmosphere gas or mixture of gases.

Singhal, R. P.↗
NASA NTRS1981

A radiometric model of an earth radiation budget radiometer optical system with diffuse-specular surfaces

The Earth Radiation Budget Experiment (ERBE) is to fly on NASA's Earth Radiation Budget Satellite (ERBS) and on NOAA F and NOAA G. Large spatial scale earth energy budget data will be derived primarily from measurements made by the ERBE nonscanning instrument (ERBE-NS). A description is given of a mathematical model capable of simulating the radiometric response of any of the ERBE-NS earth viewing channels. The model uses a Monte Carlo method to accurately account for directional distributions of emission and reflection from optical surfaces which are neither strictly diffuse nor strictly specular. The model computes radiation exchange factors among optical system components, and determines the distribution in the optical system of energy from an outside source. Attention is also given to an approach for implementing the model and results obtained from the implementation.

Luther, M. R.↗
NASA NTRS1976

CTRANS: A Monte Carlo program for radiative transfer in plane parallel atmospheres with imbedded finite clouds: Development, testing and user's guide

The program called CTRANS is described which was designed to perform radiative transfer computations in an atmosphere with horizontal inhomogeneities (clouds). Since the atmosphere-ground system was to be richly detailed, the Monte Carlo method was employed. This means that results are obtained through direct modeling of the physical process of radiative transport. The effects of atmopheric or ground albedo pattern detail are essentially built up from their impact upon the transport of individual photons. The CTRANS program actually tracks the photons backwards through the atmosphere, initiating them at a receiver and following them backwards along their path to the Sun. The pattern of incident photons generated through backwards tracking automatically reflects the importance to the receiver of each region of the sky. Further, through backwards tracking, the impact of the finite field of view of the receiver and variations in its response over the field of view can be directly simulated.

Source record↗
NASA NTRS1982

Sensitivity analysis as an aid in modelling and control of (poorly-defined) ecological systems

A literature review of the use of sensitivity analyses in modelling nonlinear, ill-defined systems, such as ecological interactions is presented. Discussions of previous work, and a proposed scheme for generalized sensitivity analysis applicable to ill-defined systems are included. This scheme considers classes of mathematical models, problem-defining behavior, analysis procedures (especially the use of Monte-Carlo methods), sensitivity ranking of parameters, and extension to control system design.

Hornberger, G. M.↗
NASA NTRS1982

Electromagnetic cascades in pulsars

The development of pair-photon cascades initiated by high energy electrons above a pulsar polar cap is simulated numerically. The calculation uses the energy of the primary electron, the magnetic field strength, and the period of rotation as parameters, and follows the curvature radiation emitted by the primary, the conversion of this radiation to e(+)e(-) pairs in the intense fields, and the quantized synchrotron radiation by the secondary pairs. A recursive technique allows the tracing of an indefinite number of generations using a Monte Carlo method. Gamma-ray and pair spectra are calculated for cascades in different parts of the polar cap and with different acceleration models. It is found that synchrotron radiation from secondary pairs makes an important contribution to the gamma-ray spectrum above 25 MeV, and that the final gamma-ray and pair spectra are insensitive to the height of the accelerating region, as long as the acceleration of the primary electrons is not limited by radiation reaction.

Daugherty, J. K.↗
NASA NTRS1981

The predicted deviation from the Z-sq dependence of the ion chamber and the Cerenkov counter response on the HEAO-3 experiment

The influence of the Mott cross section in producing a departure from a Z-sq dependence in the Heavy Nuclei Experiment instrument on board the HEAO-3 satellite is examined, along with the related problem of secondary electron energy transport through the instrument. A Monte Carlo method of electron transport and a modification of the Laulainen and Bichsel (1972) method were used to calculate energy deposition in the chamber, and corrections to the Cerenkov counter response were made. A maximum correction of a change of Z values of a -4 at Z equals 78 was found, and further calculations to determine the Mott cross sections for Z between 26-50 are indicated.

Derrickson, J. H.↗
NASA NTRS1982

Lidar clear atmosphere multiple scattering dependence on receiver range

The degree of multiple scattering encountered by nadir-directed lidar in clear atmosphere conditions was assessed by the Monte Carlo method based on a model of the vertical distribution of aerosol scattering in the atmosphere. The lowest 3 km of the atmosphere were regarded as containing 90% of the aerosol optical thickness, with the unity normalized value for the forward peak of the aerosol phase function being 4.38 at a 694.3 nm laser wavelength. Results were obtained for two lidar receiver heights for three receiver field of view (FOV) angular halfwidths. An increase in receiver height was determined to cause a significant increase in the amount of angular scattering of the lidar signal. A factor of 20 change in receiver height produced an order of magnitude change in the single to multiple scattering ratio.

Spinhirne, J. D.↗