Showing papers on "Point source published in 1992"

Dispersion and Deposition of Spherical Particles from Point Sources in a Turbulent Channel Flow

Abstract: The dispersion and deposition of particles from a point source in a turbulent channel flow are studied. An empirical mean velocity profile and the experimental data for turbulent intensities are used in the analysis. The instantaneous turbulence fluctuation is simulated as a continuous Gaussian random field, and an ensemble of particle trajectories is generated and statistically analyzed. A series of digital simulations for dispersion and deposition of aerosol particles of various sizes from point sources at different positions from the wall is performed. Effects of Brownian diffusion on particle dispersion are studied. The effects of variation in particle density and particle-surface interaction are also discussed.

A model of the 8-25 micron point source infrared sky

Abstract: We present a detailed model for the IR point-source sky that comprises geometrically and physically realistic representations of the Galactic disk, bulge, stellar halo, spiral arms (including the 'local arm'), molecular ring, and the extragalactic sky. We represent each of the distinct Galactic components by up to 87 types of Galactic source, each fully characterized by scale heights, space densities, and absolute magnitudes at BVJHK, 12, and 25 microns. The model is guided by a parallel Monte Carlo simulation of the Galaxy at 12 microns. The content of our Galactic source table constitutes a good match to the 12 micron luminosity function in the simulation, as well as to the luminosity functions at V and K. We are able to produce differential and cumulative IR source counts for any bandpass lying fully within the IRAS Low-Resolution Spectrometer's range (7.7-22.7 microns as well as for the IRAS 12 and 25 micron bands. These source counts match the IRAS observations well. The model can be used to predict the character of the point source sky expected for observations from IR space experiments.

Determining surface orientations of specular surfaces by using the photometric stereo method

Abstract: The orientation of patches on the surface of an object can be determined from multiple images taken with different illumination, but from the same viewing position. The method, referred to as photometric stereo, can be implemented using table lookup based on numerical inversion of reflectance maps. Here we concentrate on objects with specularly reflecting surfaces, since these are of importance in industrial applications. Previous methods, intended for diffusely reflecting surfaces, employed point source illumination, which is quite unsuitable in this case. Instead, we use a distributed light source obtained by uneven illumination of a diffusely reflecting planar surface. Experimental results are shown to verify analytic expressions obtained for a method employing three light source distributions.

Viscous flows down an inclined plane from point and line sources

Abstract: The flow of a viscous fluid from a point or line source on an inclined plane is analysed using the equations of lubrication theory in which surface tension is neglected. At short times, when the gradient of the interfacial thickness is much greater than that of the plane, the fluid is shown to spread symmetrically from the source, as on a horizontal plane. At long times, the flow is predominantly downslope, with some cross-slope spreading for the case of a point source. Similarity solutions for the long-time behaviour of the governing nonlinear partial differential equations are found for the case in which the volume of fluid increases with time like tα, where α is a constant. The two-dimensional equations appropriate to a line source are hyperbolic in the self-similar regime and the similarity profile is found analytically to end abruptly at a downslope position which increases like t(2α+1)/3. Inclusion of higher-order terms in the analysis resolves this frontal shock into a boundary-layer structure of width comparable to the thickness of the current. Owing to the term representing cross-slope spreading, the mathematical structure of the equations is considerably more complex for flow from a point source and the similarity form is found numerically in this case. Though the downslope and cross-slope extents of the current again increase with time according to a power-law if α > 0, they also depend on a power of In t if α = 0. The leading-order near-source structure is shown to be that of steady flow from a constant-flux source of strength given by the instantaneous flow rate. For sources with α > 1, the contact line advances at all points on the perimeter of the flow and the entire plane is eventually covered by the flow; for sources with 0 < α < 1, only a portion of the contact line is advancing at any time and only that part of the plane with |y| [les ] cx3α/(4α+3) is eventually covered, where x and y are the downslope and cross-slope coordinates and c is a constant. The theoretical spreading relationships and planforms are found to be in good agreement with experimental measurements of constant-volume and constant-flux flows of viscous fluids from a point source on a plane. At very long times, however, the experimental flows are observed to be unstable to the formation of a capillary rivulet at the nose of the current.

Waveform inversion for point source moment tensor retrieval with variable hypocentral depth and structural model

Abstract: SUMMARY In the inversion scheme of high-frequency seismograms, recorded by a local network, we have developed the unconstrained moment tensor description to study weak events which are believed to reflect small-scale complexities of the tectonic structure. In order to allow the source mechanism to change with time each moment tensor component has its own time history. In the inversion, the measure of the similarity between the synthetic seismograms and the observed records is obtained by minimizing the norm L2 of their difference. The initial synthetic seismograms are constructed by summing a set of time-dependent functions, named base functions. These base functions are computed by normal mode summation for a discrete set of source depths and for two structural models, considered to represent acceptable extremes for the region under study. In the course of the inversion, using a linear interpolation, new base functions are computed for intermediate values of the starting depths and structures. Thereafter new synthetic seismograms are computed and compared with the observed records until the convergence is reached, within some pre-assigned threshold for the norm L2. The method has been tested by treating, as ‘observed’records, synthetic seismograms (both displacement and velocity) generated for instantaneous and finite duration sources. The ‘observed’ records have been computed for two structures: one belonging to the structural range within which the base functions were computed, the other lying outside this range. Random noise has been superimposed on the records. When inverting synthetic records computed for a structure contained within the range of interpolation for the base functions, the full moment tensor is retrieved satisfactorily (provided that the noise level does not exceed about 20 per cent). For records computed for a structure lying outside this range a spurious volumetric part of the moment tensor appears. However, if the volumetric part of the moment tensor is removed, the source time function can be reconstructed satisfactorily, and the source mechanism can be determined from the deviatoric part of the moment tensor. The procedure has been applied to vertical component seismograms recorded in the Friuli (NE Italy) area for the ML= 2.9 event of 1987 December 27. The retrieved source mechanism is in agreement with the distribution of the few first arrival polarities available and the source time function indicates a possible multiple rupture process.

Discrete modes and continuous spectra in supersonic boundary layers

Abstract: The disturbance field induced due to a harmonic point source consists of discrete eigenmodes and a continuous spectrum; these are studied by using generalized Fourier transform techniques. For a supersonic boundary layer, there exist seven branches of the continuous spectrum in the complex wavenumber space, four of which (two acoustic waves, one vorticity wave and one entropy wave) contribute to the flowfield downstream of the source. The discrete eigenmodes spring off from these branches at some critical Reynolds numbers. The results for Mach 2 and 4.5 boundary layers show that the receptivity coefficients for the stable discrete modes are much larger than that for the unstable mode. Therefore, the flow very near the source is dominated by the continuous spectrum and the stable discrete modes. However, the unstable mode takes over sufficiently far away from the source. It is shown that it is only necessary to consider the first few discrete modes to construct the solution. Calculations also show that, in a supersonic boundary layer, upstream influence from a localized disturbance is minimal.

Absolute calibration of the JT‐60U neutron monitors using a 252Cf neutron sourcea)

Abstract: Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gain Q in D–D operating tokamaks. The time‐resolved neutron yield is measured with 235U and 238U fission chambers and 3He proportional counters in the JT‐60U tokamak. The in situ calibration was performed by moving the 252Cf neutron source toroidally through the JT‐60 vacuum vessel. Detection efficiencies of three 235U and two 3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.

On the field-size dependence of relative output from a linear accelerator.

Abstract: The radiation output in air on the central axis of a linac photon beam has been modeled as the sum of two components. These are a point source representing radiation direct from the target and a distributed source representing scatter in the flattening filter and primary collimator. By fitting only two parameters, the ratio of the two components for a 20 x 20 field and a width parameter for the distributed source this semi-empirical model describes the relative outputs of square, symmetric rectangular, and asymmetric rectangular fields with an average error of 0.25% for the field sizes studied.

What happens near a quasi-linear point source?

Abstract: The quasi-linear continuous point source has been central to many studies of multidimensional flow in unsaturated soil. Immediately next to the source, however, the water pressure Ψ is positive and indefinitely large, so the quasi-linear flow equation (limited to Ψ ≤ 0) fails in principle. We investigate whether this is a serious problem in practice. Exact analysis would be complicated and difficult, since the problem is of mixed saturated-unsaturated flow with an unknown free boundary between the two regions. When the source strength is not too large, the saturated region is close to spherical. A simple approximate analysis follows with exterior and interior flow velocities matched on the boundary. The correction, taking into account the saturated region, shifts the quasi-linear source a small distance above the actual source location. In practical applications the correction is usually negligibly small. Physical reasons why the approximation works well and the correction is small are indicated. In addition, these physical reasons suggest that the scope of the quasi-linear analysis is wider than supposed hitherto. The solution in potential theory for the eccentric point source needed for the study is given in the appendix.

Amplitude preprocessing of single and multicomponent seismic data

Abstract: Whenever the data acquisition is restricted to line surveys rather than areal surveys, seismic processing is necessarily in two dimensions. In this paper it is argued that two-dimensional (2-D) processing is preferably applied after transforming the point source responses into line source responses. The effect of this transformation is a correction of the amplitudes in the data. For single-component acoustic data as well as for multicomponent elastic data a line source response is nothing but a superposition of point source responses. Hence, in principle a line source response can be synthesized by integrating point source responses along the desired line source axis. In practice, however, this integration cannot be carried out due to the incompleteness of the data. It is shown that the integration along the source axis can be replaced by an integration along the receiver axis. The underlying assumption is that the wavefields exhibit a certain type of cylindrical symmetry. For horizontally layered acoustic and elastic media this assumption is fully satisfied. For 2-D inhomogeneous media this assumption is approximately satisfied, provided the data are sorted in CMP gathers. Having transformed the point source responses into line source responses, the results may be considered as 'true amplitude' 2-D data. Hence, proceeding with existing 2-D seismic processing techniques is then justified.

Dose rate distributions around 60Co, 137Cs, 198Au, 192Ir, 241Am, 125I (models 6702 and 6711) brachytherapy sources and the nuclide 99Tcm.

Abstract: Simple analytical functions derived from our point source Monte Carlo calculations on the combined attenuation and scatter factor, B exp(-mu r), for 60Co, 137Cs, 198Au, 192Ir, 241Am, 125I (models 6702 and 6711) brachytherapy sources and the nuclide 99Tcm, for water spherical geometries of radii R = 15 and 20 cm, are presented. Our results for the broadly used 60Co, 137Cs, 198Au and 192Ir brachytherapy sources can be compared directly and found in excellent agreement with the widely accepted data of Meisberger et al in the limited distance range for which the latter are valid. Our data, however, can be used with high accuracy outside this distance range. Many discrepancies observed among different data sets available in recent literature are attributable to differences in geometries used. The results for the recently introduced 241Am source are very dissimilar to those produced by any other currently used brachytherapy source. Dose rate distributions, based on the above simple functions, are proposed in accordance with the recommendations for calibration of the brachytherapy sources in terms of reference air kerma rate and were found to be in good agreement with data available in the literature. Our calculations for 125I sources (models 6702 and 6711), provided that the characteristic x-rays from titanium encapsulation are taken into account, support recent experimental and theoretical dose rate distributions indicating that currently accepted values for 125I may be overestimated.

Compact radio sources near the Galactic plane

Abstract: The results of the extension of the 20-cm Galactic-plane survey reported by Zoonematkermani to Galactic latitudes of +/- 1.8 deg over the central region of the Milky Way are reported. A total of 1457 discrete radio sources down to flux densities of less than about 5 mJy, and 95 percent completion is achieved at 20 mJy. A detailed comparison of all radio sources from the survey in this longitude range with the IRAS Point Source Catalog provides classification for 13 percent of the objects, including 159 compact H II regions, and nearly 100 planetary nebulae, over 70 of which are identified. The identity of the remaining radio sources is discussed.

A Student’s Guide to Teleseismic Body Wave Amplitudes

Abstract: We discuss the nature of the various factors contributing to the amplitude of a teleseismic body wave in the context of a geometrical ray solution, specifically: the radiation of elastic waves into an elastic medium by a point source; the radiation patterns resulting from the orientation of the double-couple in space; the effect of propagation through a radially heterogeneous Earth, known as geometrical spreading; the effect of anelastic attenuation; the contribution of depth phases to the seismogram; and finally the influence of distance on the receiver response function. For each of these parameters, we emphasize the physical arguments underlying the exact algebraic expressions of the various factors contributing to the seismic amplitude. Finally, we discuss the extension of the geometrical ray solution to deep seismic sources.

High performance optical lithography using a separated light source

Abstract: In projection lithography systems with fly’s‐eye elements, a virtual source is created as an array of approximately mutually incoherent point sources. This article describes light amplitude and phase simulations for an image of a mask illuminated by a point source. This article discusses the dependence on the point‐source location for a plane perpendicular to the optical axis. An image projected by a separated light source, rather than a ring‐shaped light source, improves due to interference effects between multiple apertures is shown. Resolution of 0.25 μm equal lines and spaces was improved experimentally at a wavelength of 365 nm and a numerical aperture of 0.54. Also, how an isolated pattern image can be improved theoretically using an optimally separated light source is shown.

Waves produced from a harmonic point source in a supersonic boundary-layer flow

Abstract: The disturbance wave pattern produced by a harmonic point source in a compressible flat-plate boundary layer is computed using linear stability theory and direct numerical integration approach. Receptivity coefficients are computed for the spectrum of spanwise modes generated at the source. The effect of boundary layer growth on the development of linear waves is determined by using the method of multiple scales. Results are presented for Mach numbers of 0, 2, 4.5, and 7. It is found that disturbances spread in wedge-shaped regions behind the source and the wedge angle decreases with Mach number. The lateral spreading angle for the instability waves turns out to be quite close to the angle found experimentally for turbulence lateral contamination.

Broadband modeling of the three-dimensional penetrable wedge

Abstract: A ray method for finding the acoustic field due to a point source in a two‐dimensional (2‐D), penetrable‐bottom wedge has been extended to the three‐dimensional (3‐D) wedge, where the receiver may lie cross slope as well as downslope or upslope from the source. The environment assumed is a simple model for a sand‐bottom ocean near a shoreline: The wedge of water and the sand bottom are isovelocity, and the sound speed in the bottom is assumed to be greater than that in the water. The total field in the 3‐D wedge is expressed as a sum of ray fields, each of which takes the form of a double integral over plane waves. As in the 2‐D wedge method, the integrals are solved using the method of steepest descent, where the plane‐wave reflection coefficients are placed in the ‘‘phase function’’ of the integrand and thus allowed to influence the location of the saddle points. The ray method is also extended to model broadband propagation: Eigenrays are found at coarsely spaced frequencies, and the eigenray characteristics are interpolated across frequency. The ray model is used to simulate the propagation of a pulse from a point source to a set of vertical arrays at various cross slope ranges. Mode extractions from the vertical array data demonstrate several interesting phenomena unique to 3‐D wedge propagation: multiple mode arrivals, modal shadow zones, and the dependence of the shadow zones on mode number. An example illustrating mode wave‐front curvature and mode capture is also given.

Modeling path effects in three-dimensional basin structures

Abstract: Path effects for seismic wave propagation within three-dimensional (3-D) basin structures are analyzed using a reciprocal source experiment. In this experiment, a numerical simulation is performed in which a point source is excited at a given location and then the wave field is propagated and recorded throughout a 3-D grid of points. Using the principle of reciprocity, source and receiver locations are reversed. This allows the modeling of path effects into a particular observation site for all possible source locations using only one simulation. The numerical technique is based on the use of paraxial extrapolators and currently tracks only acoustic waves. However, the method is capable of handling arbitrary media variations; thus, effects due to focusing, diffraction, and the generation of multiple reflections and refractions are modeled quite well. The application of this technique to model path effects for local earthquakes recorded at stations in the Los Angeles area of southern California indicates the strong influence of the 3-D crustal basins of this region on the propagation of seismic energy. The modeling results show that the Los Angeles, San Fernando, and San Gabriel basins create strong patterns of focusing and defocusing for paths into these stations from various source locations. These simulations correlate well with earthquake data recorded at both stations. By comparing these calculations with earthquake data, we can begin to evaluate the importance of these basin effects on observed patterns of strong ground motions.

The Density Evolution of IRAS Galaxies

Abstract: A maximum likelihood test for density evolution is applied to a full-sky redshift survey of 5297 galaxies selected from the IRAS data base and flux-limited at 1.2 Jy. No evidence for evolution is found. Attention is given to important systematic errors which decrease the certainty with which evolution can be measured, namely, limited knowledge of the cosmological model, the unknown intrinsic spectral energy distribution of IRAS galaxies from 16 to 140 microns, the effect of density fluctuations, a Malmquist-like bias arising from flux errors in the IRAS Point Source Catalog, and possible incompleteness of the sample at high redshifts and low Galactic latitudes. It is shown that the Malmquist bias could result in a significant overestimation of the evolution rate, especially if the catalog has a flux limit near the completion limit of the Point Source Catalog.

Superzone holographic mirror

Abstract: A diffractive mirror has a plurality of diffractive zones at least one of which has an optical step having a height equal to jλ/2 and at least one of which has an optical step having a height equal to kλ/2 where λ is a design wavelength of the mirror and j and k are unequal nonzero integers. Alternatively stated at least one of the optical steps induces a relative phase shift of 2jπ at the image point between two light rays emerging from a point source at the object point and striking the lens immediately on opposite sides of the step and at least one of the steps induces a relative phase shift of 2kπ at the image point between two light rays emerging from a point source at the object point and striking the lens immediately on opposite sides of that step.

The generalization of Rytov's method to the case of inhomogeneous media and HF propagation and scattering in the ionosphere

Abstract: Rytov's method is generalized for the case when the regularly inhomogeneous ionosphere contains local irregularities of deterministical and statistical nature. An integral representation of the point source field is constructed which takes into account the multibeam effects including strong interference of the beams field, i.e., caustics and foci formation and so on. By means of the representation developed, some problems of HF wave propagation are solved. Among these is the diffraction of a point source field on an ambipolar diffusing local inhomogeneity of the ionosphere. The calculations of the power spectra of the fluctuations of the phase and amplitude of a single-hop path field due to ionospherical electron density irregularities are also presented. The calculations are compared with the results of experimental investigations.

Thermal analysis of electrode heating and melting due to a spark

Abstract: A thermal analysis has been carried out to estimate the temperature distribution in an electrode and the shape and size of molten metal pool due to heat from a single spark. Analytical solutions for heat conduction in a semi-infinite solid subjected to various boundary heat sources (point source, disk source, and Gaussian source-all instantaneous and semicontinuous) have been compiled and listed. In all cases, but one, the latent heat of melting has been assumed to be zero. The analysis shows that the instantaneous point source results in the largest radius and depth of the molten metal pool, and the highest efficiency of melting (about 61%). The maximum error in the calculated volume of the pool, assuming zero latent heat of melting, is estimated to be less than 15%. A comparison of the calculated radius of the molten metal pool with the crater dimensions of eroded surfaces shows that the point heat source does not satisfactorily account for the crater dimensions. >

Iron K-alpha line from X-ray illuminated relativistic disks.

Abstract: The intensity and profile of the iron Kα fluorescence line from a flat, optically thick accretion disk rotating around a Schwarzschild black hole and illuminated by a central X-ray source are computed using a fully relativistic treatment of the photon intensity and shifts. The X-ray source is modelled as an isotropic point source located on the symmetry axis at a height h in units of the gravitational radius. These calculations represent a refinement and an extension of those presented in Paper I for h=20, carried out using a weak field approximation and therefore of validity limited to inclination angles <70°. Here it is shown that at high inclination angles purely relativistic effects lead to the growth of features between the two Doppler horns and that, as a consequence, the line equivalent width maintains a sizeable value, while the centroid energy and the line width go through a broad maximum at about ∼80°

Comparison of Results from a Meandering-Plume Model with Measured Atmospheric Tracer Concentration Fluctuations

Abstract: Measured wind-azimuth data are used in a simple meandering-plume model to predict observed SF6 concentration fluctuations measured downwind of a point source during a range of stability conditions. The meander component of plume diffusion is calculated as the running mean of wind-azimuth signal using travel time as the smoothing time. Instantaneous plume coefficients are derived from measured peak instantaneous concentrations in a subset of the data for distances less than 1 km. These empirical plume coefficients are shown to be a linear function of the standard deviation of the residual azimuth signal after the meander component is removed (σθI = 0.285 σ¯θR). Instantaneous concentration time series predicted with the model using the observed azimuth data are quite similar to observed time series. Sensitivity analyses indicate that the model predictions are strongly dependent upon the averaging period used to filter the azimuth signal but that the instantaneous plume width is only a weak function...

Transducer considerations for point-source/point-receiver materials measurements

Abstract: This paper reviews the requirements and operational characteristics of several sources and receivers which can be used as the basis of a point-source/point-receiver (PS/PR) materials testing system. Considered are sources and receivers whose temporal and spatial characteristics most nearly approximate those needed for making PS/PR measurements directly. Consideration is also given to means for utilizing non-ideal transducers, that is, those whose temporal response is bandlimited or which are not operating as a force source, or as a velocity or displacement receiver and those whose aperture is finite.

Variability of the Galactic center in the hard X-ray range

Abstract: The first evidence of strong flux variability in the hard X-ray range from 1E 1740.7-2942, obtained during an observation of the region on 1989 May 17, is reported. The measurement was performed with a high-sensitivity array of multiwire proportional counter operative in the energy range 15-180 keV, and with a field of view of 1.9° FWHM. The balloon-borne observation, lasting for 8100 s including background data, was pointed on 1E 1740.7-2942 as the reference source, following previous suggestions that this object was the point source responsible for the high-energy emission from the Galactic center

A new model of the vertical ground rod in two-layer earth

Abstract: A new system of integral equations (SIE) for the current distribution function in a vertical ground rod in a two-layer earth is developed. The system of integral equations is numerically solved by the point matching method using polynomial approximation of the current distribution function. The supply zone is approximated by the electric field of a point source. >