Showing papers by "Naval Postgraduate School published in 1978"

Simulation of Nonhomogeneous Poisson Processes by Thinning

Abstract: : A simple and relatively efficient method for simulating one- dimensional and two-dimensional nonhomogeneous Poisson processes is presented. The method is applicable for any rate function and is based on controlled deletion of points in a Poisson process whose rate function dominates the given rate function. In its simplest implementation, the method obviates the need for numerical integration of the rate function, for ordering of points, and for generation of Poisson variates.

Fluid forces on oscillating cylinders

Abstract: The paper presents the results of an experimental and analytical investigation of the forced oscillations of a circular cylinder in uniform flow. The transverse force has been decomposed into two components and the appropriate force-transfer coefficients have been determined experimentally through the use of a Fourier averaging techinique. The results were then incorporated into the equation of motion to predict the dynamic responsee of elastically-mounted cylinders. The numerical predictions were found to be in good agreement with those obtained experimentally.

Fluid Forces on Oscillating Cylinders

Abstract: The paper presents the results of an experimental and analytical investigation of the forced oscillations of a circular cylinder in uniform flow. The transverse force has been decomposed into two components and the appropriate force-transfer coefficients have been determined experimentally through the use of a Fourier averaging techinique. The results were then incorporated into the equation of motion to predict the dynamic responsee of elastically-mounted cylinders. The numerical predictions were found to be in good agreement with those obtained experimentally.

Formation of small metal clusters by ion bombardment of single crystal surfaces

Abstract: The mechanism for the formation of small metal clusters ejected from an ion bombarded metal surface is examined in detail. The analysis is performed by classical trajectory methods which determine the positions and momenta of all particles in a model microcrystallite as a function of time. The calculation utilizes pair potentials for Cu derived from elastic constants of the solid and is performed for 600 eV Ar+ ion at normal incidence to the crystal. The results show that cluster species do not leave the surface as intact parts of the solid but form in a region above the surface. A trajectory for Cu5 formation is traced in detail showing a typical mechanism which is valid for Cun formation where n?7.

Optically heterodyned coherent Raman spectroscopy

Abstract: A convenient variation of Raman-induced Kerr effect spectroscopy (RIKES) which overcomes many of the previous limitations of coherent Raman spectroscopy is introduced Quantum limited signal-to-noise ratios may be approached by the use of optical heterodyne detection (OHD) Detector current, linearly proportional and phase sensitive to the third-order nonlinear susceptibility, and linearly proportional to the Raman scattering cross section, is produced Heterodyne detection in coherent Raman spectroscopy enables the detection of weak Raman signals that would otherwise be obscured by noise resulting from background sources

Atomic and molecular ejection from ion-bombarded reacted single-crystal surfaces. Oxygen on copper(100)

Abstract: NSF under Grant No. MPS75-9308, and the Materials Research Program under Grant No. DMR-77-23798 and the U.S. Air Force.

Angular distributions of ejected particles from ion-bombarded clean and reacted single-crystal surfaces

Abstract: The expected angular distributions of ejected particles from ion-bombarded clean and reacted single-crystal surfaces are calculated using classical dynamics to model the momentum dissipation. For oxygen atoms adsorbed on a Cu(100) lattice in $A$-top and fourfold and twofold bridged geometries, preferred ejection angles are found for the higher-kinetic-energy particles (g20 eV). Because of scattering mechanisms which cause atoms to move through gaps in the lattice surface, we find that the patterns for different overlayer registries are easily distinguishable.

Sea-Surface Temperature Anomaly Generation in Relation to Atmospheric Storms

Abstract: An extended period of reduced surface heat and momentum fluxes due to the absence of atmospheric storms may result in upper-ocean temperature anomalies that persist for months. The predominance of either anomalously high or low temperatures is related to the ocean thermal structure that is established on the transition date between the winter and summer regimes.

An iterative algorithm to build continuous stress and displacement solutions

Abstract: An iterative algorithm is presented: it builds a continuous stress and displacement solution starting from the solution of a classical displacement finite element analysis. The modified solution satisfies the virtual work principle, and is much better than the starting solution. The algorithm is very easily included in existing programs. A number of examples shows the efficiency of the method.

Combustion Behavior of Solid-Fuel Ramjets

Abstract: AN experimental investigation was conducted of the combustion behavior in solid fuel ramjets in order to determine the effects of configuration variables and operating conditions on combustion performance. Variables considered were fuel port flow rates, bypass dump momentum and geometry, and bypass ratio. Contents To be used in a tactical situation, the solid-fuel ramjet has to demonstrate combustion stability and high efficiency over the expected operating envelope of altitudes and Mach numbers. It must also show performance comparable to that of liquid-fuel ramjets and ducted rockets. Combustion studies on the solid-fuel ramjet have been underway at United Technologies—Chemical Systems Division since 1971.l The solid-fuel ramjet has two distinct combustion zones within the fuel grain: the recirculation zone behind the sudden expansion inlet, which provides flame stabiliation; and a diffusion flame in the developing boundary layer region after flow reattachment. Unburned gaseous fuel escapes from under the flame at the aft end of the fuel grain and results in decreased combustion efficiency. Aft mixing chambers and bypass air designs are being used to increase the efficiency. A schematic of the solid-fuel ramjet is shown in Fig. 1. The apparatus employed a polymethylmeth acrylate (PMM) fuel with a fuel port to dump inlet area ratio of 9.0 and a fuel port to nozzle throat area ratio of 4.0. The exit area of the grain was held fixed at the initial port area by using a thin orifice plate. The aft mixing chamber had a length to diameter ratio of 2.93. The aft mixing chamber consisted of three interchangeable sections such that the axial location and angular orientation of the bypass dumps could be varied. A summary of the test conditions is presented in Table 1. Figure 2 presents data obtained without bypass air. The regression rate expression indicates a weaker dependence on pressure than the earlier data of Boaz and Netzer.2 Figure 3 presents data obtained with bypass air. A slightly stronger dependence on pressure was shown while regression rate indicated very little or no dependence on air flux for the values tested. In the bypass situation, the mass flux through the grain is low but the pressure is maintained high, due to the total mass flux through the nozzle throat. These conditions minimize the convective heat flux to the fuel surface. For lower mass flux through the grain, the regression rate increases relative to the air flux; thus, more gas with radiative properties (fuel rich) is present. These results indicate that using bypass with PMM fuel changes the principal wall heat flux mechanism from convection to radiation.

Observation of the temperature structure function parameter, C T 2, over the ocean

Abstract: Shipboard measurements of temperature fluctuations, mean wind, temperature, and humidity permit comparisons to be made of experimental and empirical estimates of ct2, the temperature structure function parameter. Surface flux estimates are obtained from bulk aerodynamic formulae. Temperature fluctuation data are selected to minimize a salt-contamination effect which causes increases in temperature variance. Predictions for CT2 based on surface flux scaling agree within 20%, except for near neutral and large unstable conditions where disagreement can be attributed to measurement problems.

Wave Impact Loads on Cylinders

Abstract: The design of structures that must survive in a wave environment depends on a knowledge of the forces that occur at impact, as well as on the dynamic response of the system. Two typical examples include the structural members of offshore drilling platforms at the splash zone and the often encountered slamming of ships. Two of the objectives of this investigation were (1) to examine the existing theoretical models for determining wave slam forces on circular cylinders; and (2) to furnish data about forces acting on circular cylinders subjected to impact with a sinusoidally oscillating water surface.

An Analysis of Western North Pacific Tropical Cyclone Forecast Errors

Abstract: Western North Pacific tropical cyclone position forecast errors for 10 years (1966–75) are statistically analyzed. Variations of errors versus a number of parameters are examined. It is shown that a small number of readily available parameters, such as location, maximum wind and components of motion, can, with reasonable effectiveness, classify a tropical cyclone forecast as representing a group with either markedly above or below average errors. The annual variations of forecast errors are also discussed and an attempt is made to explain those variations.

A temperature correction for Tucker's ocean rainfall estimates

Abstract: A method suggested by G. B. Tucker of estimating rainfall from present weather observations is tested on 28 coastal stations. Tucker's method was developed from a group of stations in Great Britain; at other latitudes, due to air temperature differences, the Tucker estimates may be several times too high or too low. Regression formulae are presented to adjust yearly, quarterly, and monthly mean precipitation values for air temperature. Corrected rainfall maps based upon ocean weather station data reveal a formerly undetected rainfall maximum in western mid-latitudes of the North Pacific and North Atlantic Oceans.

Entrainment characteristics of unsteady subsonic jets

Abstract: The effectiveness of jet unsteadiness in enhancing flow entrainment was assessed. It was conducted that entrainment depends on the type and amount of jet unsteadiness. Apparently, the mere introduction of jet unsteadiness by small sinusoidal flow angle variations is insufficient to enhance entrainment but, it should be noted that the results were obtained at measuring stations which are all many nozzle widths downstream of the jet nozzle. Thus, no fully conclusive statement can be made at this time about the entrainment close to the nozzle. The high entrainment of the fluidically oscillated jet was caused by the high-frequency content of this square wave type of oscillation but more detailed measurements are clearly needed, in particular for the fluidically oscillated and the pulsed jets. Practical ejector application requires the proper trade-off between entrainment and primary nozzle thrust efficiency.

Sea Surface Temperature Response to Variations in Atmospheric Wind Forcing

Abstract: Sea surface temperature changes occurring during atmospheric forcing events defined in terms of wind speed are examined with the long time series of surface data at Ocean Weather Ships P, V and N in the North Pacific Ocean. The 3 h forcing is expressed in terms of u*3, where u* is the atmospheric friction velocity, and various heat flux components. It is shown that the distributions of atmospheric forcing during the warming season are rather similar to those of the cooling season, although the intensity of the forcing is much less in the warming season. The nonlinearity in the u*3 distribution is very similar in both seasons. That is, a few of the most intense synoptic events provide a majority of the turbulent kinetic energy input to the upper layers of the ocean. During the majority of time, the winds are light and there is inadequate turbulent kinetic energy to maintain a deep layer against the stabilizing effects of near-surface heating. It is found that sea temperature increases during the w...

Model Applications to Solid-Fuel Ramjet Combustion

Abstract: A PREVIOUSLY developed model for solid-fuel ramjet combustion with negligible radiation was applied to the combustion behavior of all-hydrocarb on fuels which exhibit significant gas-phase radiation. Theoretical predictions were compared to temperature field data taken by other investigators.

Variability of longshore currents

Abstract: Simultaneous measurements were made of the offshore directional spectra of gravity waves, and longshore currents within the surf zone. The goal was to test theories which suggest a direct relationship between mean longshore currents (V) in the surf zone and offshore values of the off axis component of radiation stress (S ). Seventeen minute — xv averages of both S and V showed considerable temporal xv variation, and little or no tendency to vary together. There was also considerable longshore spatial variability of the 1ongshore"current. Attempts to measure gradients of S in the surf zone failed because of small errors in instrument orientation. The measurements suggest that considerable temporal and spatial averaging will generally be required to obtain a representative picture of longshore currents, even if no rip currents are present, due to the presence of "eddy" motions or long edge waves.

Analysis of Real-Time Systems by Data Flowgraphs

Abstract: The concept of a data flowgraph is formalized as a bipartite directed graph. Each execution sequence of a computer program has a corresponding data flowgraph which describes functionally what happens to the data if that execution sequence is followed.

Improved finite difference formulas for boundary value problems

Abstract: This paper deals with the numerical solution of linear differential equations of fourth order by finite differences It points out significant (but usually overlooked) errors which result from the conventional method of imposing the boundary conditions in such problems Revised finite difference formulas are derived which apply near the boundaries and which eliminate the above errors Three commonly encountered boundary conditions ar econsidered These correspond, in the terminology of beam analysis, to a clamped end, to a simply supported end and to a free end The improvement in accuracy that can be achieved with the revised formulas is illustrated by two representative examples The revised formulas are shown to reduce the overall error of the numerical solution by a factor of about five in a typical case

Error Bounds for the Liouville-Green Approximation to Initial-Value Problems

Abstract: New error bounds are developed for the Liouville-Green approximation for initial-value problems governed by a single second-order linear differential equation. The cases of both exponential-type and also oscillatory solutions are considered. Previous error bounds are sharpened as a consequence of our development of these new error bounds for initial-value problems. These new results are applied to an important class of linear differential equations arising in military operations research (specifically, variable-coefficient Lanchester-type equations of modern warfare for combat between two homogeneous forces). Fur die Liouville-Green-Naherung von Anfangswertproblemen, die nur durch eine lineare Differentialgleichung zweiter Ordnung bestimmt ist, wurden neue Fehlertoleranzen entwickelt. Ebenso werden sowohl exponentialartige als auch oszillatorische Losungen dargelegt. Als Folge unserer neuen Entwicklung dieser Fehlertoleranzen fur Probleme mit Anfangsbedingungen sind bisherige Toleranzen naher prazisiert. Diese neuen Ergebnisse werden in wichtigen Anwendungsbereichen fur lineare Differentialgleichungen benutzt, wie sie etwa im militarischen Operations Research (insbesondere bei Gleichungen mit veranderlichen Koeffizienten der Lanchester-Theorie moderner militarischer Duelle zweier homogener Gegner) auftreten.

Search for a Moving Target: Upper Bound on Detection Probability

Abstract: The object is to detect a randomly moving target at one of the discrete times 0,1,…,τ. The searcher determines a non- negative effort distribution, ψ(x,t), such that the total effort applied at time t does not exceed m(t). Our purpose here is to establish an upper bound on the detection probability for every effort distribution. This is intended to supplement existing iterative procedures that develop sequences of effort distributions that improve monotonically.

A Numerical Study of Barotropic Instability in a Zonally Varying Easterly Jet

Abstract: The structure and behavior of barotropically unstable and stable waves in the vicinity of a zonally varying easterly jet are studied numerically with a linearized barotropic vorticity equation on a β plane. The easterly jet is approximated by a Bickley jet with a slow zonal variation. The numerical results are also compared with a simple mechanistic analytical model using the local phase speed and growth rate concepts. In several aspects the results are grossly similar to that expected from the parallel flow theory of barotropic instability. However, in the unstable region the resultant structure of the waves causes a spatial growth rate greater than predicted by the local growth rates computed with a parallel flow model. In the stable region, the structure leads to a strong dynamic damping. When a uniform advective velocity is added to a variable mean flow, the difference between the magnitude of the growth rate of the computed waves and that implied by the parallel flow theory is somewhat reduc...