Obstacle Configurations Research Articles

Boundary element modeling of electrokinetically driven fluid flow in two-dimensional microchannels.

In recent years there has been considerable interest in fabricating electrophoretic separation systems on microchips. In this study the boundary element method is used to numerically model both the electrical charge density and the electrokinetically driven fluid flow velocity field in two-dimensional microchannels containing an arbitrary configuration of circular flow obstacles. An estimate of both the average velocity and the resolution has been determined for various obstacle configurations, obstacle sizes, area fractions, surface line lengths per unit area and for different values of the thickness of the electrical double layer. Based on the results, an optimal microchannel design is suggested. In addition, the recently observed phenomenon of recirculated flow in an open region of an otherwise packed electrochromatography column has been confirmed with the numerical model.

Deformation Behaviour of Copper Based Solid Solutions at Elevated Temperatures

The transition from discontinuous, thermally activated dislocation glide to viscous solute dragging at elevated temperatures (500 K < T< 1000 K) for polycrystals of Cu-10 at% Al and Cu-4 at% Mn has been studied by measurements of the critical flow stress, by changes from constant strain rate to deformation at constant stress (creep conditions) and by stress relaxation tests. The transition to viscous glide is found to depend on strain rate as well as on temperature and is shifted to increasing stress during deformation along the stress-strain curve. The activation energy of viscous glide determined from a joint analysis of the measurements at different strain rates and temperatures matches with the activation energy of bulk diffusion. The mobile dislocation density under conditions of viscous glide turns out to depend solely on the effective stress whereas an 'athermal' stress component accounts for hard solute obstacles. This appears to depend on temperature due to the diffusion controlled dissolution or rearrangement of such obstacle configurations. With further deformation the hardening by the total dislocation density only gives rise to an additional athermal stress contribution but leaves the effective stress unchanged. Accounting for the 'athermal' stress components reduces high stress exponents to the well known 'natural creep law'.

Hydrogen combustion loads—plexus calculations

The mechanical behaviour of the containment during severe accidents is important in order to maintain tightness of the reactor building. This integrity could be affected by combustion phenomena and, especially, hydrogen detonation. Direct detonation initiation is highly unlikely, but this combustion mode resulting from the DDT phenomenon has to be taken into account in safety analyses. In order to obtain representative pressure loads, the IPSN has supported the development of specific combustion models in the PLEXUS code. This modelling has been successfully validated on tube experiments with and without obstacles and validation work is currently underway on a selected RUT experiment. This model has also been applied in two-dimensional calculations in different geometries (upper dome, pressuriser relief tank bunker, etc.). The influence of various obstacle configurations and mixtures has been investigated in order to be representative of various dynamic pressure loads on different walls (containment, inner walls). These results will be used to study the mechanical behaviour of the previously described structures for safety analysis.

Minimal tangent visibility graphs

We prove the tight lower bound 4 n − 4 on the size of tangent visibility graphs on n pairwise disjoint bounded obstacles in the euclidean plane, and we give a simple description of the configurations of convex obstacles which realize this lower bound.

Effects of obstacles on the spread of a heavy gas—Wind tunnel simulations

The spread of a heavy gas cloud in terrain with obstacles has been studied in the wind tunnel. Observations were made at scales of 1:100 and 1:50 using video and photographic techniques; concentration measurements were obtained with a high frequency flame ionization detector. Several different obstacle configurations were examined. The results of flow visualization experiments in still air have been used to mathematically describe the spread rate using a box model approach. The addition of a term to the standard box model equation which accounted for the effects of block coverage fit the observations very well. It is argued that the major effect of the obstacles is to reduce the rate at which the hydrostatic pressure head declines and increases the path length over which the cloud must move rather than retarding the flow through friction or drag.

Distribution functions for fluids in random media

A random medium is considered, composed of identifiable interactive sites or obstacles equilibrated at a high temperature and then quenched rapidly to form a rigid structure, statistically homogeneous on all but molecular length scales. The equilibrium statistical mechanics of a fluid contained inside this quenched medium is discussed. Various particle-particle and particle-obstacle correlation functions, which differ from the corresponding functions for a fully equilibrated binary mixture, are defined through an averaging process over the static ensemble of obstacle configurations and application of topological reduction techniques. The Ornstein-Zernike equations also differ from their equilibrium counterparts.

Turbulent flame propagation and transition to detonation in large fuel-air clouds

The results of a series of flame acceleration tests performed in a top-vented channel, 1.8m× 1.8 m in cross-section and 15.5 m long, with repeated obstacles are reported. The tests were performed in order to assess the potential for flame acceleration and transition to detonation following an accidental spill of gaseous fuel in simulated industrial environments with repeated obstacles. Tests were performed with acetylene, propane and hydrogen sulphide fuels mixed with air. For near stoichiometric acetylene-air, the flame accelerates continuously as it propagates down the channel reaching speeds between 180 and 400 m/s prior to the occurrence of localized explosions which trigger the onset of detonation. The behavior of flames in lean acetylene (5.2% C2H2)-, propane- and hydrogen sulphide-air mixtures is much less dramatic. The observed flame speeds range from about 25 m/s up to 200 m/s, with associated peak overpressures typically less than 50 mbar. The continuous flame acceleration observed in stoichiometric acetylene-air is not observed in these mixtures. Based on these results it is concluded that the potential for flame acceleration and transition to detonation in the move open areas of a chemical plant is much smaller than in the heavily confined areas. The fact that flames in stoichiometric acetylene-air accelerate and produce detonations in these obstacle environments shows that the potential for damaging explosions does exist. However, higher levels of confinement, stronger ignition sources or denser obstacle configurations than in the present tests are required to produce such explosions in less sensitive mixtures. The results of numerical simulations of the turbulent flame propagation are also reported and compared with the observed results. These simulations successfully describe many of the observed phenomena. In particular, the flame acceleration in acetylene-air mixtures and the absence of similar flame acceleration in propane- and hydrogen sulphide-air mixtures are predicted.

The Effect of Obstacle Arrays on the Combustion of Large Premixed Gas/Air Clouds

Abstract A series of large-scale combustion tests has been conducted in which - 4000 m3 of premixed natural gas/air and propane/air was ignited in a wedge-shaped enclosure representing a segment of a large pancake-shaped cloud. Whereas an unobstructed natural gas/air cloud produced only a few millibars overpressure when ignited by a low-energy ignition source an obstacle array consisting of six grids of horizontal pipes, representing a segment of industrial plant, accelerated the flame to a speed in excess of 100 m/s and produced an overpressure of ∼200 mbars. A number of different obstacle configurations were investigated to demonstrate the effects of changing the obstacle parameters (height, blockage and grid spacing) on the flame speed and overpressure. The results of these tests clearly show that obstacle arrays typical of those encountered in industrial plant can lead to the production of damaging overpressures.

Mobile robot turns to colour sensing

A simple ultrasonic sensor coupled with a colour sensing system has been designed and built in the laboratory of the French National Institute of Applied Science at Rennes, and mounted on the laboratory's VESA mobile robot. It can distinguish simple configurations of obstacles and measure their distance from the robot, and within certain limits of illumination can classify obstacles to eight reference colours.

Pressure development due to turbulent flame propagation in large-scale methaneair explosions

The results of large-scale methaneair explosion tests performed at Raufoss, Norway (July–September 1980) are described and discussed. These tests were performed in a vented 50-m 3 vessel (a tube 2.5 m in diameter and 10 m long, open at one end) with regularly spaced obstacles in the form of orifice plates providing blockage ratios from 0.16 to 0.84 to the flow in the tube. It is observed that even relatively small repeated obstacles of height 0.1 m (blockage ratio 0.16) have a dramatic influence on the violence of the explosion, generating explosion overpressures larger than 1 bar in the tube. With no obstacles, the maximum overpressure observed in the tube was about 0.1 bar, to be compared with overpressures up to 8.8 bar recorded with repeated obstacles. The results obtained by a systematic variation of obstacle configurations are described and discussed in terms of a generalized venting model which takes into account the folding of the flame due to the presence of obstacles. The results are also compared with safe vent area criteria for central ignition of initially quiescent explosive mixture in near-spherical containers. The present criteria are found to be totally inadequate for large-scale explosions in obstacle environments.

Multiple scattering of elastic waves by bounded obstacles

The transition matrix method for stationary elastic waves is extended to a great class of obstacles characterized by piecewise constant properties. First, the translation properties of the basis functions is used to treat two and, then, several homogeneous obstacles, and thereafter an obstacle with consecutively enclosing layers is considered. It is then indicated how these two basic methods of combination can be applied to treat more complex cases, including an obstacle consisting of several nonenclosing parts. Finally, we give some numerical applications to configurations of spherical and nonspherical obstacles in and below the resonance region.

Thermally activated dislocation glide through a random array of point obstacles: Computer simulation

This paper reports results obtained through computer simulation of the thermally activated glide of an idealized dislocation through a random array of point obstacles. The array size is fixed at 999, and the dislocation-obstacle interaction is taken to have a simple step form. The variables governing glide are then the resolved shear stress, the temperature, and the obstacle strength, which may be phrased as dimensionless parameters. The principal subjects studied are (1) the stress required for athermal glide at a given obstacle strength and the characteristics of the obstacle configurations which determine this stress, (2) the glide path taken by the configuration and the characteristics of the obstacle configurations encountered along this glide path, (3) the velocity of glide and its dependence on the stress, the temperature, and the obstacle strength.

Analysis of Gaussian beam propagation and diffraction by inhomogeneous wave tracking

Inhomogeneous waves behave locally like A(r) exp[ikS(r)], where A and S are spatially dependent complex amplitude and phase functions, and k is the (large) free-space wavenumber. A previously developed asymptotic theory for high-frequency propagation and scattering of such waves is here applied to the propagation and scattering of paraxial Gaussian beams. Attention is given to Gaussian beams in free space, to beams in a lens-like medium with parabolic variation of the refractive index, and to beam reflection by a cylindrical obstacle. In the latter instance, the obstacle size may be comparable to the incident beamwidth, thereby introducing substantial distortion into the reflected beam. The results obtained from the asymptotic theory are verified by comparison with rigorously derived solutions, thereby confirming the validity of the theory, which can also be applied to more general medium and obstacle configurations.

Statistics of the thermally activated glide of a dislocation through a random array of point obstacles

This paper treats the statistics of thermally activated glide of a dislocation, modelled as a string of constant tension, through an array of randomly distributed identical immobile point obstacles. The parameters governing thermally activated glide are defined, and equations are developed giving the expected value and variance of the time to activate through an obstacle array or sequence of arrays in terms of the properties of the obstacle configurations encountered. The proper statistical definition of the velocity of glide is dicussed. It is shown that the velocity of glide, as determined from the strain rate, is simply proportional to the inverse of the expected time to transit the array (or sequence of arrays). Simplifying approximations are identified for use at low temperature or high stress. We finally discuss how these statistical relations and approximations may best be used in numerical simulation of thermally activated glide.

Multiple scattering of plane harmonic elastic waves in an infinite solid by an arbitrary configuration of obstacles

AbstractThe multiple scattering of plane harmonic P and S waves in an infinite elastic solid by arbitrary configurations of obstacles is considered. Integral equations relating the far-field multiple scattering amplitudes to the corresponding single scattering functions are obtained and asymptotic solutions are found by an iterative procedure. The scattering of a plane harmonic P wave by two identical rigid spheres is investigated.