Variable Mesh Size Research Articles

Characterization of Ichthyocidal Activity of Pfiesteria piscicida : Dependence on the Dinospore Cell Density

The ichthyocidal activity of Pfiesteria piscicida dinospores was examined in an aquarium bioassay format by exposing fish to either Pfiesteria-containing environmental sediments or clonal P. piscicida. The presence of Pfiesteria spp. and the complexity of the microbial assemblage in the bioassay were assessed by molecular approaches. Cell-free water from bioassays that yielded significant fish mortality failed to show ichthyocidal activity. Histopathological examination of moribund and dead fish failed to reveal the skin lesions reported elsewhere. Fish larvae within "cages" of variable mesh sizes were killed in those where the pore size exceeded that of Pfiesteria dinospores. In vitro exposure of fish larvae to clonal P. piscicida indicated that fish mortality was directly proportional to the dinospore cell density. Dinospores clustered around the mouth, eyes, and operculi, suggesting that fish health may be affected by their direct interaction with skin, gill epithelia, or mucous surfaces. Molecular fingerprinting revealed the presence of a very diverse microbial community of bacteria, protists, and fungi within bioassay aquaria containing environmental sediments. Some components of the microbial community were identified as potential fish pathogens, preventing the rigorous identification of Pfiesteria spp. as the only cause of fish death. In summary, our results strongly suggest (i) that this aquarium bioassay format, which has been extensively reported in the literature, is unsuitable to accurately assess the ichthyocidal activity of Pfiesteria spp. and (ii) that the ichthyocidal activity of Pfiesteria spp. is mostly due to direct interactions of the zoospores with fish skin and gill epithelia rather than to soluble factors.

Metal cleaning made easy: A fluidized bed system is a cost-effective option fordegreasing processes

Traditional degreasing processes employ chlorinated solvents in a liquid or vapor state in order to remove oils and oil-borne soils from previously processed parts, such as chips, metal, fines, and fluxes. Nevertheless, increased sensitivity to the environmental impacts of cleaning agents has led to alternative cleaning processes. A novel and unpublished eco-efficient degreasing technique has been analyzed in this study, namley, suface cleaning by means of a fluidized bed of hard particles. A spherical-shaped glass powder with variable mesh sizes in the range of 100 to 800 microns was fluidized. The impact of fluidized particles on the metal substrate caused accurate cleaning of the suface of the workpiece without involving the use of any solvents and without damaging the suface of the part to be degreased. Attention was paid on the correlation between process parameters and the effectiveness of the cleaning operation. By experimentation, the trend of the removal rate over time was successfully plotted for several operative conditions.

A Designed ADI Software for Solving Poisson’s Equation

In This paper, a software has been designed to perform the alternating direction implicit, ADI, method for the case of two dimensional flow represented by Poisson's equation. A square mesh with varying mesh size has been adopted. The results came out very much complying with the analytical solutions asserting the rigour of the software.

Low cost and high performance screen laminate regenerator matrix

A laminate screen matrix regenerator with 47 elements has been designed, analyzed, fabricated and tested. The laminate was fabricated from stainless steel screen sheets that were stacked on top of each other at certain angular orientation and then bonded at high temperature and pressure environment utilizing a sintering process. This laminate is a porous structure media with highly repeatable properties that can be controlled by varying mesh size, weave type, wire size and laminate sheet to sheet orientation. The flow direction in relation to the weave plan can be varied by cutting a cylindrical or rectangular laminate element along or across the weave. The regenerator flow resistance, thermal conductance losses, dead volume, surface area and heat transfer coefficient are analyzed. Regenerator cost and performance comparison data between the conventional widely used method of stacked screens and the new stacked laminate matrix regenerator is discussed. Also, a square stainless steel screen laminate was manufactured in a way which permits gas to flow along the screen wire instead of across it.

Analysis of a Nonsymmetric Discontinuous Galerkin Method for Elliptic Problems: Stability and Energy Error Estimates

In this paper we analyze a nonsymmetric discontinuous Galerkin method for elliptic problems proposed by Oden, Babuska, and Baumann. Our main results are a complete inf-sup stability analysis and, as a consequence, error estimates in a mesh dependent energy norm allowing variable meshsize and order of polynomials. The analysis is carried out in two spatial dimensions on an unstructured triangulation.

A non‐conventional watershed partitioning method for semi‐distributed hydrological modelling: the package ALADHYN

AbstractThe extraction of the river network from a digital elevation model (DEM) plays a fundamental role in modelling spatially distributed hydrological processes. The present paper deals with a new two‐step procedure based on the preliminary identification of an ideal drainage network (IDN) from contour lines through a variable mesh size, and the further extraction of the actual drainage network (AND) from the IDN using land morphology. The steepest downslope direction search is used to identify individual channels, which are further merged into a network path draining to a given node of the IDN. The contributing area, peaks and saddles are determined by means of a steepest upslope direction search. The basin area is thus partitioned into physically based finite elements enclosed by irregular polygons. Different methods, i.e. the constant and variable threshold area methods, the contour line curvature method, and a topologic method descending from the Hortonian ordering scheme, are used to extract the ADN from the IDN. The contour line curvature method is shown to provide the most appropriate method from a comparison with field surveys. Using the ADN one can model the hydrological response of any sub‐basin using a semi‐distributed approach. The model presented here combines storm abstraction by the SCS‐CN method with surface runoff routing as a geomorphological dispersion process. This is modelled using the gamma instantaneous unit hydrograph as parameterized by river geomorphology. The results are implemented using a project‐oriented software facility for the Analysis of LAnd Digital HYdrological Networks (ALADHYN). Copyright © 2002 John Wiley & Sons, Ltd.

On the importance of the effect of turbulence in cavitation inception tests of marine propellers

Current methods for predicting the inception of cavitation in marine propellers rely heavily on physical model tests in cavitation tunnels and these tests are subject to complex scale effects. The viscous nature of the flow, particularly the free–stream turbulence, is one of the main factors contributing to the scale effects on the inception of cavitation. Research to explore this factor has been grossly neglected and there is an urgent need for systematic experimental data to establish possible trends in this effect to be included in the procedures used to extrapolate small–scale experimental results to predict the inception of cavitation on the full–scale propeller. This paper presents data and reports on some interesting and important findings on the effects of different turbulence generation techniques from recent research into the scale effects on the inception of cavitation of marine propellers. Systematic measurements were taken with a five–bladed model propeller to explore the viscous scale effects on the tip vortex and sheet–type cavitation using two different methods to generate turbulence. These were a wire mesh screen with variable mesh sizes placed in front of the model propeller and several grades of carborundum applied to the leading edges of the model propeller blades. The results of the measurements displayed distinct but similar trends in the effects of the two methods on the resulting inception values, in particular for the tip vortex cavitation. This finding has an important practical implication on the current cavitation testing procedures, especially for the most widely used ‘leading–edge roughness’ technique. This technique, which has various practical disadvantages, can now be replaced by a more effective and controlled way of altering the free–stream turbulence. The paper also presents an important set of systematic data on the effect of free–stream turbulence, which is scarce in the literature, and that of leading–edge roughness and it discusses the use of these data for the development of an extrapolator to predict the cavitation inception of a marine propeller at full scale.

A study on the determination of closing level for finite element analysis of fatigue crack closure

An elastic-plastic finite element analysis is performed to investigate detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using 4-node isoparametric elements can predict fatigue crack closure behavior. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. The crack opening level for the constant mesh size increases linearly from initial crack growth. The crack opening level for variable mesh size, is almost flat after crack tip has passed the monotonic plastic zone. The prediction of crack opening level using the variable mesh size proportioning the reversed plastic zone size with the opening stress intensity factors presents a good agreement with the experimental data regardless of stress ratios.

A physically based catchment partitioning method for hydrological analysis

We propose a partitioning method for the topographic surface, which is particularly suitable for hydrological distributed modelling and shallow-landslide distributed modelling. The model provides variable mesh size and appears to be a natural evolution of contour-based digital terrain models. The proposed method allows the drainage network to be derived from the contour lines. The single channels are calculated via a search for the steepest downslope lines. Then, for each network node, the contributing area is determined by means of a search for both steepest upslope and downslope lines. This leads to the basin being partitioned into physically based finite elements delimited by irregular polygons. In particular, the distributed computation of local geomorphological parameters (i.e. aspect, average slope and elevation, main stream length, concentration time, etc.) can be performed easily for each single element. The contributing area system, together with the information on the distribution of geomorphological parameters provide a useful tool for distributed hydrological modelling and simulation of environmental processes such as erosion, sediment transport and shallow landslides. Copyright © 2000 John Wiley & Sons, Ltd.

Organic residue decomposition: The minicontainer-system a multifunctional tool in decomposition studies

The Minicontainer-test, first described by Eisenbeis (1993), was designed to study the kinetics of organic residue decomposition at a microsite level. It is derived from the litterbag technique and consists of polyethylene minicontainers (volume about 1.5 cm(3)) filled with a test substrate (litter, straw, cellulose, etc.). The minicontainers (MCs) are closed at either end with plastic gauze discs of variable mesh size (e.g. 20 microm, 250 microm, 500 microm or 2 mm). A definite number of such units are inserted into PVC-bars, which can be implanted into the soil horizontally or vertically, or be exposed on the soil surface horizontally. The bars are very stable and can be exposed in different environments for months to years. If required, the bars can be removed temporarily and stored, e.g. during soil cultivation. Should fresh litter be used, two phases of decomposition can be distinguished: a fast initial phase, which can be mainly related to the effect of leaching, and a second slow phase depending mainly on the activity of soil organisms and litter quality. Several questions can be addressed to investigations using MCs, e.g. 1) parts of the soil fauna which are involved in decomposition (nematodes, microarthropods, and smaller specimens of the macrofauna, e.g. enchytraeidae, diplopods and dipteran larvae) can be extracted from the litter substrate using a miniscale high gradient extractor, 2) the organic mass loss of litter can be determined, 3) microbial biomass (C(mic), N(mic)) can be assessed by fumigation extraction and 4) microbial activity (respiration) in the test substrate can also be assessed by use of standardised methods. Compared to litterbag studies, the larger number of small replicate units improves the statistical evaluation. Until today the Minicontainer-test has been applied in forestry and agriculture, e.g. studying the effects of liming, soil restoration and the application of insecticides, e.g. Diflubenzuron (Dimilin) and Btk (Bacillus thuringiensis var.kurstaki).

Compact finite difference schemes on non-uniform meshes. Application to direct numerical simulations of compressible flows

In this paper, the development of a fourth- (respectively third-) order compact scheme for the approximation of first (respectively second) derivatives on non-uniform meshes is studied. A full inclusion of metrics in the coefficients of the compact scheme is proposed, instead of methods using Jacobian transformation. In the second part, an analysis of the numerical scheme is presented. A numerical analysis of truncation errors, a Fourier analysis completed by stability calculations in terms of both semi- and fully discrete eigenvalue problems are presented. In those eigenvalue problems, the pure convection equation for the first derivative, and the pure diffusion equation for the second derivative are considered. The last part of this paper is dedicated to an application of the numerical method to the simulation of a compressible flow requiring variable mesh size: the direct numerical simulation of compressible turbulent channel flow. Present results are compared with both experimental and other numerical (DNS) data in the literature. The effects of compressibility and acoustic waves on the turbulent flow structure are discussed. Copyright © 1999 John Wiley & Sons, Ltd.

Localized specific absorption rate calculations in a realistic phantom leg at 1-30 MHz using a finite element method

Protection standards for radiofrequency electromagnetic radiation are principally intended to avoid detrimental thermal effects. To this end the International Commission on Non-Ionizing Radiation Protection (ICNIRP), and national bodies such as the National Radiological Protection Board (NRPB), recommend limitations on the localized specific energy absorption rate (SAR) in various parts of the body. The role of numerical dosimetry is to estimate the SAR from measurable parameters such as external field strengths and total body currents. In recent years there have been significant advances in the sophistication of the anatomical models available, and in our knowledge of the electrical properties of the body tissues. Several groups, including NRPB, have developed mathematical phantoms from medical imaging data, such as MRI scans. It has been known for some time that under certain circumstances SAR restrictions may be violated in the ankle due to the concentration of current in a small area. In this paper the author presents calculations of the SAR distribution in a human leg in the high-frequency (HF) band. This band contains the human whole-body resonance frequency and therefore gives the strongest coupling of the body to the field. The present study uses a finite element model with variable mesh size, derived from a 2 mm resolution voxel phantom of the whole body. It also uses recently acquired data on the electrical properties of the tissues. The results are discussed in the light of the exposure standards promulgated by national and international bodies such as NRPB and ICNIRP, and it is shown that the basic SAR restrictions in the leg are ensured by a current reference level of 100 mA.

The generalized TLM-based FDTD modeling of frequency-dependent and anisotropic media

A generalization of the previously proposed transmission-line matrix (TLM)-based finite-difference time-domain (FDTD) method is presented for modeling frequency-dependent and anisotropic media. The generalized scheme incorporates electric- and magnetic-flux densities in addition to variable mesh sizes. Since it is in an FD form, modeling techniques developed for the conventional FDTD can be easily adapted into the proposed TLM-based technique. In this paper, a modified z-transform technique for frequency-dependent media is implemented, and a two-dimensional (2-D) full-wave technique for guided-wave structures is developed. In all the FDTD computations, no conversions between the field quantities and TLM circuit parameters such as open- and short-circuited stubs are required.

Experimental study of turbulence in isothermal and stably stratified homogeneous shear flows: Mean flow measurements

The evolution of freestream turbulence under the combined action of linear shear and stable linear temperature profile is investigated. The experiment is carried out in a small, open circuit, low-speed test cell that uses air as working fluid. The temperature gradient formed at the entrance to the test section by means of an array of 24 horizontal, differentially heated elements is varied to get a maximum Brunt-Vaisala frequency N o [=({ g T m }{ ∂T ∂y }) 1 2 ] of 3.1 −1. Linear velocity profiles are produced using screens of variable mesh size. The Reynolds number Re M based on centre-line velocity and mesh size is varied from 80 to 175. Isothermal studies are carried out in four different experiments with varying velocity gradients. The effect of inlet turbulence level on growth of turbulence is studied in these flows by keeping the shear parameter Sh (=( x u )( ∂u ∂y )) constant. The range of shear parameters considered is 2.5–7.0. Shear and stratification combined produce a maximum gradient Richardson number Ri g (= N o 2 ( ∂u ∂y ) 2 ) of 0.0145. Results have been presented in terms of evolution of variance of velocity fluctuations, Reynolds shear stress and temperature fluctuations. Measurements show the following: In isothermal flows the growth rate of turbulence quantities depends on both shear parameter and inlet turbulence level. There are distinct stages in the evolution of the flow and that can be identified by the power-law exponent of growth of turbulence. Shear is seen to promote the growth of turbulence and accelerate it towards a fully developed equilibrium state. Stratification initially suppresses the growth of turbulence and, hence, enhances the degree of underdevelopment. Under these conditions shear becomes active and subsequently enhances the growth rate of turbulence quantities.

The Convergence Rate of Godunov Type Schemes

Godunov type schemes form a special class of transport projection methods for the approximate solution of nonlinear hyperbolic conservation laws. The authors study the convergence rate of such schemes in the context of scalar conservation laws and show how the question of consistency for Godunov type schemes can be answered solely in terms of the behavior of the associated projection operator. Namely, they prove that ${\textit{Lip}}'$-consistent projections guarantee the ${\textit{Lip}}'$-convergence of the corresponding Godunov scheme, provided the latter is ${\textit{Lip}}^+$-stable. This ${\textit{Lip}}'$-error estimate is then translated into the standard $W^{s,p} $ global error estimates $( - 1 \leq s \leq \frac{1}{p},1 \leq p \leq \infty )$ and finally to a local $L_{{\text{loc}}}^\infty $ convergence rate estimate. These convergence rate estimates are applied to a variety of scalar Godunov type schemes on a uniform grid as well as variable mesh size ones.

Proximal Renal Tubule Preparations Isolated from Rat or Rabbit Kidneys without the use of Collagenase

Kidney tubules have previously been isolated from a number of different animal species using techniques involving perfusion and digestion of the organ with collagenase, followed by centrifugation of the resulting suspension. These techniques suffer from the disadvantages of requiring prolonged incubation with a potentially damaging enzyme and contamination of the preparations with glo-meruli. We have developed a procedure for the isolation of a purified preparation of rabbit kidney tubules without collagenase digestion. Rabbit or rat kidneys are perfused with buffer, followed by a suspension of magnetic iron oxide. The cortex is then homogenized by hand and put through nylon sieves of varying mesh sizes that separate large pieces of tissue from the glomeruli and tubules, which are caught on the smaller mesh sieves. The glomeruli containing entrapped iron oxide are then separated from the tubules with the aid of a magnet. Tubules isolated in this way are morphologically intact as shown by electron microsco...

Multigrid Methods for Elliptic Problems: A Review

Multigrid methods solve a large class of problems very efficiently. They work by approximating a problem on multiple overlapping grids with widely varying mesh sizes and cycling between thew approximations, using relaxation to reduce the error on the scale of each grid. Problems solved by multigrid methods include general elliptic partial differential equations, nonlinear and eigenvalue problems, and systems of equations from fluid dynamics. The efficiency is optimal: the computational work is proportional to the number of unknowns. This paper reviews the basic concepts and techniques of multigrid methods, concentrating on their role as fast solvers for elliptic boundary-value problems. Analysis of simple relaxation schemes for the Poisson problem shows that their slow convergence is due to smooth error components; approximating these components on a coarser grid leads to a simple multigrid Poisson solver. We review the principal elements of multigrid methods for more general problems, including relaxation schemes, grids, grid transfers, and control algorithms, plus techniques for nonlinear problems and boundary conditions. Multigrid applications, current research, and available software are also discussed.

Impact of Varying Mesh Size and Depth of Fishing on the Financial Performance of an Integrated Harvesting/Processing Operation for Redfish in the Northwest Atlantic

Abstract The impact of alternative mesh sizes for codends of otter trawls on the profitability of a fishery for redfish (Sebastes spp.) in Division 2J-3K of the Northwest Atlantic Fisheries Organization area was examined. The implications of mesh size on catch rates, size of fish in the catch, and on long-term catches were translated into financial terms. Fishing with a 114-mm mesh codend in a depth range of 401-500 m was found to maximize profitability in the case examined, but a mesh size exceeding 127 mm must be used to optimize the profit margin in the 301-400 m depth range. The authors analysed the sensitivity of these results to key parameters such as vessel hold capacity, labour costs, and catch rates. Adoption of an annual fishing mortality rate of 0.05, a rate well below the current reference point of 0.14 for management of this fishery, was estimated to increase long-term profitability for the fleet studied.

Stability of multistep-methods on variable grids

This paper is concerned with the stability of multistep methods for ordinary initial-value problems on grids with variable mesh-sizes. A necessary and sufficient condition for stability is given from which generalizations of recent results by Gear et al. and by Zlatev can be obtained as special cases. As an application the stability of the variable BDF-formulas is treated.

Electromagnetic induction in the Earth in electrical contact with the oceans

Summary. Integral equations are formulated to solve the problem of electromagnetic induction in the Earth in the presence of thin finitely conducting oceans. Unlike earlier models, the oceans are assumed to be in electrical contact with the mantle and leakage of oceanic electric current is therefore possible. A powerful iterative method is developed to solve the very large system of equations and is applied successfully to a model problem. A feature of the integral equations is that they lead to a good first approximation. When fed into the iterative scheme this enhances the rate of convergence. Variable mesh size also appears possible.