Mesh Configuration Research Articles

Expansion and Fixation Properties of a New Braided Biodegradable Urethral Stent: An Experimental Study in the Rabbit

Biodegradable spiral urethral stents have been used with favorable results combined with thermal treatments of the prostate and for recurrent urethral strictures but the configuration of the helical spiral is not ideal. We developed a new tubular mesh configuration for the biodegradable urethral stent and evaluated its expansion and locking properties in the rabbit urethra. The stents were made of self-reinforced polylactic acid polymer (Bionx Implants, Ltd., Tampere, Finland) blended with BaSO (Alfa Chem, New York, New York) to achieve radiopacity. Two braiding patterns, that is 1 over 1 and 2 over 2 + 1, were used to produce a tubular mesh structure. Stainless steel stents (pattern 1 over 1) served as controls. The stents were inserted into the posterior urethra of 27 male rabbits. The animals were sacrificed after 1 week, 1 and 6 months, respectively. X-rays were done immediately after stent insertion and at sacrifice. Longitudinal movement and expansion were assessed on the x-rays.(4) All stents maintained position in the urethra without fixation. Macroscopic disorientation of the structure of the 2 over 2 + 1 braided self-reinforced polylactic acid polymer stents began before 1 month, while 1 over 1 braided stents retained their construction. At 6 months 3 of 6 biodegradable stents were degraded. Average longitudinal movement was 2 mm. (range 1 to 3) in the 1 over 1 self-reinforced polylactic acid polymer group, 2 mm. (range 0 to 7) in the 2 over 2 + 1 polylactic acid group and 3 mm. (range 3 to 3) in controls at 1 month. Biodegradable polymers are suitable materials for braided urethral stents. The expansion properties of the 2 braiding models tested in this study sufficed to fix the stents in situ in the prostatic urethra. However, the 1 over 1 braiding pattern was superior to the 2 over 2 + 1 pattern, in that it retained its macroscopic construction until the degradation of single self-reinforced polylactic acid polymer fibers.

EXPERIMENTAL STUDIES ON THE EFFECT OF ELECTRODE CONFIGURATION IN ELECTROFILTRATION

Results from the electrofiltration of anatase (TiO2) particles in a water suspension and using three different electrode configurations are presented. The three electrode configurations studied were spot, foil, and mesh. For the first two configurations, the electric field was perpendicular to the direction of flow while for the mesh configuration it was opposite to the direction of flow. The percent gain volume filtrate and power consumption were measured for each electrode configuration. For a given percent gain volume filtrate, it was observed that the foil electrode configuration consumes the least power.

Space‐time residual distribution schemes for hyperbolic conservation laws on unstructured linear finite elements

AbstractMultidimensional upwind residual distribution schemes are extended to the context of continuous linear space–time finite elements for the time accurate solution of scalar and hyperbolic systems of conservation laws. The formulation leads to a consistent discretization of the space‐time domain, thus retaining the properties of the underlying basic schemes both in space and time. We propose a particular space–time mesh configuration containing two layers of elements and three levels of nodes in time. This construction leads to unconditionally stable implicit time stepping while retaining second‐order spatial and temporal accuracy in smooth flows and monotone solution across steep gradients. The presented schemes have a strong potential in the field of moving grids, since they allow a dynamic change of the space–time mesh geometry. Numerical results demonstrate the robustness, accuracy and monotonicity of the method. Copyright © 2002 John Wiley & Sons, Ltd.

Buckling analysis for delaminated composites using plate bending elements based on higher‐order zig‐zag theory

AbstractA finite element based on the efficient higher‐order zig‐zag theory with multiple delaminations is developed. The bending part of the formulation is constructed from the concept of DKQ element. Unlike conventional elements, a developed element has its reference in the bottom surface which simplifies zig‐zag terms on formulation. Exact patch solutions are developed on elements which have the bottom reference system. The present element passes proper bending patch tests in the arbitrary mesh configurations in isotropic materials. Zig‐zag formulation is adopted to model laminated plates with multiple delaminations. To assess the accuracy and efficiency of the present element based on higher‐order zig‐zag theory with multiple delaminations, the linear buckling problem of laminated plates with multiple delaminations has been analysed. The results have been compared with three‐dimensional elasticity solutions. The present element works as an efficient tool for analysing the behaviour of the laminated composites with multiple delaminations. Copyright © 2002 John Wiley & Sons, Ltd.

Size selectivity of diamond and square mesh cod ends for four by-catch species in the crustacean fishery off the Portuguese south coast

The effects of an increase in cod end mesh size from 55 to 60 and 70 mm and a change of mesh configuration from 55 mm diamond to 55 mm square mesh on the size selectivity of four by-catch species (the red shrimp Aristeus antennatus, the European hake Merluccius merluccius, the horse mackerel Trachurus trachurus and the blue whiting Micromesistius poutassou) commonly captured in the crustacean fishery off the Portuguese south coast, were evaluated. Selectivity parameters for blue whiting, the most abundant species in the catches, were estimated taking into account between-haul variation, while for the remaining species, captured in much lower quantities, the selectivity estimates were based on pooled data by length class for all hauls within the same cod end. Length at 50% retention, L 50, was found to increase with mesh size and with the change in mesh configuration for all the studied species. For blue whiting trawling depth and cod end catch were found to play a role in between-haul variation by increasing L 50 as well. The results suggest that an increase in the current minimum mesh size of 55–70 mm would be advisable to be compatible with the minimum landing sizes (MLSs) of 29 mm carapace length and 27 cm total length for red shrimp and hake, respectively, while it would greatly reduce the amount of discards, particularly those for blue whiting, that accounted for approximately 50% of the total catch weight. Horse mackerel was the only species for which the use of a larger mesh size would result in a significant escapement of individuals above the MLS of 15 cm.

Quantifying numerical dispersion in non-orthogonal FDTD meshes

Numerical electromagnetic models such as FDTD are widely used for the design and analysis of structures, including antennas. Numerical dispersion is one of the main sources of error that degrade the accuracy of the results—for each structure of interest, the users of the model must attempt to generate a mesh that will avoid introducing high levels of dispersion. This is, however, especially difficult for non-orthogonal meshes since little information is available on the dispersion properties of the non-orthogonal FDTD algorithm on complex meshes. For the first time, the dispersion in realistic non-orthogonal FDTD models of microstrip structures is quantified directly through numerical simulations. A test structure is considered, discretised using a number of non-orthogonal mesh configurations, including single and multiple skew angles. A numerical analysis of reflections generated at the transition between two mesh regions with different skew angles is also presented. These results give a practical guide to mesh generation for users of the algorithm.

Tetrahedral composite finite elements

AbstractWe develop and analyse a composite ‘CT3D’ tetrahedral element consisting of an ensemble of 12 four‐node linear tetrahedral elements, coupled to a linear assumed deformation defined over the entire domain of the composite element. The element is designed to have well‐defined lumped masses and contact tractions in dynamic contact problems while at the same time, minimizing the number of volume constraints per element. The relation between displacements and deformations is enforced weakly by recourse to the Hu–Washizu principle. The element arrays are formulated in accordance with the ‘assumed‐strain’ prescription. The formulation of the element accounts for fully non‐linear kinematics. Integrals over the domain of the element are computed by a five‐point quadrature rule. The element passes the patch test in arbitrarily distorted configurations. Our numerical tests demonstrate that CT element has been found to possess a convergence rate comparable to those of linear simplicial elements, and that these convergence rates are maintained as the near‐incompressible limit is approached. We have also verified that the element satisfies the Babuška–Brezzi condition for a regular mesh configuration. These tests suggest that the CT3D element can indeed be used reliably in calculations involving near‐incompressible behaviour which arises, e.g., in the presence of unconfined plastic flow. Copyright © 2001 John Wiley & Sons, Ltd.

Size selectivity of diamond and square mesh cod ends for rose shrimp ( Parapenaeus longirostris) and Norway lobster ( Nephrops norvegicus) off the Portuguese south coast

The effects of an increase in cod end mesh size from 55 to 60 and 70 mm and a change of mesh configuration from diamond to square mesh on the size selectivity for rose shrimp Parapenaeus longirostris and Norway lobster Nephrops norvegicus captured off the Portuguese south coast were evaluated. The results were analysed taking into account between-haul variation in selectivity, and indicate a significant increase in L 50 for rose shrimp with an increase in mesh size or with the use of a square mesh cod end, while for Norway lobster only mesh configuration was found to affect this parameter. Two other important external variables were identified; the trawling depth and the cod end catch, which influence between-haul variation, by increasing the selection range for rose shrimp and Norway lobster, respectively. The results obtained suggest that an increase in the current minimum mesh size of 55 mm would be advisable for rose shrimp in order to respect the minimum landing size of 24 mm carapace length presently established for this species. Moreover, trawling for rose shrimp should be avoided at depths above 200 m, in order to avoid catches consisting almost exclusively of juveniles. Such an increase in mesh size would have a minor impact in terms of losses of individuals above the minimum landing size for Norway lobster and would contribute to reducing the amount of discards in this fishery.

The application of parametric 3D finite element modelling techniques to evaluate the performance of a magnetic sensor system

This paper describes the application of parametric three-dimensional (3D) finite element analysis techniques to accurately model the response of a new industrial electromagnetic sensing system. The study focuses on a magnetic transformation detector for hot steel strip, which is capable of measuring the austenite-to-ferrite phase transformation fraction of hot steel on-line as the material cools below the Curie temperature. This sensor presents several challenges with regard to modelling its response. First, small changes (∼0.3 mT) in the measured magnetic field must be detected against a much larger background value (∼160 mT). Second, the response of the sensor to variation in lift-off must be accurately determined so that changes in lift-off could be subsequently rejected using an appropriate compensation algorithm. Finally, an accurate model (error <0.2%) of the sensor was essential to avoid the need for extensive laboratory testing. The paper describes a number of approaches to address these issues. The paper presents an overview of the sensor and its application, together with a summary of the models used. A number of different approaches were used to improve the accuracy of the modelling process including, mesh configurations, model definition and output calibration, which are also described. Outputs from the model are then compared with the measurement values. This study demonstrates the feasibility of using calibrated simulations to describe the response of the sensor to the required accuracy and repeatability for symmetrical geometries. Further work is needed to assess the feasibility of the calibration method for more complex systems.

Incompressible or nearly incompressible soil dynamic behaviour—a new staggered algorithm to circumvent restrictions of mixed formulation

The restriction of the Babuska–Brezzi stability criteria for the mixed formulation requires non-uniform interpolation of displacement and pore-pressure variables, which complicates the coding in the finite element analysis. This will certainly preclude the use of many otherwise useful elements. A possible solution to overcome this problem introduced earlier by the authors requires a semi-explicit form. However, the effectiveness of the semi-explicit procedure is dependent on the time step length used, which presents difficulties for transient analysis. In this paper, an unconditionally stable staggered implicit-implicit algorithm is developed, which is generalised from the semi-explicit form. For non-uniform mesh configurations, the results indicate a significant improvement compared with those obtained by using the semi-explicit procedure.

(1-14) CFD and Combustion in Engines with an Unstructured Parallel Solver Based on KIVA((DE-4)Diesel Engine Combustion 4-Modeling)

KIFP, an hexahedral unstructured version of KMB (our present CFD code for engines), has been developed. Based on KIVA's algorithms (finite volume on staggered grids, time-splitting, SIMPLE loop, sub-cycled advection...), the new solver has been built step by step with a strong control on the numerical results. This paper shows the different phases of this work. The numerical approaches and developments are discussed. Some academic examples are shown and compared with KMB or analytical results, like scalar advection or multi-species diffusion. Better precision and convergence in the physical fields are observed. Iterative loops and advective sub-cycles are also reduced thanks to the unstructured formalism. Super-scalar machines being widely used and developed, KIFP is targeted to them. OPEN-MP paradigm is used to parallelize the code, and the paper gives results on performance and speed-up (more than 3 for 4 processors in the best case). Results on a compression of a swirl motion in Direct Injection Diesel engine are given. For a given mesh density, physical results and numerical quality (driving CPU time) seems better for KIFP than for KMB. This is due to a better mesh configuration in the KIFP case (no corner cells). At last, a pancake spark ignited engine is used to compute homogeneous combustion with the extended coherent flame model (ECFM) implemented in the new solver. We find a good agreement between the results of the two codes and experiment.

A new random walk model for PCS networks

This paper proposes a new approach to simplify the two-dimensional random walk models capturing the movement of mobile users in personal communications services (PCS) networks. Analytical models are proposed for the new random walks. For a PCS network with hexagonal configuration, our approach reduces the states of the two-dimensional random walk from (3n/sup 2/+3n-5) to n(n+1)/2, where n is the layers of a cluster. For a mesh configuration, our approach reduces the states from (2n2-2n+1) to (n/sup 2/+2n+4)/4 if n is even and to (n/sup 2/+2n+5)/4 if n is odd. Simulation experiments are conducted to validate the analytical models. The results indicate that the errors between the analytical and simulation models are within 1%. Three applications (i.e., microcell/macrocell configuration, distance-based location update, and GPRS mobility management for data routing) are used to show how our new model can be used to investigate the performance of PCS networks.

Evaluation of floating and sticking extended release delivery systems: an unconventional dissolution test

The extent to which hydrophilic matrix tablets with a propensity to stick to the dissolution apparatus and/or float are susceptible to variations in hydrodynamic conditions during dissolution testing was investigated. Furthermore the usefulness of simple alternatives to the current compendial tests is examined. Swellable hydrocolloid (guar) matrix tablets containing verapamil HCl were evaluated using USP dissolution apparatus I and II. Two additional configurations where an additional single ring and mesh device or a double mesh device was located below the paddle in the dissolution vessel were also evaluated. Tablets were placed on top of the single mesh device or in the compartment formed by the two mesh surfaces of the double mesh device. In all cases near linear (n≥0.82) release profiles were observed. When using apparatus I it was observed that the highly swellable tablets were fully constricted by the basket within 5–7 h. This prevented further independent movement and unimpeded swelling and coincided with a departure from linear release and increased variability (S.D.≤9.5%). Under standard apparatus II conditions two out of three tablets adhered to the bottom of the dissolution vessel for the duration of the experiment. Consequently their release profiles differed markedly from those obtained under apparatus I conditions (similarity factor, f2=30.5) with the release rate being approximately half of that obtained under apparatus I conditions. Adhesion to the dissolution vessel was also observed when paddle speed was doubled to 100 rpm, thus again resulting in large variability (S.D.≤34%). Whilst the averaged single and double mesh configuration profiles were similar to the apparatus I profile (f2=57.36 and 61.38, respectively), large variability (S.D.≤11%) occurred with the single mesh configuration due to floating and random adhesion of tablets to the paddle or sampling tubes. Almost superimposable profiles were obtained for the individual tablets (S.D.<3%) when the tablets were located in the compartment formed by the double mesh device. Use of a double mesh device may therefore provide an alternative to current compendial dissolution methods when the reliable determination of the true release kinetics of floating and sticking delivery systems is desired.

Coarse mesh evolution strategies in the Galerkin multigrid method with adaptive remeshing for geometrically non-linear problems

This paper addresses the strategies of evolving the coarse mesh configurations in the context of the Galerkin multi-grid (GMG) method when dealing with problems involving large deformations. A new coarse mesh evolution scheme, which continuously and in a simple manner moves the coarse mesh nodal points along with the deformation of the fine mesh, is proposed and its two implementation versions aiming at further improving the efficiency of the scheme are also developed. In addition, the practical aspects of integrating the GMG with adaptive remeshing techniques are discussed. Finally, several large strain elasto-plastic problems are presented to verify the performances of the proposed schemes and the behaviour of the combined GMG/mesh adaptivity is also illustrated. Numerical results show that up to 40 per cent reduction in the number of MG iterations has been achieved by the new coarse mesh evolution scheme. Copyright © 2000 John Wiley & Sons, Ltd.

Pressure and force distribution characteristics under the normal foot during the push-off phase in gait

The plantar pressure distribution during the push-off phase of walking was measured with the EMED-SF4 system of Novel (Munich, Germany) in 42 healthy subjects between 20 and 59 years of age. All 84 feet of the 23 women and 19 men were absolutely asymptomatic. The results showed that the big toe and the region of the sole under the second metatarsal head (MTH2) exhibited the highest pressures. After subdividing the foot sole into regions of interest, using a standard mesh configuration, forces acting in each of these areas were calculated from the pressures measured. During the push-off phase the ground force is shared mainly by the first and second metatarsal heads and by the big toe, these structures together taking 64% of the total forefoot load. The relationship of forces and peak pressures acting within the selected regions of interest were statistically analysed. There was significant negative correlation between the forces under the big toe and under each of the metatarsal heads MTH2, MTH3 and MTH4, showing, for example, that the load under MTH2–4 decreases if the force under the big toe increases. There was also significant negative correlation between the forces under MTH1 and under each of the metatarsal heads MTH3, MTH4 and MTH5, showing that the lateral load decreases when the medial load increases. No significant correlation was found between MTH1 and MTH2. Also, no significant correlation was apparent between MTH1 and the big toe.

BLOCK-JACOBI SVD ALGORITHMS FOR DISTRIBUTED MEMORY SYSTEMS II: MESHES*

This paper deals with a parallelization of the two-sided Jacobi algorithm for computation of Singular Value Decomposition (SVD) on a computer with p processors, which are organized into a two-dimensional √p × √p mesh configuration. This work represents a continuation of our paper (Part I, to appear in J. Parallel Algorithms and Applications), which described a parallelization approach by columns and efficient ordering strategies for the hypercube and ring topologies. Our parallelization approach is based on slicing the matrices by rows and columns. The orderings developed for rings and hypercubes are adopted here and we show a proper assignment of submatrices to processors that enables their efficient parallel execution. A complexity comparison to the column-based algorithm is given. Parallel computational experiments on a Paragon system are presented and discussed for two test matrices.

On the implicit time integration of semi-discrete viscous fluxes on unstructured dynamic meshes

Numerical simulations of viscous flow problems with complex moving and/or deforming boundaries commonly require the solution of the corresponding fluid equations of motion on unstructured dynamic meshes. In this paper, a systematic investigation of the importance of the choice of the mesh configuration for evaluating the viscous fluxes is performed when the semi-discrete Navier-Stokes equations are time-integrated using the popular second-order implicit backward difference algorithm. The findings are illustrated with the simulation of a laminar viscous flow problem around an oscillating airfoil

On the implicit time integration of semi‐discrete viscous fluxes on unstructured dynamic meshes

Numerical simulations of viscous flow problems with complex moving and/or deforming boundaries commonly require the solution of the corresponding fluid equations of motion on unstructured dynamic meshes. In this paper, a systematic investigation of the importance of the choice of the mesh configuration for evaluating the viscous fluxes is performed when the semi-discrete Navier–Stokes equations are time-integrated using the popular second-order implicit backward difference algorithm. The findings are illustrated with the simulation of a laminar viscous flow problem around an oscillating airfoil. Copyright © 1999 John Wiley & Sons, Ltd.

Numerical simulation of three-dimensional flow hydraulics in a braided channel

Results are presented from a numerical simulation of three-dimensional flow hydraulics around a mid-channel bar carried out using the FLUENT/UNS computational fluid dynamics (CFD) software package. FLUENT/UNS solves the three-dimensional Reynolds-averaged form of the Navier–Stokes equations. Turbulence closure is achieved using a RNG k–ϵ model. Simulated flow velocities are compared with measured two-dimensional velocities (downstream and cross-stream) obtained using an electromagnetic current meter (ECM). The results of the simulation are qualitatively consistent with the flow structures observed in the field. Quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two (r=0·88) and a mean difference of 0·09 m s−1. Deviations between simulated and measured velocities may be identified that are both random and systematic. The former may reflect a number of factors including subgrid-scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ECM orientation. Model mesh configuration, roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities. These results illustrate the potential for using CFD software to simulate flow hydraulics in natural channels with complex configurations. They also highlight the need for detailed spatially distributed datasets of three-dimensional flow variables to establish the accuracy and applicability of CFD software. Copyright © 1999 John Wiley & Sons, Ltd.

Boiling liquid nitrogen heat transfer in channels with porous copper inserts

Copper mesh, compressed and formed into porous matrices of various shapes and sizes, has been routinely used in high heat load/flux component cooling with water at the Advanced Photon Source (APS) to significantly enhance the heat transfer performance. Now the same mesh configuration is being applied to the cryogenic cooling of optical components, such as the monochromators, mirrors and multilayers with liquid nitrogen in single phase. Two-phase heat transfer is avoided to prevent flow-induced noise (vibrations and jitter) in the ultrasensitive optical components. Hence there is a great need to understand the limits of single-phase heat transfer with copper mesh using liquid nitrogen. Recently an extensive experimental program has been undertaken to investigate the heat transfer limits in conductive porous matrices with liquid nitrogen as the coolant. This paper presents the data obtained, compares the data with existing single- and two-phase correlations, and interprets the results for cryo-cooled optical component cooling applications.