Mesh Orientation Research Articles

Methodical placement of mesh during endoscopic extraperitoneal inguinal hernioplasty

Endoscopic extraperitoneal inguinal hernioplasty has been gaining in popularity worldwide recently. Placement of mesh remains a technically challenging part of the procedure. This paper describes a systematic method for the placement of mesh during endoscopic inguinal hernioplasty. To broaden the endoscopic view and maximize working space, the mesh should be kept far from the telescope within the limited extraperitoneal space. Unfolding the mesh requires prior stabilization of the mesh at one point, to avoid uncontrolled migration of the whole mesh and subsequent loss of mesh orientation. Mastering the skill of two‐handed technique is essential for coordinated manipulation and expeditious mesh placement. Before concluding the procedure, a trial of deflation can help to ensure appropriate repositioning of the peritoneum without displacement of the mesh. Chinese Abstract imageimage

Modeling delamination as a strong discontinuity with the material point method

Decohesion is an important failure mode associated with layered composite materials. Here, the energy implications of material softening are explored in a thermodynamic framework with the result that the dissipated energy (fracture energy) is greater than the plastic work of the traction on the failure surface. It is also argued that if the traction and continuum constitutive equations are solved simultaneously, the resulting algorithm is as simple as that for conventional plasticity. For numerical simulations, the material point method displays the attributes of no mesh deformation so that remeshing is not necessary and the continuous tracking of material points avoids the need for remapping history variables such as decohesion. Compatibility is invoked in a weak sense with the result that no special algorithms are needed for mesh realignment along crack surfaces or for double nodes. Example solutions exhibit no sensitivity of delamination propagation with mesh orientation.

A Fourth-Order Central WENO Scheme for Multidimensional Hyperbolic Systems of Conservation Laws

We present the first fourth-order central scheme for two-dimensional hyperbolic systems of conservation laws. Our new method is based on a central weighted nonoscillatory approach. The heart of our method is the reconstruction step, in which a genuinely two-dimensional interpolant is reconstructed from cell averages by taking a convex combination of building blocks in the form of biquadratic polynomials. Similarly to other central schemes, our new method enjoys the simplicity of the black-box approach. All that is required in order to solve a problem is to supply the flux function and an estimate on the speed of propagation. The high-resolution properties of the scheme as well as its resistance to mesh orientation, and the effectiveness of the componentwise approach, are demonstrated in a variety of numerical examples.

Mesh size sensitivity of the combined FEM/DEM fracture and fragmentation algorithms

In the context of the combined finite–discrete element method a number of algorithms aimed at modelling progressive failure, fracture and fragmentation of solids under extreme loading conditions have been proposed in the last few years. The fracture patterns obtained by recently proposed algorithms are impressive. However, sensitivity of these algorithms to both mesh size and mesh orientation remains an open question. The aim of this paper is to further investigate sensitivity to mesh size of the recently proposed so called combined single and smeared crack model. Sensitivity to mesh orientation is outside scope of this paper and is discussed only qualitatively.

Improving the reinforcement of parastomal tissues with Marlex mesh: laboratory study identifying solutions to stomal aperture distortion.

Parastomal hernia formation commonly complicates permanent stomas and represents a significant and frequently recurrent management problem, regardless of the method of repair. Prosthetic material reinforcement of parastomal tissues offers the best results. However, problems with unravelling of mesh fibers along cut margins leading to aperture enlargement and hernia recurrence may occur. Raised intra-abdominal pressure in the early postoperative period before incorporation of the mesh into surrounding tissues may result in hernia formation if the aperture size in the mesh increases. Assessment of the physical properties of Marlex mesh was performed in a materials testing laboratory, using standardized tests to simulate the stresses imposed on in situ mesh. Holes cut in Marlex mesh were found to enlarge and distort at loads simulating intra-abdominal pressure changes. Reinforcement with a polypropylene pursestring suture was found to stabilize the periaperture mesh fibers and maintain the original area throughout tensions at least double maximal intra-abdominal pressures. Distensibility of intact sheets of mesh was found to vary by up to 100 percent, depending on the direction of the applied tension, and thus, mesh orientation in hernia repair has major implications. We propose that if mesh is used to reinforce abdominal wall tissues and is cut or fashioned to size, then the cut margins must be reinforced if the intended dimensions and functional integrity of the mesh are to be maintained.

Mechanical behaviour of ferrocement composites: an experimental investigation

In-plane tension, compression and bending tests were conducted on plain mortar and ferrocement specimens with woven and welded meshes. Tension tests were also carried out on meshes. Bending tests were conducted using specimens with centre point loading. The objective of the study was to investigate the behaviour of material reinforced with varying number of mesh layers and orientations and to evolve a set of elastic and inelastic material properties. It is observed that the conventional empirical relations based on mortar crushing strength overestimate the mortar modulus. The elastic moduli obtained using the rule of mixtures compares well with the values evaluated from the tests on ferrocement specimens. The 45° orientation emerges as the weakest configuration both in terms of the Young's modulus and ultimate stress because of the lowest volume fraction of wire mesh in the direction of loading at this orientation.

Numerical modeling of water flow below dry salt lakes: effect of capillarity and viscosity

We investigate numerically the effects of capillarity and viscosity on density-dependent flows below dry salt lakes. The numerical model, MARUN (Boufadel, M.C., Suidan, M.T., Venosa, A.D., 1999a. A numerical model for density-and-viscosity-dependent flows in two-dimensional variably-saturated media. J. Contam. Hydrol., 37, 1–20) is used to assess the combined and separate effects of capillarity and concentration-engendered viscosity on the groundwater dynamics of a two-dimensional system simulating a dry lake. We found that accounting for concentration-engendered viscosity effects under saturated water flow conditions gives totally different groundwater dynamics from the case where these effects are neglected. However, concentration-dependent viscosity effects were minor when unsaturated water flow conditions were present. Under variably-saturated flow conditions, capillarity effects gave totally different hydraulics in the porous domain from the saturated flow case; upwelling regions observed under saturated flow conditions were downwelling when capillarity effects were present. A mesh orientation study revealed that density-dependent flows were susceptible to the orientation of the diagonals of the triangular elements. A discussion on the accuracy and the efficiency of the MARUN model in the context of density- and viscosity-dependent flows in two-dimensional variably-saturated media is also provided.

Stress intensity factor solutions for part-elliptical through cracks

The present analytical investigation is directed toward predicting the growth of large cracks in riveted lap-splice joints, similar to joints found in operational transport aircraft. The fatigue crack growth of large through cracks with oblique crack fronts can now be predicted using the results of this investigation. A characteristic of fatigue in riveted lap joints is crack growth under a complex stress system which, in its simplest form, consists of cyclic tension with superimposed cyclic bending due to the eccentricity in the lap joint. In reality, rivet squeezing causes hole expansion and built-in residual stresses. Rivet tilting in the rivet hole and contact problems are other complications. To predict the growth of part through cracks with a quarter elliptical shape in lap-splice joints, stress intensity solutions, such as the well-known Newman–Raju K-solutions are available. However, these cracks change into through cracks with oblique crack fronts and continue to grow due to the combined tension and bending. Since no K-solutions are available for through cracks with oblique fronts, the finite-element method and a three-dimensional virtual crack closure technique (3D VCCT) were adopted. The 3D VCCT is shown to be invariant to crack plane mesh orientation, permitting a minimal amount of pre-processing of the mesh to generate a new K solution. K-values are presented for a range of crack depth to crack length ratios ( a/ c 1=0.2, 0.3, 0.4, 0.6, 1.0, 2.0), crack depth to sheet thickness ratios ( a/ t=1.05, 1.09, 1.13, 1.17, 1.21, 2.0, 5.0, 10.0) and hole radius to sheet thickness ratios ( r/ t=0.5, 1.0, 2.0).

Moving finite element methods by use of space-time elements: I. Scalar problems

This article deals with moving finite element methods by use of the time-discontinuous Galerkin formulation in combination with oriented space–time meshes. A principle for mesh orientation in space–time based on minimization of the residual, related to adaptive error control via an a posteriori error estimate, is presented. The relation to Miller's moving finite element method is discussed. The article deals with scalar problems; systems will be treated in a companion article. © 1998 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 14:251–262, 1998

Ferrocement bolted shear joints: Failure modes and strength prediction

An experimental investigation was carried out to study the behavior of ferrocement bolted shear joints. Twenty-eight different pin-loaded ferrocement plate configurations and eight control specimens were tested. The test variables were the type of mesh (square, hexagonal, mixed), mesh orientation ( 0 90 ° or ±45 °), number of mesh layers (2 to 8 corresponding to a volume fraction from 1.68% to 6.72%), and ratio between end distance and hole diameter ( e d = 4 or 6 ). Three failure modes were observed, net tensile section failure, cleavage failure, and crushing compression failure at the bolt. Cleavage failure was predominant for e d = 4 , and net-section tension failure or compression crushing for e d = 6 . The strength of the mesh reinforcement was closely related to the strength of the specimens that failed in the cleavage and net-section modes. Prediction equations are developed based on the Limit analysis Lower Bound Theorem to predict the net tension or cleavage failure loads, and a conservative equation is proposed to predict compression crushing failure. A comparison was made with other prediction equations available in the literature.

Numerical Methods for a Nonconvex Optimization Problem Modeling Martensitic Microstructure

Using a continuum model of Ericksen and James, we compute a number of fine scale structures commonly observed as a result of martensitic phase transformations. Our results provide numerical validations of the Ericksen--James model. We show that care is needed in the discretization stage if spurious microstructures are to be avoided. A detailed description of the origin and cause of a spurious microstructure is provided. In addition, we present results showing the important effects of mesh orientation and mesh fineness on the formation of microstructure.

Reducing spurious dispersion, anisotropy and reflection in finite element analysis of time-harmonic acoustics

Galerkin and stabilized (Galerkin/least-squares and Galerkin/gradient least-squares) finite element methods for solving problems of time-harmonic acoustics are presented and analyzed. The error of discretization is related to spurious representation of physical phenomena, namely dispersion, anisotropy and reflection, all of which decrease with mesh refinement. The performance of the stabilized methods in terms of dispersion and anisotropy is identical, both are superior to the Galerkin method, relaxing wave resolution requirements. Further insight into the design of these methods is provided by the analyses. The stabilized methods differ in the degree of spurious reflection that is engendered by transitions in mesh size. The performance of Galerkin/gradient least-squares deteriorates on non-uniform meshes. However, Galerkin/least-squares maintains its enhanced performance, exhibiting low sensitivity to transitions in mesh size, thereby reducing spurious dispersion, anisotropy and reflection in finite element analysis of time-harmonic acoustics. Guidelines for discretization pertaining to mesh orientation and wave resolution are presented.

Numerical behavior of unsteady waves

Two unsteady Euler solvers using high-order upwind finite volume methods are presented and compared. The first uses the Van Leer flux vector splitting on structured moving grids and includes compatibility relations to treat boundary conditions. The second is based on Roe's approximate Riemann solver on fixed unstructured, finite element type meshes and uses a standard explicit boundary treatment. Results for one- and two-dimensional strong wave propagation and interaction show very good agreement between the two approaches and reference solutions, despite the use of different flux evaluations and integration cells. The two schemes even exhibit the same numerical sensitivity of wave propagation to mesh orientation. However, the simulation of the shock wave reflection from a wall reveals some flaws in the second solver's slip wall treatment which are avoided by the compatibility relations used in the first method.

Mass Transfer at the Wall of a Thin Channel Containing an Expanded Turbulence Promoting Structure

The paper deals with convective diffusion mass transfer at one of the planar walls of a rectangular channel containing turbulence promoters constituted by an inert expanded material. Different expanded structures are studied; particularly the influence of the mesh orientation with respect to the flow direction is put in evidence. The performance obtained with the expanded structures are compared with those observed with a SU‐grid in the same experimental conditions. The results concerning the pressure drop measurements are presented and an empirical mass‐transfer correlation is established for materials having similar geometrical characteristics.

A general topology, Godunov method

A numerical technique that utilizes a general topology mesh is described. The method employs the arbitrary Lagrangian-Eulerian procedure and explicit, finite-volume, Godunov numerics. Material interfaces are resolved to eliminate fictitious mixing and nonphysical shear impedance. Cell-centered variables, including velocity, are used to provide consistent control volumes for the advection of mass, momentum, and energy, and to allow arbitrary slip between material regions. The computational mesh is composed of arbitrary polygonal cells. The constraint of a fixed logical connectivity for the mesh is removed. Consequently, geometrical mesh limitations, which are responsible for inaccuracies and code failure during the evolution of region boundaries, are absent. Arbitrary boundaries can be resolved, and the mesh is capable of changing smoothly and rapidly from regions of high to low resolution. Lack of a coherent mesh orientation minimizes numerical anisotropy. A mesh rezoning approach, based on a dual triangulation and coupled with a global remapping algorithm, allows the mesh to evolve dynamically.

A parameter characterizing the geometry of expanded metal. II. Natural convection mass transfer at expanded metal electrodes

Free convective mass transfer rates at vertical electrodes of expanded metal were measured by the electrochemical method. Electrode height and electrolyte concentration were varied and the dependence of the expanded metal on the geometry and on the mesh orientation with respect to the vertical direction was investigated. A single equation was developed to correlate all the results. Besides the generalized dimensionless groups for natural convection the correlation includes a parameter characterizing the geometry of the expanded metal. The correlation also represents free convective mass transfer results obtained by other investigators with vertical mesh electrodes.

A General Control-Volume Formulation for Modeling Impact Ionization in Semiconductor Transport

A method for incorporating impact ionization into a general control-volume formulation of semiconductor transport is described. The methods for electric-field and current-density vector evaluation and generated charge partitioning within a two-dimensional triangular element are given. The techniques employed allow device breakdown to be accurately determined independent of mesh orientation to current flow direction. The avalanche breakdown of a 1-μm n-MOSFET illustrates the approach. Regions where the drain current is a multivalued function of drain voltage are directly and self-consistently calculated for this device.

A general control-volume formulation for modeling impact ionization in semiconductor transport

A method for incorporating impact ionization into a general control-volume formulation of semiconductor transport is described. The methods for electric-field and current-density vector evaluation and generated charge partitioning within a two-dimensional triangular element are given. The techniques employed allow device breakdown to be accurately determined independent of mesh orientation to current flow direction. The avalanche breakdown of a 1-µm n-MOSFET illustrates the approach. Regions where the drain current is a multivalued function of drain voltage are directly and self-consistently calculated for this device.

Finite Element Modeling of Crack Band Propagation

The propagation of blunt smeared crack bands through a finite element mesh in plain or reinforced concrete. structures is studied. A previous work, which demonstrated (approximate) independence of results from the chosen mesh size, is extended by demonstrating the same for the changes in mesh orientation with regard to the crack band and to the steel bars. Bond slip of bars must also be considered. This is satisfactorily accomplished by extending the previously proposed concept of free bond slip length for the cases of skew meshes and skew bars. In the presence of reinforcement, the previous formula for the equivalent strength giving approximately the same results as the energy criterion must be expanded by a term which corresponds to the third term of the asymptotic expansion of displacements near the tip of a sharp crack. The energy criterion is shown to give consistent results for different types of fracture specimens. However, the equivalent strength calibrated for the center‐cracked tensile specimen ...

Experimental cultivation of the scallops Chlamys opercularis (L.) and Pecten maximus (L.) using naturally produced spat

Spat of the scallops chlamys opercularis (L.) and Pecten maximus (L.) settled on artificial collectors made from plastic ‘Netlon’ envelopes and plastic mesh onion bags. The effect of mesh size and orientation of spat collectors has been studied, as well as the optimum period to leave collectors in the sea after settlement. Intermediate cultivation of the spat was carried out in trays and final cultivation in lantern nets. Growth of Chlamys opercularis was faster under cultivation than in the wild, with a minimum commercial size of 50 mm reached in less than two years. The feasibility of cultivating Chlamys opercularis and Pecten maximus is discussed and a schematic outline of a cultivation programme for Chlamys is produced, based on the information obtained.