Multiscale modeling of unsaturated flow in dual-scale fiber preforms of liquid composite molding I: Isothermal flows

Mold-filling simulation of unsaturated flows in LCM is important for optimizing mold design quickly and cost-effectively in the virtual space. For the first time, a true multiscale approach is developed for simulating the unsaturated flow under isothermal conditions in the dual-scale fiber-mats of RTM. To solve the coupled macro-micro equation-set, a coarse global mesh is used to solve the global flow equations over the entire domain while fine local meshes in form of the periodic unit-cells of fabrics are employed to solve the local tow-impregnation process. A multiscale algorithm based on hierarchical computational grids has been proposed to simulate the unsaturated flow in the dual-scale fiber mats under isothermal conditions. The predictions are compared with measurements for a 1-D flow experiment which indicates that the proposed approach can be used to simulate the unsaturated flow accurately through dual-scale fiber mats in LCM without the use of any fitting parameters.     

Modeling of heat transfer and unsaturated flow in woven fiber reinforcements during direct injection-pultrusion process of thermoplastic composites

This paper provides a methodology for the modeling of heat transfer and polymer flow during direct thermoplastic injection pultrusion process. Pultrusion was initially developed with thermosets which have low viscosity. But the impregnation becomes a critical point with thermoplastics which exhibit higher viscosity. There are very few reported works on direct thermoplastic impregnation with injection within the die. In addition, the rare studies have not adequately addressed the issue of unsaturated flow in woven fiber reinforcements. The solution proposed here, models the polymer flow through dual-scale porous media. A heat transfer model is coupled to a flow model enriched with a sink term. Specific changes of variables are made so as to model the steady state solution of unsaturation along a continuous process. The sink term, added to the continuity equation, represents the absorption rate of polymer by the bundles. Data were measured on a pultrusion line and micrographs confirmed the modeling strategy with an unsaturated flow approach. The flow modeling coupled to heat transfer of the thermoplastic pultrusion process aims at determining the saturation evolution through the die so as to manufacture pultruded profiles with the lowest residual porosity.     

Multiscale modeling of unsaturated flow in dual-scale fiber preforms of liquid composite molding III: reactive flows

The woven, stitched or braided fabrics used in liquid composite molding (LCM) display partial saturation behind moving flow-front in an LCM mold which is caused by delayed impregnation of fiber tows. In this part 3 of the present series of three papers, a novel multiscale approach proposed in parts 1 and 2 [1] and [2] is adapted for modeling the unsaturated flow observed in the dual-scale fabrics of LCM under non-isothermal, reactive conditions. The volume-averaged species or resin cure equation, in conjunction with volume-averaged mass, momentum and energy (temperature) equations, is employed to model the reactive resin flow in the inter-tow (gap) and intra-tow (tow) regions with coupling expressed through several sink and source terms in the governing equations. A coarse global-mesh is used to solve the global (gap) flow over the entire domain, and a fine local mesh in form of the unit-cell of periodic fabrics is employed to solve the local (tow) flows. The multiscale algorithm based on the hierarchical computational grids is then extended to solve the dual-scale flow under reactive conditions. The simulation is compared with a two-color experiment and a previously published two-layer model. Significant differences between the temperatures and cures of the gap and tow regions of the dual-scale porous medium are observed. The ratio of pore volumes in the tow and gap regions, the effective thermal conductivity in the tows, and the reaction rate are identified as the important parameters for temperature and cure distributions in the gap and tow regions.     

Multiscale modeling of unsaturated flow of dual-scale fiber preform in liquid composite molding II: Non-isothermal flows

A novel multiscale approach is developed for modeling non-isothermal flows under unsaturated conditions in the dual-scale fabrics of liquid composite molding (LCM). The flow and temperature governing equations at the global or gap or inter-tow (∼m) level and the local or intra-tow (∼mm) levels are based on a previous dual-scale volume averaging method. To solve the coupled equations at two length-scales, a coarse global mesh is used to solve the global flow over the entire domain, and a fine local mesh in form of the unit-cell of periodic fabrics is employed to solve the local tow-impregnation process. (The latter is used to compute sink terms required for solving the former.) A multiscale algorithm based on the hierarchical computational grids is then proposed to solve the dual-scale flow under non-isothermal (but non-reactive) conditions. To test the proposed multiscale model, we first carry out a validation study in which the temperature histories predicted by the multiscale method are compared with experimental data available in a publication for a simple 1-D flow. Despite the lack of information about various model parameters, a reasonably good comparison with the experimental results is achieved. Then, the non-isothermal flow through a simple 1-D flow domain is carried out and the predictions of the multiscale simulation are compared with those of a previously published two-layer model. The multiscale predictions are found to be very similar to the two-layer predictions. A significant difference between the gap and tow temperatures is observed. The ratio of pore volumes in the tow and gap regions, thermal conductivity of the tows, and fiber types are identified as the important parameters for temperature distributions in the gap and tow regions. A further comparison with the single-scale flow simulation highlights significant differences between the conventional single-scale and the proposed dual-scale modeling approaches.     

Flows of inelastic non-Newtonian fluids through arrays of aligned cylinders. Part 1. Creeping flows

Numerical simulations are presented for flows of inelastic non-Newtonian fluids through periodic arrays of aligned cylinders, for cases in which fluid inertia can be neglected. The truncated power-law fluid model is used to define the relationship between the viscous stress and the rate-of-strain tensor. The flow is shown to be dominated by shear effects, not extension. Numerical simulation results are presented for the drag coefficient of the cylinders and the velocity variance components, and are compared against asymptotically valid analytical results. Square and hexagonal arrays are considered, both for crossflow in the plane perpendicular to the alignment vector of the cylinders (flows along the axes of the array as well as off-axis flows), and for flow along the cylinders. It is shown that the observed strong dependence of the drag coefficient on the power-law index (through which the stress tensor is related to the rate-of-strain tensor) can be described at all solid area fractions by scaling the drag on a cylinder with appropriate velocity and length scales. The velocity variance components show only a weak dependence on the power-law index. The results for steady-state drag and velocity variances are used in an approximate theory for flows accelerated from rest. The numerical simulation data for unsteady flows agree very well with the approximate theory.     

Isothermal tube flow of non-linear viscoelastic fluids. Part II: Transversal field

Constitutive criteria for the existence of secondary flows, similarities and analogies, developments in the history of transversal flows, recent research on secondary flows of dilute solutions in rotating pipes and channels and related drag reduction are reviewed in depth as well as research concerning fundamental aspects of transversal flows and industrial applications relevant to secondary flows.     

Computer-based refrigerant thermodynamic properties. Part 1: Basic equations

A set of computer-based methods for calculating densities, vapour pressures, enthalpies, internal energies and entropies of refrigerants, together with simple refrigeration cycle calculations are to be described. Part 1 presents the basic equations derivations for refrigerants R 11, R 12, R 13, R 13B1, R 14, R 22, R 113, R 114 and R 502 as used by the IIR in its latest charts. Ammonia equations are also given based on Dvorek and Petrak's publications.     

A micromechanical compaction model for woven fabric preforms. Part II: Multilayer

With nesting between adjacent layers and inter-layer packing, the microstructure and the compaction behaviour of a multilayer woven fabric preform are much more complicated than those of a single layer fabric preform. A micromechanical model, based on the hierarchical structure characteristics of woven fabric preforms, was developed to investigate the elastic compaction behaviour of multilayer plain weave fabric preforms. The compaction mechanisms of fabrics at different hierarchical levels including deformation and compaction of yarn cross-section, flattening of yarn waveform, nesting between adjacent layers and inter-layer packing, are considered in an integrated approach in this predictive model. Effects of structural elements at different hierarchical levels on compaction behaviour of multilayer plain weave fabric preforms are investigated in detail. Both the number of layers and shifting are shown to have significant effects on compaction behavior, while the effect of nesting increases as the number of layers increases. The predictions by this model are correlated well with the experimental data.     

Development of a diffusion-inertia model for calculating bubble turbulent flows: Isothermal polydispersed flow in a vertical pipe

This work is dedicated to simulating polydispersed bubble turbulent flows. The work is aimed at development of an approach to simulating polydispersed bubble flows on the basis of the combination of a diffusion-inertia model with the delta-approximation method. The coagulation and fragmentation models of bubbles have been checked. The comparison of the simulation result with the experimental data has shown that the elaborated approach makes it possible to simulate the bubble polydispersed flows in wide range of gas content values.     

Development of a diffusion-inertia model for calculating bubble turbulent flows: Isothermal monodispersed flow in a vertical pipe

A diffusion-inertia model (DIM) for calculating of the monodispersed bubble flows in an isothermal flow in a turbulent liquid has been presented. The comparison of the results of the simulation with various experimental data shows that the DIM adequately describes the variety of physical phenomena occurring in bubble flows. The DIM has been incorporated in a three-dimensional computing code that allows one to use this model for engineering calculations.     

A micromechanical compaction model for woven fabric preforms. Part I: Single layer

A micromechanical model was developed to investigate elastic compression behaviour during compaction of a single layer of woven fabric preform. The compaction model describes two important deformation mechanisms at different hierarchical levels, addressing micro-deformation of yarn cross-section compaction and macro-deformation of yarn bending accompanied by yarn waveform flattening. The stress carried by the fabric is decomposed into two parts in relation to two distinct mechanisms, coupled through the requirement of deformation compatibility. With this micromechanical model the effects of microstructures of single layer woven fabric on their compaction behaviour are evaluated. It is shown that both the macro-bending stiffness of fibre and the initial fibre packing ratio of yarn affect the compaction behaviour of single layer fabric preform. The prediction is correlated with experimental data available, and satisfactory agreement is observed.     

Voltammetric and reference microelectrodes with integrated microchannels for flow through microvoltammetry. 1. The microcell

This paper describes the construction of 2 microsensor units for on-line voltammetric detection inside a cylindrical microcell (a working microsensor unit and a reference and auxiliary microsensor unit), for application to heavy-metal analysis in complex media such as natural waters. Both microsensor units include a channel for the solution renewal in the microcell after analysis. The working microsensor, a Hg-plated Ir microelectrode, is protected against fouling with an agarose gel including a hydrophobic chromatographic phase (C18). The fabrication steps and the quality tests related to long-term use and reliability, as well as to precision, are described. The application of the protective gel layer against fouling by hydrophobic or surface active small molecules is of general application, reliable, and very efficient. The reference and auxiliary unit is composed by an iridium oxide based mini reference electrode, an auxiliary Pt electrode, and a circulation channel. It is built to enable its use inside a 700-microm-diameter tubing connected to a hollow fiber supported liquid membrane lumen (volume, 5-10 microL) for heavy-metal analysis. However, it can be used in any other microanalytical system. The reference electrode is sufficiently stable for voltammetric applications (1-2 mV drift/day), and its lifetime is more than one year. The Ti/IrO2 core is immersed in a pH-buffered agarose gel, to guarantee potential stability even when the electrode is immersed in variable pH solutions.     

Methodological aspects of creep prediction for heat-resistant steels and alloys. Part 1. Analysis of the equations of state

The validity of applying the complex of constitutive equations based on the concepts of the minimum commitment method and systems analysis of known experimental data to predict creep over a wide range of temperatures and stresses is discussed. __________ Translated from Problemy Prochnosti, No. 1, pp. 55–68, January–February, 2007.     

On the capabilities of the double-layer representation for Stokes flows. Part I. Analytical solutions

Boundary-integral equations of the second kind for Stokes flows known as the completed-double-layer (CDL) representations are promising formulations for the solution of many-body problems in suspension mechanics. It is therefore important to analyze their numerical properties for geometries where accurate numerical results are not readily obtained. In this paper, several analytical and semi-analytical results are derived for the double-layer representation where particles are in close proximity to one another. In particular, the onset of spatial oscillation of the surface density is explored as a function of particle-particle separation. These oscillations are not spurious and pose a significant challenge for discretization schemes.     

On the capabilities of the double-layer representation for Stokes flows. Part II. Iterative solutions

An attractive feature of the completed-double-layer (CDL) representation for mobility problems in Stokes flows is the fact that it can be solved by fixed-point iteration. However, for particles in close contact and complex geometry, the spectral radius of the discretized CDL system approaches unity. For these cases, fixed-point iteration converges very slowly or not all. In this paper, we furnish a proof that the CDL representation for mobility problems can be solved by other iterative techniques based on residual-reduction methods.     

Boundary-element analysis of planar drop deformation in confined flow. Part 1. Newtonian fluids

A boundary element formulation is presented for the general two-dimensional simulation of confined two-phase incompressible creeping Newtonian flow (visco-elastic elaids will be considered in a second part). The method requires the solution of two simultaneous integral equations on the interface between the two fluids and the confining solid boundary. The method is illustrated through the simulation of the deformation of a drop as it is driven by the ambient flow inside a convergent channel. The accuracy and convergence of the method are assessed by varying the time increment for a given number of boundary elements. The influences of the degree of channel convergence and viscosity ratio are investigated. Circular as well as elliptic initial drops are also considered.     

Contrast enhancement in visualisation of woven composite tow architecture using a MicroCT Scanner. Part 1: Fabric coating and resin additives

MicroCT scanning is a non-destructive inspection method which was used to visualise tow architecture in woven composites with the ultimate goal of three-dimensional model generation. This has been achieved in the past for glass fabric composites, but is problematic when applied to carbon fabrics. Using X-rays, it is difficult to discriminate between elements of the composite, particularly the region between co-aligned neighbouring tows. This presents difficulty when viewing such composites using X-ray MicroCT scanning. Additives were used to enhance contrast during scanning. The most successful techniques were coating of fabrics with gold, copper, and an iodine contrast agent. Resin particle additive techniques were also trialled, with limited success. Good visualisations of glass fabrics were possible without contrast enhancement. Three-dimensional reconstructions of interior tow architectures were then made from the scans of contrast enhanced specimens. This research can be viewed as a starting point in developing methods for generating contrast between neighbouring tows within a three-dimensional woven preform using MicroCT scanning.     

Photoshop的新特色功能介绍之一——全景接片自动合成

这是新版PhotoshopCS的一项新的特色功能,适用于把分别的画面连接起来,合成一幅全景画面,影像质量很好,操作程序简便、快速而实用,本文以实际例子和图像详细阐述其具体操作过程。     

Blends of unsaturated polyester and phenolic resins for application as fire-resistant matrices in fibre-reinforced composites. Part 1: identifying compatible, co-curable resin mixtures

An unsaturated polyester resin, cured with styrene of commercial origin, has been blended and cured with several commercial phenolic resoles with a view to obtain a homogeneous polymer blend having better fire resistance than unsaturated polyester alone. It is shown that blends of the unsaturated polyester with an allylically modified resole are not only physically compatible (miscible), but also chemically compatible in which they co-cure to an extent to give, probably, a co-continuous interpenetrating polymer network.