A two-dimensional orthotropic model for simulating wood drying processes

Wood is a naturally occurring resource that must be dried before it can be manufactured. Drying is important for a number of reasons that include the protection of the wood against biological damage and the reduction of the moisture content to final equilibrium levels. The complexities involved in modelling this drying process consist of analyzing the heat and mass transfer phenomena that arise in an anisotropic, nonhomogeneous, and hygroscopic porous medium. In this work, a two-dimensional orthotropic mathematical model is formulated and a numerical code based on a structured mesh cell centered control volume approach is implemented in order to allow a more comprehensive numerical investigation of the convective drying of wood to be undertaken. A comparison is made between two different numerical solution techniques; the first numerical method solves the system of equations by treating each equation in an uncoupled form, while the second scheme solves the entire system as a completely coupled set. The most efficient numerical algorithm was obtained when the system was solved using the coupled procedure. In order to examine the important differences of the overall kinetics for both low and high temperature drying, simulation results for three different cases of convective drying of wood are presented. These cases include the drying of wood below the boiling point at a relatively low temperature of 50 °C, a moderate temperature of 80 °C, and above the boiling point at the high temperature of 120 °C. The two-dimensional model highlights the following two very important facts: for an anisotropic medium, where the ratio between longitudinal and transverse permeabilities is of the order of 10³, the moisture migration occurs in the longitudinal sense which is the most permeable direction in the wood; and the behavior of the internal gaseous pressure can have a substantial impact on moisture migration.     

A three-dimensional discrete element model for heterogeneous solids under mechanical loading

The Discrete Element Method (DEM) is used to model solids under quasi-static and dynamic loading. In order to model elastic bodies, a microscopic model must be able to represent the macroscopic properties of the material. An energy-based approach to determine the model parameters is presented for an unit cell assemblage of 13 particles in the hexagonal close packing of spheres. The stored strain energy in the unit cell is added up and the specific strain energy expression is derived with respect to the macroscopic strains. The resulting stress-strain relations can be compared to the constitutive equations of the elastic continuum. To avoid cubic anisotropy for Poisson's ratios above zero, an advanced octahedrongap-filled hexagonal close packing of spheres is presented and validated. This approach for regular lattices can be transferred to heterogeneous materials with the goal of describing porous media such as cement stone. Therefore it is possible to use the presented regular lattices with statistically distributed properties or to investigate irregular distributions of particles. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A computational model for interfaces

Decompositions of the plane into disjoint components separated by curves occur frequently. We describe a package of subroutines which provides facilities for defining, building, and modifying such decompositions and for efficiently solving various point and area location problems. Beyond the point that the specification of this package may be useful to others, we reach the broader conclusion that well-designed data structures and support routines allow the use of more conceptual or non-numerical portions of mathematics in the computational process, thereby extending greatly the potential scope of the use of computers in scientific problem solving. Ideas from conceptual mathematics, symbolic computation, and computer science can be utilized within the framework of scientific computing and have an important role to play in that area.     

A new computational approach for stochastic fluid models of multiplexers with heterogeneous sources

In this paper we derive an efficient computational procedure for the system in which fluid is produced byN ₁ on-off sources of type 1,N ₂ on-off sources of type 2 and transferred to a buffer which is serviced by a channel of constant capacity. This is a canonical model for multiservice ATM multiplexing, which is hard to analyze and also of wide interest. This paper's approach to the computation of the buffer overflow probability,G(x) = Pr{buffer content >x}, departs from all prior approaches in that it transforms the computation ofG(x) for a particularx into a recursive construction of an interpolating polynomial. For the particular case of two source types the interpolating polynomial is in two variables. Our main result is the derivation of recursive algorithms for computing the overflow probabilityG(x) and various other performance measures using their respective relations to two-dimensional interpolating polynomials. To make the computational procedure efficient we first derive a new system of equations for the coefficients in the spectral expansion formula forG(x) and then use specific properties of the new system for efficient recursive construction of the polynomials. We also develop an approximate method with low complexity and analyze its accuracy by numerical studies. We computeG(x) for different values ofx, the mean buffer content and the coefficient of the dominant exponential term in the spectral expansion ofG(x). The accuracy of the approximations is reasonable when the buffer utilization characterized by G(0) is more than 10^–2.     

A three-dimensional dynamic model for train-track interactions

This paper develops a novel three-dimensional (3-D) train-track interaction model, where the train is modelled as a group of multi-rigid-body vehicles connected by coupler and gear draft systems, and the tracks are modelled as ballastless tracks using finite element method, and finally the train and the tracks are integrated into an entire system by train-rail coupling matrices. In this model, the time-dependent coupling matrices are developed to characterize the train-track interaction in 3-D space. Mostly important the difficulties in establishing dynamic equations of motion for train-track systems by matrix representations of wheel-rail separation and infinite length computation are properly solved. The numerical results show that this model has high efficiency, stability and accuracy in the calculation of train-track interactions. Besides this model can further provide insights into a number of railway dynamics related subjects, such as train-track dynamic assessment and train derailment.     

A computational model for solid waste management with application

A model for regional solid waste management as a network flow problem is described, and a special purpose algorithm is developed. The model is applied to waste management and facility siting decisions in the Munich Metropolitan Area in the Federal Republic of Germany.     

A computational model for human eye-movements in military simulations

Models of eye movements of an observer searching for human targets are helpful in developing accurate models of target acquisition times and false positive detections. We develop a new model describing the distribution of gaze positions for an observer which includes both bottom-up (salience) and top-down (task dependent) factors. We validate the combined model against a bottom-up model from the literature and against the bottom up and top down parts alone using human performance data on stationary targets. The new model is shown to be significantly better. The new model requires a large amount of data about the terrain and the target that is obtained directly from the 3D simulation through an automated process.     

A mathematical model for fungal development in heterogeneous environments

A mathematical model for the development of fungal mycelia in heterogeneous environmental conditions is presented. The validity of this model is tested by comparison of numerical simulations with experimental observations.     

A simple one-dimensional model for the three-dimensional vorticity equation

A simple qualitative one-dimensional model for the 3-D vorticity equation of incompressible fluid flow is developed. This simple model is solved exactly; despite its simplicity, this equation retains several of the most important structural features in the vorticity equations and its solutions exhibit some of the phenomena observed in numerical computations for breakdown for the 3-D Euler equations.     

A duopoly model with heterogeneous congestion-sensitive customers

This paper analyzes a model with two firms (providers), and two classes of customers. These customers classes are characterized by their attitude towards ‘congestion’ (caused by other customers using the same resources); a firm is selected on the basis of both the prices charged by the firms, and the ‘congestion levels’. The model can be represented by a two-stage game: in the first providers set their prices, whereas in the second the customers choose the provider (or to not use any service at all) for given prices. We explicitly allow the providers to split their resources, in order to serve more than just one market segment. This enables us to further analyze the Paris metro pricing (Pmp) proposal for service differentiation in the Internet.     

A diffusive predator-prey model in heterogeneous environment

In this paper, we demonstrate some special behavior of steady-state solutions to a predator-prey model due to the introduction of spatial heterogeneity. We show that positive steady-state solutions with certain prescribed spatial patterns can be obtained when the spatial environment is designed suitably. Moreover, we observe some essential differences of the behavior of our model from that of the classical Lotka-Volterra model that seem to arise only in the heterogeneous case.     

A computational toy model for shallow landslides: Molecular dynamics approach

The aim of this paper is to propose a 2D computational algorithm for modeling the triggering and propagation of shallow landslides caused by rainfall. We used a molecular dynamics (MD) approach, similar to the discrete element method (DEM), that is suitable to model granular material and to observe the trajectory of a single particle, so to possibly identify its dynamical properties. We consider that the triggering of shallow landslides is caused by the decrease of the static friction along the sliding surface due to water infiltration by rainfall. Thence the triggering is caused by the two following conditions: (a) a threshold speed of the particles and (b) a condition on the static friction, between the particles and the slope surface, based on the Mohr–Coulomb failure criterion. The latter static condition is used in the geotechnical model to estimate the possibility of landslide triggering. The interaction force between particles is modeled, in the absence of experimental data, by means of a potential similar to the Lennard-Jones one. The viscosity is also introduced in the model and for a large range of values of the model’s parameters, we observe a characteristic velocity pattern, with acceleration increments, typical of real landslides. The results of simulations are quite promising: the energy and time triggering distribution of local avalanches show a power law distribution, analogous to the observed Gutenberg–Richter and Omori power law distributions for earthquakes. Finally, it is possible to apply the method of the inverse surface displacement velocity [4] for predicting the failure time.     

A computational model for multi-agent E-commerce negotiations with adaptive negotiation behaviors

This paper presents a computational model to organize multi-agent E-commerce negotiations with adaptive negotiation behaviors aiming at enhancing the negotiation flexibilities of software agents. Firstly, the computational E-commerce negotiation model covering negotiation protocol, negotiation issues and negotiation strategies is specified to assist agents’ computing functions. Then, a three-staged adaptive negotiation behavior configuration mechanism is proposed to tackle the negotiation dynamics. In the pre-negotiation stage, agents’ negotiation behaviors are deployed by the case-based strategy assignment mechanism; in the on-going negotiation stage, opponents’ negotiation behaviors are tracked through the neural network learning model; in the post-negotiation stage, opponents’ concession functions are recorded using the time series measure. Finally, the computational negotiation model is tested through hypothetical negotiation cases. The outcomes show that the adaptive negotiation behavior configuration mechanism can benefit an agent to win more in the E-commerce negotiation.     

A multiphase single relaxation time lattice Boltzmann model for heterogeneous porous media

The lattice Boltzmann (LB) method has been shown to be a highly efficient numerical method for solving fluid flow in confined domains such as pipes, irregularly shaped channels or porous media. Traditionally the LB method has been applied to flow in void regions (pores) and no flow in solid regions. However, in a number of scenarios, this may not suffice. That is partial flow may occur in semi-porous regions. Recently gray-scale LB methods have been applied to model single phase flow in such semi-porous materials. Voxels are no longer completely void or completely solid but somewhere in between. We extend the single relaxation time LB method to model multiphase, immiscible flow (e.g., gas and liquid or water and oil) in a semi-porous medium. We compare the solution to test cases and find good agreement of the model as compared to analytical solutions. We then apply the model to real porous media and recover both capillary and viscous flow regimes. However, some deficiencies in the single relaxation time LB method applied to multiphase flow are uncovered and we describe methods to overcome these limitations.     

A fracture angle based three-dimensional damage model for unidirectional fibre-reinforced plastics

Since the failure analysis of fibre-reinforced plastics is not limited to the first-ply failure, it is mandatory to use adequate damage models to simulate the failure process. The paper describes a damage model for three-dimensional stress states, which uses the crack orientation of the inter-fibre fracture (IFF). The fracture angle describes the crack orientation and can be obtained by using Puck's IFF criterion. The fracture angle enables the possibility to take the causal stress situation for the IFF into account. This means that each Young's or shear modulus has its own damage function, which depends on the stress state and the fracture angle. Therefore, the stress-strain extrapolation method of Schürmann and Weber for two dimensional stress states was advanced and modified for the three-dimensional stress space. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anisotropic moisture depending viscoelastic material model for wood

A moisture depending anisotropic viscoelastic material model is presented in this paper. The necessity of consideration cylindrical anisotropy is caused by the growing process. Wood exhibits different creep characteristics depending on the state of stress. Therefore, the consideration of anisotropic viscoelasticity is required. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)