Showing papers by "Helsinki University of Technology published in 2008"
TL;DR: In this paper, the authors defined severe plastic deformation (SPD) as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal.
Abstract: Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal The objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by using high strength metal for the safety and reliability of micro-parts and for environmental harmony In this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative roll-bonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals processed by the SPD processes are shown Moreover, the combined processes developed recently are also explained Finally, the applications of the ultra-fine grained (UFG) metals are discussed
849 citations
Book•
12 Jan 2008TL;DR: In this article, a historical review of spinors is given together with a construction of spinor spaces as minimal left ideals of Clifford algebras, which induce bilinear forms or scalar products on spinors.
Abstract: A historical review of spinors is given together with a construction of spinor spaces as minimal left ideals of Clifford algebras. Spinor spaces of euclidean spaces over reals have a natural linear structure over reals, complex numbers or quaternions. Clifford algebras have involutions which induce bilinear forms or scalar products on spinor spaces. The automorphism groups of these scalar products of spinors are determined and also classified.
749 citations
TL;DR: In this paper, the differences between the tools are discussed and the current situation within the tools is critically analyzed, and the benefits of using the tools should be analyzed. But the comparison of the tools and their results is difficult, if not impossible.
673 citations
TL;DR: In this paper, simple analytical formulas are introduced for the grid impedance of electrically dense arrays of square patches and for the surface impedance of high-impedance surfaces based on the dense array of metal strips or square patches over ground planes.
Abstract: Simple analytical formulas are introduced for the grid impedance of electrically dense arrays of square patches and for the surface impedance of high-impedance surfaces based on the dense arrays of metal strips or square patches over ground planes. Emphasis is on the oblique-incidence excitation. The approach is based on the known analytical models for strip grids combined with the approximate Babinet principle for planar grids located at a dielectric interface. Analytical expressions for the surface impedance and reflection coefficient resulting from our analysis are thoroughly verified by full-wave simulations and compared with available data in open literature for particular cases. The results can be used in the design of various antennas and microwave or millimeter wave devices which use artificial impedance surfaces and artificial magnetic conductors (reflect-array antennas, tunable phase shifters, etc.), as well as for the derivation of accurate higher-order impedance boundary conditions for artificial (high-) impedance surfaces. As an example, the propagation properties of surface waves along the high-impedance surfaces are studied.
636 citations
TL;DR: Paakko et al. as mentioned in this paper demonstrate that such aqueous nanofibrillar gels are unexpectedly robust to allow formation of highly porous aerogels by direct water removal by freeze-drying.
Abstract: Recently it was shown that enzymatic and mechanical processing of macroscopic cellulose fibers lead to disintegration of long and entangled native cellulose I nanofibers in order to form mechanically strong aqueous gels (Paakko et al., Biomacromolecules, 2007, 8, 1934). Here we demonstrate that (1) such aqueous nanofibrillar gels are unexpectedly robust to allow formation of highly porous aerogels by direct water removal by freeze-drying, (2) they are flexible, unlike most aerogels that suffer from brittleness, and (3) they allow flexible hierarchically porous templates for functionalities, e.g. for electrical conductivity. No crosslinking, solvent exchange nor supercritical drying are required to suppress the collapse during the aerogel preparation, unlike in typical aerogel preparations. The aerogels show a high porosity of ∼98% and a very low density of ca. 0.02 g cm−3. The flexibility of the aerogels manifests as a particularly high compressive strain of ca. 70%. In addition, the structure of the aerogels can be tuned from nanofibrillar to sheet-like skeletons with hierarchical micro- and nanoscale morphology and porosity by modifying the freeze-drying conditions. The porous flexible aerogel scaffold opens new possibilities for templating organic and inorganic matter for various functionalities. This is demonstrated here by dipping the aerogels in an electrically conducting polyaniline–surfactant solution which after rinsing off the unbound conducting polymer and drying leads to electrically conducting flexible aerogels with relatively high conductivity of around 1 × 10−2 S cm−1. More generally, we foresee a wide variety of functional applications for highly porous flexible biomatter aerogels, such as for selective delivery/separation, tissue-engineering, nanocomposites upon impregnation by polymers, and other medical and pharmaceutical applications.
595 citations
TL;DR: In this article, the methods of small-footprint airborne laser scanning for extracting forest inventory data, mainly in the boreal forest zone, are reviewed and some recommendations for future research are given to foster the methodology development.
Abstract: Experiences from Nordic countries and Canada have shown that the retrieval of the stem volume and mean tree height of a tree or at stand level from laser scanner data performs as well as, or better than, photogrammetric methods, and better than other remote sensing methods. This paper reviews the methods of small-footprint airborne laser scanning for extracting forest inventory data, mainly in the boreal forest zone. The methods are divided into the following categories: extraction of terrain and canopy height model; feature extraction approaches (canopy height distribution and individual-tree-based techniques, techniques based on the synergetic use of aerial images and lidar, and other new approaches); tree species classification and forest growth using laser scanner; and the use of intensity and waveform data in forest information extraction. Despite this, the focus is on methods, some review of quality obtained, especially in the boreal forest area, is included. Several recommendations for future research are given to foster the methodology development.
573 citations
533 citations
TL;DR: A detailed study of the energetics of periodic graphene antidot lattices is presented, the level structure of a single defect is analyzed, the exchange coupling between a pair of spin qubits is calculated, and possible avenues for further developments are identified.
Abstract: Antidot lattices, defined on a two-dimensional electron gas at a semiconductor heterostructure, are a well-studied class of man-made structures with intriguing physical properties. We point out that a closely related system, graphene sheets with regularly spaced holes ("antidots"), should display similar phenomenology, but within a much more favorable energy scale, a consequence of the Dirac fermion nature of the states around the Fermi level. Further, by leaving out some of the holes one can create defect states, or pairs of coupled defect states, which can function as hosts for electron spin qubits. We present a detailed study of the energetics of periodic graphene antidot lattices, analyze the level structure of a single defect, calculate the exchange coupling between a pair of spin qubits, and identify possible avenues for further developments.
483 citations
TL;DR: Computer simulations use computer simulations to explore the translocation of fullerene clusters through a model lipid membrane and the effect of high fulleanne concentrations on membrane properties, suggesting that mechanical damage is an unlikely mechanism for membrane disruption and fullerenes toxicity.
Abstract: Recent toxicology studies suggest that nanosized aggregates of fullerene molecules can enter cells and alter their functions, and also cross the blood-brain barrier However, the mechanisms by which fullerenes penetrate and disrupt cell membranes are still poorly understood Here we use computer simulations to explore the translocation of fullerene clusters through a model lipid membrane and the effect of high fullerene concentrations on membrane properties The fullerene molecules rapidly aggregate in water but disaggregate after entering the membrane interior The permeation of a solid-like fullerene aggregate into the lipid bilayer is thermodynamically favoured and occurs on the microsecond timescale High concentrations of fullerene induce changes in the structural and elastic properties of the lipid bilayer, but these are not large enough to mechanically damage the membrane Our results suggest that mechanical damage is an unlikely mechanism for membrane disruption and fullerene toxicity
483 citations
TL;DR: This paper argues from a critical perspective that it is important to be aware of the inherent problems associated with dominant discourses as well as to actively advance the use of alternative ones and shows that strategy as practice involves alternative and even competing discourses that have fundamentally different kinds of implications for participation in strategy work.
Abstract: We still know little of why strategy processes often involve participation problems. In this paper, we argue that this crucial issue is linked to fundamental assumptions about the nature of strategy work. Hence, we need to examine how strategy processes are typically made sense of and what roles are assigned to specific organizational members. For this purpose, we adopt a critical discursive perspective that allows us to discover how specific conceptions of strategy work are reproduced and legitimized in organizational strategizing. Our empirical analysis is based on an extensive research project on strategy work in 12 organizations. As a result of our analysis, we identify three central discourses that seem to be systematically associated with nonparticipatory approaches to strategy work: “mystification,” “disciplining,” and “technologization.” However, we also distinguish three strategy discourses that promote participation: “self-actualization,” “dialogization,” and “concretization.” Our analysis shows that strategy as practice involves alternative and even competing discourses that have fundamentally different kinds of implications for participation in strategy work. We argue from a critical perspective that it is important to be aware of the inherent problems associated with dominant discourses as well as to actively advance the use of alternative ones.
413 citations
26 May 2008
TL;DR: A movement model for delay-tolerant network simulations that is able to produce inter-contact time and contact time distributions that follow closely the ones found in the traces from the real-world measurement experiments is presented.
Abstract: movement models, such as Random Waypoint, do not capture reliably the properties of movement in the real life scenarios. We present and analyse a movement model for delay-tolerant network simulations that is able to produce inter-contact time and contact time distributions that follow closely the ones found in the traces from the real-world measurement experiments. We validate the movement model using the ONE simulator.
TL;DR: In this article, a turbocharged charge air cooled common rail heavy duty diesel engine was used to test different fuel injection timings in a common rail diesel engine and showed that there is potential for optimizing engine settings together with enhanced fuel composition.
Abstract: Hydrotreating of vegetable oils or animal fats is an alternative process to esterification for producing biobased diesel fuels. Hydrotreated products are also called renewable diesel fuels. Hydrotreated vegetable oils (HVO) do not have the detrimental effects of ester-type biodiesel fuels, like increased NO x emission, deposit formation, storage stability problems, more rapid aging of engine oil or poor cold properties. HVOs are straight chain paraffinic hydrocarbons that are free of aromatics, oxygen and sulfur and have high cetane numbers. In this paper, NO x ‐ particulate emission trade-off and NO x ‐ fuel consumption trade-off are studied using different fuel injection timings in a turbocharged charge air cooled common rail heavy duty diesel engine. Tested fuels were sulfur free diesel fuel, neat HVO, and a 30% HVO + 70% diesel fuel blend. The study shows that there is potential for optimizing engine settings together with enhanced fuel composition. HVO could be used in optimized low emission diesel power trains in captive fleet applications like city buses, indoor fork-lift trucks, or mine vehicles.
TL;DR: In this paper, a modified microfibrillated cellulose (MFC) was prepared by disintegration of bleached softwood sulphite pulp through mechanical homogenization, and the surface of the MFC was modified using different chemical treatments, using reactions both in aqueous and organic solvents.
Abstract: Microfibrillated cellulose (MFC) was prepared by disintegration of bleached softwood sulphite pulp through mechanical homogenization. The surface of the MFC was modified using different chemical treatments, using reactions both in aqueous- and organic solvents. The modified MFC was characterized with fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Epoxy functionality was introduced onto the MFC surface by oxidation with cerium (IV) followed by grafting of glycidyl methacrylate. The length of the polymer chains could be varied by regulating the amount of glycidyl methacrylate added. Positive charge was introduced to the MFC surface through grafting of hexamethylene diisocyanate, followed by reaction with the amines. Succinic and maleic acid groups could be introduced directly onto the MFC surface as a monolayer by a reaction between the corresponding anhydrides and the surface hydroxyl groups of the MFC.
TL;DR: Based on an analysis of the most active acquirers in seven industry sectors in the United States in the 1990s, it is found that both a high rate of acquisitions and a high variability of the rate are negatively related to performance.
Abstract: Based on an analysis of the most active acquirers in seven industry sectors in the United States in the 1990s, we find that both a high rate of acquisitions and a high variability of the rate are negatively related to performance. An acquirer's size, the scope of its acquisition program, and acquisition experience moderate the relationship by weakening the negative effects. Our findings contribute to an improved understanding of acquisition capabilities and program-level acquisition performance, thereby adding to an emerging stream of research that is building an acquisition program perspective.
TL;DR: This article estimates the changes in parameters of the Tonle Sap flood pulse due to the flow alterations in the Mekong River and the impacts on the flooded area and loss of gallery forest and protected areas were analyzed using geographic information system–based methods.
Abstract: Rapid development in the upper reaches of the Mekong River, in the form of construction of large hydropower dams and reservoirs, large irrigation schemes, and rapid urban development, is putting water resources under stress. Recent studies have concluded that these developments will lead to flow alterations in the Mekong River. These flow alterations would threaten the sensitive ecosystems downstream, particularly Tonle Sap River, Tonle Sap Lake, its floodplain, and its gallery forest and protected areas, by changing the flood-pulse system of the lake. This article estimates the changes in parameters of the Tonle Sap flood pulse due to the aforementioned flow alterations. The impacts on the flooded area and loss of gallery forest and protected areas were analyzed using geographic information system–based methods. Relatively small rises in the dry-season lake water level would permanently inundate disproportionately large areas of floodplain, rendering it inaccessible to floodplain vegetation and eroding the productivity basis of the ecosystem. It is highly important to maintain the natural hydrological pattern of the Mekong River, particularly the dry-season water levels, to preserve Tonle Sap Lake’s ecosystem productivity.
TL;DR: A new Rauch-Tung-Striebel type form of the fixed-interval unscented Kalman smoother is derived, which is not based on running two independent filters forward and backward in time, but on a separate backward smoothing pass which recursively computes corrections to the forward filtering result.
Abstract: This note considers the application of the unscented transform to optimal smoothing of nonlinear state-space models. In this note, a new Rauch-Tung-Striebel type form of the fixed-interval unscented Kalman smoother is derived. The new smoother differs from the previously proposed two-filter-formulation-based unscented Kalman smoother in the sense that it is not based on running two independent filters forward and backward in time. Instead, a separate backward smoothing pass is used, which recursively computes corrections to the forward filtering result. The smoother equations are derived as approximations to the formal Bayesian optimal smoothing equations. The performance of the new smoother is demonstrated with a simulation.
TL;DR: In this paper, the authors studied a pulp mill construction project in Uruguay to identify the different strategies project stakeholders use to increase their salience, including direct withholding, indirect withholding, resource building, coalition building, conflict escalation, creditability building, communication, and direct action.
TL;DR: This paper describes a matrix decomposition formulation for Boolean data, the Discrete Basis Problem, and gives a simple greedy algorithm for solving it and shows how it can be solved using existing methods.
Abstract: Matrix decomposition methods represent a data matrix as a product of two factor matrices: one containing basis vectors that represent meaningful concepts in the data, and another describing how the observed data can be expressed as combinations of the basis vectors. Decomposition methods have been studied extensively, but many methods return real-valued matrices. Interpreting real-valued factor matrices is hard if the original data is Boolean. In this paper, we describe a matrix decomposition formulation for Boolean data, the Discrete Basis Problem. The problem seeks for a Boolean decomposition of a binary matrix, thus allowing the user to easily interpret the basis vectors. We also describe a variation of the problem, the Discrete Basis Partitioning Problem. We show that both problems are NP-hard. For the Discrete Basis Problem, we give a simple greedy algorithm for solving it; for the Discrete Basis Partitioning Problem we show how it can be solved using existing methods. We present experimental results for the greedy algorithm and compare it against other, well known methods. Our algorithm gives intuitive basis vectors, but its reconstruction error is usually larger than with the real-valued methods. We discuss about the reasons for this behavior.
TL;DR: The authors conducted an exploratory study of Sussex University, a medium-sized, research-oriented university, which launched technology transfer activities in the mid-1990s and found that a considerable number of researchers engage in knowledge exchange processes with industry and other non-academic partners.
Abstract: This paper seeks to contribute to the growing literature on the entrepreneurial university by mapping knowledge exchange relationships of entrepreneurial faculty in a comprehensive manner and also capturing faculty attitudes towards measures taken by the university administration to promote entrepreneurship. We report on an exploratory study of Sussex University, a medium-sized, research-oriented university, which launched technology transfer activities in the mid-1990s. Our results show that, in spite of a comparatively late start, a considerable number of researchers engage in knowledge exchange processes with industry and other non-academic partners. We present evidence that suggests faculty in the social sciences and humanities as well as natural sciences and engineering maintain links to industrial partners, including multinational corporations. Furthermore, our observations indicate that schools differ in the way their faculty engage in university–industry collaborations. Further differences can be observed with respect to faculty attitudes towards technology transfer and awareness of the university’s respective codes of practice.
TL;DR: Alkanethiols and oleic acid were found to improve air stability and oxidation resistance of thiol-capped copper nanoparticles was found to increase with the chain length of the thiol, however, excess thiol caused etching of the particles under nitrogen.
TL;DR: In this article, cellulose nanofibrils were used together with a cationic polylelectrolyte, poly(amideamine) epichlorohydrin (PAE), to enhance the wet and dry strength of paper.
Abstract: In this paper cellulose nanofibrils were used together with a cationic polylelectrolyte, poly(amideamine) epichlorohydrin (PAE), to enhance the wet and the dry strength of paper. The adsorption of nanofibrils and PAE on cellulose model surfaces was studied using quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). The differences in fibril and polyelectrolyte adding strategies onto cellulose fibres were studied by comparing layer-structures and nano-aggregates formed by the nanofibrils and PAE. The results showed that when PAE was first adsorbed on the model fibre surface a uniform and viscous layer of nanofibrils could be adsorbed. When PAE and nanofibrils were adsorbed as cationic aggregates a non-uniform and more rigid layer was adsorbed. Paper sheets were prepared using both the bi-layer and nano-aggregate adding strategy of the nanofibrils and PAE. When PAE and nanofibrils were adsorbed on pulp fibres as a bi-layer system significant increase in both wet and dry tensile strength of paper could be achieved even at low added amounts of PAE. When the substances were added as nano-aggregates the improvements in paper strength properties were not as significant. Bulk and surface nitrogen content analyses of the paper samples showed that the adding strategy does not affect the total adsorbed amount of PAE but it has a strong effect on distribution of substances in the paper matrix which has a crucial effect on paper wet and dry strength development.
TL;DR: A sequential clique percolation algorithm (SCP) is presented to do fast community detection in weighted and unweighted networks, for cliques of a chosen size and can simultaneously produce a dendrogram representation of hierarchical community structure.
Abstract: In complex network research clique percolation, introduced by Palla, Der\'enyi, and Vicsek [Nature (London) 435, 814 (2005)], is a deterministic community detection method which allows for overlapping communities and is purely based on local topological properties of a network. Here we present a sequential clique percolation algorithm (SCP) to do fast community detection in weighted and unweighted networks, for cliques of a chosen size. This method is based on sequentially inserting the constituent links to the network and simultaneously keeping track of the emerging community structure. Unlike existing algorithms, the SCP method allows for detecting $k$-clique communities at multiple weight thresholds in a single run, and can simultaneously produce a dendrogram representation of hierarchical community structure. In sparse weighted networks, the SCP algorithm can also be used for implementing the weighted clique percolation method recently introduced by Farkas et al. [New J. Phys. 9, 180 (2007)]. The computational time of the SCP algorithm scales linearly with the number of $k$-cliques in the network. As an example, the method is applied to a product association network, revealing its nested community structure.
TL;DR: This study employs atomistic simulations and experiments to characterize a cholesterol compound with fluorescent boron dipyrromethene difluoride linked to sterol carbon‐24 (BODIPY‐cholesterol), and concludes that this new probe closely mimics the membrane partitioning and trafficking of cholesterol and enables the direct monitoring of sterol movement by time‐lapse imaging using trace amounts of the probe.
Abstract: Analysis of sterol distribution and transport in living cells has been hampered by the lack of bright, photostable fluorescent sterol derivatives that closely resemble cholesterol. In this study, we employed atomistic simulations and experiments to characterize a cholesterol compound with fluorescent boron dipyrromethene difluoride linked to sterol carbon-24 (BODIPY-cholesterol). This probe packed in the membrane and behaved similarly to cholesterol both in normal and in cholesterol-storage disease cells and with trace amounts allowed the visualization of sterol movement in living systems. Upon injection into the yolk sac, BODIPY-cholesterol did not disturb zebrafish development and was targeted to sterol-enriched brain regions in live fish. We conclude that this new probe closely mimics the membrane partitioning and trafficking of cholesterol and, because of its excellent fluorescent properties, enables the direct monitoring of sterol movement by time-lapse imaging using trace amounts of the probe. This is, to our knowledge, the first cholesterol probe that fulfills these prerequisites.
TL;DR: It is demonstrated that the main effect of monovalent salt on a phospholipid membrane is determined by cations binding to the carbonyl region of a membrane, while chloride anions mostly stay in the water phase.
Abstract: To gain a better understanding of how monovalent salt under physiological conditions affects plasma membranes, we have performed 200 ns atomic-scale molecular dynamics simulations of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) lipid bilayers. These two systems provide representative models for the outer and inner leaflets of the plasma membrane, respectively. The implications of cation−lipid interactions in these lipid systems have been considered in two different aqueous salt solutions, namely NaCl and KCl, and the sensitivity of the results on the details of interactions used for ions is determined by repeating the simulations with two distinctly different force fields. We demonstrate that the main effect of monovalent salt on a phospholipid membrane is determined by cations binding to the carbonyl region of a membrane, while chloride anions mostly stay in the water phase. It turns out that the strength and character of the cation−lipid interactions are quite different for different types...
TL;DR: The extended RPM framework permits more comprehensive modeling of portfolio problems and provides support for advanced benefit-cost analyses and it retains the key features of RPM by providing robust project and portfolio recommendations and by identifying projects on which further attention should be focused.
TL;DR: Native cellulose model films containing both amorphous and crystalline cellulose I regions were prepared by spin-coating aqueous cellulose nanofibril dispersions onto silica substrates and showed the best coverage for the low charged fibrils was achieved by using 3-aminopropyltrimethoxysilane (APTS).
TL;DR: In this article, the authors investigate the relationship between project portfolio control techniques and portfolio management performance and how this relationship is moderated by situational idiosyncrasies of internal and external dynamics, industries, governance types, and geographic location.
Abstract: This article investigates the nature and relationship of project portfolio control techniques and portfolio management performance, and how this relationship is moderated by situational idiosyncrasies of internal and external dynamics, industries, governance types, and geographic location. A worldwide questionnaire with 242 responses was used, of which 136 high-performing responses were filtered out for quantitative analysis of best practices. Three portfolio control factors were identified: portfolio selection, portfolio reporting, and decision-making style. Two measures for portfolio management performance were identified: achievement of desired portfolio results and achievement of project and program purpose. The results indicate that different portfolio control mechanisms are associated with different performance measures. A contingency model was developed, including moderating effects by contextual variables.
TL;DR: Numerical results indicate that the optical loss of the Gaussian guided mode can be made very small by tuning the thickness of the dielectric layer and that the refractive-index resolution for aqueous analytes is 1x 10(-4).
Abstract: We propose a novel surface-plasmon-resonance sensor design based on coating the holes of a three-hole microstructured optical fiber with a low-index dielectric layer on top of which a gold layer is deposited. The use of all three fiber holes and their relatively large size should facilitate the fabrication of the inclusions and the infiltration of the analyte. Our numerical results indicate that the optical loss of the Gaussian guided mode can be made very small by tuning the thickness of the dielectric layer and that the refractive-index resolution for aqueous analytes is 1×10-4.
TL;DR: In this paper, the authors investigated the transport properties of superconductor-normal-metal-superconductor (S-N-S) junctions at low temperatures by measuring the electron temperature in the normal metal.
Abstract: We investigate hysteresis in the transport properties of superconductor-normal-metal-superconductor (S-N-S) junctions at low temperatures by measuring directly the electron temperature in the normal metal. Our results demonstrate unambiguously that the hysteresis results from an increase of the normal-metal electron temperature once the junction switches to the resistive state. In our geometry, the electron temperature increase is governed by the thermal resistance of the superconducting electrodes of the junction.
TL;DR: In this paper, a detailed analysis of the real data of the International Trade Network (ITN) is presented, and it is shown that the scaled link weight distribution has an approximate log-normal distribution which remains robust over a period of 53 years.
Abstract: Tools of the theory of critical phenomena, namely the scaling analysis and universality, are argued to be applicable to large complex web-like network structures. Using a detailed analysis of the real data of the International Trade Network we argue that the scaled link weight distribution has an approximate log-normal distribution which remains robust over a period of 53 years. Another universal feature is observed in the power-law growth of the trade strength with gross domestic product, the exponent being similar for all countries. Using the 'rich-club' coefficient measure of the weighted networks it has been shown that the size of the rich-club controlling half of the world's trade is actually shrinking. While the gravity law is known to describe well the social interactions in the static networks of population migration, international trade, etc, here for the first time we studied a non-conservative dynamical model based on the gravity law which excellently reproduced many empirical features of the ITN.