Bistability of buoyancy-driven exchange flows in vertical tubes

Buoyancy-driven exchange flows are common to a variety of natural and engineering systems, ranging from persistently active volcanoes to counterflows in oceanic straits. Laboratory experiments of exchange flows have been used as surrogates to elucidate the basic features of such flows. The resulting data have been analysed and interpreted mostly through core–annular flow solutions, the most common flow configuration at finite viscosity contrasts. These models have been successful in fitting experimental data, but less effective at explaining the variability observed in natural systems. In this paper, we demonstrate that some of the variability observed in laboratory experiments and natural systems is a consequence of the inherent bistability of core–annular flow. Using a core–annular solution to the classical problem of buoyancy-driven exchange flows in vertical tubes, we identify two mathematically valid solutions at steady state: a solution with fast flow in a thin core and a solution with relatively slow flow in a thick core. The theoretical existence of two solutions, however, does not necessarily imply that the system is bistable in the sense that flow switching may occur. Through direct numerical simulations, we confirm the hypothesis that core–annular flow in vertical tubes is inherently bistable. Our simulations suggest that the bistability of core–annular flow is linked to the boundary conditions of the domain, which implies that is not possible to predict the realized flow field from the material parameters of the fluids and the tube geometry alone. Our finding that buoyancy-driven exchange flows are inherently bistable systems is consistent with previous experimental data, but is in contrast to the underlying hypothesis of previous analytical models that the solution is unique and can be identified by maximizing the flux or extremizing the dissipation in the system. Our results have important implications for data interpretation by analytical models and may also have interesting ramifications for understanding volcanic degassing

Graph Transformations for the Resource Description Framework

The Resource Description Framework (RDF) is a standard developed by the World Wide Web Consortium (W3C) to facilitate the representation and exchange of structured (meta-)data in the "SemanticWeb". While there is a large body of work dealing with inference on RDF, a concept for transformation and manipulation is still missing. Since RDF uses graphs as a formal basis, this paper proposes the use of algebraic graph transformations with their wealth of well-known constructions and results for this purpose. It turns out that RDF graphs are an interesting application area for graph transformation methods, where some significant differences to classical graphs yield practically relevant solutions for features like attribution, typing and globally unique nodes

Managing and Presenting Digital Content in the ARHiNET System

ARHiNET is a network information system for describing, processing and managing archival records created in 2006 by the Croatian State Archives and Avicena Software Company. It is a national archival system in Croatia, recognized by the Ministry of Culture as national project, as well as part of the e- Croatia program, the operational plan of the Government of the Republic of Croatia. Development of the archival information and institutions network is a long-term strategic archival service project and ARHiNET implementation enhanced the standardization of the archival institutions work, and enabled establishment of a unique system of processing and description of archival material, as well as data integration and exchange between the institutions that keep archival records. All archives in Croatia are included in the implementation of this unique archival information system that comprises all business processes in archival institutions, together with some other records holders under the state archives supervision. Currently, there are about 700 registered users from more than 150 institutions. Designing, realization, introduction, use, maintenance and development of such a complex program solution enclose permanent activities on system improvement, finding new functionalities and solutions, as well as upgrading of the present ones. During the three years of the system operating, more than 300 versions of program solutions have been developed and put in production, and experiences gained from work and user education led to the development of the version 2.0 that was released in February 2009. In this article authors present solutions and functionalities concerning managing, indexing and presentation of digital content developed and implemented within the ARHiNET program solution

Time machines and the Principle of Self-Consistency as a consequence of the Principle of Stationary Action (II): the Cauchy problem for a self-interacting relativistic particle

We consider the action principle to derive the classical, relativistic motion of a self-interacting particle in a 4-D Lorentzian spacetime containing a wormhole and which allows the existence of closed time-like curves. In particular, we study the case of a pointlike particle subject to a `hard-sphere' self-interaction potential and which can traverse the wormhole an arbitrary number of times, and show that the only possible trajectories for which the classical action is stationary are those which are globally self-consistent. Generically, the multiplicity of these trajectories (defined as the number of self-consistent solutions to the equations of motion beginning with given Cauchy data) is finite, and it becomes infinite if certain constraints on the same initial data are satisfied. This confirms the previous conclusions (for a non-relativistic model) by Echeverria, Klinkhammer and Thorne that the Cauchy initial value problem in the presence of a wormhole `time machine' is classically `ill-posed' (far too many solutions). Our results further extend the recent claim by Novikov et al. that the `Principle of self-consistency' is a natural consequence of the `Principle of minimal action.'Comment: 39 pages, latex fil

PRODUCT LIFECYCLE DATA SHARING AND VISUALISATION: WEB-BASED APPROACHES

Both product design and manufacturing are intrinsically collaborative processes. From conception and design to project completion and ongoing maintenance, all points in the lifecycle of any product involve the work of fluctuating teams of designers, suppliers and customers. That is why companies are involved in the creation of a distributed design and a manufacturing environment which could provide an effective way to communicate and share information throughout the entire enterprise and the supply chain. At present, the technologies that support such a strategy are based on World Wide Web platforms and follow two different paths. The first one focuses on 2D documentation improvement and introduces 3D interactive information in order to add knowledge to drawings. The second one works directly on 3D models and tries to extend the life of 3D data moving these design information downstream through the entire product lifecycle. Unfortunately the actual lack of a unique 3D Web-based standard has stimulated the growing up of many different proprietary and open source standards and, as a consequence, a production of an incompatible information exchange over the WEB. This paper proposes a structured analysis of Web-based solutions, trying to identify the most critical aspects to promote a unique 3D digital standard model capable of sharing product and manufacturing data more effectively—regardless of geographic boundaries, data structures, processes or computing environmen

Compartmental analysis of dynamic nuclear medicine data: models and identifiability

Compartmental models based on tracer mass balance are extensively used in clinical and pre-clinical nuclear medicine in order to obtain quantitative information on tracer metabolism in the biological tissue. This paper is the first of a series of two that deal with the problem of tracer coefficient estimation via compartmental modelling in an inverse problem framework. Specifically, here we discuss the identifiability problem for a general n-dimension compartmental system and provide uniqueness results in the case of two-compartment and three-compartment compartmental models. The second paper will utilize this framework in order to show how non-linear regularization schemes can be applied to obtain numerical estimates of the tracer coefficients in the case of nuclear medicine data corresponding to brain, liver and kidney physiology