Computers have transformed how we think, discuss and learn—as individuals, in groups, within cultures and globally. However, social media are problematic, fostering flaming, culture wars and fake news. This volume presents an alternative paradigm for computer support of group thinking, collaborative learning and joint knowledge construction. This requires expanding concepts of cognition to collectivities, like collaborative groups of networked students. Theoretical Investigations explores the conditions for group cognition, supplying a philosophical foundation for new models of pedagogy and methods to analyze group interaction. Twenty-five self-contained investigations document progress in research on computer-supported collaborative learning (CSCL)—both in Stahl’s own research and during the first decade of the CSCL journal. The volume begins with two new reflections on the vision and theory that result from this research. Representing both ethnomethodological and social-constructivist research paradigms, the investigations within this volume comprise a selection of seminal and influential articles and critical commentaries that contribute to an understanding of concepts and themes central to the CSCL field. The book elaborates an innovative theory of group cognition and substantiates the pedagogical potential of CSCL. Theoretical Investigations: Philosophical Foundations of Group Cognition is essential as a graduate text for courses in educational theory, instructional design, learning and networked technologies. The investigations will also appeal to researchers and practitioners in those areas.

Metapopulation Dynamics: Empirical and Theoretical Investigations covers the 1989 proceedings of a metapopulation dynamics workshop held at Lammi Biological Station, Helsinki, Finland. It is divided into 18 chapters that cover various approaches to spatially structured population and community dynamics. After briefly discussing the history of metapopulation ideas and the major conceptual links, the book covers types of studies that have been conducted on single-species and multispecies metapopulations. Then, it examines the relationships between metapopulation dynamics, the equilibrium theory of island biogeography, and the dynamics of populations living in patchy environments. It further tackles practical issues and the links between metapopulation dynamics and landscape ecology, and between metapopulation dynamics and conservation biology. Chapters 4 and 5 present structured models describing changes in the number of individuals within patches and an empirical evaluation of local extinction in metapopulation studies. The subsequent chapters discuss several aspects of metapopulation, including dispersal and connectivity, colonization, conspecific attraction, extinction and isolation, and forest fragmentation. The latter chapters describe the concept of habitat fragmentation, the diversity and competition in metapopulations, the community collapse, and the effects of metapopulation studies in predator-prey systems.

This document covers the topic of propulsion systems that include rocket propulsion and jet propulsion, including some gas turbine material.

Metal screens are commonly used as components for fluid handling in spacecraft and rocket tank designs. In most cases, the screens perform a passive separation of the propellant phases. The separation of the liquid from the gaseous propellant phase, is a special challenge. Liquid-gas phase separation means that the gaseous phase is allowed to enter a phase separation device while the liquid phase is blocked. The technical application of this process is the depressurization in a propellant tank. A certain amount of the gaseous propellant phase is vented from the tank through the gas port. The liquid propellant phase remains in the tank in order to be stored for the engine. However, if the tank causes a liquid movement during the depressurization, a part of the liquid can potentially enter the gas port. In order to prevent the unwanted liquid outflow, a separation of the liquid from the gas is necessary. This is possible with the aid of a double screen element and has already been performed for storable liquids in Earth’s gravity and microgravity as well as for cryogenic liquids in Earth’s gravity. At the current state of the art, the separation of the liquid from the gaseous phase of the cryogenic propellant hydrogen using a double screen element has not been performed in microgravity. However, with regard to a possible application, it is mandatory to investigate the function of the double screen element for the real propellant under relevant environmental conditions. In this work, a cryogenic test facility has been developed and operated successfully under Earth’s gravity and microgravity conditions using the drop tower at the University of Bremen. Hereby, the original, cryogenic propellant phases: liquid and gaseous hydrogen, have been used. The experiments show the appearance of the physical processes which are related to the retention capability of a double screen element against liquid hydrogen. Furthermore, these physical processes can obviously be influenced by an unknown boundary condition at the screens: the screen saturation. This unknown boundary condition in turn can obviously be influenced by a certain stimulus which causes a special, fluid mechanical process. A simplified mathematical and two numerical models have been developed which combine the observed, physical processes in the experiments. Two fitting parameters are introduced which influence the flow through screen pressure loss of the liquid and the gaseous hydrogen phase. After the fitting to experimental data, the two fitting parameters have been interpreted with respect to a possible screen saturation. The results lead to a prediction of the unknown boundary condition and indicate that a partial saturation of the screens with liquid could be present in each considered experiment. This can possibly lead to a major influence of the overall resistance of the double screen element against liquid hydrogen.

Concrete is still the most widely used construction material since it has the lowest ratio between cost and strength as compared to other available materials. However, it has two undesirable properties, namely: low tensile strength and large brittleness that cause the collapse to occur shortly after the formation of the first crack. To improve these two negative properties and to achieve a partial substitute of conventional reinforcement, an addition of short discontinuous randomly oriented steel fibres can be practiced among others. In spite of positive properties, fibrous concrete did not find such acknowledgment and application as usual concrete. There do not still exist consistent dimensioning rules due to the lack sufficient large-scale static and dynamic experiments taking into account the effect of the fibre orientation. The intention of the book is twofold: first to summarize the most important mechanical and physical properties of steel-fibre-added concrete and reinforced concrete on the basis of numerous experiments described in the scientific literature, and second to describe a quasi-static fracture process at meso-scale both in plain concrete and fibrous concrete using a novel discrete lattice model. In 2D and 3D simulations of fibrous concrete specimens under uniaxial tension, the effect of the fibre volume, fibre distribution, fibre orientation, fibre length, fibrous bond strength and specimen size on both the stress-strain curve and fracture process was carefully analyzed.

Robert Harbison offers a novel interpretation of what architectural theory might look like. The title is based on Wallace Stevens' "Thirteen Ways of Looking at a Blackbird", and like the poem, Harbison's work is a composite structure built of oblique meanings and shifts that give a portrait of architecture in which symbol and metaphor coexist. 10 illustrations.