5 research outputs found

    Effect of polydispersity and bubble clustering on the steady shear viscosity of semidilute bubble suspensions in Newtonian media

    Get PDF
    In this work, we examine the steady shear rheology of semidilute polydisperse bubble suspensions to elucidate the role of polydispersity on the viscosity of these systems. We prove theoretically that the effect of polydispersity on suspension viscosity becomes apparent only if the bubble size distribution is bimodal, with very small and very large bubbles having similar volume fractions. In any other case, we can consider the polydisperse suspension as monodisperse, with the average bubble diameter equal to the De Brouckere mean diameter (d43). To confirm the theoretical results, we carried out steady shear rheological tests. Our measurements revealed an unexpected double power-law decay of the suspension relative viscosity at average capillary numbers between 0.01 and 1. To investigate this behavior further, we visualized the produced bubble suspensions under shear. The visualization experiments revealed that bubbles started forming clusters and threads at an average capillary number around 0.01, where we observed the first decay of viscosity. Clustering and alignment have been associated with shear-thinning behavior in particle suspensions. We believe that the same holds for bubble suspensions, where bubble clusters and threads align with the imposed shear flow, reducing the streamline distortions and, in turn, resulting in a decrease in the suspension viscosity. Consequently, we can attribute the first decay of the relative viscosity to the formation of bubble clusters and threads, proving that the novel shear-thinning behavior we observed is due to a combination of bubble clustering and deformation

    “Kallipos”, the first open academic textbooks initiative during the years of crisis in Greece and its sustainable continuation

    No full text
    In this paper, we explore the challenges towards the adoption/use of Open Educational Resources (OER) within the Greek Higher Education (HE). To achieve this objective, first we present “Kallipos”, a large-scale open access textbook publishing initiative that was launched in 2013 by the Hellenic Academic Libraries Link (HEAL-Link) consortium. Then, we analyse and interpret the results of two surveys performed at the outskirts of this project. The first survey, committed among faculty members that authored or peer-reviewed an open textbook, highlights the barriers they faced, as well as the teaching and learning benefits from OER adoption/use. The second survey, addressed to university administrative executives (Deans of Schools), identifies their views about the OER usefulness and further development. The empirical data were collected by the use of two questionnaires. The findings align with the results of the current European and international research, thus are usable not only by the national but also by the international policy makers and educational leaders. We conclude by summarising the main lessons learned by “Kallipos”, and by suggesting specific proposals for the sustainable continuation of the action guided by strategies for its potential international scope expansion

    HEAL-Link Advanced Services for Hellenic Academic Libraries

    No full text
    Libraries play an important role in supporting learning and research. To better withstand in current financial crisis libraries are being required to collaborate. Being ahead of his time NTUA Professor Vassilios J. Papazoglou formed the Hellenic Academic Libraries Link, a consortium which today consists of thirty six (36) academic libraries. As a consortium the libraries were able to get funded more easily so as to create and offer services available to all of its members
    corecore