title: Evaluation of Cavitation Erosion Behavior of Commercial Steel Grades Used in the Design of Fluid Machinery

authors: Tzanakis, I.

abstract: The erosion response under cavitation of different steel grades was assessed by studying the erosion rate, the volume removal, the roughness evolution, and the accumulated strain energy. A 20 kHz ultrasonic transducer with a probe diameter of 5 mm and peak-to-peak amplitude of 50 μm was deployed in distilled water to induce damage on the surface of commercial chromium and carbon steel samples. After a relatively short incubation period, cavitation induced the formation of pits, cracks, and craters whose features strongly depended on the hardness and composition of the tested steel. AISI 52100 chromium steel showed the best performance and is, therefore, a promising design candidate for replacing the existing fluid machinery materials that operate within potential cavitating environments.

text: Evaluation of Cavitation Erosion Behavior of Commercial Steel Grades Used in the Design of Fluid Machinery The erosion response under cavitation of different steel grades was assessed by studying the erosion rate, the volume removal, the roughness evolution, and the accumulated strain energy. A 20 kHz ultrasonic transducer with a probe diameter of 5 mm and peak-to-peak amplitude of 50 μm was deployed in distilled water to induce damage on the surface of commercial chromium and carbon steel samples. After a relatively short incubation period, cavitation induced the formation of pits, cracks, and craters whose features strongly depended on the hardness and composition of the tested steel. AISI 52100 chromium steel showed the best performance and is, therefore, a promising design candidate for replacing the existing fluid machinery materials that operate within potential cavitating environments.

year: 2017

title: Current singularities at finitely compressible three-dimensional magnetic null points

authors: Dmitruk, Pablo

subjects: mathematics

abstract: The formation of current singularities at line-tied two- and three-dimensional (2D and 3D, respectively) magnetic null points in a nonresistive magnetohydrodynamic environment is explored. It is shown that, despite the different separatrix structures of 2D and 3D null points, current singularities may be initiated in a formally equivalent manner. This is true no matter whether the collapse is triggered by flux imbalance within closed, line-tied null points or driven by externally imposed velocity fields in open, incompressible geometries. A Lagrangian numerical code is used to investigate the finite amplitude perturbations that lead to singular current sheets in collapsing 2D and 3D null points. The form of the singular current distribution is analyzed as a function of the spatial anisotropy of the null point, and the effects of finite gas pressure are quantified. It is pointed out that the pressure force, while never stopping the formation of the singularity, significantly alters the morphology of the current distribution as well as dramatically weakening its strength. The impact of these findings on 2D and 3D magnetic reconnection models is discussed.

text: Current singularities at finitely compressible three-dimensional magnetic null points The formation of current singularities at line-tied two- and three-dimensional (2D and 3D, respectively) magnetic null points in a nonresistive magnetohydrodynamic environment is explored. It is shown that, despite the different separatrix structures of 2D and 3D null points, current singularities may be initiated in a formally equivalent manner. This is true no matter whether the collapse is triggered by flux imbalance within closed, line-tied null points or driven by externally imposed velocity fields in open, incompressible geometries. A Lagrangian numerical code is used to investigate the finite amplitude perturbations that lead to singular current sheets in collapsing 2D and 3D null points. The form of the singular current distribution is analyzed as a function of the spatial anisotropy of the null point, and the effects of finite gas pressure are quantified. It is pointed out that the pressure force, while never stopping the formation of the singularity, significantly alters the morphology of the current distribution as well as dramatically weakening its strength. The impact of these findings on 2D and 3D magnetic reconnection models is discussed.

year: 2005-07-15

title: Fanning the divine spark: Gaining understandings of micro-interactions in New Zealand classrooms

authors: Eley, Elizabeth Martha Ann

subjects: micro-interactions

abstract: New Zealand’s Treaty of Waitangi was signed by representatives of Iwi Māori (Indigenous tribal groups) and the British Crown.  Inherent in the heritages of both signatory groups is the ontological positioning that children are born of greatness; carriers of the divine spark from Tama-nui-te-rā (the central sun) and created in the image of God.  Almost all New Zealand children attend school.  For some, their ongoing classroom experiences with educators fans their divine spark and works towards the realisation of their innate potential.  For others, schooling is not a positive experience and their divine spark may be neither recognised nor celebrated.  

This research explores the relationships and pedagogical practices that underpin the classroom micro-interactions experienced by children.  Children’s schooling experience is made up of small interactions in classrooms that are situated in the social, political and legal contexts of schooling.  Therefore, this research applies both a centripetal and a centrifugal examination of the macro and micro contexts of classroom interactions.  

In examining the macro-context, ontological views of the child and the theoretical frameworks that guide pedagogy are considered.  The Education Acts governing New Zealand schooling have successively contained an ontological contradiction that enforces both a factory model as the preferred means of schooling for New Zealand and a requirement to honour the Treaty of Waitangi, including the worldviews of both signatories.  I conclude that educators within the compulsory schooling sector are not provided with specific guidance on how to navigate these contradictory worldviews.  Neither ontological views of how children are perceived within the system, nor theoretical frameworks that could and should inform views of knowledge and how learning occurs, are made explicit for educators.  

At the micro-context level, relationships and pedagogical practices are explored through case studies in three different New Zealand classrooms.  In each case, from video-recordings of classroom interactions, critical incidents were extracted and used to support interviews with participating teachers, students, parents and members of the wider school community.  Employing culturally responsive research methodologies, the interactions with participants were relational and dialogic.  The research found that classroom micro-interactions are pervasive and complex.  All participants, students, teachers and whānau, draw from the knowledge, understandings and positionings of the individual’s own cultural toolkits in classroom interactions.  Their cultural toolkits contain a range of factors to support learning, all of which were necessary and interdependent.  

The thesis calls for all children to receive an education that fans their inherent divine spark.  While this experience is not yet the case for all, there is opportunity in the rhetoric and structures already within New Zealand’s education system for this to occur.  Some schools and communities have begun to successfully challenge and address the prevailing theoretical and pedagogical status quo.  Therefore, there is reason for radical hope for a system-wide fanning of the divine spark in every child.

text: Fanning the divine spark: Gaining understandings of micro-interactions in New Zealand classrooms New Zealand’s Treaty of Waitangi was signed by representatives of Iwi Māori (Indigenous tribal groups) and the British Crown.  Inherent in the heritages of both signatory groups is the ontological positioning that children are born of greatness; carriers of the divine spark from Tama-nui-te-rā (the central sun) and created in the image of God.  Almost all New Zealand children attend school.  For some, their ongoing classroom experiences with educators fans their divine spark and works towards the realisation of their innate potential.  For others, schooling is not a positive experience and their divine spark may be neither recognised nor celebrated.  

This research explores the relationships and pedagogical practices that underpin the classroom micro-interactions experienced by children.  Children’s schooling experience is made up of small interactions in classrooms that are situated in the social, political and legal contexts of schooling.  Therefore, this research applies both a centripetal and a centrifugal examination of the macro and micro contexts of classroom interactions.  

In examining the macro-context, ontological views of the child and the theoretical frameworks that guide pedagogy are considered.  The Education Acts governing New Zealand schooling have successively contained an ontological contradiction that enforces both a factory model as the preferred means of schooling for New Zealand and a requirement to honour the Treaty of Waitangi, including the worldviews of both signatories.  I conclude that educators within the compulsory schooling sector are not provided with specific guidance on how to navigate these contradictory worldviews.  Neither ontological views of how children are perceived within the system, nor theoretical frameworks that could and should inform views of knowledge and how learning occurs, are made explicit for educators.  

At the micro-context level, relationships and pedagogical practices are explored through case studies in three different New Zealand classrooms.  In each case, from video-recordings of classroom interactions, critical incidents were extracted and used to support interviews with participating teachers, students, parents and members of the wider school community.  Employing culturally responsive research methodologies, the interactions with participants were relational and dialogic.  The research found that classroom micro-interactions are pervasive and complex.  All participants, students, teachers and whānau, draw from the knowledge, understandings and positionings of the individual’s own cultural toolkits in classroom interactions.  Their cultural toolkits contain a range of factors to support learning, all of which were necessary and interdependent.  

The thesis calls for all children to receive an education that fans their inherent divine spark.  While this experience is not yet the case for all, there is opportunity in the rhetoric and structures already within New Zealand’s education system for this to occur.  Some schools and communities have begun to successfully challenge and address the prevailing theoretical and pedagogical status quo.  Therefore, there is reason for radical hope for a system-wide fanning of the divine spark in every child.

year: 2020