Ilias Mavroidis

Numerical investigation of concentrations and concentration fluctuations around isolated obstacles of different shapes. Comparison with wind tunnel results

This paper presents computational simulations of atmospheric dispersion experiments conducted around isolated obstacles in a wind tunnel. The tool used for the simulations is the computational fluid dynamics (CFD) code ADREA-HF, which was especially developed for the calculation of flow and dispersion of positively or negatively buoyant gases in complicated geometries. The wind tunnel experiments simulated involve a cube normal to the flow, a taller rectangular building—comprising of two stacked cubes and a right circular cylinder. Three different gas source locations are examined: two cube heights upwind, at the upwind face and at the downwind face of each obstacle. The experimental data in all cases consist of mean concentrations and concentration fluctuations downwind of the obstacle. In the first part of the study, a computational assessment is performed to examine the influence of factors such as turbulence model, grid resolution, boundary conditions and numerical scheme, on the results of the CFD model in the case of dispersion around an isolated cube. Following this assessment the model parameters are optimized. The model is then used so that computed flow fields and concentration patterns around the three obstacles and for the three different source positions are inter-compared and analysed. Along-wind profiles of computed and measured concentrations and concentration standard deviations have been compared to examine the differences between simulations and wind tunnel experiments. Finally, a statistical performance evaluation of the model is carried out by comparing computed and experimental concentrations and concentration fluctuations. In most cases there is a good level of agreement between calculated and measured quantities, while the model has a general tendency to over-predict concentration fluctuations. In conclusion, the wind tunnel data together with the detailed spatial results that the CFD model produces, give the opportunity to study in detail the flow fields and concentration patterns and to reveal the different behaviours associated with the different obstacle shapes and gas source locations.

Transactional Distance among Open University Students: How Does it Affect the Learning Process?

Abstract This study examines the presence of transactional distance among students, the factors affecting it, as well as the way it influences the learning process of students in a blended distance learning setting in Greece. The present study involved 12 postgraduate students of the Hellenic Open University (HOU). A qualitative research was conducted, using information collected via individual semi-structured interviews. Content analysis of the gathered information provided evidence regarding the existence of student-student transactional distance for several reasons, such as geographical and relatively limited face to face interaction. The role of the tutor as well as of the course provider were also examined in this respect. Finally the study indicated that the existence of perceived transactional distance among the students has a negative effect on their learning process.

Transactional Distance and Autonomy in a Distance Learning Environment.

Abstract This paper studies the transactional distance between the students and the tutor as well as the autonomy of students in a postgraduate course of the Hellenic Open University (HOU). The aim of the paper is to examine how the relation between autonomy and transactional distance evolves during an academic year and how this relation is affected by demographic parameters, such as gender, age group and student experience (year of studies). For this purpose an empirical research was conducted and a sample of postgraduate students responded to a questionnaire (which included a scale for transactional distance, a scale for autonomy and demographic questions) during two different time periods within an academic (2012-2013). The results of the statistical analysis showed that during the evolution of the learning process there was a small change in transactional distance, especially for the dimension of co-understanding, while there was also a change in one of the dimensions of autonomy, namely sensitivity to others. However, no relation was observed between transactional distance and autonomy, ether in the first or the second measurement during the academic year. This finding is also regarded in the overall framework of the different critiques and interpretations of Moore’s theory.

Numerical modelling of odour dispersion around a cubical obstacle using large eddy simulation

In the present work two different large eddy simulation (LES) approaches, namely the Dynamic Smagorinsky model and the Wale model, are used to simulate the air flow and pollutant dispersion around a cubical obstacle. Results are compared with wind tunnel data (WT) and with results from the Smagorinsky LES model. Overall agreement was good between the different LES approaches and the WT results, both for the mean and fluctuating flow and concentration patterns. LES models can provide good estimates of concentration fluctuation intensity and enable the calculation of the intermittency factor. The model results indicate that LES is a viable tool for odour impact assessment.

Learner Autonomy as a Factor of the Learning Process in Distance Education

Abstract The present study aims to empirically examine the relation between learner autonomy and specific aspects of the learning process, such as student-student interaction and tutor-student interaction, in a distance learning environment in Greece. An empirical study was conducted at the Hellenic Open University (HOU) using data gathered via a four-section questionnaire, completed by 100 postgraduate students. The correlation analysis yielded a positive correlation between learner autonomy and both student-student and tutor-student interaction. In particular, the results revealed the existence of a positive correlation between all three subscales of autonomy, namely sensibility to others, ability to manage new situations and self-awareness, and student-student interaction. A significant positive correlation was also observed between selfawareness and tutor-student interaction. Moreover, the results suggested that there are no statistically significant differences of the above parameters in relation to demographic features, such as gender, age and the number of Counselling Group Sessions (CGS) in which students had participated. Yet, results suggested that there is an effect of the number of course modules attended by students on the levels of student-student interaction.

Response From Giossos, Koutsouba, and Mavroidis

After studying carefully Professor Farhad Saba’s constructive comments and arguments on our article, we considered that a reply to them would offer us the opportunity to provide some further clarifications to the article from our point of view. Let’s start with our belief that transactional distance as a topic is open to many and different approaches. In fact, this is one of the reasons for its being at the core of distance education’s debate until recently. Furthermore, in one of our previous articles (Giossos et al. 2009), by developing a series of arguments we had concluded that “transactional distance is nothing more than the lack of common or mutual perception of knowledge, thoughts, approaches, needs and emotions between teacher and learner.” On this basis, we conceptualize transactional distance in an alternative way. By proposing this conceptual definition, we believe that the problem of tautology that Gorsky and Caspi (2005) highlighted is overcome. In any case, our focus is not on the theory ad hoc. From our point of view, transactional distance is a key concept that can be used for explaining psychological aspects of the relation between learner and teacher in distance education. In this respect, transactional distance can be used not only as a tool for developing and managing distance education programs (the main aim of the theory) but also as a way to measure and explain emotional and psychological responses in the context of distance education courses (our proposal for the concept). Regarding the critique of Professor Saba on the positivist paradigm in science, we have different epistemological assumptions because it seems that Professor Saba rejects it as a method to be used in the context of complex concepts such as learning and transactional distance. In addition, as teachers in distance education, we think that it is necessary to have tools so as to comprehend the interaction between learner and teacher. In this respect, transactional distance is an important conceptual tool to understand and measure this interaction. As all such tools, it has its own assumptions and limitations, but this is not a reason to reject its use (of course under the precondition that its assumptions and limitations are taken into consideration, a precondition that we are aware of). Finally, we thank Professor Farhad Saba for his comments and arguments on our article that gave us the opportunity for these fruitful, as we believe, clarifications.

Development of an Instrument for Measuring Learner-Teacher Transactional Distance.

ABSTRACT An instrument was developed to assess the perceived learner–teacher transactional distance in the context of the Hellenic Open University and to provide psychometric evidence of reliability and validity of the interpretation of the scores derived from the scale. First, an elicitation study gathered information on students’ beliefs and feelings about their teachers’ behavior. Two dimensions of learner–teacher transactional distance, namely, co-understanding and awareness, were revealed. Then, an item pool was developed and the preliminary questionnaires (one for each dimension) were field tested on 60 postgraduate students who commented on their clarity. Reliability and validity were then tested using exploratory and confirmatory factor analysis with two samples of 220 and 201 postgraduate students. The confirmatory factor analysis results indicated good model fit for the dimensions of co-understanding and awareness. Univariate F tests showed no significant difference between males and females for (a) co-understanding of cognitive skills and aptitudes, (b) co-understanding of emotional state, (c) awareness of cognitive skills and aptitudes, and (d) awareness of emotional state. A final questionnaire included ten items for the co-understanding dimension and fifteen items for the awareness dimension.