Open University Learning Analytics dataset

Learning Analytics focuses on the collection and analysis of learners’ data to improve their learning experience by providing informed guidance and to optimise learning materials. To support the research in this area we have developed a dataset, containing data from courses presented at the Open University (OU). What makes the dataset unique is the fact that it contains demographic data together with aggregated clickstream data of students’ interactions in the Virtual Learning Environment (VLE). This enables the analysis of student behaviour, represented by their actions. The dataset contains the information about 22 courses, 32,593 students, their assessment results, and logs of their interactions with the VLE represented by daily summaries of student clicks (10,655,280 entries). The dataset is freely available at https://analyse.kmi.open.ac.uk/open_dataset under a CC-BY 4.0 license

Pseudonymization risk analysis in distributed systems

In an era of big data, online services are becoming increasingly data-centric; they collect, process, analyze and anonymously disclose growing amounts of personal data in the form of pseudonymized data sets. It is crucial that such systems are engineered to both protect individual user (data subject) privacy and give back control of personal data to the user. In terms of pseudonymized data this means that unwanted individuals should not be able to deduce sensitive information about the user. However, the plethora of pseudonymization algorithms and tuneable parameters that currently exist make it difficult for a non expert developer (data controller) to understand and realise strong privacy guarantees. In this paper we propose a principled Model-Driven Engineering (MDE) framework to model data services in terms of their pseudonymization strategies and identify the risks to breaches of user privacy. A developer can explore alternative pseudonymization strategies to determine the effectiveness of their pseudonymization strategy in terms of quantifiable metrics: i) violations of privacy requirements for every user in the current data set; ii) the trade-off between conforming to these requirements and the usefulness of the data for its intended purposes. We demonstrate through an experimental evaluation that the information provided by the framework is useful, particularly in complex situations where privacy requirements are different for different users, and can inform decisions to optimize a chosen strategy in comparison to applying an off-the-shelf algorithm

Versuchsanlage ROCOM zur Untersuchung der Kühlmittelvermischung in Druckwasserreaktoren - Ergebnisse quasistationärer Vermischungsexperimente

The test facility ROCOM (Rossendorf Coolant Mixing Model) has been built for the investigation of coolant mixing processes in the reactor pressure vessel of pressurised water reactors (PWR). ROCOM is a 1:5 model of the German PWR KONVOI and has been designed for a wide range of different mixing scenarios. ROCOM disposes of four loops with fully controllable coolant pumps. The test facility is operated with demineralised water. For the investigation of mixing, tracer solution (water labelled with salt) is injected into the facility. The transient distribution of the electrical conductivity is is measured at different positions of the flow path by means of wire-mesh sensor technique with high resolution in space and time. The measured conductivity is transformed into a dimensionless mixing scalar. The mixing at quasi-stationary conditions (constant loop mass flow rates) has been investigated in the presented experiments. That concerned nominal operation conditions, the operation with a reduced number of loops and the investigation of cold-water transients with running pumps and conditions of developed natural circulation. In special experimental series, the reproducibility of the results at identicla boundary conditions within the confidence intervalls has been shown. Further, the influence of various factors on the mixing has been investigated. This included the pressure losses at the core bottom plate, the global coolant flow level and the influence of the loop flow rate on the perturbed sector at the core inlet. An analysis of the measurement error of the used measurement technique completes the report

Construction and execution of experiments at the multi-purpose thermal hydraulic test facility TOPFLOW for generic investigations of two-phase flows and the development and validation of CFD codes - Final report

The works aimed at the further development and validation of models for CFD codes. For this reason, the new thermal-hydraulic test facility TOPFLOW was erected and equipped with wire-mesh sensors with high spatial and time resolution. Vertical test sections with nominal diameters of DN50 and DN200 operating with air-water as well as steam-water two-phase flows provided results on the evaluation of flow patterns, on the be¬haviour of the interfacial area as well as on interfacial momentum and heat transfer. The validation of the CFD-code for complex geometries was carried out using 3D void fraction and velocity distributions obtained in an experiment with an asymmetric obstacle in the large DN200 test section. With respect to free surface flows, stratified co- and counter-current flows as well as slug flows were studied in two horizontal test channels made from acrylic glass. Post-test calculations of these experiments succeeded in predicting the slug formation process. Corresponding to the main goal of the project, the experimental data was used for the model development. For vertical flows, the emphasis was put on lateral bubble forces (e.g. lift force). Different constitutive laws were tested using a Multi Bubble Size Class Test Solver that has been developed for this purpose. Basing on the results a generalized inhomogeneous Multiple Size Group (MUSIG) Model has been proposed and implemented into the CFD code CFX (ANSYS). Validation calculations with the new code resulted in the conclusion that particularly the models for bubble coalescence and fragmentation need further optimisation. Studies of single effects, like the assessment of turbulent dissipation in a bubbly flow and the analysis of trajectories of single bubbles near the wall, supplied other important results of the project

Aufbau und Durchführung von Experimenten an der Mehrzweck-Thermohydraulikversuchsanlage TOPFLOW für generische Untersuchungen von Zweiphasenströmungen und die Weiterentwicklung und Validierung von CFD-Codes - Abschlussbericht

Ziel der Arbeiten war die Weiterentwicklung und Validierung von Modellen in CFD-Codes. Hierzu wurde am FZD die thermohydraulische Versuchsanlage TOPFLOW aufgebaut und mit räumlich und zeitlich hochauflösenden Gittersensoren ausgestattet. Vertikale Teststrecken mit Nenndurchmessern von DN50 bzw. DN200 für Luft/Wasser- sowie Dampf/Wasser-Strömungen lieferten Ergebnisse zur Entwicklung von Strömungsformen, zum Verhalten der Zwischenphasengrenzfläche sowie zum Wärme- und Impulsaustausch zwischen den Phasen. Die Validierung des CFD-Codes in komplexen Geometrien erfolgte anhand von 3D Gasgehalts- und Geschwindigkeitsfeldern, die bei Umströmung eines asymmetrischen Hindernisses auftreten, das in der Teststrecke DN200 eingebaut war. Im Hinblick auf Strömungen mit freier Oberfläche untersuchte das FZD in zwei horizontalen Acrylglas-Kanälen geschichtete Zweiphasenströmungen im Gleich- bzw. Gegenstrom sowie Schwallströmungen. Bei den Nachrechnungen dieser Versuche gelang die Simulation der Schwallentstehung. Entsprechend des Projektziels wurden die experimentellen Ergebnisse zur Modellentwicklung genutzt. Bei vertikalen Strömungen stand die Wirkung der lateralen Blasenkräfte (z.B. Liftkraft) im Vordergrund. Zum Test unterschiedlicher Modellansätze wurde hierzu ein Mehrblasenklassen-Testsolver entwickelt und genutzt. Darauf aufbauend wurde ein neues Konzept für ein Mehrblasenklassenmodell, das Inhomogene MUSIG Modell erarbeitet und in den kommerziellen CFD Code CFX (ANSYS) implementiert. Bei Validierungsrechnungen zeigte sich, dass vor allem die Blasenkoaleszenz- und -zerfallsmodelle weiter optimiert werden müssen. Untersuchungen zu Einzeleffekten, wie z.B. die Abschätzung von Turbulenzkoeffizienten und die Analyse der Trajektoren von Einzelblasen in unmittelbarer Wandnähe, lieferten weitere wichtige Ergebnisse des Projekts