Computational Study of Turbulent-Laminar Patterns in Couette Flow

Turbulent-laminar patterns near transition are simulated in plane Couette flow using an extension of the minimal flow unit methodology. Computational domains are of minimal size in two directions but large in the third. The long direction can be tilted at any prescribed angle to the streamwise direction. Three types of patterned states are found and studied: periodic, localized, and intermittent. These correspond closely to observations in large aspect ratio experiments.Comment: 4 pages, 5 figure

Evolution of turbulent spots in a parallel shear flow

The evolution of turbulent spots in a parallel shear flow is studied by means of full three-dimensional numerical simulations. The flow is bounded by free surfaces and driven by a volume force. Three regions in the spanwise spot cross-section can be identified: a turbulent interior, an interface layer with prominent streamwise streaks and vortices and a laminar exterior region with a large scale flow induced by the presence of the spot. The lift-up of streamwise streaks which is caused by non-normal amplification is clearly detected in the region adjacent to the spot interface. The spot can be characterized by an exponentially decaying front that moves with a speed different from that of the cross-stream outflow or the spanwise phase velocity of the streamwise roll pattern. Growth of the spots seems to be intimately connected to the large scale outside flow, for a turbulent ribbon extending across the box in downstream direction does not show the large scale flow and does not grow. Quantitatively, the large scale flow induces a linear instability in the neighborhood of the spot, but the associated front velocity is too small to explain the spot spreading.Comment: 10 pages, 10 Postscript figure

The rise of fully turbulent flow

Over a century of research into the origin of turbulence in wallbounded shear flows has resulted in a puzzling picture in which turbulence appears in a variety of different states competing with laminar background flow. At slightly higher speeds the situation changes distinctly and the entire flow is turbulent. Neither the origin of the different states encountered during transition, nor their front dynamics, let alone the transformation to full turbulence could be explained to date. Combining experiments, theory and computer simulations here we uncover the bifurcation scenario organising the route to fully turbulent pipe flow and explain the front dynamics of the different states encountered in the process. Key to resolving this problem is the interpretation of the flow as a bistable system with nonlinear propagation (advection) of turbulent fronts. These findings bridge the gap between our understanding of the onset of turbulence and fully turbulent flows.Comment: 31 pages, 9 figure

Monitoring, Awareness and Reflection in Blended Technology Enhanced Learning: a Systematic Review

Education is experiencing a paradigm shift towards blended learning models in technology-enhanced learning (TEL). Despite the potential benefits of blended learning, it also entails additional complexity in terms of monitoring, awareness and reflection, as learning happens across different spaces and modalities. In recent years, literature on Learning Analytics (LA) and Educational Data Mining (EDM) has gained momentum and started to address the issue. To provide a clear picture of the current state of the research on the topic and to outline open research gaps, this paper presents a systematic literature review of the state-of-the-art of research in LA and EDM on monitoring, awareness and reflection in blended TEL scenarios. The search included six main academic databases in TEL that were enriched with the proceedings of the workshop on ’Awareness and Reflection in TEL’ (ARTEL), resulting in 1089 papers out of which 40 papers were included in the final analysis

Threshold Amplitudes for Transition in Channel Flows

The threshold amplitude for subcritical transition in Poiseuille and Couette flows is investigated. For Poiseuille flow we present examples where the threshold scales like ß R \Gamma7=4 for 1500 R 5000, where R is the Reynolds number. For Couette flow we present an example where the threshold scales like ß R \Gamma5=4 for 500 R 2000. 1. Introduction It has been demonstrated that turbulence can appear in plane Poiseuille and Couette flows at Reynolds numbers R as low as ß 1000 (Patel and Head, 1969; Carlson, Widnall and Peeters, 1982) and ß 360 (Lundbladh and Johansson, 1991; Tillmark and Alfredsson 1992), respectively. Here R is based on the channel half-height and the difference between the wall and centerline velocities. On the other hand, linear analysis predicts that Poiseuille flow can be unstable only if R ? R c ß 5772 (Orszag, 1971) and that Couette flow is stable for all Reynolds numbers (Romanov, 1973). At supercritical Reynolds numbers an infinitesimally small dis..