Statistical Classification of Cascading Failures in Power Grids

We introduce a new microscopic model of the outages in transmission power grids. This model accounts for the automatic response of the grid to load fluctuations that take place on the scale of minutes, when the optimum power flow adjustments and load shedding controls are unavailable. We describe extreme events, initiated by load fluctuations, which cause cascading failures of loads, generators and lines. Our model is quasi-static in the causal, discrete time and sequential resolution of individual failures. The model, in its simplest realization based on the Directed Current description of the power flow problem, is tested on three standard IEEE systems consisting of 30, 39 and 118 buses. Our statistical analysis suggests a straightforward classification of cascading and islanding phases in terms of the ratios between average number of removed loads, generators and links. The analysis also demonstrates sensitivity to variations in line capacities. Future research challenges in modeling and control of cascading outages over real-world power networks are discussed.Comment: 8 pages, 8 figure

Analytical and Numerical Solutions of the Rotor Flow in Tesla Turbines

This paper summarises the numerical and theoretical studies of the incompressible, laminar airflow through a single flow passage of a blade-less radial turbine. Furthermore, it yields the numerical validation of the simplified theoretical model for incompressible rotor flows without the consideration of mechanical losses. It exposes the accuracy of the simplified, analytical performance prediction and flow field for a given geometry, which is based on an optimisation of performance by solving the simplified and incompressible Navier-Stokes-Equations in cylindrical coordinates. The influences of the dimensionless machine parameters on performance and efficiency are obtained from a theoretical analysis. The stream-lines of the bulk flow are derived by analytical means. The inflow conditions for maximum performance and efficiency are theoretically determined and later compared to laminar CFD. In order to quantify the error of the simplified theoretical analysis, different inflow conditions and their influences on shaft power and flow behavior are examined by means of CFD. The de-velopment of the axial velocity distribution at the inlet zone is compared to the one from the theoretical inflow assumption. The influences of Reynolds number and revolution speed on the velocity profiles are investigated. In addition to that, a compressible flow model is introduced. Numerical results are obtained  and compared to the incompressible solution. More-over, compressibility effects on turbine performance are derived

Phase change with local thermal non-equilibrium in a two-phase mixture model

CFD solutions of multi-phase, single-component flow through a vertical channel, filled with a porous medium, heated from one side, are shown. Steady-state solutions are presented for Darcian flow through capillary porous media. Local thermal non- equilibrium is used with a two-phase mixture. The effects of variation of Peclet-number and dimensionless heat input are shown. The displacement effect of the super-heated vapor reduces the free cross-section of flow of the liquid fluid and accelerates the liquid fluid flowing past the evaporation front. The temperatures, liquid saturations, liquid and vapor heat transfer coefficients are shown for cases with super-heated vapor

Time-resolved temperature and velocity field measurements in gas turbine film cooling flows with mainstream turbulence

Gas turbine film cooling strategies must provide adequate cooling performance under high levels of mainstream turbulence. Detailed information about the structure, dynamics and transport of the cooling films is needed to understand the flow physics and develop suitable numerical simulation tools. Here, we study film cooling flows in a wind tunnel with mainstream turbulence generated by an active turbulence grid. Gas temperature and velocity fields are measured using a laser-imaging method based on thermographic phosphor tracer particles. By replacing the previously used tracer BaMgAl10O17: Eu2+ with ZnO, significant gains in accuracy and precision could be achieved. The increased sensitivity (~ 1%/K) of ZnO led to a threefold improvement in the single-shot, single-pixel temperature precision to ± 5 K. The smaller particle size (dp,v ~ 600 nm) and agglomerated nanoparticle structure also reduced the tracing response time to ~ 5 μs allowing accurate tracking of turbulent fluctuations approaching 10 kHz. Moreover, no uncertainty arising from multiple scattering effects were observed using ZnO particles in this enclosed wind tunnel geometry at an estimated average seeding density of 2 × 1011 particles/m3. Timeaverage, fluctuation and single-shot temperature–velocity fields are presented for two mainstream turbulence levels ( u /um= 7% and 14%) and two momentum ratios (IR = 4.7 and 9.3) at a fixed density ratio of 1.55. These flow conditions produce a cooling jet which is detached from the surface. High main flow turbulence causes faster mixing with the surrounding hot gas, increasing the wall-normal spreading of the cooling jet. The instantaneous flow fields show that mainstream turbulence has a significant effect on the shear layer velocity fluctuations and consequently on the streamwise and wall-normal turbulent heat flux, which is derived from the simultaneously acquired temperature–velocity data. We found that high mainstream turbulence reduces the heat flux away from the wall, suggesting that mainstream turbulence can act to diminish cooling performance. Sets of instantaneous measurements recorded at a 6 kHz repetition rate also reveal the dynamic interactions between the main flow turbulence and the cooling jet. These findings and the recorded data can be used to advance turbulence modelling for numerical simulations.Projekt DEAL 202

Aufgabenkultur im Sportunterricht

Die Sportpädagogik und die Sportdidaktik haben sich mit einem gewissen zeitlichen Verzug gegenüber den Didaktiken der naturwissenschaftlichen Fächer und der Mathematik den Anforderungen einer „neuen“ Aufgabenkultur zugewendet. Die Lernaufgabe wird als ergänzendes Aufgabenformat zur Bewegungsaufgabe diskutiert. Zur konzeptionellen Ausdifferenzierung in eine sportorientierte, eine künstlerisch-pädagogische und eine bewegungspädagogische Auslegung der Bewegungsaufgabe finden sich eine Reihe empirischer Studien zur aufgabenorientierten Gestaltung des Sportunterrichts. Jüngst wird die Aufgabenanalysefähigkeit von Sportlehrkräften in den Blick genommen. Zielebenen der entfalteten Aktivitäten sind die Entwicklung guter Aufgaben und die Qualifizierung von Sportlehrkräften für die Erteilung guten Sportunterrichts