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

A Consistent Hybrid LES-RANS PDF Method for Non-premixed Flames

AbstractThe computational demanding LES methods have widely demonstrated their reliability in the description of large scale unsteady phenomena in turbulent reactive flows. RANS Transported Probability Density Function (TPDF) methods treat the nonlinear chemical reactions in closed form on relatively coarse grids and using a smaller number of stochastic particles. Combining the two approaches, a hybrid LES-RANS PDF method to predict non-premixed turbulent flames is presented. In this method a LES, based on Smagorinsky's model and steady flamelet, is performed; subsequently, the calculated flow-field is used to drive the RANS-TPDF equation, which is closed at the joint scalar level and based on a Lagrangian Monte Carlo scheme. The required velocity and turbulent quantities for RANS simulation are estimated from the resolved LES and an algebraic model based on dimensional analysis and the mixing length hypothesis. The results of the velocity, turbulent kinetic energy and mixture fraction show that the consistency of the method is achieved

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

Professionalisierung von Grundschullehrkräften für das Unterrichtsfach Sport

Die Autoren arbeiten die Anforderungen an eine Professionalisierung von Grundschullehrkräften im Unterrichtsfach Sport heraus. Zur Konturierung dieser Anforderungen wird zum einen auf der Grundlage empirischer Befunde das Bewegungs-, Spiel- und Sportverhalten von Grundschulkindern näher beschrieben. Zum anderen wird das Konzept des Schulsports umrissen und die didaktischen Konzepte für den Sportunterricht in der Grundschule werden mit Blick auf daraus resultierende Anforderungen reflektiert. Auch die Biografie von Sportlehrkräften wird hinsichtlich der Bedeutung für Professionalisierungsprozesse in den Blick genommen. (DIPF/Orig.

The Eulerian Stochastic Fields Method Applied to Large Eddy Simulations of a Piloted Flame with Inhomogeneous Inlet

Large Eddy Simulations of the Sydney mixed-mode flame with inhomogeneous inlet (FJ200-5GP-Lr75-57) are performed using the Eulerian Stochastic Fields (ESF) transported probability functions method to account for the sub-grid scale turbulence–chemistry interaction, to demonstrate the suitability of the ESF method for mixed-mode combustion. An analytically reduced 19-species methane mechanism is used for the description of the chemical reactions. Prior to the reactive case, simulation results of the non-reactive setup with cold and hot pilot stream are presented, which show differences in the jet breakup and radial species mass fluxes. The reactive case simulations are compared to experimental data and a recently conducted model free quasi-DNS (qDNS), showing very good agreement with the qDNS in terms of scatter data and radial mean values of temperature and species distribution, as well as mixture fraction conditional statistics. Further analysis is dedicated to sub-grid scale statistics, showing that mixture fraction and reaction progress variable are strongly correlated in this flame. The impact of the number of stochastic fields on the filtered temperature and species distribution is investigated; it reveals that the ESF method in conjunction with finite-rate chemistry is very insensitive to the number of employed fields to obtain highly accurate simulation results