57,069 research outputs found
Gruppen lokaler Charakteristik - eine Kennzeichnung von Gruppen vom Lie-Typ in Charakteristik 2
Full text linkDiese Arbeit ist Teil eines Projekts von U. Meierfrankenfeld, B. Stellmacher und G. Stroth zur Klassifikation endlicher Gruppen in lokaler Charakteristik p, welches einen wesentlichen Beitrag zur Revision der Klassifikation der endlichen einfache Gruppen bilden soll. Ausgehend von einer Gruppe G in lokaler Charakteristik 2, die eine große 2-Untergruppe besitzt, wird mittels des Struktursatzes sowie weiterer Resultate aus dem vorgenannten Projekt eine fast einfache Untergruppe H von G konstruiert. Wir betrachten den Fall, dass H vom Lie-Typ in Charakteristik 2 ist. Durch genaue Untersuchung von einfachen Gruppen, ihrer 2-lokalen Struktur und ihrer Operation auf gewissen Modulen wird nun H mit G identifiziert.This thesis is part of an ongoing project of U. Meierfrankenfeld, B. Stellmacher and G. Stroth on groups of local characteristic p which shall be a fundamental contribution to the revision of the classification of finite simple groups. Consider a group of local characteristic 2 with a large 2-subgroup. Some theorems of the above-mentioned project like the structure theorem show how to construct an almost simple subgroup H of G. We study the case that H is a simple group of Lie type in characteristic 2. By analyzing finite simple groups, their 2-local structure and their operation on modules we show H = G.von Gerald Pientk
A comparative study of transport in stellarators and tokamaks
No full textA first extensive comparison was published four years ago in (Wagner and Stroth, 1993) and is briefly summarized in the next section. This work is an extension and specifically builds upon the experimental findings collected since then. It concentrates on aspects where the author believes most progress in understanding has been made. Accordingly, the article treats the dependences of the energy confinement time on global (Section 5) and local parameters such as magnetic shear (Section 6), isotopic mass (Section 7), density (Section 8) and temperature (Sections 9, 10 and 11). An interesting question related is whether transport is exclusively determined by local plasma parameters or whether global effects can be important too (Section 12). An alternative access to anomalous transport, which is discussed in Section 13, is to measure the fluctuation amplitudes of plasma parameters. (orig.)163 refs.Available from TIB Hannover: RA 71(III/216) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
A Dynamic Subfilter-scale Stress Model for Large Eddy Simulations Based on Physical Flow Scales
No full textWe propose a new definition of the length scale in an eddy-viscosity model for large-eddy simulations (LES). This formulation extends and generalizes a previous proposal [Piomelli, Rouhi and Geurts, Proc. ETMM10, 2014], in which the LES length scale was expressed in terms of the integral length-scale of turbulence determined by the flow characteristics and explicitly decoupled from the simulation grid; this approach was named Integral Length-Scale Approximation (ILSA). As in the original ILSA, the model coefficient was determined by the user, and required to maintain a desired contribution of the unresolved, subfilter scales (SFS) to the global transport. We propose a local formulation (local ILSA) in which the model coefficient is local in space, allowing a precise control over SFS activity as a function of location. This new formulation preserves the properties of the global model; application to channel flow and backward-facing step verifies its features and accuracy
Large-eddy simulation of a separated flow with a sub-filter scale model based on the integral length-scale
No full textA new sub-filter scale model for large-eddy simulations, which uses a length-scale proportional to the integral scale of the turbulence instead of the grid resolution to parametrize the modelled stresses, will be assessed in the prediction of the flow of a boundary-layer over a rough surface, which includes separation and reattachment
Near Wall PIV-Measurements on the Windward Slope of a Hill
No full textThe turbulent flow over periodic hills was measured near to the wall, using planar Particle-Image-Velocimetry (PIV) at high spatial resolution. Our focus is on the near wall turbulence structure on the windward slope of the hill. For large-eddy simulation (LES) we suspect that, if this was not predicted accurately, it affects the prediction of the velocity profiles over the hill crest which in turn will affect the recirculation length downstream of the hill. Regarding the time averaged velocities, we were able to resolve the linear viscous region of the boundary layer. The velocity distribution and also the Reynolds stress does not comply with the law of the wall as it is valid for a turbulent boundary layer at equilibrium
Energy dissipation and flux laws for unsteady turbulence
No full textDirect Numerical Simulations of spatially periodic unsteady turbulence show that the high Reynolds number scalings of the instantaneous energy dissipation rate and interscale energy flux at intermediate wavenumbers are qualitatively different from the well-known cornerstone scalings of equilibrium turbulence where and are time-dependent rms velocity and integral length-scales. Instead, they both scale as where and are length and velocity scales characterizing initial/overall unsteady turbulence conditions
Direct numerical simulation of turbulent Couette-Poiseuille flow with zero skin friction
No full textThe near-wall scaling of mean velocity U(y) is addressed for the case of zero skin friction on one wall of a fully turbulent channel flow. The present DNS results can be added to the evidence in support of the conjecture that U is proportional to √yw in the region just above the wall at which the mean shear dU/dy = 0
Optimized tomography methods for plasma emissivity reconstruction at the ASDEX Upgrade tokamak
No full textThe soft X-ray (SXR) emission provides valuable insight into processes happening inside of high-temperature plasmas. A standard method for deriving the local emissivity profiles of the plasma from the line-of-sight integrals measured by pinhole cameras is the tomographic inversion. Such an inversion is challenging due to its ill-conditioned nature and because the reconstructed profiles depend not only on the quality of the measurements but also on the inversion algorithm used. This paper provides a detailed description of several tomography algorithms, which solve the inversion problem of Tikhonov regularization with linear computational complexity in the number of basis functions. The feasibility of combining these methods with the minimum Fisher information regularization is demonstrated, and various statistical methods for the optimal choice of the regularization parameter are investigated with emphasis on their reliability and robustness. Finally, the accuracy and the capability of the methods are demonstrated by reconstructions of experimental SXR profiles, featuring poloidal asymmetric impurity distributions as measured at the ASDEX Upgrade tokamak
Real-space Manifestations of Bottlenecks in Turbulence Spectra
No full textAn energy-spectrum bottleneck, a bump in the turbulence spectrum between the inertial and dissipation ranges, is shown to occur in the non-turbulent, one-dimensional, hyperviscous Burgers equation and found to be the Fourier-space signature of oscillations in the real-space velocity, which are explained by boundary-layer-expansion techniques. Pseudospectral simulations are used to show that such oscillations occur in velocity correlation functions in one- and three-dimensional hyperviscous hydrodynamical equations that display genuine turbulence
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