Bifurcation in electrostatic resistive drift wave turbulence

The Hasegawa-Wakatani equations, coupling plasma density and electrostatic potential through an approximation to the physics of parallel electron motions, are a simple model that describes resistive drift wave turbulence. We present numerical analyses of bifurcation phenomena in the model that provide new insights into the interactions between turbulence and zonal flows in the tokamak plasma edge region. The simulation results show a regime where, after an initial transient, drift wave turbulence is suppressed through zonal flow generation. As a parameter controlling the strength of the turbulence is tuned, this zonal flow dominated state is rapidly destroyed and a turbulence-dominated state re-emerges. The transition is explained in terms of the Kelvin-Helmholtz stability of zonal flows. This is the first observation of an upshift of turbulence onset in the resistive drift wave system, which is analogous to the well-known Dimits shift in turbulence driven by ion temperature gradients.Comment: 21 pages, 11 figure

Nationalsozialismus in Göttingen (1933-1945)

In dieser Dissertation wird erstmals der Versuch unternommen, "Nationalsozialismus in Göttingen" in einer Gesamtdarstellung als eine mit der Stadt eng verwobene, komplexe Lebenswirklichkeit darzustellen. Dabei versteht sich die hier vorgelegte Arbeit, die sich methodisch als eine um alltagsgeschichtliche Fragestellungen erweiterte, sozial- und gesellschaftshistorische Studie charakterisieren läßt, nicht nur als ein Beitrag zur Geschichte der Stadt Göttingen, sondern auch zur Gesellschaftsgeschichte des politischen Verhaltens

Air-water flow properties in step cavity down a stepped chute

For the last three decades, the research into skimming flows down stepped chutes was driven by needs for better design guidelines. The skimming flow is characterised by some momentum transfer from the main stream to the recirculation zones in the shear layer developing downstream of each step edge. In the present study some physical modelling was conducted in a relatively large facility and detailed air-water flow measurements were conducted at several locations along a triangular cavity. The data implied some self-similarity of the main flow properties in the upper flow region, at step edges as well as at all locations along the step cavity. In the developing shear layer and cavity region (i.e. y/h < 0.3), the air-water flow properties presented some specific features highlighting the development of the mixing layer downstream of the step edge and the strong interactions between cavity recirculation and mainstream skimming flows. Both void fraction and bubble count rate data showed a local maximum in the developing shear layer, although the local maximum void fraction was always located below the local maximum bubble count rate. The velocity profiles had the same shape as the classical mono-phase flow data. The air-water flow properties highlighted some intense turbulence in the mixing layer that would be associated with large shear stresses and bubble-turbulence interactions. (c) 2011 Elsevier Ltd. All rights reserved

Linear stability of the flow of a second order fluid past a wedge

The linear stability analysis of Rivlin–Ericksen fluids of second order is investigated for boundary layer flows, where a semi-infinite wedge is placed symmetrically with respect to the flow direction. Second order fluids belong to a larger family of fluids called order fluids, which is one of the first classes proposed to model departures from Newtonian behavior. Second order fluids can model non-zero normal stress differences, which is an essential feature of viscoelastic fluids. The linear stability properties are studied for both signs of the elasticity number K, which characterizes the non-Newtonian response of the fluid. Stabilization is observed for the temporal and spatial evolution of two-dimensional disturbances when K > 0 in terms of increase of critical Reynolds numbers and reduction of growth rates, whereas the flow is less stable when K 0 and diminished when K < 0

General stability criterion of inviscid parallel flow

A more restrictively general stability criterion of two-dimensional inviscid parallel flow is obtained analytically. First, a sufficient criterion for stability is found as either $-\mu_1<\frac{U''}{U-U_s}<0$ or $0<\frac{U''}{U-U_s}$ in the flow, where $U_s$ is the velocity at inflection point, $\mu_1$ is the eigenvalue of Poincar\'{e}'s problem. Second, this criterion is generalized to barotropic geophysical flows in $\beta$ plane. Based on the criteria, the flows are are divided into different categories of stable flows, which may simplify the further investigations. And the connections between present criteria and Arnol'd's nonlinear criteria are discussed. These results extend the former criteria obtained by Rayleigh, Tollmien and Fj{\o}rtoft and would intrigue future research on the mechanism of hydrodynamic instability.Comment: Revtex4, 4 pages, 2 figures, extends the first part of physics/0512208, Accepted, to be continue

Numerical Experiments on the Stability of Leading Edge Boundary Layer Flow

A numerical study is performed in order to gain insight to the stability of the infinite swept attachment line boundary layer. The basic flow is taken to be of the Hiemenz class with an added cross-flow giving rise to a constant thickness boundary layer along the attachment line. The full Navier-Stokes equations are solved using an initial value problem approach after two-dimensional perturbations of varying amplitude are introduced into the basic flow. A second-order-accurate finite difference scheme is used in the normal-to-the-wall direction, while a pseudospectral approach is employed in the other directions; temporally, an implicit Crank-Nicolson scheme is used. Extensive use of the efficient fast Fourier transform (FFT) algorithm has been made, resulting in substantial savings in computing cost. Results for the two-dimensional linear regime of perturbations are in very good agreement with past numerical and theoretical investigations, without the need for specific assumptions used by the latter, thus establishing the generality of our method

The influence of the spatial frequency content of discrete roughness distributions on the development of the crossflow instability

An experimental investigation on the influence of the spatial frequency content of roughness distributions on the development of crossflow instabilities has been carried out. From previous research it is known that micro roughness elements can have a large influence on the crossflow development. When the spanwise spacing is chosen such that it is the most unstable wavelength (following linear stability analysis), stationary crossflow waves are amplified. While in earlier studies the focus was on the height or spanwise spacing of roughness elements, in the present study it is chosen to vary the shape of the elements. Through the modification of the shape the forcing at the critical wavelength is increased, while the forcing at the harmonics of the critical wavelength is damped. Experiments were carried in the low turbulence wind tunnel at City University London (Tu=0.006%) on a swept flat plate in combination with displacement bodies to create a sufficiently strong favourable pressure gradient. Hot wire measurements across the plate tracked the development of stationary and travelling crossflow waves. Initially, stronger crossflow waves were found for the elements with stronger forcing, while further downstream the effect of forcing diminished. Spatial frequency spectra showed that the stronger forcing at the critical wavelength (via the roughness shape) dominates the response of the flow while low forcing at the harmonics has no notable effect. Additionally, high resolution streamwise hot wire scans showed that the onset of secondary instability is not significantly influenced by the spatial frequency content of the roughness distribution