144 research outputs found
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
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 or
in the flow, where is the velocity at inflection
point, is the eigenvalue of Poincar\'{e}'s problem. Second, this
criterion is generalized to barotropic geophysical flows in 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
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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
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NACA Technical Memorandums
The present paper describes the results of a stability investigation on symmetrical velocity profiles in a channel and of boundary-layer profiles. The effect of friction was assumed to be vanishing and did not occur in the stability consideration so far as it had been resorted to for preparatory asymptotic considerations. Proceeding on very general premises as regards the form of the velocity distribution, a proof was deduced of the elementary theorem that velocity profiles with inflection points are unstable
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Charakteristikentheorie
The theory of characteristics will be presented generally for quasilinear differential equations of the second order in two variables. This is necessary because of the manifold requirements to be demanded from the theory of characteristics
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NACA Technical Memorandums
This report presents an introduction to the extensive literature on turbulence, with especial consideration of L. Prandtl's theories
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