418 research outputs found
Relation between shear parameter and Reynolds number in statistically stationary turbulent shear flows
Studies of the relation between the shear parameter S^* and the Reynolds
number Re are presented for a nearly homogeneous and statistically stationary
turbulent shear flow. The parametric investigations are in line with a
generalized perspective on the return to local isotropy in shear flows that was
outlined recently [Schumacher, Sreenivasan and Yeung, Phys. Fluids, vol.15, 84
(2003)]. Therefore, two parameters, the constant shear rate S and the level of
initial turbulent fluctuations as prescribed by an energy injection rate
epsilon_{in}, are varied systematically. The investigations suggest that the
shear parameter levels off for larger Reynolds numbers which is supported by
dimensional arguments. It is found that the skewness of the transverse
derivative shows a different decay behavior with respect to Reynolds number
when the sequence of simulation runs follows different pathways across the
two-parameter plane. The study can shed new light on different interpretations
of the decay of odd order moments in high-Reynolds number experiments.Comment: 9 pages, 9 Postscript figure
Fairness in the organic food chain – practical experiences from Bio Suisse
In the last years Suisse has introduced a fairness strategy to enhance trade relations between organic market partners in supply chains within Switzerland. A code of conduct was elaborated, round table discussions on fairness issues were initiated between market partners and a fairness survey was conducted to evaluate the current level of fairness in trade relations. Results are satisfying but also show future challenges
Three-dimensional spontaneous magnetic reconnection in neutral current sheets
Magnetic reconnection in an antiparallel uniform Harris current sheet
equilibrium, which is initially perturbed by a region of enhanced resistivity
limited in all three dimensions, is investigated through compressible
magnetohydrodynamic simulations. Variable resistivity, coupled to the dynamics
of the plasma by an electron-ion drift velocity criterion, is used during the
evolution. A phase of magnetic reconnection amplifying with time and leading to
eruptive energy release is triggered only if the initial perturbation is
strongly elongated in the direction of current flow or if the threshold for the
onset of anomalous resistivity is significantly lower than in the corresponding
two-dimensional case. A Petschek-like configuration is then built up for \sim
100 Alfven times, but remains localized in the third dimension. Subsequently, a
change of topology to an O-line at the center of the system (``secondary
tearing'') occurs. This leads to enhanced and time-variable reconnection, to a
second pair of outflow jets directed along the O-line, and to expansion of the
reconnection process into the third dimension. High parallel current density
components are created mainly near the region of enhanced resistivity.Comment: 22 pages, 14 figures (Figs. 3,9,10, and 14 as external GIF-Files
Statistics of the Energy Dissipation Rate and Local Enstrophy in Turbulent Channel Flow
Using high-resolution direct numerical simulations, the height and Reynolds
number dependence of higher-order statistics of the energy dissipation rate and
local enstrophy are examined in incompressible, fully-developed turbulent
channel flow. The statistics are studied over a range of wall distances,
spanning the viscous sublayer to the channel flow centerline, for friction
Reynolds numbers and . The high resolution of
the simulations allows dissipation and enstrophy moments up to fourth order to
be calculated. These moments show a dependence on wall distance, and Reynolds
number effects are observed at the edge of the logarithmic layer. Conditional
analyses based on locations of intense rotation are also carried out in order
to determine the contribution of vortical structures to the dissipation and
enstrophy moments. Our analysis shows that, for the simulation at the larger
Reynolds number, small-scale fluctuations of both dissipation and enstrophy
become relatively constant for .Comment: Accepted by Physica
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