12,310 research outputs found
Sub-Kolmogorov-Scale Fluctuations in Fluid Turbulence
We relate the intermittent fluctuations of velocity gradients in turbulence
to a whole range of local dissipation scales generalizing the picture of a
single mean dissipation length. The statistical distribution of these local
dissipation scales as a function of Reynolds number is determined in numerical
simulations of forced homogeneous isotropic turbulence with a spectral
resolution never applied before which exceeds the standard one by at least a
factor of eight. The core of the scale distribution agrees well with a
theoretical prediction. Increasing Reynolds number causes the generation of
ever finer local dissipation scales. This is in line with a less steep decay of
the large-wavenumber energy spectra in the dissipation range. The energy
spectrum for the highest accessible Taylor microscale Reynolds number
R_lambda=107 does not show a bottleneck.Comment: 8 pages, 5 figures (Figs. 1 and 3 in reduced quality
Yang-Mills equation for stable Higgs sheaves
We establish a Kobayashi-Hitchin correspondence for the stable Higgs sheaves
on a compact Kaehler manifold. Using it, we also obtain a Kobayashi-Hitchin
correspondence for the stable Higgs G-sheaves, where G is any complex reductive
linear algebraic group
Asymmetry of temporal cross-correlations in turbulent shear flows
We investigate spatial and temporal cross-correlations between streamwise and
normal velocity components in three shear flows: a low-dimensional model for
vortex-streak interactions, direct numerical simulations for a nearly
homogeneous shear flow and experimental data for a turbulent boundary layer. A
driving of streamwise streaks by streamwise vortices gives rise to a temporal
asymmetry in the short time correlation. Close to the wall or the bounding
surface in the free-slip situations, this asymmetry is identified. Further away
from the boundaries the asymmetry becomes weaker and changes character,
indicating the prevalence of other processes. The systematic variation of the
asymmetry measure may be used as a complementary indicator to separate
different layers in turbulent shear flows. The location of the extrema at
different streamwise displacements can be used to read off the mean advection
speed; it differs from the mean streamwise velocity because of asymmetries in
the normal extension of the structures.Comment: 10 pages, 7 Postscript figures (low quality due to downsizing
Die Kinetik der thermischen Reaktion zwischen Pentafluorschwefelhypofluorid und Kohlenmonoxid
[no abstract
Die Kinetik des thermischen Zerfalles von Pentafluorschwefelhypofluorid (SF_5OF) in Gegenwart von Chlor
[no abstract
Moist turbulent Rayleigh-Benard convection with Neumann and Dirichlet boundary conditions
Turbulent Rayleigh-Benard convection with phase changes in an extended layer
between two parallel impermeable planes is studied by means of
three-dimensional direct numerical simulations for Rayleigh numbers between
10^4 and 1.5\times 10^7 and for Prandtl number Pr=0.7. Two different sets of
boundary conditions of temperature and total water content are compared:
imposed constant amplitudes which translate into Dirichlet boundary conditions
for the scalar field fluctuations about the quiescent diffusive equilibrium and
constant imposed flux boundary conditions that result in Neumann boundary
conditions. Moist turbulent convection is in the conditionally unstable regime
throughout this study for which unsaturated air parcels are stably and
saturated air parcels unstably stratified. A direct comparison of both sets of
boundary conditions with the same parameters requires to start the turbulence
simulations out of differently saturated equilibrium states. Similar to dry
Rayleigh-Benard convection the differences in the turbulent velocity
fluctuations, the cloud cover and the convective buoyancy flux decrease across
the layer with increasing Rayleigh number. At the highest Rayleigh numbers the
system is found in a two-layer regime, a dry cloudless and stably stratified
layer with low turbulence level below a fully saturated and cloudy turbulent
one which equals classical Rayleigh-Benard convection layer. Both are separated
by a strong inversion that gets increasingly narrower for growing Rayleigh
number.Comment: 19 pages, 13 Postscript figures, Figures 10,11,12,13, in reduced
qualit
- …