220 research outputs found
Fluctuations of statistics among subregions of a turbulence velocity field
To study subregions of a turbulence velocity field, a long record of velocity
data of grid turbulence is divided into smaller segments. For each segment, we
calculate statistics such as the mean rate of energy dissipation and the mean
energy at each scale. Their values significantly fluctuate, in lognormal
distributions at least as a good approximation. Each segment is not under
equilibrium between the mean rate of energy dissipation and the mean rate of
energy transfer that determines the mean energy. These two rates still
correlate among segments when their length exceeds the correlation length. Also
between the mean rate of energy dissipation and the mean total energy, there is
a correlation characterized by the Reynolds number for the whole record,
implying that the large-scale flow affects each of the segments.Comment: 7 pages, accepted by Physics of Fluids (see http://pof.aip.org/
Two-point velocity average of turbulence: statistics and their implications
For turbulence, although the two-point velocity difference u(x+r)-u(x) at
each scale r has been studied in detail, the velocity average [u(x+r)+u(x)]/2
has not thus far. Theoretically or experimentally, we find interesting features
of the velocity average. It satisfies an exact scale-by-scale energy budget
equation. The flatness factor varies with the scale r in a universal manner.
These features are not consistent with the existing assumption that the
velocity average is independent of r and represents energy-containing
large-scale motions alone. We accordingly propose that it represents motions
over scales >= r as long as the velocity difference represents motions at the
scale r.Comment: 8 pages, accepted by Physics of Fluids (see http://pof.aip.org/
Laboratory experiments for intense vortical structures in turbulence velocity fields
Vortical structures of turbulence, i.e., vortex tubes and sheets, are studied using one-dimensional velocity data obtained in laboratory experiments for duct flows and boundary layers at microscale Reynolds numbers from 332 to 1934. We study the mean velocity profile of intense vortical structures. The contribution from vortex tubes is dominant. The radius scales with the Kolmogorov length. The circulation velocity scales with the rms velocity fluctuation. We also study the spatial distribution of intense vortical structures. The distribution is self-similar over small scales and is random over large scales. Since these features are independent of the microscale Reynolds number and of the configuration for turbulence production, they appear to be universal
On Landau's prediction for large-scale fluctuation of turbulence energy dissipation
Kolmogorov's theory for turbulence in 1941 is based on a hypothesis that
small-scale statistics are uniquely determined by the kinematic viscosity and
the mean rate of energy dissipation. Landau remarked that the local rate of
energy dissipation should fluctuate in space over large scales and hence should
affect small-scale statistics. Experimentally, we confirm the significance of
this large-scale fluctuation, which is comparable to the mean rate of energy
dissipation at the typical scale for energy-containing eddies. The significance
is independent of the Reynolds number and the configuration for turbulence
production. With an increase of scale r above the scale of largest
energy-containing eddies, the fluctuation becomes to have the scaling r^-1/2
and becomes close to Gaussian. We also confirm that the large-scale fluctuation
affects small-scale statistics.Comment: 9 pages, accepted by Physics of Fluids (see http://pof.aip.org
Vortex Tubes in Turbulence Velocity Fields at Reynolds Numbers 300-1300
The most elementary structures of turbulence, i.e., vortex tubes, are studied
using velocity data obtained in a laboratory experiment for boundary layers
with microscale Reynolds numbers 295-1258. We conduct conditional averaging for
enhancements of a small-scale velocity increment and obtain the typical
velocity profile for vortex tubes. Their radii are of the order of the
Kolmogorov length. Their circulation velocities are of the order of the
root-mean-square velocity fluctuation. We also obtain the distribution of the
interval between successive enhancements of the velocity increment as the
measure of the spatial distribution of vortex tubes. They tend to cluster
together below about the integral length and more significantly below about the
Taylor microscale. These properties are independent of the Reynolds number and
are hence expected to be universal.Comment: 8 pages, to appear in Physical Review
APPLICATION OF BLENDED CEMENT IN SHOTCRETE TO
The application of blended cements in shotcrete is investigated to reduce the environmental burden. Reduction of the environmental burden and meeting the required properties of quick setting and strength development are achieved using blended cements with blast furnace slag and a powder accelerator with a main component of calcium sulfo-aluminate. The powder accelerator is hydro-reactive and promotes quick setting and strength development in shotcrete. The reduced environmental burden of shotcrete with blended cement and powder accelerator are calculated
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