57 research outputs found
Dissipative Effects on Inertial-Range Statistics at High Reynolds numbers
Using the unique capabilities of the Variable Density Turbulence Tunnel at
the Max Planck Institute for Dynamics and Self-Organization, G\"{o}ttingen, we
report experimental result on classical grid turbulence that uncover fine, yet
important details of the structure functions in the inertial range. This was
made possible by measuring extremely long time series of up to
samples of the turbulent fluctuating velocity, which corresponds to
large eddy turnover times. These classical grid
measurements were conducted in a well-controlled environment at a wide range of
high Reynolds numbers from up to , using both
traditional hot-wire probes as well as NSTAP probes developed at Princeton
University. We found that deviations from ideal scaling are anchored to the
small scales and that dissipation influences the inertial-range statistics at
scales larger than the near-dissipation range.Comment: 6 pages, 5 figure
Control of long-range correlations in turbulence
The character of turbulence depends on where it develops. Turbulence near
boundaries, for instance, is different than in a free stream. To elucidate the
differences between flows, it is instructive to vary the structure of
turbulence systematically, but there are few ways of stirring turbulence that
make this possible. In other words, an experiment typically examines either a
boundary layer or a free stream, say, and the structure of the turbulence is
fixed by the geometry of the experiment. We introduce a new active grid with
many more degrees of freedom than previous active grids. The additional degrees
of freedom make it possible to control various properties of the turbulence. We
show how long-range correlations in the turbulent velocity fluctuations can be
shaped by changing the way the active grid moves. Specifically, we show how not
only the correlation length but also the detailed shape of the correlation
function depends on the correlations imposed in the motions of the grid. Until
now, large-scale structure had not been adjustable in experiments. This new
capability makes possible new systematic investigations into turbulence
dissipation and dispersion, for example, and perhaps in flows that mimic
features of boundary layers, free streams, and flows of intermediate character.Comment: This paper has been accepted to Experiments in Fluids. 25 pages, 10
figure
Variable Density Turbulence Tunnel Facility
The Variable Density Turbulence Tunnel (VDTT) at the Max Planck Institute for
Dynamics and Self-Organization in G\"ottingen, Germany produces very high
turbulence levels at moderate flow velocities, low power consumption and
adjustable kinematic viscosity between and . The
Reynolds number can be varied by changing the pressure or flow rate of the gas
or by using different non-flammable gases including air. The highest kinematic
viscosities, and hence lowest Reynolds numbers, are reached with air or
nitrogen at 0.1 bar. To reach the highest Reynolds numbers the tunnel is
pressurized to 15 bar with the dense gas sulfur hexafluoride (SF).
Turbulence is generated at the upstream ends of two measurement sections with
grids, and the evolution of this turbulence is observed as it moves down the
length of the sections. We describe the instrumentation presently in operation,
which consists of the tunnel itself, classical grid turbulence generators, and
state-of-the-art nano-fabricated hot-wire anemometers provided by Princeton
University [Vallikivi et al. (2011) Exp. Fluids 51, 1521]. We report
measurements of the characteristic scales of the flow and of turbulent spectra
up to Taylor Reynolds number , higher than any other
grid-turbulence experiment. We also describe instrumentation under development,
which includes an active grid and a Lagrangian particle tracking system that
moves down the length of the tunnel with the mean flow. In this configuration,
the properties of the turbulence are adjustable and its structure is resolvable
up to .Comment: 45 pages, 31 figure
Experimental study of the influence of anisotropy on the inertial scales of turbulence
We ask whether the scaling exponents or the Kolmogorov constants depend on
the anisotropy of the velocity fluctuations in a turbulent flow with no shear.
According to our experiment, the answer is no for the Eulerian second-order
transverse velocity structure function. The experiment consisted of 32
loudspeaker-driven jets pointed toward the centre of a spherical chamber. We
generated anisotropy by controlling the strengths of the jets. We found that
the form of the anisotropy of the velocity fluctuations was the same as that in
the strength of the jets. We then varied the anisotropy, as measured by the
ratio of axial to radial root-mean-square (RMS) velocity fluctuations, between
0.6 and 2.3. The Reynolds number was approximately constant at around
= 481. In a central volume with a radius of 50 mm, the turbulence
was approximately homogeneous, axisymmetric, and had no shear and no mean flow.
We observed that the scaling exponent of the structure function was , independent of the anisotropy and regardless of the direction in which
we measured it. The Kolmogorov constant, , was also independent of
direction and anisotropy to within the experimental error of 4%.Comment: 29 pages, 21 figure
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