20 research outputs found
Freely-Decaying, Homogeneous Turbulence Generated by Multi-scale Grids
We investigate wind tunnel turbulence generated by both conventional and
multi-scale grids. Measurements were made in a tunnel which has a large
test-section, so that possible side wall effects are very small and the length
assures that the turbulence has time to settle down to a homogeneous shear-free
state. The conventional and multi-scale grids were all designed to produce
turbulence with the same integral scale, so that a direct comparison could be
made between the different flows. Our primary finding is that the behavior of
the turbulence behind our multi-scale grids is virtually identical to that
behind the equivalent conventional grid. In particular, all flows exhibit a
power-law decay of energy, , where is very close to the
classical Saffman exponent of . Moreover, all spectra exhibit
classical Kolmogorov scaling, with the spectra collapsing on the integral
scales at small , and on the Kolmogorov micro-scales at large . Our
results are at odds with some other experiments performed on similar
multi-scale grids, where significantly higher energy decay exponents and
turbulence levels have been reported.Comment: 19 pages, 18 figure
Evolution of turbulence and in-plane vortices in the near field flow behind multi-scale planar grids
In this experimental work, we carry out detailed two-dimensional particle image velocimetry investigations for the near field wakes behind a conventional and two multi-scale planar grids, using stitched camera fields of view. Statistical independent measurements are conducted focusing on the first few mesh distances downstream of the grid. It is found that the multiple integral length scales originated from the grids loose their importance on the turbulence development after about three mesh distances downstream, much earlier than the distance where the turbulence becomes homogeneous. The largest eddy size, represented by the integral length scales, does not show clear differences in its growth rate among the three grids after an initial development of three times the largest grid size downstream. Nevertheless, when examining individual vortex behaviours using conditional averaging and filtering processes, clear differences are found. The grids are found to have different decay rates of peak vorticity and projected vortex strengths. Despite these differences, the in-plane vorticity correlation function reveals that the mean vortex shape of all the grids shows a universal near-Gaussian pattern which does not change much as the turbulence decays
Evolution of turbulence and in-plane vortices in the near field flow behind multi-scale planar grids
Probability density function of turbulent velocity fluctuations in rough-wall boundary layer
The probability density function of single-point velocity fluctuations in
turbulence is studied systematically using Fourier coefficients in the
energy-containing range. In ideal turbulence where energy-containing motions
are random and independent, the Fourier coefficients tend to Gaussian and
independent of each other. Velocity fluctuations accordingly tend to Gaussian.
However, if energy-containing motions are intermittent or contaminated with
bounded-amplitude motions such as wavy wakes, the Fourier coefficients tend to
non-Gaussian and dependent of each other. Velocity fluctuations accordingly
tend to non-Gaussian. These situations are found in our experiment of a
rough-wall boundary layer.Comment: 6 pages, to appear in Physical Review
Renormalization group in the infinite-dimensional turbulence: third-order results
The field theoretic renormalization group is applied to the stochastic
Navier-Stokes equation with the stirring force correlator of the form
k^(4-d-2\epsilon) in the d-dimensional space, in connection with the problem of
construction of the 1/d expansion for the fully developed fluid turbulence
beyond the scope of the standard epsilon expansion. It is shown that in the
large-d limit the number of the Feynman diagrams for the Green function (linear
response function) decreases drastically, and the technique of their analytical
calculation is developed. The main ingredients of the renormalization group
approach -- the renormalization constant, beta function and the ultraviolet
correction exponent omega, are calculated to order epsilon^3 (three-loop
approximation). The two-point velocity-velocity correlation function, the
Kolmogorov constant C_K in the spectrum of turbulent energy and the
inertial-range skewness factor S are calculated in the large-d limit to third
order of the epsilon expansion. Surprisingly enough, our results for C_K are in
a reasonable agreement with the existing experimental estimates.Comment: 30 pages with EPS figure