2,945 research outputs found
Effect of Initial Conditions on the Scalar Decay in Grid Turbulence at Low Rλ
Decaying grid turbulence is considered at low Reynolds number (Rλ ~ 50) for different initial conditions. Three different grid geometries are used. Heat is injected via a mandoline at a distance of 1.5 M from the grid. The amount of heating is such that temperature may be treated as a passive scalar. A small contraction (1.36:1) is added at a distance of 11M downstream of the grid. The power-law exponents for the scalar variance are compared with those for the turbulent kinetic energy. These exponents depend on the grid geometry. For the isotropic dissipation rate 〈χ〉iso, the power-law exponent agrees with that inferred from the temperature variance transport equation. Restricting the range of validity of the decay law affects the magnitudes of the origin and decay exponent. Secondorder temperature structure functions collapse when the normalization is based on the local temperature variance and the Corrsin microscale but the asymptotic form of this collapse depends on the initial conditions
Effect of large-scale intermittency and mean shear on scaling-range exponents in a turbulent jet
The present study investigates the combined impact of the intermittency associated with the turbulent-nonturbulent interface and the mean shear rate in an axisymmetric jet on the structure of turbulence in the scaling range, where the spectrum exhibits a power-law behavior. Second-order structure functions, autocorrelations of the dissipation rate, and spectra of both the longitudinal velocity fluctuation and the passive temperature fluctuation are measured at a distance of 40 diameter downstream from the nozzle exit. All the scaling range exponents are influenced by the large-scale intermittency and the mean shear. The scalar fluctuation is much more sensitive to the variation in large-scale intermittency than the velocity fluctuation.J. Mi and R. A. Antoni
Joint statistics between temperature and its dissipation rate components in a round jet
J. Mi, R. A. Antonia, and F. Anselme
Isotropy of small scale turbulence
The degree to which local isotropy is satisfied has been examined using direct numerical simulations for a fully developed channel flow. Attention is mainly given to the high wavenumber part of vorticity and temperature derivative spectra. The ratio of these spectra and their isotropic values depends on the particular quantity considered, the departure from isotropy being more pronounced for the temperature derivative than for the vorticity. When the Kolmogorov-normalized wavenumber is sufficiently large, isotropy is satisfied provided the (Kolmogorov-normalized) mean strain rate is sufficiently small. This result appears to be independent of the quantity considered and of the Reynolds number
Experimental assessment of a new form of scaling law for near-wall turbulence
Scaling laws and intermittency in the wall region of a turbulent flow are
addressed by analyzing moderate Reynolds number data obtained by single
component hot wire anemometry in the boundary layer of a flat plate. The paper
aims in particular at the experimental validation of a new form of refined
similarity recently proposed for the shear dominated range of turbulence, where
the classical Kolmogorov-Oboukhov inertial range theory is inappropriate. An
approach inspired to the extended self-similarity allows for the extraction of
the different power laws for the longitudinal structure functions at several
wall normal distances. A double scaling regime is found in the logarithmic
region, confirming previous experimental results. Approaching the wall, the
scaling range corresponding to the classical cascade-dominated range tends to
disappear and, in the buffer layer, a single power law is found to describe the
available range of scales. The double scaling is shown to be associated with
two different forms of refined similarity. The classical form holds below the
shear scale L s . The other, originally introduced on the basis of DNS data for
a turbulent channel, is experimentally confirmed to set up above L s . Given
the experimental diffulties in the evaluation of the instantaneous dissipation
rate, some care is devoted to check that its one-dimensional surrogate does not
bias the results. The increased intermittency as the wall is approached is
experimentally found entirely consistent with the failure of the refined
Kolmogorov-Oboukhov similarity and the establishment of its new form near the
wall.Comment: 27 pages, 9 figure
Observation of inertial energy cascade in interplanetary space plasma
We show in this article direct evidence for the presence of an inertial
energy cascade, the most characteristic signature of hydromagnetic turbulence
(MHD), in the solar wind as observed by the Ulysses spacecraft. After a brief
rederivation of the equivalent of Yaglom's law for MHD turbulence, we show that
a linear relation is indeed observed for the scaling of mixed third order
structure functions involving Els\"asser variables. This experimental result,
confirming the prescription stemming from a theorem for MHD turbulence, firmly
establishes the turbulent character of low-frequency velocity and magnetic
field fluctuations in the solar wind plasma
Perturbation of a turbulent boundary layer by spatially-impulsive dynamic roughness
First experimental measurements of manipulation of the structure of a canonical zero pressure gradient
turbulent boundary layer using a low frequency (compared to the viscous frequency) mechanical dynamic
roughness are presented. “Dynamic” (or time-dependent) surface roughness is proposed as a method for both
control and diagnosis of turbulent boundary layers
Length Scales of Acceleration for Locally Isotropic Turbulence
Length scales are determined that govern the behavior at small separations of
the correlations of fluid-particle acceleration, viscous force, and pressure
gradient. The length scales and an associated universal constant are quantified
on the basis of published data. The length scale governing pressure spectra at
high wave numbers is discussed. Fluid-particle acceleration correlation is
governed by two length scales; one arises from the pressure gradient, the other
from the viscous force.Comment: 2 figures, 4 pages. Physical Review Letters, accepted August 200
Intermittency and structure functions in channel flow turbulence
We present a study of intermittency in a turbulent channel flow. Scaling
exponents of longitudinal streamwise structure functions, ,
are used as quantitative indicators of intermittency.
We find that, near the center of the channel the values of
up to are consistent with the assumption of homogeneous/isotropic
turbulence. Moving towards the boundaries, we observe a growth of intermittency
which appears to be related to an intensified presence of ordered vortical
structures. In fact, the behaviour along the normal-to-wall direction of
suitably normalized scaling exponents shows a remarkable correlation with the
local strength of the Reynolds stress and with the \rms value of helicity
density fluctuations. We argue that the clear transition in the nature of
intermittency appearing in the region close to the wall, is related to a new
length scale which becomes the relevant one for scaling in high shear flows.Comment: 4 pages, 6 eps figure
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