73 research outputs found
Two-dimensionalization of the flow driven by a slowly rotating impeller in a rapidly rotating fluid
We characterize the two-dimensionalization process in the turbulent flow
produced by an impeller rotating at a rate in a fluid rotating at a
rate around the same axis for Rossby number down to
. The flow can be described as the superposition of a large-scale
vertically invariant global rotation and small-scale shear layers detached from
the impeller blades. As decreases, the large-scale flow is subjected to
azimuthal modulations. In this regime, the shear layers can be described in
terms of wakes of inertial waves traveling with the blades, originating from
the velocity difference between the non-axisymmetric large-scale flow and the
blade rotation. The wakes are well defined and stable at low Rossby number, but
they become disordered at of order of 1. This experiment provides insight
into the route towards pure two-dimensionalization induced by a background
rotation for flows driven by a non-axisymmetric rotating forcing.Comment: Accepted for publication in Physical Review Fluid
Influence of the multipole order of the source on the decay of an inertial wave beam in a rotating fluid
We analyze theoretically and experimentally the far-field viscous decay of a
two-dimensional inertial wave beam emitted by a harmonic line source in a
rotating fluid. By identifying the relevant conserved quantities along the wave
beam, we show how the beam structure and decay exponent are governed by the
multipole order of the source. Two wavemakers are considered experimentally, a
pulsating and an oscillating cylinder, aiming to produce a monopole and a
dipole source, respectively. The relevant conserved quantity which
discriminates between these two sources is the instantaneous flowrate along the
wave beam, which is non-zero for the monopole and zero for the dipole. For each
source the beam structure and decay exponent, measured using particle image
velocimetry, are in good agreement with the predictions
Disentangling inertial waves from eddy turbulence in a forced rotating turbulence experiment
We present a spatio-temporal analysis of a statistically stationary rotating
turbulence experiment, aiming to extract a signature of inertial waves, and to
determine the scales and frequencies at which they can be detected. The
analysis uses two-point spatial correlations of the temporal Fourier transform
of velocity fields obtained from time-resolved stereoscopic particle image
velocimetry measurements in the rotating frame. We quantify the degree of
anisotropy of turbulence as a function of frequency and spatial scale. We show
that this space-time-dependent anisotropy is well described by the dispersion
relation of linear inertial waves at large scale, while smaller scales are
dominated by the sweeping of the waves by fluid motion at larger scales. This
sweeping effect is mostly due to the low-frequency quasi-two-dimensional
component of the turbulent flow, a prominent feature of our experiment which is
not accounted for by wave turbulence theory. These results question the
relevance of this theory for rotating turbulence at the moderate Rossby numbers
accessible in laboratory experiments, which are relevant to most geophysical
and astrophysical flows
Eckhaus-like instability of large scale coherent structures in a fully turbulent von K\'arm\'an flow
The notion of instability of a turbulent flow is introduced in the case of a
von K\'arm\'an flow thanks to the monitoring of the spatio-temporal spectrum of
the velocity fluctuations, combined with projection onto suitable Beltrami
modes. It is shown that the large scale coherent fluctuations of the flow obeys
a sequence of Eckhaus instabilities when the Reynolds number is
varied from to . This sequence results in modulations of
increasing azimuthal wavenumber. The basic state is the laminar or
time-averaged flow at an arbitrary , which is axi-symmetric, i.e.
with a azimuthal wavenumber. Increasing leads to
non-axisymmetric modulations with increasing azimuthal wavenumber from to
. These modulations are found to rotate in the azimuthal direction. However
no clear rotation frequency can be established until . Above, they become periodic with an increasing frequency. We
finally show that these modulations are connected with the coherent structures
of the mixing shear layer. The implication of these findings for the turbulence
parametrization is discussed. Especially, they may explain why simple eddy
viscosity models are able to capture complex turbulent flow dynamics
Experimental study of the effect of disorder on subcritical crack growth dynamics
The growth dynamics of a single crack in a heterogeneous material under
subcritical loading is an intermittent process; and many features of this
dynamics have been shown to agree with simple models of thermally activated
rupture. In order to better understand the role of material heterogeneities in
this process, we study the subcritical propagation of a crack in a sheet of
paper in the presence of a distribution of small defects such as holes. The
experimental data obtained for two different distributions of holes are
discussed in the light of models that predict the slowing down of crack growth
when the disorder in the material is increased; however, in contradiction with
these theoretical predictions, the experiments result in longer lasting cracks
in a more ordered scenario. We argue that this effect is specific to
subcritical crack dynamics and that the weakest zones between holes at close
distance to each other are responsible both for the acceleration of the crack
dynamics and the slightly different roughness of the crack path.Comment: 4 pages, 5 figures, accepted in Physical Review Letters
(http://prl.aps.org
Strong dynamical effects during stick-slip adhesive peeling
We consider the classical problem of the stick-slip dynamics observed when
peeling a roller adhesive tape at a constant velocity. From fast imaging
recordings, we extract the dependencies of the stick and slip phases durations
with the imposed peeling velocity and peeled ribbon length. Predictions of
Maugis and Barquins [in Adhesion 12, edited by K.W. Allen, Elsevier ASP,
London, 1988, pp. 205--222] based on a quasistatic assumption succeed to
describe quantitatively our measurements of the stick phase duration. Such
model however fails to predict the full stick-slip cycle duration, revealing
strong dynamical effects during the slip phase.Comment: Soft Matter 201
Turbulent drag in a rotating frame
What is the turbulent drag force experienced by an object moving in a
rotating fluid? This open and fundamental question can be addressed by
measuring the torque needed to drive an impeller at constant angular velocity
in a water tank mounted on a platform rotating at a rate . We
report a dramatic reduction in drag as increases, down to values as
low as \% of the non-rotating drag. At small Rossby number , the decrease in drag coefficient follows the approximate
scaling law , which is predicted in the framework of nonlinear
inertial wave interactions and weak-turbulence theory. However, stereoscopic
particle image velocimetry measurements indicate that this drag reduction
rather originates from a weakening of the turbulence intensity in line with the
two-dimensionalization of the large-scale flow.Comment: To appear in Journal of Fluid Mechanics Rapid
Euler-like modelling of dense granular flows: application to a rotating drum
General conservation equations are derived for 2D dense granular flows from
the Euler equation within the Boussinesq approximation. In steady flows, the 2D
fields of granular temperature, vorticity and stream function are shown to be
encoded in two scalar functions only. We checked such prediction on steady
surface flows in a rotating drum simulated through the Non-Smooth Contact
Dynamics method. This result is non trivial because granular flows are
dissipative and therefore not necessarily compatible with Euler equation.
Finally, we briefly discuss some possible ways to predict theoretically these
two functions using statistical mechanics
Multiscale Stick-Slip Dynamics of Adhesive Tape Peeling
Using a high-speed camera, we follow the propagation of the detachment front
during the peeling of an adhesive tape from a flat surface. In a given range of
peeling velocity, this front displays a multiscale unstable dynamics,
entangling two well-separated spatiotemporal scales, which correspond to
microscopic and macroscopic dynamical stick-slip instabilities. While the
periodic release of the stretch energy of the whole peeled ribbon drives the
classical macro-stick-slip, we show that the micro-stick-slip, due to the
regular propagation of transverse dynamic fractures discovered by Thoroddsen et
al. [Phys. Rev. E 82, 046107 (2010)], is related to a high-frequency periodic
release of the elastic bending energy of the adhesive ribbon concentrated in
the vicinity of the peeling front.Comment: to appear in Physical Review Letters (2015
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