56 research outputs found
A zero-mode mechanism for spontaneous symmetry breaking in a turbulent von K\'arm\'an flow
We suggest that the dynamical spontaneous symmetry breaking reported in a
turbulent swirling flow at by Cortet et al., Phys. Rev. Lett., 105,
214501 (2010) can be described through a continuous one parameter family
transformation (amounting to a phase shift) of steady states and could be the
analogue of the Goldstone mode of the vertical translational symmetry in an
ideal system. We investigate a possible mechanism of emergence of such
spontaneous symmetry breaking in a toy model of our out-equilibrium system,
derived from its equilibrium counterpart. We show that the stationary states
are solution of a linear differential equation. For a specific value of the
Reynolds number, they are subject to a spontaneous symmetry breaking through a
zero-mode mechanism. These zero-modes obey a Beltrami property and their
spontaneous fluctuations can be seen as the "phonon of turbulence".Comment: 17 pages, 4 figures, submitted to New. J. Phy
Boundary-induced inhomogeneity of particle layers in the solidification of suspensions
When a suspension freezes, a compacted particle layer builds up at the
solidification front with noticeable implications on the freezing process. In a
directional solidification experiment of monodispersed suspensions in thin
samples, we evidence a link between the thickness of this layer and the sample
depth. We attribute it to an inhomogeneity of particle density induced by the
sample plates. A mechanical model enables us to relate it to the layer
thickness with a dependency on the sample depth and to select the distribution
of particle density that yields the best fit to our data. This distribution
involves an influence length of sample plates of about nine particle diameters.
These results clarify the implications of boundaries on suspension freezing.
They may be useful to model polydispersed suspensions since large particles
could play the role of smooth boundaries with respect to small ones.Comment: 16 pages, 13 figure
Local Oscillatory Rheology from Echography
Local Oscillatory Rheology from Echography (LORE) consists in a traditional
rheology experiment synchronized with high-frequency ultrasonic imaging which
gives access to the local material response to oscillatory shear. Besides
classical global rheological quantities, this method provides quantitative
time-resolved information on the local displacement across the entire gap of
the rheometer. From the local displacement response, we compute and decompose
the local strain in its Fourier components and measure the spatially-resolved
viscoelastic moduli. After benchmarking our method on homogeneous Newtonian
fluids and soft solids, we demonstrate that this technique is well suited to
characterize spatially heterogeneous samples, wall slip, and the emergence of
nonlinearity under large amplitude oscillatory stress in soft materials.Comment: 10 pages, 5 figures, submitted to Phys. Rev. Applie
Evidence for Forcing-Dependent Steady States in a Turbulent Swirling Flow
We study the influence on steady turbulent states of the forcing in a von Karman flow, at constant impeller speed, or at constant torque. We find that the different forcing conditions change the nature of the stability of the steady states and reveal dynamical regimes that bear similarities to low-dimensional systems. We suggest that this forcing dependence may be applicable to other turbulent systems
A statistical mechanics framework for the large-scale structure of turbulent von K{\'a}rm{\'a}n flows
In the present paper, recent experimental results on large scale coherent
steady states observed in experimental von K{\'a}rm{\'a}n flows are revisited
from a statistical mechanics perspective. The latter is rooted on two levels of
description. We first argue that the coherent steady states may be described as
the equilibrium states of well-chosen lattice models, that can be used to
define global properties of von K{\'a}rm{\'a}n flows, such as their
temperatures. The equilibrium description is then enlarged, in order to
reinterpret a series of results about the stability of those steady states,
their susceptibility to symmetry breaking, in the light of a deep analogy with
the statistical theory of Ferromagnetism. We call this analogy
"Ferro-Turbulence
Imagerie ultrasonore de suspensions de particules non-browniennes
Nous présentons un système nouveau d'étude du comportement de fluides complexes, s'appuyant sur des mesures rhéologiques globales ainsi que des mesures ultrasonores à deux dimensions résolues en temps. Ces mesures permettent de récupérer une composante du champ de vitesses, mais également la concentration en particules. Nous détaillons ses avantages et ses limitations lors d'expériences effectuées avec des suspensions de particules non-Browniennes
Uncovering instabilities in the spatiotemporal dynamics of a shear-thickening cornstarch suspension
Recent theories predict that discontinuous shear-thickening (DST) involves an
instability, the nature of which remains elusive. Here, we explore unsteady
dynamics in a dense cornstarch suspension by coupling long rheological
measurements under constant shear stresses to ultrasound imaging. We
demonstrate that unsteadiness in DST results from localized bands that travel
along the vorticity direction with a specific signature on the global shear
rate response. These propagating events coexist with quiescent phases for
stresses slightly above DST onset, resulting in intermittent, turbulent-like
dynamics. Deeper into DST, events proliferate, leading to simpler, Gaussian
dynamics. We interpret our results in terms of unstable vorticity bands as
inferred from recent model and numerical simulations
A statistical mechanics framework for the large-scale structure of turbulent von Kármán flows
In the present paper, recent experimental results on large scale coherent steady states observed in experimental von Kármán flows are revisited from a statistical mechanics perspective. The latter is rooted on two levels of description. We first argue that the coherent steady states may be described as the equilibrium states of well-chosen lattice models, that can be used to define global properties of von Kármán flows, such as their temperatures. The equilibrium description is then enlarged, in order to reinterpret a series of results about the stability of those steady states, their susceptibility to symmetry breaking, in the light of a deep analogy with the statistical theory of Ferromagnetism. We call this analogy ''Ferro-Turbulence'
- …