90 research outputs found
Local waiting time fluctuations along a randomly pinned crack front
The propagation of an interfacial crack along a heterogeneous weak plane of a
transparent Plexiglas block is followed using a high resolution fast camera. We
show that the fracture front dynamics is governed by local and irregular
avalanches with very large size and velocity fluctuations. We characterize the
intermittent dynamics observed, i.e. the local pinnings and depinnings of the
crack front which trigger a rich burst activity, by measuring the local waiting
time fluctuations along the crack front during its propagation. The local front
line velocity distribution deduced from the waiting time analysis exhibits a
power law behavior, with , for
velocities larger than the average front speed . The burst size
distribution is also a power law,
with . Above a characteristic
length scale of disorder , the avalanche clusters become
anisotropic, and the scaling of the anisotropy ratio provides an estimate of
the roughness exponent of the crack front line, , in close agreement
with previous independent estimates.Comment: Phys. Rev. Lett., accepte
Direct velocity measurement of a turbulent shear flow in a planar Couette cell
In a plane Couette cell a thin fluid layer consisting of water is sheared
between a transparent band at Reynolds numbers ranging from 300 to 1400. The
length of the cells flow channel is large compared to the film separation. To
extract the flow velocity in the experiments a correlation image velocimetry
(CIV) method is used on pictures recorded with a high speed camera. The flow is
recorded at a resolution that allows to analyze flow patterns similar in size
to the film separation. The fluid flow is then studied by calculating flow
velocity autocorrelation functions. The turbulent pattern that arise on this
scale above a critical Reynolds number of Re=360 display characteristic
patterns that are proven with the calculated velocity autocorrelation
functions. The patterns are metastable and reappear at different positions and
times throughout the experiments. Typically these patterns are turbulent rolls
which are elongated in the stream direction which is the direction the band is
moving. Although the flow states are metastable they possess similarities to
the steady Taylor vortices known to appear in circular Taylor Couette cells
Local dynamics of a randomly pinned crack front during creep and forced propagation: An experimental study
We have studied the propagation of a crack front along the heterogeneous weak
plane of a transparent poly(methyl methacrylate) (PMMA) block using two
different loading conditions: imposed constant velocity and creep relaxation.
We have focused on the intermittent local dynamics of the fracture front for a
wide range of average crack front propagation velocities spanning over four
decades. We computed the local velocity fluctuations along the fracture front.
Two regimes are emphasized: a depinning regime of high velocity clusters
defined as avalanches and a pinning regime of very low-velocity creeping lines.
The scaling properties of the avalanches and pinning lines (size and spatial
extent) are found to be independent of the loading conditions and of the
average crack front velocity. The distribution of local fluctuations of the
crack front velocity are related to the observed avalanche size distribution.
Space-time correlations of the local velocities show a simple diffusion growth
behavior.Comment: Physical Review E (2011); 62.20.mt, 46.50.+a, 68.35.C
Dynamic aerofracture of dense granular packings
International audienceA transition in hydraulically induced granular displacement patterns is studied by means of discrete numerical molecular dynamics simulations. During this transition the patterns change from fractures and fingers to finely dispersed bubbles. The dynamics of the displacement patterns are studied in a rectangular Hele-Shaw cell filled with a dense but permeable two-dimensional granular layer. At one side of the cell the pressure of the compressible interstitial gas is increased. At the opposite side from the inlet of the cell a semipermeable boundary is located. This boundary is only permeable towards the gas phase while preventing grains from leaving the cell. The imposed pressure gradient compacts the grains. In the process we can identify and describe a mechanism that controls the transition of the emerging displacement patterns from fractures and fingers to finely dispersed bubbles as a function of the interstitial gas's properties and the characteristics of the granular phase
The non-Gaussian nature of fracture and the survival of fat-tail exponents
4 pagesInternational audienceWe study the fluctuations of the global velocity Vl(t), computed at various length scales l, during the intermittent Mode-I propagation of a crack front. The statistics converge to a non-Gaussian distribution, with an asymmetric shape and a fat tail. This breakdown of the Central Limit Theorem (CLT), is due to the diverging variance of the underlying local crack front velocity distribution, displaying a power law tail. Indeed, by the application of a generalized CLT, the full shape of our experimental velocity distribution at large scale is shown to follow the stable Levy distribution, which preserves the power law tail exponent under upscaling. This study aims to demonstrate in general for Crackling Noise systems, how one can infer the complete scale dependence of the activity- and extreme event distributions, by measuring only at a global scale
Numerical approach to frictional fingers
Experiments on confined multiphase flow systems, involving air and a dense suspension, have revealed a diverse set of flow morphologies. As the air displaces the suspension, the beads that make up the suspension can accumulate along the interface. The dynamics can generate “frictional fingers” of air coated by densely packed grains. We present here a simplified model for the dynamics together with a new numerical strategy for simulating the frictional finger behavior. We further make theoretical predictions for the characteristic width associated with the frictional fingers, based on the yield stress criterion, and compare these to experimental results. The agreement between theory and experiments validates our model and allows us to estimate the unknown parameter in the yield stress criterion, which we use in the simulations
Family-Vicsek scaling of detachment fronts in Granular Rayleigh Taylor Instabilities during sedimenting granular/fluid flows
When submillimetric particles are confined in a fluid such that a compact
cluster of particles lie above the clear fluid, particles will detach from the
lower boundary of the cluster and form an unstable separation front giving rise
to growing fingers of falling particles. We study this problem using both
experiments and hybrid granular/fluid mechanics models. In the case of
particles from 50 to 500 microns in diameter falling in air, we study the
horizontal density fluctuations at early times: the amplitude of the density
difference between two points at a certain horizontal distance grows as a power
law of time. This happens up to a saturation corresponding to a power law of
the distance. The way in which the correlation length builds up to this
saturation also follows a power law of time. We show that these decompaction
fronts in sedimentation problems follow a Family-Vicsek scaling, characterize
the dynamic and Hurst exponent of the lateral density fluctuations,
respectively z \sim 1 and \zeta \sim 0.75, and show how the prefactors depend
on the grain diameter. We also show from similar simulations with a more
viscous and incompressible fluid, that this feature is independent of the fluid
compressibility or viscosity, ranging from air to water/glycerol mixtures
Average crack-front velocity during subcritical fracture propagation in a heterogeneous medium
We study the average velocity of crack fronts during stable interfacial fracture experiments in a heterogeneous quasibrittle material under constant loading rates and during long relaxation tests. The transparency of the material (polymethylmethacrylate) allows continuous tracking of the front position and relation of its evolution to the energy release rate. Despite significant velocity fluctuations at local scales, we show that a model of independent thermally activated sites successfully reproduces the large-scale behavior of the crack front for several loading conditions
Pattern formation of frictional fingers in a gravitational potential
Aligned finger structures, with a characteristic width, emerge during the slow drainage of a liquid-granular mixture in a tilted Hele-Shaw cell. A transition from vertical to horizontal alignment of the finger structures is observed as the tilting angle and the granular density are varied. An analytical model is presented, demonstrating that the alignment properties are the result of the competition between fluctuating granular stresses and the hydrostatic pressure. The dynamics is reproduced in simulations. We also show how the system explains patterns observed in nature, created during the early stages of a dike formation
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