arXiv subjects

Irene Ippolito

Publications and source records attributed to Irene Ippolito.

7 recordsLinked to original sources

arXiv2012

Stability of a granular layer on an inclined "fakir plane"

We present here experimental results on the effect of a forest of cylinder obstacles (nails) on the stability of a granular layer over a rough incline, in a so-called "fakir plane" configuration. The nail forest is found to increase the stability of the layer, the more for the densest array, and such an effect is recovered by a simple model taking into account the additional friction force exerted by the pillar forest onto the granular layer.

cond-mat.soft↗

arXiv2009

Miscible transfer of solute in different types of rough fractures: from random to multiscale fracture walls heights

Miscible tracer dispersion measurements in transparent model fractures with different types of wall roughness are reported. The nature (Fickian or not) of dispersion is determined by studying variations of the mixing front as a function of the traveled distance but also as a function of the lateral scale over which the tracer concentration is averaged. The dominant convective dispersion mechanisms (velocity profile in the gap, velocity variations in the fracture plane) are established by comparing measurements using Newtonian and shear thinning fluids. For small monodisperse rugosities, front spreading is diffusive with a dominant geometrical dispersion (dispersion coefficient $D \propto Pe$) at low Péclet numbers $Pe$; at higher $Pe$ values one has either $D \propto Pe^2$ ({\it i.e.} Taylor dispersion) for obstacles of height smaller than the gap or $D \propto Pe^{1.35}$ for obstacles bridging the gap. For a self affine multiscale roughness like in actual rocks and a relative shear displacement $\vecδ$ of complementary walls, the aperture field is channelized in the direction perpendicular to $δ$. For a mean velocity $\vec{U}$ parallel to the channels, the global front geometry reflects the velocity contrast between them and is predicted from the aperture field. For $\vec{U}$ perpendicular to the channels, global front spreading is much reduced. Local spreading of the front thickness remains mostly controlled by Taylor dispersion except in the case of a very strong channelization parallel to $\vec U$.

physics.flu-dyn↗

arXiv2008

Anisotropy of tracer dispersion in rough model fractures with sheared walls

Dispersion experiments are compared for two transparent model fractures with identical complementary rough walls but with a relative shear displacement $\vecδ$ parallel ($\vecδ\parallel \vec{U}$) or perpendicular ($\vecδ \perp \vec{U}$) to the flow velocity $\vec{U}$. The structure of the mixing front is characterized by mapping the local normalized local transit time $\bar t(x,y)$ and dispersivity $α(x,y)$. For $\vecδ \perp \vec{U}$, displacement fronts display large fingers: their geometry and the distribution of $\bar t(x,y)U/x$ are well reproduced by assuming parallel channels of hydraulic conductance deduced from the aperture field. For $\vecδ \parallel \vec{U}$, the front is flatter and $α(x,y)$ displays a narrow distribution and a Taylor-like variation with $Pe$.

physics.geo-ph↗

arXiv2007

Pentagon deposits unpack under gentle tapping

We present results from simulations of regular pentagons arranged in a rectangular die. The particles are subjected to vertical tapping. We study the behavior of the packing fraction, number of contacts and arch distributions as a function of the tapping amplitude. Pentagons show peculiar features as compared with disks. As a general rule, pentagons tend to form less arches than disks. Nevertheless, as the tapping amplitude is decreased, the typical size of the pentagon arches grows significantly. As a consequence, a pentagon packing reduces its packing fraction when tapped gently in contrast with the behavior found in rounded particle deposits.

cond-mat.soft↗

arXiv2007

Velocity contrasts enhancement for shear thinning solutions flowing in a rough fracture

Flow and transport are studied in transparent model fractures with rough complementary self-affine walls with a relative shear displacement $\vec{u}$. The aperture field is shown to display long range correlations perpendicular to $\vec{u}$: for flow in that direction, the width and geometry of the front of a dyed shear-thinning polymer solution displacing a transparent one have been studied as a function of the fluid rheology and flow rate. The front width increases linearly with distance indicating a convection of the fluids with a low transverse mixing between the flow paths. The width also increases with the flow-rate as the fluid rheology shifts from Newtonian at low shear rates $\dot γ$ towards a shear thinning behaviour at higher $\dot γ$ values. The width also increases with the polymer concentration at high flow-rates. These results demonstrate the enhancement of the flow velocity contrasts between different flow channels for shear thinning fluids. The relative widths at low and high $\dot γ$ values for different polymer concentrations are well predicted by an analytical model considering the fracture as a set of parallel ducts of constant hydraulic apertures. The overall geometry of the experimental front geometry is also predicted by the theoretical model from the aperture map.

physics.flu-dyn↗

arXiv2006

Influence of the disorder on tracer dispersion in a flow channel

Tracer dispersion is studied experimentally in periodic or disordered arrays of beads in a capillary tube. Dispersion is measured from light absorption variations near the outlet following a steplike injection of dye at the inlet. Visualizations using dye and pure glycerol are also performed in similar geometries. Taylor dispersion is dominant both in an empty tube and for a periodic array of beads: the dispersivity $l\_d$ increases with the Péclet number $Pe$ respectively as $Pe$ and $Pe^{0.82}$ and is larger by a factor of 8 in the second case. In a disordered packing of smaller beads (1/3 of the tube diameter) geometrical dispersion associated to the disorder of the flow field is dominant with a constant value of $l\_d$ reached at high Péclet numbers. The minimum dispersivity is slightly higher than in homogeneous nonconsolidated packings of small grains, likely due heterogeneities resulting from wall effects. In a disordered packing with the same beads as in the periodic configuration, $l\_d$ is up to 20 times lower than in the latter and varies as $Pe^α$ with $α= 0.5$ or $= 0.69$ (depending on the fluid viscosity). A simple model accounting for this latter result is suggested.

physics.flu-dyn↗

arXiv2006

Miscible displacement fronts of shear thinning fluids inside rough fractures

The miscible displacement of a shear-thinning fluid by another of same rheological properties is studied experimentally in a transparent fracture by an optical technique imaging relative concentration distributions. The fracture walls have complementary self-affine geometries and are shifted laterally in the direction perpendicular to the mean flow velocity {\bf U} : the flow field is strongly channelized and macro dispersion controls the front structure for Péclet numbers above a few units. The global front width increases then linearly with time and reflects the velocity distribution between the different channels. In contrast, at the local scale, front spreading is similar to Taylor dispersion between plane parallel surfaces. Both dispersion mechanisms depend strongly on the fluid rheology which shifts from Newtonian to shear-thinning when the flow rate increases. In the latter domain, increasing the concentration enhances the global front width but reduces both Taylor dispersion (due to the flattening of the velocity profile in the gap of the fracture) and the size of medium scale front structures.

physics.flu-dyn↗