71 research outputs found
Low self-affine exponents of fracture surfaces of glass ceramics
The geometry of post mortem rough fracture surfaces of porous glass ceramics
made of sintered glass beads is shown experimentally to be self-affine with an
exponent zeta=0.40 (0.04) remarkably lower than the 'universal' value zeta=0.8
frequently measured for many materials. This low value of zeta is similar to
that found for sandstone samples of similar micro structure and is also
practically independent on the porosity phi in the range investigated (3% < phi
< 26%) as well as on the bead diameter d and of the crack growth velocity. In
contrast, the roughness amplitude normalized by d increases linearly with phi
while it is still independent, within experimental error, of d and of the crack
propagation velocity. An interpretation of this variation is suggested in terms
of a transition from transgranular to intergranular fracture propagation with
no influence, however, on the exponent zeta.Comment: 4 page
Turning bacteria suspensions into a "superfluid"
The rheological response under simple shear of an active suspension of
Escherichia coli is determined in a large range of shear rates and
concentrations. The effective viscosity and the time scales characterizing the
bacterial organization under shear are obtained. In the dilute regime, we bring
evidences for a low shear Newtonian plateau characterized by a shear viscosity
decreasing with concentration. In the semi-dilute regime, for particularly
active bacteria, the suspension display a "super-fluid" like transition where
the viscous resistance to shear vanishes, thus showing that macroscopically,
the activity of pusher swimmers organized by shear, is able to fully overcome
the dissipative effects due to viscous loss
Flow channelling in a single fracture induced by shear displacement
The effect on the transport properties of fractures of a relative shear
displacement of rough walls with complementary self-affine surfaces
has been studied experimentally and numerically. The shear displacement induces an anisotropy of the aperture field with a correlation length
scaling as and significantly larger in the direction perpendicular to . This reflects the appearance of long range channels perpendicular to resulting in a higher effective permeability for flow in the direction
perpendicular to the shear. Miscible displacements fronts in such fractures are
observed experimentally to display a self affine geometry of characteristic
exponent directly related to that of the rough wall surfaces. A simple model
based on the channelization of the aperture field allows to reproduces the
front geometry when the mean flow is parallel to the channels created by the
shear displacement
2D Electrical Resistivity Tomography surveys optimisation of solutes transports in porous media
International audienceThe purpose of this study is to quantify experimentally the evolution of dissolved species in porous media from 2D resistivity models. Transport experiments are carried out at the laboratory scale by performing flow in a model porous medium obtained by filling a transparent container with mono disperse glass beads. A tracer made by mixing a dissolved of blue dye and a known NaCl concentration is injected with a constant flow rate through the porous medium already saturated by a transparent fluid. ERT measurements are acquired during the fluid flow. The measurement conditions and the inversion parameters are estimated so that the relation between spatial and temporal resolutions is optimised. A video follow-up is also carried out during the upward tracer propagation. The comparison of the temporal evolution of the NaCl concentration distribution estimated from ERT models with Video analysis shows remarkable agreement
Stokes flow paths separation and recirculation cells in X-junctions of varying angle
Fluid and solute transfer in X-junctions between straight channels is shown
to depend critically on the junction angle in the Stokes flow regime.
Experimentally, water and a water-dye solution are injected at equal flow rates
in two facing channels of the junction: Planar Laser Induced fluorescence
(PLIF) measurements show that the largest part of each injected fluid "bounces
back" preferentially into the outlet channel at the lowest angle to the
injection; this is opposite to the inertial case and requires a high curvature
of the corresponding streamlines. The proportion of this fluid in the other
channel decreases from 50% at 90\degree to zero at a threshold angle. These
counterintuitive features reflect the minimization of energy dissipation for
Stokes flows. Finite elements numerical simulations of a 2D Stokes flow of
equivalent geometry con rm these results and show that, below the threshold
angle 33.8\degree recirculation cells are present in the center part of the
junction and separate the two injected flows of the two solutions. Reducing
further leads to the appearance of new recirculation cells with lower flow
velocities
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