134 research outputs found
Experimental velocity fields and forces for a cylinder penetrating into a granular medium
We present here a detailed granular flow characterization together with force
measurements for the quasi-bidimensional situation of a horizontal cylinder
penetrating vertically at a constant velocity in dry granular matter between
two parallel glass walls. In the velocity range studied here, the drag force on
the cylinder does not depend on the velocity V_0 and is mainly proportional to
the cylinder diameter d. Whereas the force on the cylinder increases with its
penetration depth, the granular velocity profile around the cylinder is found
stationary with fluctuations around a mean value leading to the granular
temperature profile. Both mean velocity profile and temperature profile exhibit
strong localization near the cylinder. The mean flow perturbation induced by
the cylinder decreases exponentially away from the cylinder on a characteristic
length \lambda, that is mainly governed by the cylinder diameter for large
enough cylinder/grain size ratio d/d_g: \lambda ~ d/4 + 2d_g. The granular
temperature exhibits a constant plateau value T_0 in a thin layer close to the
cylinder of extension \delta_{T_0} ~ \lambda/2 and decays exponentially far
away with a characteristic length \lambda_T of a few grain diameters (\lambda_T
~ 3d_g). The granular temperature plateau T_0 that scales as (V_0^2 d_g/d) is
created by the flow itself from the balance between the "granular heat"
production by the shear rate V_0/\lambda over \delta_{T_0} close to the
cylinder and the granular dissipation far away
Competitive dynamics of two erosion patterns around a cylinder
We investigate experimentally the local erosion of a granular bed near a
fixed vertical cylinder that emerges from the bed. The onset of erosion arising
at the base of the cylinder and usually ascribed to the wrapping horseshoe
vortex is determined and rationalized by a flow contraction effect. We report a
new erosion pattern visible downstream of the cylinder that consists of two
side-by-side elongated holes. This pattern is observed for flow regimes close
to the horseshoe scour onset, whose growth usually inhibits its spatiotemporal
development.Comment: 6 pages, 6 figure
Wall effects on granular heap stability
We investigate the effects of lateral walls on the angle of movement and on
the angle of repose of a granular pile. Our experimental results for beads
immersed in water are similar to previous results obtained in air and to recent
numerical simulations. All of these results, showing an increase of pile angles
with a decreasing gap width, are explained by a model based on the redirection
of stresses through the granular media. Two regimes are observed depending on
the bead diameter. For large beads, the range of wall effects corresponds to a
constant number of beads whereas it corresponds to a constant characteristic
length for small beads as they aggregate via van der Waals forces
Experimental investigation of tsunami waves generated by granular collapse into water
The generation of a tsunami wave by an aerial landslide is investigated
through model laboratory experiments. We examine the collapse of an initially
dry column of grains into a shallow water layer and the subsequent generation
of waves. The experiments show that the collective entry of the granular
material into water governs the wave generation process. We observe that the
amplitude of the wave relative to the water height scales linearly with the
Froude number based on the horizontal velocity of the moving granular front
relative to the wave velocity. For all the different parameters considered
here, the aspect ratio and the volume of the column, the diameter and density
of the grains, and the height of the water, the granular collapse acts like a
moving piston displacing the water. We also highlight that the density of the
falling grains has a negligible influence on the wave amplitude, which suggests
that the volume of grains entering the water is the relevant parameter in the
wave generation.Comment: 23 pages, 16 figure
Dynamics of grain ejection by sphere impact on a granular bed
The dynamics of grain ejection consecutive to a sphere impacting a granular
material is investigated experimentally and the variations of the
characteristics of grain ejection with the control parameters are
quantitatively studied. The time evolution of the corona formed by the ejected
grains is reported, mainly in terms of its diameter and height, and favourably
compared with a simple ballistic model. A key characteristic of the granular
corona is that the angle formed by its edge with the horizontal granular
surface remains constant during the ejection process, which again can be
reproduced by the ballistic model. The number and the kinetic energy of the
ejected grains is evaluated and allows for the calculation of an effective
restitution coefficient characterizing the complex collision process between
the impacting sphere and the fine granular target. The effective restitution
coefficient is found to be constant when varying the control parameters.Comment: 9 page
Granular Avalanches in Fluids
Three regimes of granular avalanches in fluids are put in light depending on
the Stokes number St which prescribes the relative importance of grain inertia
and fluid viscous effects, and on the grain/fluid density ratio r. In gas (r >>
1 and St > 1, e.g., the dry case), the amplitude and time duration of
avalanches do not depend on any fluid effect. In liquids (r ~ 1), for
decreasing St, the amplitude decreases and the time duration increases,
exploring an inertial regime and a viscous regime. These regimes are described
by the analysis of the elementary motion of one grain
Parallel flow in Hele-Shaw cells with ferrofluids
Parallel flow in a Hele-Shaw cell occurs when two immiscible liquids flow
with relative velocity parallel to the interface between them. The interface is
unstable due to a Kelvin-Helmholtz type of instability in which fluid flow
couples with inertial effects to cause an initial small perturbation to grow.
Large amplitude disturbances form stable solitons. We consider the effects of
applied magnetic fields when one of the two fluids is a ferrofluid. The
dispersion relation governing mode growth is modified so that the magnetic
field can destabilize the interface even in the absence of inertial effects.
However, the magnetic field does not affect the speed of wave propagation for a
given wavenumber. We note that the magnetic field creates an effective
interaction between the solitons.Comment: 12 pages, Revtex, 2 figures, revised version (minor changes
Expanding Duplication of Free Fatty Acid Receptor-2 (GPR43) Genes in the Chicken Genome
International audienceFree fatty acid receptors (FFAR) belong to a family of five G-protein coupled receptors that are involved in the regulation of lipidmetabolism, so that their loss of function increases the risk of obesity. The aim of this study was to determine the expansion of genesencoding paralogs of FFAR2 in the chicken, considered as amodel organism for developmental biology and biomedical research. Byestimating the gene copy number using quantitative polymerase chain reaction, genomic DNA resequencing, and RNA sequencingdata, we showed the existence of 23 ±1.5 genes encoding FFAR2 paralogs in the chicken genome. The FFAR2 paralogs shared anidentity from 87.2%up to 99%. Extensive gene conversion was responsible for this high degree of sequence similarities betweenthese genes, and this concerned especially the four amino acids known to be critical for ligand binding. Moreover, elevated nonsynonymous/synonymous substitutionratios onsomeamino acids withinor inclose-vicinity of the ligand-bindinggroove suggest thatpositive selectionmay have reduced the effective rate of gene conversion in this region, thus contributing to diversify the function ofsome FFAR2 paralogs. All the FFAR2 paralogs were located on a microchromosome in a same linkage group. FFAR2 genes wereexpressed in different tissues and cells such as spleen, peripheral blood mononuclear cells, abdominal adipose tissue, intestine, andlung, with the highest rate of expression in testis. Further investigations are needed to determine whether these chicken-specificevents along evolution are the consequence of domestication and may play a role in regulating lipid metabolism in this species
Thixotropy in macroscopic suspensions of spheres
An experimental study of the viscosity of a macroscopic suspension, i.e. a
suspension for which Brownian motion can be neglected, under steady shear is
presented. The suspension is prepared with a high packing fraction and is
density-matched in a Newtonian carrier fluid. The viscosity of the suspension
depends on the shear rate and the time of shearing. It is shown for the first
time that a macroscopic suspension shows thixotropic viscosity, i.e.
shear-thinning with a long relaxation time as a unique function of shear. The
relaxation times show a systematic decrease with increasing shear rate. These
relaxation times are larger when decreasing the shear rates, compared to those
observed after increasing the shear. The time scales involved are about 10000
times larger than the viscous time scale and about 1000 times smaller than the
thermodynamic time scale. The structure of the suspension at the outer cylinder
of a viscometer is monitored with a camera, showing the formation of a
hexagonal structure. The temporal decrease of the viscosity under shear
coincides with the formation of this hexagonal pattern
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