143 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
Relevant heating of the quiet solar corona by Alfvén waves: a result of adiabaticity breakdown
International audienceIon heating by Alfvén waves has been considered for long as the mechanism explaining why thesolar corona has a temperature several orders of magnitude higher than the photosphere. Unfortu-nately, as the measured wave frequencies are much smaller than the ion cyclotron frequency, particleswere expected to behave adiabatically, impeding a direct wave-particle energy transfer to take place,except through decorrelating stochastic mechanisms related to broadband wave spectra. This paperproposes a new paradigm for this mechanism by showing it is actually much simpler, more general,and very efficient. Indeed, for measured wave amplitudes in the quiet corona, ion orbits are shownto cross quasi-periodically one or several slowly pulsating separatrices in phase space. Now, a sepa-ratrix is an orbit with an infinite period, thus much longer than the pulsation one. Therefore, eachseparatrix crossing cancels adiabatic invariance, and yields a very strong energy transfer from thewave, and thus particle heating. This occurs whatever be the wave spectrum, even a monochromaticone. The proposed mechanism is so efficient that it might lead to a self-organized picture of coronalheating: all Alfvén waves exceeding a threshold are immediately quenched and transfer their energyto the waves
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
Shock Wave Structure in a Strongly Nonlinear Granular Lattice with Viscous Dissipation
The shock wave structure in a one-dimensional lattice (e.g. granular chain)
with a power law dependence of force on displacement between particles with
viscous dissipation is considered and compared to the corresponding long wave
approximation. A dissipative term depending on the relative velocity between
neighboring particles is included in the discrete model to investigate its
influence on the shape of steady shock profiles. The critical viscosity
coefficient is obtained from the long-wave approximation for arbitrary values
of the exponent n and denotes the transition from an oscillatory to a monotonic
shock profile in stronly nonlinear systems. The expression for the critical
viscosity coefficient converges to the known equation for the critical
viscosity in the weakly nonlinear case. Values of viscosity based on this
expression are comparable to the values obtained in the numerical analysis of a
discrete particle lattice with a Herzian contact interaction corresponding to n
= 3/2. An initial disturbance in a discrete system approaches a stationary
shock profile after traveling a short distance that is comparable to the width
of the leading pulse of a stationary shock front. The shock front width is
minimized when the viscosity is equal to its critical value.Comment: 20 pages, 6 figure
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
Embryo vitrification in rabbits: Consequences for progeny growth
[EN] The objective of this research is to examine if there are any effects of the rederivation procedures on rabbit growth pattern and on weight of different organ in adults. For this purpose, three experiments were conducted on two different groups of animals (control group and vitrified transferred group) to evaluate the possible effect of embryo manipulation (vitrification and transfer procedures) on future growth traits. The first experiment studies body weight from 1 to 9 weeks of age from the two groups. The second experiment describes the growth curve of progeny from experimental groups and analyzes their Gompertz curve parameters, including the estimation of adult body weight. The third experiment has been developed to study if there are any differences in different organ weight in adult males from the two experimental groups. In general, the results indicate that rederivation procedures had effect on the phenotypic expression of growth traits. The results showed that rabbit produced by vitrification and embryo transfer had higher body weight in the first four weeks of age than control progeny. Results from body weight (a parameter) and b parameter estimated by fitting the Gompertz growth curve did not show any difference between experimental groups. However, differences related with growth velocity (k parameter of the Gompertz curve) were observed among them, showing that the control group had higher growth velocity than the vitrified transferred group. In addition, we found that liver weight at 40th week of age exhibits significant differences between the experimental groups. The liver weight was higher in the control males than in the VF males. Although the present results indicate that vitrification and transfer procedures might affect some traits related with growth in rabbits, further research is needed to assess the mechanisms involved in the appearance of these phenotypes and if these phenotypes could be transferred to the future progeny.This study was supported by the Generalitat Valenciana research program (Prometeo II 2014/036) and Spanish Research Projects (CICYT AGL2011-29831-C03-01; AGL2014-53405-C2-1-P). Lavara R. acknowledges the partial support received from Generalitat Valenciana under VALid+ program (APOST/2014/034) and from Ministry of Economy and Competitiveness under subprogramme "Formacion posdoctoral" (FPDI-2013-16707).Lavara García, R.; Baselga Izquierdo, M.; Marco Jiménez, F.; Vicente Antón, JS. (2015). Embryo vitrification in rabbits: Consequences for progeny growth. Theriogenology. 84(5):674-680. https://doi.org/10.1016/j.theriogenology.2015.04.025S67468084
Dense flow around a sphere moving into a cloud of grains
A bidimensional simulation of a sphere moving at constant velocity into a cloud of smaller spherical grains without gravity is presented with a non-smooth contact dynamics method. A dense granular “cluster” zone of about constant solid fraction builds progressively around the moving sphere until a stationary regime appears with a constant upstream cluster size that increases with the initial solid fraction ϕ0 of the cloud. A detailed analysis of the local strain rate and local stress fields inside the cluster reveals that, despite different spatial variations of strain and stresses, the local friction coeffcient μ appears to depend only on the local inertial number I as well as the local solid fraction ϕ, which means that a local rheology does exist in the present non parallel flow. The key point is that the spatial variations of I inside the cluster does not depend on the sphere velocity and explore only a small range between about 10−2 and 10−1. The influence of sidewalls is then investigated on the flow and the forces
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