46 research outputs found
Scale-free channeling patterns near the onset of erosion of sheared granular beds
Erosion shapes our landscape and occurs when a sufficient shear stress is
exerted by a fluid on a sedimented layer. What controls erosion at a
microscopic level remains debated, especially near the threshold forcing where
it stops. Here we study experimentally the collective dynamics of the moving
particles, using a set-up where the system spontaneously evolves toward the
erosion onset. We find that the spatial organization of the erosion flux is
heterogeneous in space, and occurs along channels of local flux whose
distribution displays scaling near threshold and follows , where is the mean erosion flux. Channels are strongly correlated
in the direction of forcing but not in the transverse direction. We show that
these results quantitatively agree with a model where the dynamics is governed
by the competition of disorder (which channels mobile particles) and particle
interactions (which reduces channeling). These observations support that for
laminar flows, erosion is a dynamical phase transition which shares similarity
with the plastic depinning transition occurring in dirty superconductors. The
methodology we introduce here could be applied to probe these systems as well.Comment: 8 pages, 6 figure
Study of the ejection of grains out of a pressurized cylinder
Nous consideÌrons un sceÌnario hypotheÌtique dâaccident initieÌ par une insertion de reÌactiviteÌ nucleÌaire entrainant la rupture de la gaine dâun crayon combustible. La violence du contact avec le fluide caloporteur deÌpendrait fortement du deÌbit de fragment combustible, initialement confinĂ© avec un gaz pressurisĂ©, sortant du crayon. Ce travail de theÌse a consisteÌ aÌ eÌtudier cette dynamique en la modeÌlisant par la vidange dâun milieu granulaire hors dâun silo pressuriseÌ. Nous nous sommes dâabord concentreÌs sur le roÌle de la geÌomeÌtrie interne du silo grĂące Ă un fond inclinĂ©. Pour un faible angle dâinclinaison, lâorientation de lâeÌcoulement granulaire (donc le dĂ©bit) est controÌleÌe par la friction parieÌtale, tandis quâune grande inclinaison deÌtermine cette orientation. Nous nous sommes ensuite inteÌresseÌs au roÌle du gaz pressuriseÌ dâabord en imposant une surpression dâair constante puis en consideÌrant un cas transitoire dans lequel une surpression initiale provoque la rupture de lâorifice. Dans les deux configurations, le deÌbit granulaire est mis aÌ lâeÌchelle par une loi de Beverloo modifieÌe ouÌ le gradient de pression du gaz preÌs de lâorifice agit comme une force motrice suppleÌmentaire qui peut ĂȘtre deÌcrite par la loi de Forchheimer. Nous proposons un modeÌle quasi-stationnaire pour le deÌbit granulaire ainsi quâune simulation continue baseÌe sur un modeÌle multiphasique qui reproduisent bien les rĂ©sultats expĂ©rimentaux. A forts deÌbits, nous observons une instabiliteÌ du jet, initieÌe par des oscillations de pression Ă lâorifice. Enfin nous avons montrĂ© que le milieu environnant agit sur la condition de pression Ă la sortie du siloWe consider a hypothetical scenario of reactivity initiated accident in a nuclear power plant. The violence of the so-called fuel-coolant interaction phenomena depends strongly on the flow rate of particles out of the gas pressurized rod. The aim of this thesis was to study how this discharge rate is driven by the internal geometry and the pressurized gas. We focused firstly on the discharge of a rectangular silo with an inclined bottom. For a small inclination angle, the granular flow orientation is controlled by the wall friction, whereas a large inclination angle fully determines this orientation. Secondly, we focused on two configurations with pressurized gas : a case with constant gas overpressure at the top of the silo and a more transient case for which an initial larger overpressure initiates the rupture of an orifice. The granular flow rate increases significantly with the gas flow, especially for the finer particles and the large overpressures. In both cases, the flow rate scales with a modified Beverloo law where the gas pressure gradient near the outlet acts as an additional driving force. The pressure gradient is well described by a Forchheimer resistance law through the granular medium. We therefore propose a quasi-steady model for the transient description of the granular flow rate. The two configurations were successfully reproduced by numerical simulations based on a continuum multiphase model. For the larger flow rates, instabilities of the granular jet were found to be initiated by pressure oscillations in the outlet region. The presence of water surrounding the silo only acts through an additional hydrostatic pressure effec
Etude de l'éjection de grains hors d'un cylindre pressurisé
We consider a hypothetical scenario of reactivity initiated accident in a nuclear power plant. The violenceof the so-called fuel-coolant interaction phenomena, that we arbitrarily assume to occur during the accident, dependsstrongly on the flow rate of particles out of the gas pressurized rod in which they were initially confined. The aimof this thesis was to study how this discharge rate is driven by the internal geometry and the pressurized gas. Fuelparticles were experimentally simulated by a dense granular material discharging out of a confined and pressurizedsilo. The controlled parameters were the particle size, density and shape, the outlet size and the gas pressure, whereasthe granular and gas flow rates and the pressure along the silo are measured. Discrete and continuous numericalsimulations were performed on similar configurations.We focused firstly on the discharge of a rectangular silo of thickness W, with a lateral outlet of size D and aninclined bottom with a parametric angle. For a small inclination angle, the granular flow orientation depends on theaspect ratio D/W due to the wall friction, whereas a large inclination angle fully determines this orientation. Theseresults were successfully reproduced by numerical simulations.Secondly, we focused on two configurations with pressurized gas : a case with constant gas overpressure at thetop of the silo and a more transient case for which an initial larger overpressure initiates the rupture of an orifice. Thegranular flow rate increases significantly with the gas flow, especially for the finer particles and the large overpressures.In both cases, the flow rate scales with a modified Beverloo law where the gas pressure gradient near the outlet actsas an additional driving force. The pressure gradient is well described by a Forchheimer resistance law through thegranular medium. We therefore propose a quasi-steady model for the transient description of the granular flow rate.The two configurations were successfully reproduced by numerical simulations based on a continuum multiphasemodel. For the larger flow rates, instabilities of the granular jet were found to be initiated by pressure oscillationsin the outlet region. The presence of water surrounding the silo only acts through an additional hydrostatic pressureeffect.- Nous considĂ©rons un scĂ©nario hypothĂ©tique dâaccident initiĂ© par une insertion de rĂ©activitĂ© dans unecentrale nuclĂ©aire entrainant la rupture de la gaine dâun crayon de combustible. La violence du contact entre lecombustible et le fluide caloporteur dĂ©pendrait alors fortement du dĂ©bit de fragment combustible sortant hors ducrayon qui est initialement confinĂ© avec le gaz pressurisĂ©. Ce travail de thĂšse a consistĂ© Ă Ă©tudier cette dynamique enla modĂ©lisant par la vidange dâun milieu granulaire hors dâun silo pressurisĂ©. Les paramĂštres de contrĂŽle sont la taille,la densitĂ© et la forme des particules ainsi que la taille de lâorifice et la pression initiale dâair imposĂ©e dans le silo, tandisque les dĂ©bits dâair et du milieu granulaire et la pression dâair le long du silo sont mesurĂ©s. Les rĂ©sultats expĂ©rimentauxont Ă©tĂ© confrontĂ©s avec des simulations numĂ©riques discrĂštes et continues avec une rhĂ©ologie visco-plastique pour lemilieu granulaire.Afin dâĂ©tudier le rĂŽle du confinement dans la gĂ©omĂ©trie dâintĂ©rĂȘt, nous nous sommes dâabord concentrĂ©s sur lavidange dâun silo rectangulaire dâĂ©paisseurW, avec une sortie latĂ©rale de taille D et un fond dont nous varions lâinclinaison.Pour un faible angle dâinclinaison, lâorientation de lâĂ©coulement granulaire (et donc le dĂ©bit) est contrĂŽlĂ©e parla friction pariĂ©tale et dĂ©pend du rapport dâaspect de lâorifice D/W, tandis quâun grand angle dâinclinaison dĂ©termineentiĂšrement cette orientation.Nous nous sommes ensuite intĂ©ressĂ©s au rĂŽle du gaz pressurisĂ© dâabord en imposant une surpression dâair constanteen haut du silo puis en considĂ©rant un cas plus transitoire dans lequel une surpression initiale importante provoque larupture de lâorifice. Le dĂ©bit granulaire augmente significativement avec le dĂ©bit dâair, dâautant plus que les particulessont fines. Dans les deux configurations, le dĂ©bit granulaire est mis Ă lâĂ©chelle par une loi de Beverloo modifiĂ©e oĂčle gradient de pression du gaz prĂšs de lâorifice agit comme une force motrice supplĂ©mentaire. De plus le gradientde pression est bien dĂ©crit par une loi de rĂ©sistance de Forchheimer Ă travers le milieu granulaire. Nous proposonsdonc un modĂšle quasi-stationnaire pour la description transitoire du dĂ©bit granulaire. Les deux configurations ont Ă©tĂ©reproduites avec succĂšs par la simulation continue basĂ©e sur un modĂšle multiphasique. Pour les plus forts dĂ©bits, nousobservons une instabilitĂ© du jet granulaire, initiĂ©e par des oscillations de pression dans la zone prĂšs de lâorifice. Nousavons variĂ© le milieu environnant et montrĂ© que la prĂ©sence dâeau autour du silo nâagit que par un effet de pressionhydrostatique supplĂ©mentaire
Non-steady discharge of granular media from silo driven by a pressurized gas
International audienceWe studied experimentally and numerically the effect of an imposed gas pressure on the discharge flow of granular media, through a circular outlet located at the center of the bottom of a cylindrical silo. This study is motivated by the nuclear safety related phenomenology of fuel fragments displacement, idealized by a granular media, within a fuel rod, idealized by an elongated silo, under several accidental conditions, the flow being potentially driven by pressurized fission gases within the rod. We imposed a moderate constant air pressure at the top of the granular column (~ 3000 Pa) and we varied the size and type of the particles and the surrounding fluid where the discharge occurs, using air and water to test the role of the coolant fluid in the nuclear safety problem. The measured parameters are the particle mass flow rate, the volumetric flow rate of air and the pressure along the silo. The particle and air flow rates are found to be non-steady and to increase with time.To model these behaviors, following Zhou et al. [1], we use a two-phase continuum model with africtional rheology to describe particle-particle interactions and we propose a simple quasi-steadyanalytical model considering the air-pressure gradient at the orifice as an additional driving forceto the gravity. We implemented numerically the two-phase continuum model in an axi-symmetricconfiguration which reproduces the experimental results