Vibration-induced dynamical weakening of pyroclastic flows: Insights from rotating drum experiments

Pyroclastic flows are characterized by their high mobility, which is often attributed to gas fluidization of the usually fine and/or low-density particles. However, the physical mechanism that might drive sustained fluidization of pyroclastic flows over extraordinarily long runout distances is elusive. In this letter it is proposed that a powerful mechanism to weaken the frictional resistance of pyroclastic flows would arise from the prolonged and intense mechanical vibrations that commonly accompany these dense gravitational fluid-particle flows. The behavior of fine powders in a slowly rotating drum subjected to vibrations suggests that fluid-particle relative oscillations in granular beds can effectively promote the pore gas pressure at reduced shear rates. Dynamical weakening, as caused by the enhancement of pore fluid pressure, may be an important mechanism in any geophysical process that involves vibrations of granular beds in a viscous fluid. This is particularly relevant for granular flows involving large amounts of fine and/or light particles such as pyroclastic density currents.Ministerio de Economia y Competitividad CTQ2014-52763-C2-2-RJunta de Andalucía FQM-573

Dynamical weakening of pyroclastic flows by mechanical vibrations

Dynamical weakening of dense granular flows plays a critical role on diverse geological events such as seismic faulting and landslides. A common feature of these processes is the development of fluid-solid relative flows that could lead to fluidization by hydrodynamic viscous stresses. Volcanic ash landslides (pyroclastic flows) are characterized by their high mobility often attributed to fluidization of the usually fine and/or low-density particles by their interaction with the entrapped gas. However, the physical mechanism that might drive sustained fluidization of these dense granular flows over extraordinarily long runout distances is elusive. The behavior of volcanic ash in a slowly rotating drum subjected to mechanical vibrations shown in this work suggests that fluid-particle relative oscillations in dense granular flows present in volcanic eruption events can promote pore gas pressure at reduced shear rates as to sustain fluidization.Ministerio de Economía y Competitividad CTQ2014-52763-C2-2-

Modeling the electrophoretic deposition of colloidal particles

This letter presents the results of numerical simulations of the buildup of a layer of colloidal particles on an electrode. In a low-frequency electric field, particles suspended in a low-conductivity liquid migrate to one electrode and then to the other. During each cycle, deposits are formed and dissipated. The current-voltage characteristics of the process reflect properties of the suspension and the deposited layer. Using a flux corrected transport (FCT) algorithm, the transport equation for the particle phase is solved simultaneously with equations describing the electric field. The model reproduces the main features of the current-voltage relation.Ministerio de Ciencia y Tecnología BFM2000-105

The charged bouncing ball: An experimental model for period-doubling bifurcation

This paper presents an experimental and theoretical study of the dynamics of a conducting ball in a poorly conducting liquid subjected to an electric field. When the applied voltage is constant the ball bounces regularly on the lower electrode. If an AC voltage is superimposed, with a period equal to the unperturbed time between impacts, the ball undergoes a period-doubling bifurcation when increasing the amplitude of the AC signal. The non-linear map which describes the dynamics of the ball is closely related to the standard map and to the classical problem of a bouncing ball on a moving table.Dirección General de Investigación Científica y Técnica (DGICYT) PB93-118

Mechanical Strength of Humidified Glass Beads

In this work we measure the mechanical strength of a bed of glass beads that has been previously subjected to a controlled flow of humid air by using a fluidized bed setup. The material is first initialized in a reproducible state by subjecting it to a high flow of dry air that drives the bed to bubbling. Then the gas flow is abruptly turned off and, once the bed is settled, it is subjected to a small flow of humid air during a certain period of time. Water moisture is added to the fluidizing air by bubbling it through water. The relative humidity and dew point of the air are monitored on-line by using humidity sensors upstream and downstream of the bed. After the stationary condition is reached, the bed is slowly tilted and the avalanche angle and depth are measured. From Coulomb’s wedge model and Rumpf’s equation using the attractive capillary force, the predicted angle of avalanche of the humidified samples is shown to be close to the experimental value.Ministerio de Ciencia y Tecnología (MCYT). España FIS2006-03645Junta de Andalucía FQM 42

Cohesion and Internal Friction of Fine Glass Beads as Affected by Small Intensity Vertical Vibration

We have used a novel centrifuge powder tester to obtain the angle of internal friction and cohesion of fine glass beads as affected by previous vibration in the vertical direction. In the experimental procedure we use a small amount of mass, typically between 2 and 4 grams, contained in a rectangular cell. The bed is initialized and subjected to low intensity vertical vibrations of controlled frequency and amplitude for a fixed period of time. By means of pre-vibration the material becomes compacted. Then the cell is taken to the centrifugal powder tester, in which it is rotated around its vertical axis at increasing values of the rotation velocity. At a critical point the shear stress caused by the action of the centrifugal force is large enough to drive material avalanches. From a theoretical analysis of these avalanches based on the Coulomb’s method of wedges we derive the angle of internal friction and cohesion of the glass beads. Measurements have been performed using different masses pre-vibrated at different frequencies and amplitudes. Results from the tests are fitted to a single trend when they are plotted as a function of the effective consolidation stress imposed on the bed by means of pre-vibration. Basically, the data indicate a significant increase of cohesion and a slight decrease of the angle of internal friction as the effective consolidation on the sample is increased. The interparticle cohesion force has been estimated from the cohesion measured, and using the averaging Rumpf’s equation. For the unconsolidated samples, the value estimated agrees with the expected force due to the sum of van der Waals and capillary forces for undeformed contacts between surface asperities. However, the interparticle cohesion force increases as pre-vibration intensity is increased, being this the main reason for the increase of cohesion at the bulk level. According to theoretical estimations, the increase of the interparticle cohesion force is attributable to the plastic yield of the surface asperities at contact. The rate of increase of the interparticle cohesion force with the interparticle consolidation force is in accordance with the results predicted by a theoretical model on plastic contacts between surface asperities. It can be concluded that fine powder flowability is seriously hindered by compaction due to pre-vibratio

Multi-species simulation of Trichel pulses in oxygen

A multi-species model consisting of seven species has been implemented to simulate the generation and development of Trichel pulses in oxygen between a sphere (the cathode) and a plane (the anode). The spatial and temporal evolution of species is obtained by solving the continuity equations of species using a classical one-dimensional model of negative corona discharge. The chemical kinetics of corona discharge includes electron impact reactions (ionization, dissociative and non-dissociative electron attachment, molecular dissociation, etc.), charge transfer reactions and reactions between neutral species

Procedimiento para mejorar la fluidización de polvos finos cohesivos adsorbentes de CO2

La presente invención tiene por objeto un procedimiento destinado a mejorar la fluidizabilidad de polvos finos cohesivos (tipo Geldart C) empleados en la adsorción de CO2 y en técnicas avanzadas de combustión basadas en lechos fluidos. El procedimiento propuesto consiste en mezclar el polvo fino cohesivo adsorbente de CO2 con un polvo nanoestructurado cuyos aglomerados son muy porosos, preferentemente Aerosil R974, y que se fluidizan de manera homogénea. Tras someter la mezcla a un tratamiento de agitación, por ejemplo mediante rotación en un tambor durante algunos minutos o por agitación magnética, los aglomerados porosos del polvo nanoestructurado quedan recubiertos por una capa de partículas adsorbentes de CO2 . Esta mezcla es fluidizable uniformemente, con la ventaja de que las partículas adsorbentes de CO2, que recubren los aglomerados porosos del polvo nanoestructurado, quedan expuestas a la corriente de gas en el lecho fluidizado. Con ello se consigue incrementar el área efectiva de contacto entre el gas de poscombustión y la superficie de las partículas adsorbentes de CO2 en un lecho fluido de cara a s u aplicación en la tecnología de adsorción de CO2.Españ

Non-linear hybrid kinetic-MHD simulations of ELMs in the ASDEX Upgrade tokamak

EUROfusion Consortium 63305

Dependence on plasma shape and plasma fueling for small ELM regimes in TCV and ASDEX Upgrade

Within the EUROfusion MST1 Work Package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (ne,sep/nG ∼ 0.3), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a Double Null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened.EURATOM 63305