C. Soria-Hoyo

Particle-in-cell simulation of Trichel pulses in pure oxygen

The development and propagation of Trichel pulses in oxygen have been numerically simulated using an improved fluid particle-in-cell (PIC) method. The numerical method has been optimized to accurately reproduce sequences of about 100 Trichel pulses (~1 ms). A classical one-dimensional model of negative corona in sphere-to-plane geometry has been used to formulate the continuity equations for electrons and ions. The effects of ionization, attachment and secondary-electron emission from the cathode have all been considered. The electric field has been obtained from the solution of Poissons equation in two dimensions. Using this model, the temporal and electrical characteristics of Trichel pulses have been investigated, in particular, the relation between applied voltage, pulse frequency and time-averaged current intensity and charge.

A PIC based procedure for the integration of multiple time scale problems in gas discharge physics

A efficient PIC technique has been implemented to study the development of electrical discharges during long periods of time. Special motivation is provided by electrical pulsations that develop in very short times but whose repetition period is much longer. The method exploits the existence of different time scales in the electrical discharge to implement a long time-step particle pushing technique both at particle and at mesh levels. The development of a train of hundreds of Trichel pulses, which is a prohibitively long computation with a conventional PIC, has been used to test the validity of the method.

Two dimensional numerical simulation of gas discharges: comparison between particle-in-cell and FCT techniques

Two different numerical techniques have been applied to the numerical integration of equations modelling gas discharges: a finite-difference flux corrected transport (FD-FCT) technique and a particle-in-cell (PIC) technique. The PIC technique here implemented has been specifically designed for the simulation of 2D electrical discharges using cylindrical coordinates. The development and propagation of a streamer between two parallel electrodes has been used as a convenient test to compare the performance of both techniques. In particular, the phase velocity of the cathode directed streamer has been used to check the internal consistency of the numerical simulations. The results obtained from the two techniques are in reasonable agreement with each other, and both techniques have proved their ability to follow the high gradients of charge density and electric field present in this type of problems. Moreover, the streamer velocities predicted by the simulation are in accordance with the typical experimental values.

Novel instrument to characterize dry granular materials at low consolidations

The performance of traditional instruments for measuring the flow properties of dry granular materials at small consolidation stresses is not fully satisfactory. Generally, commercial quick tests, as, for example, the angle repose method, do not yield intrinsic material properties. This difficulty is solved in currently available ring shear testers, in which the externally applied torque necessary for shearing the sample is measured as a function of the normal stress previously applied through an annular lid. In this article we show a novel device in which the shear stress is caused by the action of a centrifugal force on a vertical layer of unconsolidated material, which is rotated around its vertical axis. At a critical point the shear stress is large enough to drive material avalanches. From a theoretical analysis of these avalanches based on Coulombs method of wedges, we derive the angle of internal friction and cohesion of the granular material. To illustrate the functioning of the instrument, measurements on steel, ferrite, and magnetite beads of different particle size are presented. The data obtained are used to analyze the gravity-driven avalanches of these materials in a slowly rotated drum.

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.

Design of a conductivity meter for highly insulating liquids

The accurate measurement of the electrical conductivity of low conducting liquids is an important issue in many industrial applications. The lack of repeatability is a common problem to the available procedures and commercial techniques. In this paper, we present a device to measure the electrical conductivity of low conducting liquids. The variable inter-electrode gap allows us to assure the existence of an ohmic regime, since only under ohmic regime conditions is the value of the conductivity meaningful.

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...

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.

Experimental study of conductivity of suspensions of silica particles in propanol and toluene-ethanol mixture

Two sets of silica particles have been synthesized and dispersed in both 1-propanol and toluene-ethanol mixture. The conductivity of the suspensions has been measured as a function of their concentration, by means of a device and experimental procedure developed by the authors. It has been observed that the conductivity increases linearly with the solid fraction for values smaller than 5%. Additionally, the water content has also been measured, to evaluate its effect on the conductivity of suspensions.