Simulation of the effect of bond strength on the breakage pattern of agglomerates by Distinct Element Method

Chemical, pharmaceutical and food industries amongst many others use agglomerates either as intermediate or manufactured products. The mechanical strength of agglomerates under impact or shear deformation during handling and processing is of great interest to these industries for optimising product specification and functionality. The effect of surface energy on agglomerate behaviour under impact has been investigated using Distinct Element Method (DEM). Four different agglomerates were formed and impacted against a target along the direction of gravity for three different values of the surface energy (0.35, 3.5 and 35.0 J/m²). The agglomerate breakage pattern was influenced by the surface energy and a transition in the mode of failure of agglomerates was observed when the surface energy was varied. Based on the previous work, the surface energy is expressed in terms of Weber Number, We=(V-V0²)ρD/γ. Agglomerates showed extensive deformation under impact at the lowest value of surface energy (0.35 J/m²) and no evidence of fragmentation was found for any value of impact velocity. In this case the agglomerates behaved macroscopically in a ductile mode. At values of surface energy larger than 3.5 J/m² the agglomerates fragmented at the same time as local damage around the impact site occurred. This type of behaviour is typical of semi-brittle material failure. Therefore, the breakage pattern of agglomerates is influenced by the surface energy

Analysis of the flowability of cohesive powders using Distinct Element Method

Computer simulations using Distinct Element Method (DEM) have been carried out to investigate the effect of cohesion on the flowability of polydisperse particulate systems. For this purpose, two assemblies with different values of surface energy and made of 3000 spheres with the mechanical properties of glass beads were considered. The analysis of the flowability of the powders is presented in terms of the unconfined yield stress as a function of strain rate for different pre-consolidation loads. For values of the surface energy of 1.0 J/m2 and strain rates lower than 6 s− 1, the unconfined yield stress does not change significantly indicating a quasi-static behaviour of the particulate assemblies during the compression process. For larger strain rates, the unconfined yield stress varies with the power index of 1.2 of the strain rate. The influence of the pre-consolidating stress on the powder behaviour has also been investigated and a flow factor was obtained from the linear relationship between the unconfined yield stress and pre-consolidation stress. The computer simulations show qualitatively a good agreement with the experimental trends on highly cohesive powder flow behaviour

Analysis of the effect of cohesion and gravity on the bulk behaviour of powders using Discrete Element Method

Computer simulations using Distinct Element Method have been carried out to analyse the bulk behaviour of a polydisperse assembly of glass beads. For this purpose an assembly made of 3000 spheres were generated to which the mechanical properties of glass beads were assigned. The system was initially compressed isotropically at a strain rate of 1 s-1 in the absence of gravity and surface energy. Once the assembly reached a packing fraction of about 0.62, the effects of cohesion and gravity on the bulk behaviour were analysed for two different cases. In the first case only gravity was applied, whilst in the second case both gravity and surface energy were acting on the particles. The evolution of the components of the stress tensor for the case in which only gravity was applied indicated that the gravity did not appreciably affect the isotropy of the system. In contrast, the system in which surface energy was introduced became anisotropic. The concept of unconfined yield stress of bulk cohesive powders was used to analyse the effect of surface energy and strain rate. For values of surface energy of 1.0 J/m2 and of strain rate lower than 1 s-1 the unconfined yield strength did not change significantly indicating a quasi-static behaviour for the compression process. However, for values of strain rates larger than 1 s-1 the unconfined yield strength increased with the strain rate, following a power law trend with an index of 1.7

Mechanistic analysis and computer simulation of impact breakage of agglomerates: Effect of surface energy

Agglomerates are ubiquitous as intermediate or manufactured products in chemical, pharmaceutical and food industries. During handling and processing they may suffer breakage if they are weak. On the other hand, if they are too strong, their dispersion and disintegration could be difficult. The control of their mechanical strength is therefore highly desirable. However, the analysis of agglomerate strength is complex due to the large number of parameters that influence agglomerate behaviour, such as the primary particle size, density and elastic modulus, and the interparticle bond strength. A simple mechanistic model is presented here which relates the number of broken contacts in agglomerate due to impact velocity, interparticle adhesion energy and the particle properties of the particles forming the agglomerate. The model is based on the hypothesis that the energy used to break contacts during impact is proportional to the incident kinetic energy of the agglomerate. The damage ratio defined as the ratio of broken contacts to the initial number of bonds is shown to depend on the dimensionless group, Δ, in the form (ρV2D5/3E2/3)/ Γ5/3, where V is the impact velocity, E the elastic modulus, D the particle diameter, ρ the particle density and Γ the interface energy. This dimensionless group, Δ, incorporates the Weber number, (ρDV2/Γ), which was previously shown to be influential in agglomerate breakage, and may be presented in the form, Δ=WeIe2/3 , where Ie = ED/ Γ. The predicted dependency of the damage ratio on the surface energy has been tested using Distinct Element Method (DEM). Four different agglomerates have been formed and impacted against a target for three different values of the surface energy of the primary particles. The simulation results show that the effect of surface energy is better described by the above mechanistic model than by the Weber number alone, as previously used to characterise the impact strength of agglomerates

Direct determination of granular pressure in liquid fluidized beds using a DEM-based simulation approach

In this paper, the fluidization of 8 mm glass particles in water has been simulated using a new methodology developed within the DEM framework. In this methodology, random liquid fluctuating velocities are used as direct input into the drag model. The specific aim of this study is to directly compute the granular pressure in a liquid fluidized bed. The granular pressure is defined using the particle-wall collision frequency and the corresponding particle momentum transport during the collision. Initially, we validated our model by comparing the relationship between superficial fluid velocity and bed expansion against the well-known Richardson-Zaki [1] equation. The results demonstrated a good agreement of our model. The granular pressure and temperature, as well as the particle-wall collision frequency, in the liquid fluidized bed were determined for superficial fluid velocities in the range between 0.08 and 0.32 m/s. The granular pressure exhibited a maximum (between 0.3-0.4 solid fraction) that matched the experimental measurements of Zenit et al. [2] for high inertia particles. The granular temperature also revealed a peak at a solid concentration of around 0.2 which is in line with the experimental measurements of Zivkovic et al. [3] and the model of Gervin et al. [4]. The set of the results presented in this study suggests that the approach used here is valid for obtaining the granular pressure and temperature for a wide range of volume fractions in liquid fluidized beds

Impact Fracture of Composite and Homogeneous Nanoagglomerates

It is not yet clear on whether the fracture characteristics of structured composite capsules and homogeneous nanoagglomerates differ significantly under impact loading conditions. Experimental measurement of impact fracture properties of such small agglomerates is difficult, due to the length and time scales associated with this problem. Using computer simulations, here we show that nanoagglomerates are subjected to normal impact loading fracture within a few nanoseconds in a brittle manner. The restitution coefficient of the nanoagglomerates varies nonlinearly with initial kinetic energy. The fracture of nanoagglomerates does not always happen at the moment when they experience the maximum wall force, but occurs after a time lag of a few nanoseconds as characterised by impact survival time (IST) and IST index. IST is dependant on the initial kinetic energy, mechanical and geometric properties of the nanoagglomerates. For identical geometries of the capsules, IST index is higher for capsules with a soft shell than for these with a hard shell, an indication of the enhanced ability of the soft nanocapsules to dissipate impact energy. The DEM simulations reported here based on theories of contact mechanics provide fundamental insights on the fracture behaviour of agglomerates—at nanoscale, the structure of the agglomerates significantly influences their breakage behaviour

Valoración de la fuerza isométrica de la musculatura de la columna cervical en sujetos con latigazo cervical

Introducción. Para la valoración de la capacidad funcional de la columna cervical se utilizan pruebas biomecánicas, ya que las pruebas complementarias clásicas proporcionan una información insuficiente. Se diseñó un trabajo para valorar la fiabilidad test retest de la valoración de la fuerza de columna cervical mediante el dinamómetro Biodex System 3 Pro®, determinar cifras normativas en una población sana y compararla con otra población patológica, y por último determinar criterios de magnificación de la patología cervical mediante el test isométrico cervical. Metodología. Se incluyeron 336 sujetos en un estudio transversal, reclutados desde mayo de 2010 hasta diciembre de 2014. Ciento veintiuno fueron sanos voluntarios y doscientos quince fueron sujetos con dolor cervical agudo. El grupo de pacientes fue subdividido en dos grupos: un grupo de "sinceros" y otro grupo de "no sinceros". En el subgrupo de "no sinceros" fueron incluidos aquellos pacientes que eran remitidos por sus médicos con alta sospecha de magnificación de la patología, por las incongruencias que explicaban en la exploración física, así como por los resultados obtenidos en las diferentes pruebas complementarias. Se valoró la fuerza isométrica de la columna cervical en sedestación. Resultados. La fiabilidad intraobservador para la valoración de la fuerza isométrica varió de 0.93 a 0.97.La fuerza máxima flexora y extensora en sujetos sanos dependen de forma significativa del sexo del sujeto en la flexión y extensión (P .001), así como de la edad (flexión, P =0.006; extensión, P =. 0.002). No se observaron efectos de interacción entre la edad y el sexo para la flexión (P = 0.688) y extensión (P = 0.258). En los pacientes observamos un descenso en la fuerza máxima y media de la musculatura flexora y extensora de la columna cervical al compararlos con los sujetos sanos (P 0.001). Los pacientes "no sinceros" tuvieron coeficientes de variación un 50% más elevado que los pacientes "sinceros", lo que nos demuestra la gran variabilidad en las diferentes valoraciones en los pacientes "no sinceros". Conclusiones. La valoración de la fuerza isométrica de la columna cervical tiene una alta fiabilidad intraobservador e interobservador, hecho que valida los resultados obtenidos. Los pacientes "sinceros" presentan un descenso de la fuerza flexora y extensora de la columna cervical en comparación con los sanos. El coeficiente de variación y el ratio flexores/extensores pueden ser variables útiles para detectar pacientes magnificadores.Introduction. For the assessment of the functional capacity of the cervical spine we carry out biomechanical tests. The traditional tests we performed provided insufficient information. This study was designed to assess the test retest reliability of the assessment of the strength of the cervical spine by Biodex System 3 Pro ® , to determine the average figures in a healthy population and to compare it with the pathological population , and finally to determine magnification criteria of cervical pathology by doing an isometric cervical test. Methodology. 336 subjects in a cross-sectional study , recruited from May 2010 to December 2014. One hundred and twenty were healthy volunteers and two hundred and fifteen were subjects with acute neck pain. The patient group was subdivided into two groups : a group of " sincere " and another group " insincere " . In the subgroup of "insincere " we included patients who were referred by their doctors with a high suspicion of magnification of the pathology, because of inconsistencies related to physical examination as well as the results of various tests . Isometric strength of the cervical spine was assessed in a sitting position Results . The intraobserver reliability for assessing isometric strength ranged from 0.93 to 0.97. The maximum flexor and extensor strength in healthy subjects significantly depends on the sex of the subject , on flexion and extension ( P .001 ) and on age (flexion , P = 0.006 ; extension , P = 0.002 ) . We didn't observe any effects between age and sex for flexion ( P = 0.688 ) and extension ( P = 0.258 ) . In patients we observed a decrease in maximum strength and in the average on the flexor and extensor muscles of the cervical spine when compared to healthy subjects ( P 0.001). The " insincere " patients had coefficients of variation 50% higher than the " honest " patients which demonstrates the large variability in the different assessments of the " insincere " patients. Conclusions . The assessment of the isometric strength of the cervical spine has a high intraobserver and interobserver reliability , that validates the results. The "sincere " patients have a decrease of the flexor and extensor strength of the cervical spine compared with healthy people. The variation coefficient and the ratio flexor / extensor may be useful in detecting the variables of magnifiers patients

Effect of Structure on Strength of Agglomerates using Distinct Element Method

Knowledge of agglomerate strength is highly desirable for compression and tableting, dissolution and dispersion and mitigation of dust formation. The behaviour of agglomerates is affected by parameters such as density, agglomerate size, primary particle size, and interparticle bond strength. The method of agglomeration influences the evolution of structure, and this in turn affects its strength. Furthermore, the methods of strength characterisation, i.e. quasi-static or impact produce different results. To understand the role of structure and the influence of test method, agglomerate failure behaviour has been analysed by the use of the Distinct Element Method (DEM). We report on our work on the simulation of the breakage of the agglomerates for different porosities and impact conditions, where the role of impact speed and angle and type of contact bonding model have been evaluated. The adhesive contact model of JKR is used to form an agglomerate. The effect of the bonding level on the strength and size distribution of the clusters released as a result of failure has been investigated. This work also evaluates the effect of structure (porosity) on the strength of the agglomerates

El episodio de polvo de febrero de 2016: una mirada a través de la radiación solar

El pasado 21 y 22 de febrero, tuvo lugar de forma generalizada en gran parte de la Península Ibérica un episodio de intrusión de polvo sahariano, inusual tanto por su intensidad como por la época del año en la que tuvo lugar, dado que estos fenómenos son más propios de los meses de verano o próximos a dicha estación. En este artículo pretendemos mostrar de forma resumida el efecto que dicho episodio tuvo sobre la radiación solar, y a su vez mostrar un ejemplo del seguimiento de estos episodios mediante el uso de fotómetros solares, una herramienta de teledetección terrestre cuyos productos son de gran utilidad para diversos campos y sectores, como su utilización en modelos numéricos de predicción del tiempo y de calidad del aire, herramienta de apoyo para la navegación aérea o elemento de vigilancia de la calidad del aire

Numerical analysis of strain rate sensitivity in ball indentation on cohesive powder Beds

In the shear deformation of powder beds beyond the quasi-static regime the shear stress is dependent on the strain rate. Extensive work has been reported on the rapid chute flow of large granules but the intermediate regime has not been widely addressed particularly in the case of cohesive powders. However in industrial powder processes the powder flow is often in the intermediate regime. In the present work an attempt is made to investigate the sensitivity of the stresses in an assembly of cohesive spherical particles to the strain rate in ball indentation using the Distinct Element Method. This technique has recently been proposed as a quick and easy way to assess the flowability of cohesive powders. It is shown that the hardness, deviatoric and hydrostatic stresses within a bed, subjected to ball indentation on its free surface, are dependent on the indentation strain rate. These stresses are almost constant up to a dimensionless strain rate of unity, consistent with trends from traditional methods of shear cell testing, though fluctuations begin to increase from a dimensionless strain rate of 0.5. For dimensionless strain rates greater than unity, these stresses increase, with the increase in hardness being the most substantial. These trends correlate well with those established in the literature for the Couette device. However the quantitative value of the strain rate boundary of the regimes differs, due to differences in the geometry of shear deformation bands. Nevertheless, this shows the capability of the indentation technique in capturing the dynamics of cohesive powder flow