Magnetic field induced inversion in the effect of particle size on powder cohesiveness

Experimental measurements are reported on the tensile yield stress of magnetofluidized beds of fine magnetic powders operated in the cross-flow configuration. In the absence of externally applied magnetic field the yield stress of the powder depends on particle size as expected, i.e., it increases as bead size is decreased. This trend is however inverted when an external magnetic field is applied. It is suggested that the average orientation of interparticle contacts relative to the direction of the field as affected by particle size plays a relevant role on the magnetic yield stress of these systems.Ministerio de Ciencia y Tecnología de España-FIS2006-0364

Dual-rate background subtraction approach for estimating traffic queue parameters in urban scenes

This study proposes traffic queue-parameter estimation based on background subtraction. An appropriate combination of two background models is used: a short-term model, very sensitive to moving vehicles, and a long-term model capable of retaining as foreground temporarily stopped vehicles at intersections or traffic lights. Experimental results in typical urban scenes demonstrate the suitability of the proposed approach. Its main advantage is the low computational cost, avoiding specific motion detection algorithms or post-processing operations after foreground vehicle detection.Ministerio de Educación y Ciencia DPI2010-19154Consejería de Innovación, Ciencia y Empresa P07-TIC-0262

Magnetic stabilization of fluidized beds: Effect of magnetic field orientation

Fluidized beds of granular materials can be stabilized by interparticle attractive forces which confer the expanded bed an elastic modulus that stabilizes it against flow perturbations. Stabilization in a structure of enduring contacts is seen to occur naturally due to the universal van der Waals forces for ∼50 μm particle size beds albeit in a quite reduced interval of gas velocities over the minimum fluidization velocity vmf. As shown in this work, a magnetic field may induce attractive forces between magnetizable particles thus extending the stable fluidization interval well beyond vmf. The structure of the magnetically stabilized bed is however markedly anisotropic since attractive magnetic forces are maximum along the direction of the externally imposed field which leads to the formation of chain particle aggregates. This paper shows experimental measurements on the magnetic yield stress, gas velocity at the transition to marginal stability and microstructure of magnetostabilized beds as affected by the direction of the magnetic field. Data shows that magnetic stabilization is optimized for co-flow fields as chain aggregates are preferentially orientated parallel to the magnetic field and along the direction of minimum drag. As the magnetic field is tilted, particle chains become tilted according to a balance between the magnetic attractive force between the particles and the vertical drag force, which reduces the magnetic yield stress and therefore shortens the interval of magnetic stabilization.Junta de Andalucía FQM-5735Ministerio de Economia y Competitividad CTQ2014-52763-C2-2-R, FIS2014-54539-

Pardela cenicienta atlántica – Calonectris borealis (Cory, 1881)

Aves - Orden Procellariiformes - Familia Procellariidae en la Enciclopedia Virtual de Vertebrados Españoles, http://www.vertebradosibericos.org/.A comprehensive review of the natural history of the Cory's Shearwater Calonectris borealis in Spain.Peer reviewe

Pardela cenicienta – Calonectris diomedea (Scopoli, 1769)

Aves - Orden Procellariiformes - Familia Procellariidae en la Enciclopedia Virtual de Vertebrados Españoles, http://www.vertebradosibericos.org/. Versiones anteriores: 20-02-2012A comprehensive review of the natural history of the Cory's Shearwater Calonectris diomedea in Spain.Peer reviewe

Magnetofluidization of fine magnetite powder

The behavior of a fluidized bed of fine magnetite particles as affected by a cross-flow magnetic field is investigated. A distinct feature of this naturally cohesive powder, as compared to noncohesive magnetic grains usually employed in magnetofluidized beds, is that the fluidized bed displays a range of stable fluidization even in the absence of an external magnetic field. Upon application of the magnetic field, the interval of stable fluidization is extended to higher gas velocities and bed expansion is enhanced. We have measured the tensile strength as affected by application of the external magnetic field according to two different operation modes. In the H off-on operation mode, the bed is driven to bubbling in the absence of external magnetic field. Once the gas velocity is decreased below the bubbling onset and the bed has returned to stable fluidization due to natural cohesive forces, the field is applied. In the H on-on mode, the field is maintained during the whole process of bubbling and return to stable fluidization. It is found that the tensile strength of the naturally stabilized bed is not essentially changed by application of the field _H off-on_ since the magnetic field cannot alter the bed structure once the particles are jammed in the stable fluidization state. Magnetic forces within the bulk of the jammed bed are partially canceled as a result of the anisotropic nature of the dipole-dipole interaction between the particles, which gives rise to just a small increment of the tensile strength. On the other hand, when the field is held on during bubbling and transition to stable fluidization _H on-on mode_, the tensile strength is appreciably increased. This suggests the formation of particle chains when the particles are not constrained due to the dipole-dipole attractive interaction which affects the mechanical strength of the stably fluidized bed. Experimental data are analyzed in the light of theoretical models on magnetic surface stresses

Computer Vision Techniques for Background Modeling in Urban Traffic Monitoring

Jose Manuel Milla, Sergio Luis Toral, Manuel Vargas and Federico Barrero (2010). Computer Vision Techniques for Background Modeling in Urban Traffic Monitoring, Urban Transport and Hybrid Vehicles, Seref Soylu (Ed.), ISBN: 978-953-307-100-8, InTech, DOI: 10.5772/10179. Available from: http://www.intechopen.com/books/urban-transport-and-hybrid-vehicles/computer-vision-techniques-for-background-modeling-in-urban-traffic-monitoringIn this chapter, several background modelling techniques have been described, analyzed and tested. In particular, different algorithms based on sigma-delta filter have been considered due to their suitability for embedded systems, where computational limitations affect a real-time implementation. A qualitative and a quantitative comparison have been performed among the different algorithms. Obtained results show that the sigma-delta algorithm with confidence measurement exhibits the best performance in terms of adaptation to particular specificities of urban traffic scenes and in terms of computational requirements. A prototype based on an ARM processor has been implemented to test the different versions of the sigma-delta algorithm and to illustrate several applications related to vehicle traffic monitoring and implementation details

Rheology of magnetofluidized fine powders

Usually a bed of solid particles °uidized by a gas is inherently unstable. Gas bubbles are rapidly formed at the onset of °uidization, which hinders the e±ciency of gas-solid contact. In the case of magnetizable particles, gas bubbles may be suppressed by means of an externally applied ¯eld that magnetizes the particles. In general, magnetized particles are assumed to behave as point dipoles that organize in chainlike structures oriented along ¯eld lines due to dipole-dipole attraction. The physical mechanism responsible for stabilization is however unclear. In particular, rheological characterization of magnetically stabilized beds (MSBs) has been a subject of controversy and there is not a widely accepted explanation to the empirical fact that magneto°uidized beds can be stabilized by a horizontal ¯eld. Several experimental approaches have been used mainly aimed to observe the °uidity of MSBs. Generally, MSBs are reported to behave as a °uid up to a critical magnetic ¯eld strength at which the bed is frozen and there appears an appreciable yield stress. Most of these techniques are invasive, which sheds doubts on the mechanism responsible for the apparition of the yield stress. In this work, we have measured the yield stress of MSBs of ¯ne magnetic powders by means of a noninvasive technique that uses gas °ow to put the bed under tension. It is shown that the MSB behave as a plastic solid. The yield stress of the stabilized bed, which is developed just at marginal stability, arises as a consequence of the existence of the magnetic attraction between particles at contact. Fine magnetic powders of di®erent aggregative nature have been used in our work. Direct visual observation of mesoscopic structures have revealed that naturally nonaggregated particles organize in quasivertical local linear chains when the ¯el

Fluid to solid transition in magnetofluidized beds of fine powders

Experimental observations on the fluid to solid transition in beds of magnetized fine particles fluidized by gas are reported for different particle sizes dp. Contrarily to stability analysis prediction, the fluidized bed is stabilized by a sufficiently strong magnetic field in the cross-flow configuration. As the strength H of the horizontally applied magnetic field is increased, particle chaining in the bubbling bed becomes apparent due to the induced attractive magnetostatic forces between the particles. In close analogy with magnetorheological fluids chain stability is determined by the balance between gas flow shear and the interparticle magnetostatic force. The jamming transition occurs at a gas velocity scaling proportionally to dp 2 H2 when the length of the stable chains reaches a critical size which is independent of particle size.Gobierno de España. Ministerio de Ciencia y Tecnología-FIS2006-03645Junta de Andalucía-FQM 42

Electromechanics of fluidized beds of nanoparticles

The electromechanical behavior of a gas-fluidized bed of insulating silica nanoparticles is investigated. When fluidized by gas, these nanoparticles form highly porous agglomerates of size of the order of hundreds of microns, which gives rise to a nonbubbling fluidization regime. Bed expansion is enhanced by an imposed alternating electric field for oscillation frequencies in the range between tens and hundreds of hertzs and field strengths of about 1 kVcm. Nanoparticle agglomerates are naturally charged and experience forced oscillations that cause an increase of the gas flow shear on their surface. As a consequence, the agglomerate size is expected to decrease, which can explain the observed behavior. A model based on the balance between attractive and flow shear forces is presented that accounts for agglomerate size reduction as the strength of the field is increased