Salt, as with rice, cement, sand, are finely divided solids which we call granular media. They are groups of similar particles that can behave as a liquid, flowing through orifices; or as a solid, given that they can maintain a constant shape and volume (see Fig A, sand sculptures). This is why the behaviour of granular media is curious and, on many occasions, problematic.
Figures A, B, C and D
One of the intrinsic phenomena on handling granular media is the spontaneous formation of arches as shown in figure B. Arches are structures as used in architecture in order to construct bridges and aqueducts (figure C). These arches are referred to as being vaulted when formed in three dimensions and have the common feature that the particles forming them stabilize each other. That is, if one of the arcs making up the vaulted arch is eliminated, the whole structure will collapse.
When a granular medium flows through an orifice, the formation of the vaulting can cause a blockage. The flow of particles stops and the arch supports the weight of all the material on top of it, in the same way that the arches of a bridge support the weight of the vehicles crossing over it. The blockages in the flow of a granular medium cause serious problems in certain industrial processes. The plastics, cement and pharmaceutical industries are example of where granular media are the main players.
The blockages in discharging silos or dosifiers have physical properties in common with other kinds of hold-ups. Who has not been in a traffic jam, with the start of their holiday delayed? An example closer to home – and certainly more dangerous – is that of the bull running in the fiestas of San Fermín in Pamplona (figure D). When the street narrows and the runners run into each other, a spectacular accumulation of bodies occurs.
Over the past decade numerous scientists have been trying to understand the properties of obstructions of this nature, as well as the factors that are most influential in their formation. Nevertheless, there are many questions left unanswered. In this thesis Iker Zuriguel has investigated the simplest example of blockages that can be studied in the laboratory: a small silo full of spherical particles and with a circular orifice at its base. The thesis was presented at the University of Navarra.
Despite the apparent simplicity of the phenomenon, the unresolved questions are many. For example, what controls the phenomenon of blockages? The particle size? The size of the orifice? This thesis shows that the really important factor is the relationship between the radius of the orifice and that of the particle. Another important question: with the same size of particle and orifice, is it always the same number of grains that fall in an avalanche before the system blocks up? This research is a resounding proof that this is not so. For the same experimental conditions, we can find avalanches of 10 to 10,000 spheres!
The most important result of this thesis was the discovery that, in a 3-dimensional silo, when spherical particles are used, it suffices for the radius of the orifice to be five times greater than that of the particles in order that obstruction does not occur. And, as the saltcellar has holes less than this size, it is necessary to shake it in order to break up the arches formed and that impede the salt to free-flow on to our food.
Irati Kortabitarte | alfa
Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory
SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute
More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.
Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...
Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.
"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
28.04.2017 | Event News
20.04.2017 | Event News
18.04.2017 | Event News
28.04.2017 | Medical Engineering
28.04.2017 | Earth Sciences
28.04.2017 | Life Sciences