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Granular flow in silos - observations and comments

SAMSI Workshop on Fluctuations and continuum Equations for Granular flow , April 16-17, 2004. Granular flow in silos - observations and comments. Jørgen Nielsen Danish Building and Urban Research Jn@dbur.dk. Silo versus hydrostatic pressure. Focus on understanding phenomena.

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Granular flow in silos - observations and comments

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  1. SAMSI Workshop onFluctuations and continuum Equations for Granular flow, April 16-17, 2004 Granular flow in silos - observations and comments Jørgen Nielsen Danish Building and Urban Research Jn@dbur.dk

  2. Silo versus hydrostatic pressure

  3. Focus on understanding phenomena • Observations from silo tests • Comments related to • Physical and mathematical modelling – Continuum / discrete particles • Phenomena observed in silos • Stochastic approach

  4. Physical modelling versus mathematical modelling • Mathematical modelling is needed to generalise our understanding of physical phenomena and to predict behaviour under specified circumstances • Physical modelling is wanted for controlled experiments in order to systematically observe and explore phenomena as a basis for mathematical modelling - and to verify such models

  5. Silo scales

  6. A good scientific physical model is more than just a small scale structure The creation of a model law calls for some considerations: • Which phenomena to cover? • Discrete particles or continuum approach? • Which mathematical model to be based on? – Must be precisely formulated, but you may not be able to solve the equations Leads to the model law: Model Requirements and a Scaling Law Ref: J. Nielsen ”Model laws for granular media and powders with special view to silo models”, Archives of Mechanics, 29, 4, pp 547-560, Warzawa, 1977

  7. Particle history

  8. Discrete particles

  9. Model requirements Kx (scaled particles) Kg = 1/ Kx (centrifuge) …….. Scaling law K = 1 K = 1 Kt = Kx (Forces of inertia) Kt = 1 (Time dep. Konst. rel.) Kt = 1 (Pore flow) Model law – discrete, particles

  10. The centrifuge model - filling

  11. Centrifuge, continuum approach

  12. Stacking the particles

  13. Landslide

  14. Cone squeeze

  15. Distributed filing

  16. Fluidized powder

  17. Anisotropy from inclined filling

  18. Preferred orientation - anisotropy

  19. Outcomes of filling from the stacking process • Density • Pore pressure • Homogeneity • Anisotropy - and thus strength, stiffness and rupture mode of the ensiled solids

  20. From contact forces to pressure

  21. Relative standard deviation Test Diameter of particle Pressure cell diameter Surface area of pressure cell From contact forces to pressure

  22. Pressure cell reading -fluctuations

  23. Pressure distribution with time and height 24 4 0

  24. Circumferential distribution of maximum discharge pressures – Wheat, eccentric inlet and outlet

  25. Circumferential distribution of maximum discharge pressures – Barley, eccentric inlet and outlet

  26. Large pressure gradients

  27. Geometrical wall imperfections

  28. Load consequences of geometrical wall imperfections

  29. Dilating boundary layer

  30. Dilating boundary layer, details

  31. Rotational symmetrical pressure distribution – almost(Jørgen Munch-Andersen)

  32. Formation of rupture planes in dense materials

  33. Dynamics

  34. On the search of a suitable model for the stress-strain relationship in granular materials

  35. The modelling challenges P.S. Time dependent material behaviour may cause scale errors

  36. A ”friendly” silo problem - may be characterised by: • A non-cohesive powder • Aerated filling • Low wall friction • Mass flow

  37. A ”bad” silo problem - may be characterised by: • Coarse-grained sticky particles • Eccentric filling • High wall friction • Pipe flow expanding upwards until the full cross section has become involved

  38. Items for a stochastic/statistic treatment • Redistribution of pressure due to imperfections of wall geometry • The value of material parameters for the (future) stored material • The wall friction coefficient • The formation of unsymmetrical flow patterns in symmetrical silos – and their load implications • Wall pressure fluctuations - load redistributions • The formation of rupture planes in dense materials

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