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|Title: ||Numerical investigation of granular flow and dynamic pressure in silos|
|Authors: ||Wang, Yin|
|Supervisor(s): ||Ooi, Jin|
|Issue Date: ||25-Jun-2012|
|Publisher: ||The University of Edinburgh|
|Abstract: ||Although the flow of granular material in silos and the pressure acting on the silo walls
have been studied for over a century, many challenges still remain in silo design. In
particular, during the discharge process some dynamic phenomena in silos can often be
observed to display large, self-induced and dynamic pulsations which may endanger the
stability of the silo structure. The aim of this thesis is to study the flow and pressure in
silos using numerical modelling and analytical methods, and to further understand the
mechanical behaviour of granular material and mechanism of dynamic phenomena
during silo discharge.
The Finite Element (FE) method can be used to analyse the behaviour of the granular
material in silos by considering the material as a continuum. In this thesis, FEM
modelling of silo flow was developed using the Arbitrary Lagrangian-Eulerian (ALE)
formulation in the Abaqus/Explicit program and the key parameters that affect the
predictions of the flow and pressure during discharge were identified.
Using the ALE technique, almost the entire silo discharge process can be simulated
without mesh distortion problems. The mass flow rate and temporally averaged
discharge pressure predicted by the FE model were first investigated in a conical hopper
and were found to be in good agreement with those from the most commonly quoted
theoretical solutions. The transient dynamic pressure fluctuations during incipient silo
discharge were predicted and the causes for these dynamic events have been investigated
which led to the conclusion that the stress wave propagation and the moving shear zone
phenomena within the bulk solid were responsible for the dominant higher and lower
frequencies effects respectively.
A one-dimensional dynamic model of granular columns subject to Coulomb wall friction
was developed to investigate the propagation of stress waves, focusing on the effect of
geometry by examining converging and diverging tapered columns. The analytical
solutions of this model are compared to the FE model based on the ALE formulation.
This FE model was first validated using the known behaviour for cylindrical columns. In
all cases, the stress impulse set off by incipient discharge at the silo outlet grew with the
distance travelled up the column, however the rate was shown to depend on the halfangle
of the taper. Over a range of small angles, the proposed analytical model was
found to accurately predict this behaviour.
After the successful application of the ALE technique for a conical hopper, the FE
model was extended to simulate the granular flow in a flat-bottomed model silo. The FE
predictions were compared with the silo pressure measurements in a model silo (Rotter
et al, 2004). Pressure cells mounted along a vertical line on the silo walls were used to
measure the pressure distribution in the silo tests using dry sand.
The FE model was further extended to simulate the granular flow in a model silo
consisting of a cylindrical section with a conical hopper. The prediction was compared
with the experimental observations from a model silo (Munch-Andersen et al, 1992),
together with the well-known theoretical solutions. Two numerical issues were
addressed in some detail: one is the numerical treatment of the abrupt transition between the cylinder section and the conical hopper, the other is the interaction between the
granular solid and the silo walls that was modelled using a dynamic friction model. In
addition, the dynamic pressure events during discharge were examined and plausible
explanations were given.
Finally, this thesis deployed a non-coaxial elastoplastic constitutive model to explore the
effect of non-coaxiality on silo phenomena. The non-coaxial FE modelling was
performed on three problems: a simple shear test under various initial conditions, a steep
hopper and a flat-bottomed silo. The results show that non-coaxiality did not influence
the prediction of wall pressure during filling and storing, on the other hand, the
discharge pressure was predicted to be larger when non-coaxiality is considered.|
|Keywords: ||Arbitrary Lagrangian-Eulerian|
|Appears in Collections:||Engineering thesis and dissertation collection|
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