C.M. Wensrich

Experimental behaviour of quaking in tall silos

This paper examines silo quaking from within using experimental data obtained from two small Perspex models in which the conditions experienced could be carefully controlled. In these experiments, the acceleration of the material was measured at various depths in the silo column and by doing so, not only could the variation of the amplitude of the quakes be observed, but also the propagation of waves could clearly be seen. Similarities were seen between the two sets of results; the most striking of which is that the amplitude of the quakes is observed to grow exponentially with height in the silo.

Flow dynamics or ‘quaking’ in gravity discharge from silos

Abstract This paper reviews the characteristics of pulsating or cyclic flow of bulk solids during gravity discharge in bins and silos. The dynamic load phenomenon is often referred to as “silo quaking” and is influenced by various factors related to the type of flow pattern developed in the bin and the flow properties of the bulk material. Of particular relevance is the influence of ‘slip-stick’ during shear flow, and the velocity at critical sections in the silo during discharge. An overview of recent and current research on this subject is presented.

Dissipation, dispersion, and shocks in granular media

Abstract An understanding of the propagation of pressure waves in granular bodies is of significant importance to many aspects of industrial handling of bulk materials. In this paper the propagation of longitudinal pressure waves in one-dimensional granular bodies is investigated using a hypoplastic framework. From a one-dimensional simplification of a hypoplastic model, a dynamic version of Janssens equation for the pressure within a silo is constructed. The effect of friction on the propagation of small disturbances in this continuum is studied. Dispersion and dissipation are observed in this system, with the rate of dissipation being related to the Janssen characteristic depth. The paper then addresses the propagation of waves with larger amplitudes and the formation of shock waves is analysed. By examining characteristic solutions to the governing equations, a Hugoniot for shock waves in hypoplastic granular materials is found as well as an expression for the velocity with which shock waves propagate.

Numerical modelling of quaking in tall silos

Quaking in tall mass flow silos is a significant problem faced by industry, and can be treated as a simple one-dimensional system. In this paper, quaking in tall silos is examined using a dynamic version of Janssens equation, based on a one-dimensional version of a hypoplastic constitutive model. This model is used to examine the motion of rarefaction waves such as those present at the beginning of each quake cycle, and the numerical solutions for this system show many features in common to experimental findings from previous studies. In particular, the model shows the same exponential growth of the amplitude of quakes with the height of the silo, as that observed experimentally. Through this model the role of wall friction in the quaking process can be understood.

Bragg-edge elastic strain tomography for in situ systems from energy-resolved neutron transmission imaging

Technological developments in high resolution time-of-flight neutron detectors have raised the prospect of tomographic reconstruction of elastic strain fields from Bragg-edge strain images. This approach holds the potential to provide a unique window into the full triaxial stress field within solid samples. While general tomographic reconstruction from these images has been shown to be ill-posed, an injective link between measurements and boundary deformations exists for systems subject to in situ applied loads in the absence of residual stress. Recent work has provided an algorithm to achieve tomographic reconstruction for this class of mechanical system. This letter details an experimental proof-of-concept for this algorithm involving the full reconstruction of a biaxial strain field within a non-trivial steel sample. This work was carried out on the RADEN energy resolved neutron imaging instrument within the Japan Proton Accelerator Research Complex, with validation through Digital Image Correlation and constant wavelength neutron strain scans.

The Prediction of Permeability with the Aid of Computer Simulations

The particular area of focus for this study is the use of theory to predict the permeability of a material through the use of the Ergun equation and the Kozeny-Carman equation along with computer simulations. The Ergun equation is well known for estimating permeability, and the Kozeny-Carman equation has also been used to a lesser extent. Existing literature extensively covers the use of these equations with homogeneous materials containing mono-size particles. In this study, an alternative way is sought to characterize mixtures that is based on the structure of the porosity, or void size, rather than the traditional method of using mean particle diameter. In doing so, this allows the Ergun and Kozeny-Carman equations to be rewritten to provide for an expansion in the type of mixtures that they can be applied to. Results are presented in this article on the application of these equations to mixtures including mono-size particles, then modified to include binary and distributed mixtures.

Novel cellular perlite-epoxy foams: Effects of particle size:

The micro-structure and mechanical properties of lightweight porous foams synthesized by dispersing expanded perlite particles (expanded siliceous volcanic glass) in a matrix of epoxy resin were examined. Foams were fabricated with three distinct particle size ranges and, within each size, samples covered a density range of 0.15–0.45 g/cm3. The effects of particle size variation on compressive strength, effective elastic modulus, and modulus of toughness were investigated. An upper and a lower bound were estimated for the elastic modulus of particles in EP/epoxy foams. EP/epoxy foams showed Reuss-like behaviour similar to metals but atypical of non-plastic materials. In addition, results illustrated the significant contribution of the expanded perlite particles in the effective elastic modulus of the foams. Micro-structure of expanded perlite particles was examined and related to their macroscopic properties via two geometrical relationships. Post-test microscopic observations coupled with macroscopic observations taken during the test were used to understand the effect of particle size on the behaviour of the foams under compressive load. Observations revealed the presence of three different failure modes for all foams regardless of their particle size and density; however, the strain to activate each mode was different for each foam type.

Stress Distribution in Iron Powder during Die Compaction

The unique and unusual state of matter represented by granular materials has historically made it very difficult to develop models of stress distributions and was previously not able to be explored experimentally in the required detail. This paper reports the application of the neutron diffraction strain scanning method, originally developed for residual stress measurements within engineering components, to the problem of the stress distribution in granular Fe under a consolidating pressure. Strains were measured in axial, radial, circumferential and an oblique direction using the neutron strain scanning diffractometer KOWARI at ANSTO (Sydney). The full stress tensor as a function of position was able to be extracted for both straight walled, converging and stepped dies.

Probabilistic modelling and reconstruction of strain

Abstract This paper deals with modelling and reconstruction of strain fields, relying upon data generated from neutron Bragg-edge measurements. We propose a probabilistic approach in which the strain field is modelled as a Gaussian process, assigned a covariance structure customised by incorporation of the so-called equilibrium constraints. The computational complexity is significantly reduced by utilising an approximation scheme well suited for the problem. We illustrate the method on simulations and real data. The results indicate a high potential and can hopefully inspire the concept of probabilistic modelling to be used within other tomographic applications as well.

On the Use of the Uniaxial Shear Test for DEM Calibration

Discrete Element Modelling (DEM) is a widely used and useful tool in the materials handling field. It is commonly used in the design of transfer chutes, bins, hoppers and feeders. However, there is still uncertainty in the translation of material characteristics that are measured experimentally in the laboratory to DEM parameters. There is a vast array of literature on this topic, with most of this work focusing on physical tests in the laboratory such as the angle of repose test, shear tests, compression tests and sliding friction tests. This paper investigates the use of a uniaxial shear test, conducted on a large scale with gravity switched off, for the development of flow functions to allow ease of comparison to experimental tests. A linear cohesion contact model was used in the simulations presented in this study.