Rheometry of granular materials in cylindrical Couette cells: Anomalous stress caused by gravity and shear

The cylindrical Couette device is commonly employed to study the rheology of fluids, but seldom used for dense granular materials. Plasticity theories used for granular flows predict a stress field that is independent of the shear rate, but otherwise similar to that in fluids. In this paper we report detailed measurements of the stress as a function of depth, and show that the stress profile differs fundamentally from that of fluids, from the predictions of plasticity theories, and from intuitive expectation. In the static state, a part of the weight of the material is transferred to the walls by a downward vertical shear stress, bringing about the well-known Janssen saturation of the stress in vertical columns. When the material is sheared, the vertical shear stress changes sign, and the magnitudes of all components of the stress rise rapidly with depth. These qualitative features are preserved over a range of the Couette gap and shear rate, for smooth and rough walls and two model granular materials. To explain the anomalous rheological response, we consider some hypotheses that seem plausibleapriori, but showthat none survive after careful analysis of the experimental observations. We argue that the anomalous stress is due to an anisotropic fabric caused by the combined actions of gravity, shear, and frictional walls, for which we present indirect evidence from our experiments. A general theoretical framework for anisotropic plasticity is then presented. The detailed mechanics of how an anisotropic fabric is brought about by the above-mentioned factors is not clear, and promises to be a challenging problem for future investigations. (C) 2013 AIP Publishing LLC.

Analysis of shear bands in slow granular flows using a frictional Cosserat model

The slow flow of granular materials is often marked by the existence of narrow shear layers, adjacent to large regions that suffer little or no deformation. This behaviour, in the regime where shear stress is generated primarily by the frictional interactions between grains, has so far eluded theoretical description. In this paper, we present a rigid-plastic frictional Cosserat model that captures thin shear layers by incorporating a microscopic length scale. We treat the granular medium as a Cosserat continuum, which allows the existence of localised couple stresses and, therefore, the possibility of an asymmetric stress tensor. In addition, the local rotation is an independent field variable and is not necessarily equal to the vorticity. The angular momentum balance, which is implicitly satisfied for a classical continuum, must now be solved in conjunction with the linear momentum balances. We extend the critical state model, used in soil plasticity, for a Cosserat continuum and obtain predictions for flow in plane and cylindrical Couette devices. The velocity profile predicted by our model is in qualitative agreement with available experimental data. In addition, our model can predict scaling laws for the shear layer thickness as a function of the Couette gap, which must be verified in future experiments. Most significantly, our model can determine the velocity field in viscometric flows, which classical plasticity-based model cannot.

STUDIES ON THE STRESS FLUCTUATIONS IN SHEARED STOKESIAN SUSPENSIONS USING CHAOS THEORY AND NONLINEAR DYNAMICS

The thesis report results obtained from a detailed analysis of the fluctuations of the rheological parameters viz. shear and normal stresses, simulated by means of the Stokesian Dynamics method, of a macroscopically homogeneous sheared suspension of neutrally buoyant non-Brownian suspension of identical spheres in the Couette gap between two parallel walls in the limit of vanishingly small Reynolds numbers using the tools of non-linear dynamics and chaos theory for a range of particle concentration and Couette gaps. The thesis used the tools of nonlinear dynamics and chaos theory viz. average mutual information, space-time separation plots, visual recurrence analysis, principal component analysis, false nearest-neighbor technique, correlation integrals, computation of Lyapunov exponents for a range of area fraction of particles and for different Couette gaps. The thesis observed that one stress component can be predicted using another stress component at the same area fraction. This implies a type of synchronization of one stress component with another stress component. This finding suggests us to further analysis of the synchronization of stress components with another stress component at the same or different area fraction of particles. The different model equations of stress components for different area fraction of particles hints at the possible existence a general formula for stress fluctuations with area fraction of particle as a parameter

Numerical study of bifurcation solutions of spherical Taylor-Couette flow

The steady bifurcation flows in a spherical gap (gap ratio sigma=0.18) with rotating inner and stationary outer spheres are simulated numerically for Re(c1)less than or equal to Re less than or equal to 1 500 by solving steady axisymmetric incompressible Navier-Stokes equations using a finite difference method. The simulation shows that there exist two steady stable flows with 1 or 2 vortices per hemisphere for 775 less than or equal to Re less than or equal to 1 220 and three steady stable flows with 0, 1, or 2 vortices for 1 220

Cell exclusion in couette flow:evaluation through flow visualisation and mechanical forces

Cell exclusion is the phenomenon whereby the hematocrit and viscosity of blood decrease in areas of high stress. While this is well known in naturally occurring Poiseuille flow in the human body, it has never previously been shown in Couette flow, which occurs in implantable devices including blood pumps. The high-shear stresses that occur in the gap between the boundaries in Couette flow are known to cause hemolysis in erythrocytes. We propose to mitigate this damage by initiating cell exclusion through the use of a spiral-groove bearing (SGB) that will provide escape routes by which the cells may separate themselves from the plasma and the high stresses in the gap. The force between two bearings (one being the SGB) in Couette flow was measured. Stained erythrocytes, along with silver spheres of similar diameter to erythrocytes, were visualized across a transparent SGB at various gap heights. A reduction in the force across the bearing for human blood, compared with fluids of comparable viscosity, was found. This indicates a reduction in the viscosity of the fluid across the bearing due to a lowered hematocrit because of cell exclusion. The corresponding images clearly show both cells and spheres being excluded from the gap by entering the grooves. This is the first time the phenomenon of cell exclusion has been shown in Couette flow. It not only furthers our understanding of how blood responds to different flows but could also lead to improvements in the future design of medical devices.

Automated Feedback for 'Fill in the Gap' Programming Exercises

Timely feedback is a vital component in the learning process. It is especially important for beginner students in Information Technology since many have not yet formed an effective internal model of a computer that they can use to construct viable knowledge. Research has shown that learning efficiency is increased if immediate feedback is provided for students. Automatic analysis of student programs has the potential to provide immediate feedback for students and to assist teaching staff in the marking process. This paper describes a “fill in the gap” programming analysis framework which tests students’ solutions and gives feedback on their correctness, detects logic errors and provides hints on how to fix these errors. Currently, the framework is being used with the Environment for Learning to Programming (ELP) system at Queensland University of Technology (QUT); however, the framework can be integrated into any existing online learning environment or programming Integrated Development Environment (IDE)

Bridging the gap between citizens and local authorities via e-government

Among the many promises of the digital revolution is its potential to strengthen social equality and make governments more responsive to the needs of their citizens. E-government is the use of information and communications technologies (ICTs) to transform governments by making them more accessible, effective, accountable, and making the most of the new technologies to deliver better quality and more accessible public services. This paper provides an overview of recent literature addressing e-government issues, and includes a discussion of its implications at the municipal level. It also covers Australian experiences in establishing and managing e-government services.

Impeding good governance implementation in Indonesian regional government : the effect of a bureaucratic generation gap

The implementation of ‘good governance’ in Indonesia’s regional government sector became a central tenet in governance research following the introduction of the national code for governance in 2006. The code was originally drafted in 1999 as a response to the Asian financial crises and many cases of unearthed corruption, collusion, and nepotism. It was reviewed in 2001 and again in 2006 to incorporate relevant political, economical, and social developments. Even though the national code exists along with many regional government decrees on good governance, the extent of implementation of the tenets of good governance in Indonesia’s regional government is still questioned. Previous research on good governance implementation in Indonesian regional government (Mardiasmo, Barnes and Sakurai, 2008) identified differences in the nature and depth of implementation between various Indonesian regional governments. This paper analyses and extends this recent work and explores key factors that may impede the implementation and sustained application of governance practices across regional settings. The bureaucratic culture of Indonesian regional government is one that has been shaped for over approximately 30 years, in particular during that of the Soeharto regime. Previous research on this regime suggests a bureaucratic culture with a mix of positive and negative aspects. On one hand Soeharto’s regime resulted in strong development growth and strong economic fundamentals, resulting in Indonesia being recognised as one of the Asian economic tigers prior to the 1997 Asian financial crises. The financial crises however revealed a bureaucratic culture that was rife with corruption, collusion, and nepotism. Although subsequent Indonesian governments have been committed to eradicating entrenched practices it seems apparent that the culture is ingrained within the bureaucracy and eradication of it will take time. Informants from regional government agree with this observation, as they identify good governance as an innovative mechanism and to implement it will mean a deviation from the “old ways.” Thus there is a need for a “changed” mind set in order to implement sustained governance practices. Such an exercise has proven to be challenging so far, as there is “hidden” resistance from within the bureaucracy to change its ways. The inertia of such bureaucratic cultures forms a tension against the opportunity for the implementation of good governance. From this context an emergent finding is the existence of a ‘bureaucratic generation gap’ as an impeding variable to enhanced and more efficient implementation of governance systems. It was found that after the Asian financial crises the Indonesian government (both at national and regional level) drew upon a wider human resources pool to fill government positions – including entrants from academia, the private sector, international institutions, foreign nationals and new graduates. It suggested that this change in human capital within government is at the core of this ‘inter-generational divide.’ This divergence is exemplified, at one extreme, by [older] bureaucrats who have been in-position for long periods of time serving during the extended Soeharto regime. The “new” bureaucrats have only sat in their positions since the end of Asian financial crisis and did not serve during Soeharto’s regime. It is argued that the existence of this generation gap and associated aspects of organisational culture have significantly impeded modernising governance practices across regional Indonesia. This paper examines the experiences of government employees in five Indonesian regions: Solok, Padang, Gorontalo, Bali, and Jakarta. Each regional government is examined using a mixed methodology comprising of on-site observation, document analysis, and iterative semi-structured interviewing. Drawing from the experiences of five regional governments in implementing good governance this paper seeks to better understand the causal contexts of variable implementation governance practices and to suggest enhancements to the development of policies for sustainable inter-generational change in governance practice across regional government settings.