A Terradynamics of Legged Locomotion on Granular Media

The theories of aero- and hydrodynamics predict animal movement and device design in air and water through the computation of lift, drag, and thrust forces. Although models of terrestrial legged locomotion have focused on interactions with solid ground, many animals move on substrates that flow in response to intrusion. However, locomotor-ground interaction models on such flowable ground are often unavailable. We developed a force model for arbitrarily-shaped legs and bodies moving freely in granular media, and used this "terradynamics" to predict a small legged robot's locomotion on granular media using various leg shapes and stride frequencies. Our study reveals a complex but generic dependence of stresses in granular media on intruder depth, orientation, and movement direction and gives insight into the effects of leg morphology and kinematics on movement

Nonlocal Granular Rheology: Role of Pressure and Anisotropy

We probe the secondary rheology of granular media, by imposing a main flow and immersing a vane-shaped probe into the slowly flowing granulate. The secondary rheology is then the relation between the exerted torque T and rotation rate \omega of our probe. In the absence of any main flow, the probe experiences a clear yield-stress, whereas for any finite flow rate, the yield stress disappears and the secondary rheology takes on the form of a double exponential relation between \omega and T. This secondary rheology does not only depend on the magnitude of T, but is anisotropic --- which we show by varying the relative orientation of the probe and main flow. By studying the depth dependence of the three characteristic torques that characterize the secondary rheology, we show that for counter flow, the dominant contribution is frictional like --- i.e., T and pressure are proportional for given \omega --- whereas for co flow, the situation is more complex. Our experiments thus reveal the crucial role of anisotropy for the rheology of granular media.Comment: 6 pages, 5 figure

Dynamic viscoplastic granular flows: A persistent challenge in gas-solid fluidization

Fluidization is a prime example of complex granular flows driven by fluid-solid interactions. The interplay of gravity, particle-particle and fluid-particle forces leads to a rich spectrum of hydrodynamic behavior. A number of complex mathematical formulations exist to describe granular flows. At a macroscopic scale, Eulerian models based on the Kinetic Theory of Granular Flow (KTGF) have been successfully employed to simulate dilute and moderately dense systems, such as circulating fluidized bed reactors. However, their applications to dense flows are challenging, because sustained particle contacts are important. As solid fraction rises, the behavior of granular media responds dramatically to particle properties and changes in concentration. Lacking a coherent transition between formulations of dilute, dense and quasi-static flow behavior, kinetic models are incapable of describing how microstructure emerges and affects the rheology. The behavior of transitional granular flows, such as pulsed fluidized beds, for which the particulate phase transitions between the viscous and plastic regimes, are good reminders of this limitation. In recent years, tremendous effort has been devoted to finding new ways to describe the effects of sustained solids friction and dense flow rheology. This article provides a perspective on this matter from the viewpoint of gas-solid fluidization and discusses advances in describing the dilute-to-dense transition in a continuum framework. Four innovative approaches prevail to extend or supersede the existing kinetic theory: (i) including effective restitution coefficients, (ii) coupling local granular rheological correlations, (iii) introducing rotational granular energy, and (iv) combining non-local laws. While their reliability is still far from that of a Eulerian-Lagrangian approach, they lay a promising foundation for developing a rigorous description of granular media that merges the classical frameworks of continuous fluid and soil mechanics. The progress of continuum formulations does not compete with multi-scale modeling platforms with an applied focus. Ultimately, combining both is a prerequisite to developing new solid stress models that will improve not only the performance of macroscopic models, but also our understanding of granular physics

Capillary Effects on Fluid Transport in Granular Media

Fluid transport phenomena in granular media are of great importance due to various natural and industrial applications, including CO2 sequestration, enhanced oil recovery, remediation of contamination, and water infiltration into soil. Although numerous studies exist in the literature with aims to understand how fluid properties and flow conditions impact the transport process, some key mechanisms at microscale are often not considered due to simplifications of physical phenomenon and geometry, limited computational resources, or limited temporal/spatial resolution of existing imaging techniques. In this Thesis, we investigate fluid transport phenomena in granular media with a focus on the capillary effects. We move from relatively simple scenario on patterned surfaces to more complex granular media, tackling a variety of liquid-transport related problems that all have extensive industrial applications. The bulk of this Thesis is composed of six published papers. Each chapter is prefaced by an introductory section presenting the motivation for the corresponding paper and its context within the greater body of work. This Thesis reveals the impact of some previously neglected physical phenomena at microscale on the fluid transport in granular materials, providing new insights and methodology for describing and modelling fluid transport process in porous media

Flow behaviour of grains through the dosing station of spacecraft under low gravity environments

For the design of the grain-processing stations of spacecrafts, such as EXOMARS 2020, reliable estimates are required on the internal and bulk flow characteristics of granular media under the low gravitational environments. Using theoretical and computational modelling, here we present results on the generic flow behaviour of granular materials through flow channels under different gravity levels. For this, we use three approaches, viz., (i) a simple one-dimensional discrete layer approach (DLA) based on hybrid-Lagrange continuum analysis (ii) three dimensional Kirya structural continuum model and (iii) three dimensional discrete element modelling (DEM). Each model has its merits and limitations. For the granular simulant considered here, a good level of agreement is obtained between the results of Kirya model and DEM simulations on the flow properties of the grains. Some qualitative comparisons are also reported favourably on the flow characteristics of grains between the results of the experimental parabolic flight campaign and the DEM simulations. The theoretical and DEM simulations presented here could help to minimise relying on the complex experimental programmes, such as the parabolic flight campaign, for evaluating the processing behaviour of grains under low gravitational environments in future

Flow behaviour of grains through the dosing station of spacecraft under low gravity environments

For the design of the grain-processing stations of spacecrafts, such as EXOMARS 2020, reliable estimates are required on the internal and bulk flow characteristics of granular media under the low gravitational environments. Using theoretical and computational modelling, here we present results on the generic flow behaviour of granular materials through flow channels under different gravity levels. For this, we use three approaches, viz., (i) a simple one-dimensional discrete layer approach (DLA) based on hybrid-Lagrange continuum analysis (ii) three dimensional Kirya structural continuum model and (iii) three dimensional discrete element modelling (DEM). Each model has its merits and limitations. For the granular simulant considered here, a good level of agreement is obtained between the results of Kirya model and DEM simulations on the flow properties of the grains. Some qualitative comparisons are also reported favourably on the flow characteristics of grains between the results of the experimental parabolic flight campaign and the DEM simulations. The theoretical and DEM simulations presented here could help to minimise relying on the complex experimental programmes, such as the parabolic flight campaign, for evaluating the processing behaviour of grains under low gravitational environments in future

FEM-DEM simulation of two-way fluid-solid interaction in fibrous porous media

Fluid flow through particulate media is pivotal in many industrial processes, e.g. in fluidized beds, granular storage, industrial filtration and medical aerosols. Flow in these types of media is inherently complex and challenging to simulate, especially when the particulate phase is mobile. The goals of this paper are twofold: (i) the derivation of accurate correlations for the drag force, taking into account the effect of microstructure, to improve the higher scale macro-models and (ii) incorporating such closures into a “compatible” monolithic multi-phase/scale model that uses a (particle-based) Delaunay triangulation (DT) of space as basis – in future, possibly, involving also multiple fields

Dynamically structured fluidization: Oscillating the gas flow and other opportunities to intensify gas-solid fluidized bed operation

Various approaches to structure gas-solid fluidized beds are reviewed, followed by detailed discussion on the use of gas pulsation to induce dynamic structuring. Granular media are dissipative systems, which develop complex spatiotemporal patterns when excited by an oscillating energy source. Here, we discuss how such perturbations initiate surface patterns and how these could propagate into a macroscopically organized flow. We call this dynamically structured fluidization. Vibrated shallow granular layers form ordered surface waves. The hydrodynamics of pulsed gas-fluidized layers are related, but more complex: Under appropriate conditions, surface waves transition into a three-dimensionally ordered bubbling flow. This occurs in much deeper granular beds than under vibration, indicating distinct physics. In this dynamically structured state, bubbles organize into a scalable sub-harmonic, triangular lattice that is highly predictable and responsive to changes in oscillation parameters, allowing for an unprecedented level of control. Structured bubbling is observed only under sufficiently dense conditions; thus, a dynamically structured fluidized bed sits between fixed and fluidized beds, offering opportunities for process intensification, due to less macromixing than traditional fluidization, but a higher level of control through micromixing. This informs new intensified designs for processes that are highly exothermic, involve particle formation, thermally sensitive or high-value materials