Tomasz Koziara

Kinematic flow patterns in slow deformation of a dense granular material

The kinematic flow pattern in slow deformation of a model dense granular medium is studied at high resolution using in situ imaging, coupled with particle tracking. The deformation configuration is indentation by a flat punch under macroscopic plane-strain conditions. Using a general analysis method, velocity gradients and deformation fields are obtained from the disordered grain arrangement, enabling flow characteristics to be quantified. The key observations are the formation of a stagnation zone, as in dilute granular flow past obstacles; occurrence of vortices in the flow immediately underneath the punch; and formation of distinct shear bands adjoining the stagnation zone. The transient and steady state stagnation zone geometry, as well as the strength of the vortices and strain rates in the shear bands, are obtained from the experimental data. All of these results are well-reproduced in exact-scale non-smooth contact dynamics simulations. Full 3D numerical particle positions from the simulations allow extraction of flow features that are extremely difficult to obtain from experiments. Three examples of these, namely material free surface evolution, deformation of a grain column below the punch and resolution of velocities inside the primary shear band, are highlighted. The variety of flow features observed in this model problem also illustrates the difficulty involved in formulating a complete micromechanical analytical description of the deformation.

Fast DEM Collision Checks on Multicore Nodes

Many particle simulations today rely on spherical or analytical particle shape descriptions. They find non-spherical, triangulated particle models computationally infeasible due to expensive collision detections. We propose a hybrid collision detection algorithm based upon an iterative solve of a minimisation problem that automatically falls back to a brute-force comparison-based algorithm variant if the problem is ill-posed. Such a hybrid can exploit the vector facilities of modern chips and it is well-prepared for the arising manycore era. Our approach pushes the boundary where non-analytical particle shapes and the aligning of more accurate first principle physics become manageable.

Characterisation of pattern formation in constrained multiblock assembly subjected to horizontal harmonic excitation

Pattern formation for a constrained one-dimensional assembly comprising a number of blocks undergoing external harmonic excitation of the basin boundary is investigated. The importance of the relationship between the excitation amplitude and the basin size which leads to repeatable response patterns of the single and multiple blocks is established, considering in addition the role of the energy dissipation due to inter block collisions and collisions with the moving boundary. Experimental results for the four- and eight-block assembly control problems, exposed to a range of amplitudes and frequencies, are included and compared with the two sets of computational simulation frameworks, the explicit discrete element method and the non-smooth contact dynamics. In the characterisation of the dynamic sensitivity response of the multiblock assemblies, several attributes and indices (kinetic energy index, block assembly mass centroid and mass inertia index, presented through time histories and phase planes) are extracted from both the experimental and computational simulation results and successfully compared.

DYNAMIC SENSITIVITY OF MULTI-BLOCK STACKS SUBJECTED TO BASE PULSE EXCITATION - EXPERIMENTAL EVIDENCE AND NON SMOOTH CONTACT DYNAMIC SIMULATIONS

Experimental and computational dynamic sensitivity study of Multi-Block Stacks subjected to Pulse Base Excitation is considered. Advanced non contact optical measuring technique based on the GOM Aramis and Pontos systems, as well as the corresponding processing software (displacement history of control sensor points, with a high resolution high speed cameras) have been applied to replace conventional displacement measuring systems and accelerometers. The Non Smooth Contact Dynamics (NSCD) time integration simulation framework SOLFEC http://code.google.com/p/solfec/ is adopted here for comparative NSCD analyses, including a sensitivity study on interface characteristics, as a validation process. Series of test experiments were conducted and recorded on a bespoke platform with and without lateral constraints in the Oxford Impact Engineering Laboratory and Rijeka University Structural Dynamics Laboratory for an extensive series of controlled pulse base excitation tests of multi block stacks configurations. Impact is generated by a pin-ball mechanism with spring and a wooden projectile, attached to an optical bench. For the NSCD simulations the base was subjected to a constant acceleration over a finite time, thereby facilitating the characterisation of multi block stacks tumbling modes of failures (global or partial), as a function of stop gap distance. Creation of well documented benchmarks for the validation of simulation paradigms for discontinuous media will be extremely valuable for researchers and code developers (non smooth contact dynamics, discrete elements, discontinuous deformation analysis), as well as for safety case engineers and industry regulators. N. Čeh, A. Pellegrino, J. F. Camenen, N. Petrinić, G. Jelenić, T. Koziara and N. Bićanić __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

On Large Scale Implicit Multibody Contact Dynamics Modeling with SOLFEC

SOLFEC is a multibody contact/impact simulation code. Unlike most of the currently used explicit multibody codes, it does not use repulsive springs to prevent interpenetration. Instead, it exploits an implicit formulation of contact conditions, where physically plausible impact resolution follows from momentum balance and deformations of contacting bodies. As a result, stiff springs are eliminated and larger stable time steps are allowed. In this contribution, we outline the basic principles behind the computational approach employed in SOLFEC. We give examples relevant to dynamic analysis of AGR graphite cores, demonstrating that our approach can effectively deal with multibody structures composed of thousands of loosely keyed together bodies with tight geometrical features and small clearances. Multiple impacts, disengagement of interacting features, and fragmentation of bodies are all handled in our implementation. These aspects, as well as the parallel efficiency will also be exemplified. SOLFEC can be downloaded from: http://code.google.com/p/solfec/