Tomaž Rodič

Shape sensitivity analysis of large deformation frictional contact problems

Sensitivity analysis of large displacement multi-body two-dimensional contact problems with friction is developed in the paper. The incremental (path-dependent) sensitivity problem is derived by direct differentiation of the discretized equations governing the direct problem. In view of finite deformations, due attention is paid to spatial and nominal contact tractions and to proper formulation of the contact laws within the penalty approach. For these reasons an extended node-to-segment contact element is used to model the frictional contact interactions. As the finite elasto-plastic deformations of the contacting bodies are considered, the numerical procedures for computation of all the necessary characteristic formulae of the solid elements (for both the direct and the sensitivity problem) are automatically derived and generated using the symbolic algebra package AceGen. Numerical examples of shape and parameter sensitivity analysis illustrate the approach.

Shape optimization of two-phase inelastic material with microstructure

Purpose – Proposes a methodology for dealing with the problem of designing a material microstructure the best suitable for a given goal. Design/methodology/approach – The chosen model problem for the design is a two-phase material, with one phase related to plasticity and another to damage. The design problem is set in terms of shape optimization of the interface between two phases. The solution procedure proposed herein is compatible with the multi-scale interpretation of the inelastic mechanisms characterizing the chosen two-phase material and it is thus capable of providing the optimal form of the material microstructure. The original approach based upon a simultaneous/sequential solution procedure for the coupled mechanics-optimization problem is proposed. Findings – Several numerical examples show a very satisfying performance of the proposed methodology. The latter can easily be adapted to other choices of design variables. Originality/value – Confirms that one can thus achieve the optimal design of the nonlinear behavior of a given two-phase material with respect to the goal specified by a cost function, by computing the optimal form of the shape interface between the phases.

A Numerical Framework for Wall Dissolution Modeling

Scallops and flutes are common dissolution rock forms encountered in karst caves and surface streams. Their evolution is only partially understood and no numerical model that simulates their formation has been presented. This work at least partially fills the gap by introducing a numerical approach to simulate the evolution of different initial forms of soluble surfaces embedded in a turbulent fluid. The aim is to analyze wall dissolution phenomena from basic principles and to identify stable profiles. The analysis is based on a finite volume moving boundary method. The underlying mathematical model is a

Time-Dependent Finite Element Analysis of In Vivo Electrochemotherapy Treatment

k-\epsilon

Optimal design of preform geometry and tribological conditions in can forming

k-ϵ turbulent model for fluid flow coupled with turbulent scalar transport. The rock wall is treated as a moving boundary, where the normal wall retreat velocity is proportional to the under-saturation of the boundary fluid cells with respect to the mineral comprising the wall. As the flow time scale is several orders of magnitude smaller than the dissolution time scale, stationary flow field, concentration field and wall propagation velocity are calculated for each iteration. The boundary at all points is then moved by distracting minimal velocity along the entire boundary from the actual velocity at a certain location, and then normalized to the maximum allowed shift, which is equal to half the height of the boundary cell. In this way only deformation of the initial wall is calculated. The method was applied to several different initial profiles. During the evolution, the profiles progressively converged towards stable forms. In this work, a framework is proposed for a computation of the moving boundary problem related to slow dissolution of a soluble surface.

Damage driven crack initiation and propagation in ductile metals using XFEM

Purpose – The purpose of this paper is to evaluate the effect that polymer coat has on the impact behavior of grinding sphere and to find possible subsection of parameter space in which grinding sphere wear could be reduced.Design/methodology/approach – Numerical analysis is based on axisymmetric finite elements that were developed using symbolic tool AceGen. Comparing stress response of elastic and visco‐elastic material revealed that for high strain rates observed in impacts both behave the same and that is why elastic elements were used in simulations.Findings – Impact velocity, coat thickness and polymer material properties were varied in a parametric case study of polymer‐coated sphere impact. Decrease of the pressure on the surface of grinding sphere indicates that polymer layer can be effective in reducing grinding media wear, but in order to maintain adequate impact pressure to do the grinding the impact velocity has to be increased. Both upper and lower limit for impact velocity were determined f...

PROCESS DESIGN AND SENSITIVITY ANALYSIS IN METAL FORMING

Electrochemotherapy and irreversible electroporation are gaining importance in clinical practice for the treatment of solid tumors. For successful treatment, it is extremely important that the coverage and exposure time of the treated tumor to the electric field are within the specified range. In order to ensure successful coverage of the entire target volume with sufficiently strong electric fields, numerical treatment planning has been proposed and its use has also been demonstrated in practice. Most of numerical models in treatment planning are based on charge conservation equation and are not able to provide time course of electric current, electrical conductivity, or electric field distribution changes established in the tissue during pulse delivery. Recently, a model based on inverse analysis of experimental data that delivers time course of tissue electroporation has been introduced. The aim of this study was to apply the previously reported time-dependent numerical model to a complex in vivo example of electroporation with different tissue types and with a long-term follow-up. The model, consisting of a tumor placed in the liver with 2 needle electrodes inserted in the center of the tumor and 4 around the tumor, was validated by comparison of measured and calculated time course of applied electric current. Results of simulations clearly indicated that proposed numerical model can successfully capture transient effects, such as evolution of electric current during each pulse, and effects of pulse frequency due to electroporation effects in the tissue. Additionally, the model can provide evolution of electric field amplitude and electrical conductivity in the tumor with consecutive pulse sequences.

Some numerical issues on the use of XFEM for ductile fracture

Purpose – The purpose of this paper is to describe the micro fluid flow analysis in a micro thruster of micro‐/nano‐ satellite propulsion system and to propose the algorithm for the fluid flow simulations with the open boundary based on moving boundary method.Design/methodology/approach – The analysis is based on a finite volume moving boundary method. Underlying mathematical model is the system of Navier‐Stokes‐Fourier partial differential equation describing compressible gas model. Propellant under the study is pure nitrogen gas. First, the static geometry velocity vector field is calculated and the information of the velocity at the outflow boundary is obtained; then, with the moving boundary method the outlet boundary is evolved. Evolution of the boundary is stopped when the continuum model ceases to hold. The criteria of the continuum model failure are based on the local Knudsen number.Findings – The validations of the flow with respect to the Knudsen number showed that the continuum model is valid i...