This is a past event
Title1 : Low dimensional models for stick-slip vibration of drill-strings (Short talk)
Speaker : Marcos Silveira, University of Aberdeen
Abstract: Effective reduction of drill-string vibration is still a major problem in drilling industry and therefore robust predictive tools need to be developed. Here we study two low dimensional nonlinear models. The first is a torsional model of the bottom-hole assembly (BHA). The second model is a torsional system having in addition to the BHA a rotary table, which allows simulation of interactions for which there is experimental evidence. Three friction models with increasing levels of complexity are applied to determine their influence in the dynamical responses. Comparison between the dynamic responses for these friction models shows that the dangerous stick-slip limit-cycles do not change qualitatively. Simulations show that, if appropriately controlled, large amplitude stick-slip limit-cycles can change to small amplitude limit-cycles in Model 2. In Model 1, with constant velocity of the rotary table, it goes from a large amplitude stick-slip limit-cycle to a fixed point. Bifurcation diagrams confirm the existence of a set of parameters in which the system operates without stick-slip vibration.
Title 2: Multi-scale methods and the determination of Representative Volume Elements in Solid Mechanics
Speaker: Dr Inna Gitman, University of Sheffield
Abstract: Several different approaches are available in order to describe material behaviour. Considering material on the higher (macro) level of observation constitutes the macroscopic approach. However, the key to understand a macro materials behaviour lies in its meso-structure. As such the meso-scopic approach can be used, which is based on the detailed material description of the lower (meso) observational level. The main focus of this presentation is the combination of the two above techniques: the multi-scale approach. The idea is, by means of a hierarchical multi-scale procedure, to bring the homogenised information of the detailed meso-structural description to the macro-level in the form of effective properties. Thus, the homogeneous macro-structural behaviour is driven by the heterogeneous meso-structure.
Traditionally, the size of a Representative Volume Element (RVE) of the material on the meso-level is chosen as a model parameter within the multi-scale framework. Two questions arise: what should this size be and how stable is this multi-scale model based on an RVE? As an answer to the first question, we propose a unique procedure to determine the RVE size. With knowledge of the RVE size, the multi-scale procedure can be introduced, in which the meso-level RVE plays the role of a macro-level length-scale parameter. However, the answer to the second question is not always positive. As an example the material behaviour due to mechanical loading can be considered. Although the results are stable and reliable in the linear-elastic and hardening regimes, the picture changes in softening. This is caused by the material developing strain localisation and as a consequence losing its statistical homogeneity. For such a material a Representative Volume cannot be found and as an inference cannot be used in the multi-scale framework. A conceptually new so-called coupled-volume multi-scale approach is introduced, based on abandoning the separation of scales principle. This approach does not require an RVE be a model parameter. The idea of the approach is to uniquely link the size of the meso-structural unit cell and element size of the discretised macro-structure. The results of this coupled-volume approach show stable and reliable behaviour in all mechanical regimes.
- Speaker
- Marcos Silveira (University of Aberdeen ) & Dr Inna Gitman (University of Sheffield)
- Venue
- Rm 113