28 December 2016, 11:00

Modelling of lotus-type porous structures: bi- and multi- axial loading

Abstract: A computational modelling of the low-cycle fatigue behaviour of lotus-type porous material subjected to multiaxial loading cycles is presented. The considered computational models have the same porosity but different pore topology patterns. Multiaxial loading conditions in the direction perpendicular to the longitudinal axis of pores are assumed to be proportional (in-phase) and non-proportional (out-of-phase) loading paths in numerical simulation. The fatigue life analysis is performed using a damage initiation and evolution law, based on the inelastic strain energy approach. The computational results show that a different fatigue life is obtained in the models with the same porosity but with different pore topology at the same loading level. Furthermore, the results of computational simulations show a qualitative understanding of the loading path on low-cycle fatigue failures of lotus-type porous material under multiaxial loading conditions.

7 December 2016, 10:00

Iron. Static and "dynamic" phase diagrams and transformation kinetics.

Abstract: Our civilization is closely acquainted with iron for some 4500 years, iron polymorphism is known for some 100 years, and it is some 50 years since the iron static phase (P,T) diagram has been established with reasonable accuracy.

The talk describes some recent experimental results aimed to establishing the borders of existence of iron phases when the iron is compressed by shock. Such "dynamic" phase diagram is found to differ strongly from the static one, i.e. the shock-generated metastable phase can subsist for a time longer than the experiment duration (microseconds) while the time required for the phase formation (transformation kinetics) is extremely short, few tens of nanoseconds.

9 November 2016, 10:00

Regularized Models for Softening Materials

Abstract: For many materials, the deformation process at some stage leads to propagation and coalescence of existing defects and to initiation of new ones. If the defects grow sufficiently fast, the material can exhibit, on the macroscopic scale, a decrease of the averaged stress even at increasing strain. This phenomenon, referred to as softening, is one of the destabilizing factors that can, under certain conditions, lead to localization of inelastic deformation processes into narrow bands. An objective description of localized strain patterns in the framework of continuum mechanics requires special adjustments of material models, because for traditional models the width of the localized band can become arbitrarily small and, consequently, the numerical solution exhibits a pathological sensitivity to the discretization (e.g., to the size of finite elements).
This lecture provides an overview of various regularization techniques that can serve as localization limiters. In view of their diversity, localized solutions will be analyzed for a one-dimensional model problem only. We will identify which specific regularization techniques are suitable for elastoplastic models with softening and for damage models, and we will compare the corresponding localization conditions and localized profiles of plastic strain or damage, including their subsequent evolution. Such analysis will reveal why certain specific formulations based on nonlocal averaging or on gradient enhancements serve as reliable localization limiters while other formulations fail or suffer by various deficiencies. 
 

5 October  2016, 10:00

Experimental study and simulation on localization of phase transformation in shape memory alloys

Abstract: The remarkable properties of shape memory alloys (SMA) that are utilized in a number of useful applications are due to a phase transformation between austenite and martensite. In many situations, the transformation does not proceed homogenously, but in the form of macroscopic transformation bands, i.e. in a highly localized manner. Since these inhomogeneities influence the mechanical response and reduce the fatigue performance of SMA products, the roots and mechanisms of localization have been investigated by material scientists and engineers for many years. In a unique experiment utilizing the advanced technique of three-dimensional X-ray diffraction (3D-XRD), complete strain and stress states of the polycrystalline grains close to the macroscopic transformation band front have been resolved on a grain-by-grain basis for the first time. Results show substantial heterogeneity of stress between grains – implied by anisotropy of both elastic and transformation properties – and a striking redistribution of macroscopic (homogenized) stress near the interface. Analysis of the experimental data allowed the team to adapt an established constitutive model tailored for NiTi SMA so that non-local, gradient effects could be included. Consequent numerical simulation of the propagating transformation band demonstrated how the internal stress redistributes close to the phase interface within the wire causing the macroscopic localization to occur. 
 

15 September  2016, 10:00

Effects of the nozzle exit boundary layer on hot-jet mixing

Abstract: The main motivation of the research effort reported here was passive protection of airplanes against an enemy’s heat seeking missiles. The stated goal was to reduce the temperature of a jet engine exit flow by means of the hot-jet rapid mixing with the ambient air, and no penalty of the jet engine thrust was allowed. The focus of the first phase of this project was to explore the properties of high-speed free jets and demonstrate that it is possible to enhance jet flow mixing by flow excitation. Inconsistencies in hot-jet responses to flow excitations were investigated in the second phase of this project. It was shown that the jet receptivity to flow excitation is strongly dependent on a character of the nozzle exit boundary layer. Finally, it was proven that the decisive factor controlling the jet receptivity is the velocity gradient across the exit boundary layer. There were also side byproducts of this research effort. First, it was an improvement in the high-frequency stroboscopic visualization of large-scale turbulent structures in free jet flows. The second innovation was the development of a new methodology for conditional sampling of random laser velocimeter data. 
 

17 August  2016, 10:00

Rotation-free parametrization and isogeometric analysis of shear deformable plates and shells

Abstract: Structural theories for static and dynamic analysis of shear deformable plates and shells (Reissner-Mindlin type) usually employ independent degrees of freedom for displacements and rotations. It is shown how equivalent models can be developed based on displacement degrees of freedom only. In the context of finite element formulations this has the advantage that transverse shear locking can be intrinsically avoided within a standard displacement-based concept, regardless of the underlying function spaces used for discretization.

As in this context higher continuity of the shape functions is required, a natural way is to incorporate such theories into the isogeometric concept, using NURBS (non-uniform rational B-splines) as shape functions. Corresponding shear-deformable shell finite element formulations for geometrically linear and non-linear applications are presented and their performance is demonstrated with the help of numerical examples. 
 

2 June 2016, 10:00

Mathematics of fluids in motion

Abstract: We discuss the recent development of the mathematical theory of fluid dynamics, classical and new open problems and possibilities of their solution. Special attention is paid to the recent results on well and/or ill-posedness of problems describing inviscid fluids. In particular, several modern concepts of solutions are examined: Weak, very weak, measure-valued etc.

4 May 2016, 10:00

Development of Human Artificial Vocal Folds

Abstract: Voice production is a complex physical process, which involves airflow coming from the lungs, selfoscillating vocal folds and acoustics of the resonance cavities of the human vocal tract. The vocal folds, excited by the airflow, generate a primary sound which propagates in the airways of the vocal tract modifying its spectrum and producing the final acoustic signal radiated from the mouth. Understanding basic principles of voice production is important for detection of laryngeal pathologies and treatment of laryngeal disorders. The physical models of voice production are important tools not only for experimental verification of computational 3D finite element models of phonation, but also for development of the vocal folds prosthesis. The study compares results of in vitro measurements of phonation characteristics performed on originally developed 1:1 scaled artificial models of human vocal folds. The measured aerodynamic, vibration and acoustic characteristics of the last models are comparable with the values found in humans..

6 April 2016, 10:00

Stochastic Self-Organization in Inner Structure of Vehicular Systems

Abstract: History of traffic science is relatively short. Generally, it is meant that the first scientific article is the essay of Professor Bruce Greenshields dated to 1934. The factual beginning of systematic scientific discipline (called Transportation Science) is the year 1992, when the field accelerated by many famous publications. Nowadays Transportation Science is very well anchored in the portfolio of scientific disciplines (associated MIF is 1.377). For the purposes of this seminar talk we choose a theme of predictions for statistical properties of traffic flows and detection of surprising relations in traffic microstructure.
In this talk we will show that macroscopic self-organization of traffic streams (e.g. spontaneous traffic congestions) is projected into evolution of stochastic properties detected for vehicular micro-quantities. Furthermore, we will demonstrate that there exists a smart and uncomplicated way how to predict such microscopic effects.

Animal Flight

Abstract: Unlike airplanes, animals must have either flapping wings (birds, bats) or oscillating wings (insects). Only such wings can produce both lift and thrust, provided the animal has sufficient muscle power.
To fly, wings impart downward momentum to the surrounding air and obtain lift by reaction. How this is achieved under various flight situations (cruise flight, hovering, landing, etc.), and what is the role of the wing-generated vortices in producing lift and thrust is discussed (both for birds and insects).
Bird wings have several possibilities how to obtain the same functions as airplane wings. Birds have the capabilities of adjusting the shape of the wing according to what the immediate flight situation demands, as well as of responding almost immediately to conditions the flow environment dictates.

FLUIDIC OSCILLATORS FOR ALGAE CULTIVATION and their role in geopolitic stability

Abstract:
Our civilisation is extremely dependent on cheap liquid fuel used for transportation. Until roughly the end of 19th century people used to work in their respective dwelling places. Now they commute in huge numbers every day. Food and other goods travels hundreds (if not thousands) of kilometres between production and use. This model is increasingly adopted by developing most populated countries (China, India). Fossil fuel sources, on which this all depends, is produced – at an increasingly high cost – in politically unstable regions. No wonder research grant providers are willing to support financially the research promising renewable petrol as its result. The starting point are algae – primitive, often unicellular plants capable to produce by photosynthesis - from H in water and CO2 taken from air - hydrocarbon compounds, processing of which into biofuels brings no difficulty in principle – after all, the fossil oil was produced the same way from algae millions of years ago. Additional benefit would be the whole process being carbon neutral so that removal of CO2 from the atmosphere would suppress the global warming. Algae may be also a starting point of a food chain, solving another global problem.
The difficulty is so far the price of the crude oil from algae being higher than the fossil one. The key factor for success is making more efficient every step in the process. One of perhaps small but nevertheless important contribution towards the goal is more efficient diffusion transport of CO2 into the algae in bioreactors. Suggested solution is generation of sub-millimetre sized microbubbles by placing a fluidic oscillator into the gas inlet. The research grant project investigated in the Institute of Thermomechnaics enabled recently testing a number of alternative oscillator designs.

Discontinuous Galerkin method for the solution of elasto-dynamic, compressible flow and fluid-structure interaction problems

Abstract:
This lecture will be concerned with the numerical solution of dynamic elasticity and compressible flow. We consider the linear case as well as the nonlinear St. Venant-Kirchhoff model. The space Discretizat on is carried out by the discontinuous Galerkin method (DGM). For the time discretization several techniques are proposed and tested. As the best method the DG discretization both in space and time appears. The discontinuous Galerkin method is also used for the numerical solution of compressible flow in time-dependent domains, formulated with the aid of the arbitrary Lagrangian-Eulerian (ALE) method. It will be shown that this method allows the solution of compressible flow with a large range of the Mach number. Then the developed methods are combined and used for the numerical simulation of vibrations of elastic bodies induced by compressible flow. The applicability of the developed techniques will be demonstrated by several numerical experiments.
The results were obtained in cooperation with M. Balázsová, J. Česenek, M. Hadrava, A. Kosík and J. Horáček.