Přijďte k nám ve dnech 12., 14. a 16. listopadu 2019, ať vidíte, na čem děláme! Více zde.
6. 12. 2017, 10:00  
Research on synthetic jets 

Assoc. Prof. Zdeněk Trávníček, Institute of Thermomechanics, v.v.i., CAS, Prague


Synthetic jets are fluid flows which are generated from periodically oscillating fluid. In spite of zero timemean flux at the actuator, a nonzero timemean jet flow can be generated (synthesized) from a train of individual fluid “puffs”. These flows have many perspective applications such as active control of flowfields and thermal fields (external and internal aerodynamics, cooling, mixing, etc.). The basic advantage is the simplicity – neither fluid source (compressor, blower, pump) nor supply piping is required. Therefore, the synthetic jet has been subject of intensive investigations recently.
The topic has been investigated at the Institute of Thermomechanics since 2001. For example, the following particular tasks have been solved: (1) Impinging synthetic jet and heat transfer enhancement, (2) newly proposed principle: "hybrid synthetic jet", (3) formation criterion of synthetic jets and identification of flow regimes, and (4) geometry optimization. 

15. 11. 2017, 10:00  
Modelling of yield surface distortion in the finite strain range 

Prof. A.V. Shutov, Lavrentyev Institute of Hydrodynamics, Novosibirsk State University  
The talk is devoted to the phenomenological modelling of the stress response of metallic materials subjected to nonproportional loading conditions. As a preliminary step, a class of twodimensional rheological models is introduced, capable of capturing the initial and straininduced anisotropies of the analyzed material. The rheological models mimic the effect of a combined isotropickinematicdistortional hardening; the essential part of the approach is a directiondependent friction element, which allows us to describe an arbitrary sharpening of the yield surface in the loading direction, accompanied by arbitrary flattening on the opposite side. Two different specific definitions of the directiondependent friction are provided. The first approach is based on a certain interpolation between the initial yield surface of the von Mises type and a fully saturated yield surface exhibiting maximum distortion. The second approach allows interpolating between a sequence of predefined symmetric yield surfaces. Both approaches are practical and flexible. They guarantee that the yield surface remains convex and smooth at any stage of the deformation process, which is important for stable and robust computations. Next, basing on these results, a system of constitutive equations is constructed for a general multiaxial loading. The description of the finite strain kinematics is based on the nested multiplicative split of the deformation gradient. The resulting model is objective, thermodynamically consistent, winvariant; it is free from shear stress oscillations. Finally, an efficient and robust numerical implementation of the model is discussed.  
10. 11. 2017, 10:00 

Cellular structures and materials – fabrication, properties characterisation and applications 

Zoran Ren, Srečko Glodež, Matej Vesenjak and Nejc Novak, University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia 

The presentation will give a short overview of cellular materials in general. Initially, their properties, fabrication procedures and application possibilities will be discussed. Then their geometrical characterization, experimental testing and computational modelling within the finite element method of various cellular metal types will be described. The geometrical characterisation is based on the analysis of micro computed tomography scans and proper recognition of their internal cellular structure, taking into account the statistical distribution of morphological and topological properties. The results of conducted geometrical analysis provided means to develop methodology for proper 2D and 3D geometrical modelling of irregular cellular materials and consequent formation of computational models. The numerical models were validated by quasistatic and dynamic mechanical experimental tests supported by infrared thermography. In the next part of the presentation, auxetic cellular structures, which exhibit negative Poisson’s ratio, will be discussed. Negative Poisson’s ratio is a consequence of internal structure deformation. This effect is useful for many different applications to enhance properties in density, stiffness, fracture toughness, energy absorption and damping. Several 2D and 3D auxetic structures will be introduced. Experimental results of some selected auxetic structures, tested under quasistatic and dynamic loading conditions, will be presented. Furthermore, representative discrete computational models built with the beam finite elements and homogenised computational models that were validated by experimental data will be shown as well. They were developed to explore the auxetic response at different loading conditions and material distribution (including porosity variation). 

1. 11. 2017, 11:00 

On gravitational waves and 2017 Nobel Prize for Physics 

Prof. Jiří Chýla, Institute of Physics of the Czech Academy of Sciences


Four weeks ago Nobel Prize for Physics had been awarded to three leading scientists from LIGOVIRGO Collaboration “for decisive contributions to the LIGO detector and the observation of gravitational waves”. In this seminar I will first recall basic facts about the origin and detection of gravitational waves in general and then discuss in nontechnical terms the construction and amazing sensitivity of LIGO detector as well as the way how the gravitational waves are recorded and presented. 

2. 10. 2017, 11:00 

Internal Variables associated with Microstructure 

Dr. Arkadi Berezovski, Department of Cybernetics, School of Science, Tallinn University of Technology


Prediction of the response of microstructured materials on an external loading can be achieved by means of various methods. In (quasi)statics, homogenization methods are suitable in most situations, but this is not the case for functionally graded materials, e.g. Strain gradient models are quite sufficient if only the influence of a microscale length is taken into account. The most general approach is provided by generalized continuum theories, which include microdeformation into consideration. One more possibility is the introduction of internal variables for the description of microstructure.
In the paper, we compare different descriptions of microstructured solids on the simple example of wave propagation in the onedimensional setting. In the classical continuum mechanics the existence of a microstructure is neglected. Thus, the classical wave equation needs to be modified to include the observed dispersive effects due to the microstructure. We consider modifications of the wave equation which follow from homogenization, continualization of lattice models, and from generalized continuum theories. The linear version of the Boussinesq equation for elastic crystals, the LoveRayleigh equation for rods accounting for lateral inertia, the MaxwellRayleigh model of anomalous dispersion, etc., are compared with dispersive wave equations obtained by means of single and dual internal variables.


2. 10. 2017, 10:00 

2D Discrete Spectral Analysis – A Tool for Examining of omplicated Wave Structures 

Prof. Andrus Salupere, Department of Cybernetics, School of Science, Tallinn University of Technology
in collaboration with Mart Ratas 

In case of 1D wave propagation the discrete spectral analysis is very helpful method in order to analyze the spacetime behavior of different wave structures. Here we generalize the method to 2D case. The Kadomtsev–Petviashvili equation is applied as a model equation. For numerical integration the pseudospectral method is applied. We demonstrate how 2D spectral characteristics can be applied for analysis of complicated wave structures that can be formed from different initial pulses in case of the Kadomtsev–Petviashvili equation. Recurrence phenomenon, temporal periodicity and temporal symmetry of the solution will be discussed.


17. 8. 2017, 11:00 

Recent advances in reciprocal mass matrices 

Dr. Anton Tkachuk, Institute for Structural Mechanics, University of Stuttgart, Stuttgart, Germany
in collaboration with Anne Schäuble, Prof. Manfred Bischoff 

Standard explicit dynamic simulation relies on diagonal or lumped mass matrices. Lumped mass enables a trivial computation of the nodal accelerations from the total force vector. Moreover, critical time step estimators and contactimpact algorithms for such mass types are well understood and developed. A disadvantage of the exlicit time integration with the lumped mass is huge number of the time steps even for short time dynamics. Recently, several approaches for reciprocal mass matrix that allows higher time steps and reduction of the total computational cost were proposed. A reciprocal mass is a sparse inverse of mass matrix that usually has a mask/structure of consistent mass or stiffness matrix. It can be constructed directly and cheaply either with variational or with algebraic methods. Achievable speedup with respect to lumped mass is from 20% to 50%. In this talk, an overview of existing approaches of construction reciprocal mass matrices is given and recent advances in reciprocal mass matrices for impact algorithms, time step estimation and assessment of the error in heterogeneous materials are presented. 

17. 8. 2017, 10:00 

MultiScale Structural Gradients Optimize the BioMechanical Functionality of the Spider Fang 

Dr. Benny BarOn, Laboratory for the Mechanics of Complex Materials, Department of Mechanical Engineering, BenGurion University of the Negev


The spider fang is a natural injection needle, built as a multiscale composite material with outstanding mechanical properties. In this study we introduce a hierarchical modeling for the spider fang, based on computer tomography and SAXS measurement, and analyze the correlation between the fang architectural motifs and its macroscopic elastic behavior. Analytical methods and FiniteElement simulations are used for the mechanical analysis and the effects of small and largescale structural gradients on the macroscopic mechanical properties are investigated. It is found that the multiscale structural gradients of the spider fang optimize its performances in term of loadbearing stiffness and strength, and that the naturally evolved fang architecture provides optimal mechanical properties compared to other alternative structural configurations.


26. 6. 2017, 10:00 

Modelling extreme deformation and dynamic behaviour of materials using meshless methods 

Dr. Raj Das, Sir Lawrence Wackett Aerospace Research Centre, School of Engineering, RMIT University, Australia


The seminar will present overview of computational mechanics research at the Centre for Multifunctional and Composite Materials of RMIT University, Australia. Our research covers both fundamental and applied aspects of material behaviour and failure processes. This presentation will encompass computational modelling of material deformation, damage and fracture using multiscale techniques in conjunction with meshless methods, novel composite materials development and damage tolerance structural optimisation. Multiscale modelling of damage and fracture progression linking nano to macro scales and associated development of coupled computational modelling tools will be highlighted. The strengths of meshless methods will be illustrated with reference to both low to highspeed impact induced fractures and small to large scale problems. These include several dynamic fracture and fragmentation processes, such as hypervelocity impact fracture, nanoscale machining, large scale geomechanical failures (magma intrusion, caving, slope stability, etc). One of our core areas to be presented is novel impact and blast resistant, light weight composite material developments for aerospace components subjected to highspeed loading and extreme deformations, as occurs in the cases of debris impact on spacecrafts, bird strike on aircraft engines, blast induced failures, etc. Lastly novel shape and topology optimisation methodologies for damage tolerance optimisation, i.e. maximising the residual strength and fatigue life, of aerostructures will be highlighted. Case studies from projects with Royal Australian Air Force and Defence Science and Technology Organisation will be presented to demonstrate the practical implementation and utilities of the developed design and analysis methodologies.


19. 6. 2017, 10:00 

Additive Manufacturing of metals: Past, today and tomorrow 

Dr. Edson Costa Santos, SENAI Innovation institute in Laser Processing, Joinville, Santa Catarina, Brazil 

· Draw some observations from various attitudes to AM worldwide. Dr. Edson Costa Santos spent more than a decade in various laboratories related to Additive Manufacturing in Europe, South America and Japan. In the presentation, it will be drawn from his experience, and present a view of current AM layout – technologies, directions and main leaders. 

15. 6. 2017, 10:00 

Quasibrittle Failure Probability and Scaling 

Prof. Zdeněk P. Bažant, Northwestern University, Evanston, Illinois, USA 



30. 5. 2017, 15:00  
ISGIsrael Smart Grid consortium and LargeScale Power System Dynamics


3. 5. 2017, 10:00 

Nonstandard damped oscillators 

Prof. Dalibor Pražák, Department of Mathematical Analysis, Faculty of Mathematics and Physics, Charles University in Prague 

Damped oscillators of the form x'' + a(x)x' + b(x) = f(t) are classical models in mechanics and for regular enough a(.), b(.), say C1 or Lipschitz, the mathematical theory is very well understood. Nonstandard analysis (NSA), on the other hand, is a rather strong and abstract logical framework. Using NSA, various mathematical theories can be embedded into larger universes with nonstandard ("ideal") elements. The simplest and most famous examples are infinitely large and small numbers (which are thought by some advocates of NSA to be fatally missing from Calculus for nearly 200 years by now.) Curiously enough, some nonstandard choices of the functions a(.) and b(.), taking infinitely large values, or with infinitely steep growth, are natural models of some "nonstandard" mechanical elements: damper with Coulomb's friction, inextensible string, or more generally, collision of a moving mass with a wall. 

5. 4. 2017, 10:00 

Implosive magnetocumulative generator for effective energy conversion 

Dr. Jiří Šonský, Institute of Thermomechanics of the CAS, v. v. i. 

History of magnetohydrodynamic generators goes back to 1832 when Michael Faraday tried first experiments. Magnetocumulative generators were developed by Andrei Sakharov at the start of the 1950s, but up to these days such devices are not used for public energetics and remain in experimental, often military development. Therefore we have developed new thermal plasma source for magnetohydrodynamic or magnetocumulative generator suitable for general energetics. The thermal plasma is created from combustible mixture by implosion  spherical compression driven by convergent detonation wave. The detonation wave is initiated by a weak spark and by means of deflagration to detonation transition in detonation tube. Convergent polyhedral shape of the detonation wave is formed by large number of vents opened to hemispherical combustion chamber. Propagation of the detonation wave and its multiple branches is tracked by an array of ionization probes. Resulting high velocity plasma is ejected from a nozzle near the geometrical center of the device. The plasma is observed by capturing emitted light by hispeed camera to determine plasma velocity. Construction of the implosion plasma source and possible variants of extraction of electrical energy from kinetic energy of the plasma by interaction of high velocity plasma with seed magnetic field will be also discussed in this presentation. 

1. 3. 2017, 10:00 

Atmospheric Boundary Layer: main characteristics and methods of the research in context of continuum mechanics 

Prof. Zbyněk Jaňour, Institute of Thermomechanics of the CAS, v. v. i. 



14. 2. 2017, 10:00 

Modelling of complex processes in nanopowder fabrication using thermal plasma flows 

prof. Masaya SHIGETA, Joining and Welding Research Institute, Osaka University, Japan 

Thermal plasmas have been expected as a promising tool for massproduction of nanopowders [1] because thermal plasmas offer a distinctive thermalfluid field involving high temperature, high chemical reactivity and variable properties. Furthermore, thermal plasmas have steep temperature gradients at their fringes where many small nanoparticles are produced rapidly from the material vapour as a result of the highly supersaturated state. However, it is still difficult to investigate the formation mechanism of nanoparticles generated in/around a thermal plasma because the process involves remarkably intricate mass transfer of phase conversions in microsecond scales. Moreover, the plasma fringe is fluiddynamically unstable and consequently it forms a turbulent mixing field composed of multiscale eddies [2]. The growing nanoparticles are transported by the complicated convection as well as diffusion and thermophoresis. In this lecture, several modelling works to simulate those complex processes are explained.


1. 2. 2017, 10:00 

Highlights of plasma spraying in the life of one researcher 

Dr. Tomáš Chráska, Institute of Plasma Physics of the CAS, v. v. i. 

Thermal spraying techniques are coating processes in which melted or heated materials are sprayed onto a surface. There is a great variety of feedstock materials that can be thermally sprayed including solid powders and suspension liquids. There is also a great variety of thermally sprayed coatings used for many different applications including for example the thermal barrier coatings in jet engines. Plasma spraying process is a member of the thermal spraying family of techniques. It uses plasma gun to generate a plasma jet that melts feedstock materials. This talk is not going to present a complete overview of plasma sprayed coatings and their applications. It will rather present a set of interesting and sometimes intriguing examples of what can achieved by plasma spraying. The examples will include nanopowders, epitaxial growth of crystals in plasma sprayed coatings, amorphous and nanocomposite coatings, spraying of suspension and more. 

4.1. 2017, 10:00 

Usage of time reversal signal processing in nondestructive diagnostics of materials and structures 

Dr. Zdeněk Převorovský, Institute of Thermomechanics of the CAS, v. v. i. 

Time reversal processing of acoustic and ultrasonic signals (TRA) is very effective tool for complicated problems solution in many fields like nondestructive testing and evaluation (NDT/NDE) of materials and structures. TRA enables waves focusing in time and space and therefore precise localization and reconstruction of wave sources in strongly inhomogeneous, anisotropic, and dispersive media. Properties of TRA may be used to signal processing in acoustic emission (AE), nonlinear elastic wave spectroscopy (NEWS), and also e.g. in seismology, medicine, telecommunications, etc. TRA principles will be mentioned in the talk, and its potentials in AE source location and identification will be discussed. Outlined will be also some questions of a new approach to that inverse problems solution by using the ultrasonic signal transfer from a real body onto its laboratory and/or numerical model where they can be analyzed more easily. 