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| 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
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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 contact-impact 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 speed-up 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.
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| 17. 8. 2017, 10:00 |
Multi-Scale Structural Gradients Optimize the Bio-Mechanical Functionality of the Spider Fang
Dr. Benny Bar-On, Laboratory for the Mechanics of Complex Materials, Department of Mechanical Engineering, Ben-Gurion University of the Negev
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The spider fang is a natural injection needle, built as a multi-scale 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 Finite-Element simulations are used for the mechanical analysis and the effects of small- and large-scale structural gradients on the macroscopic mechanical properties are investigated.
It is found that the multi-scale structural gradients of the spider fang optimize its performances in term of load-bearing stiffness and strength, and that the naturally evolved fang architecture provides optimal mechanical properties compared to other alternative structural configurations.
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| 26. 6. 2017, 10:00 |
Modelling extreme deformation and dynamic behaviour of materials using mesh-less methods
Dr. Raj Das, Sir Lawrence Wackett Aerospace Research Centre, School of Engineering, RMIT University, Australia
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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 multi-scale techniques in conjunction with mesh-less methods, novel composite materials development and damage tolerance structural optimisation.
Multi-scale 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 mesh-less methods will be illustrated with reference to both low to high-speed impact induced fractures and small to large scale problems. These include several dynamic fracture and fragmentation processes, such as hypervelocity impact fracture, nano-scale machining, large scale geo-mechanical 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 high-speed 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 aero-structures 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.
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| 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
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The lecture will be addressed the following topics in Additive Manufacturing (AM) of metals:
· Draw some observations from various attitudes to AM world-wide.
· Review shortly various additive manufacturing technologies, their virtues and drawbacks.
· Address cases, in which additive can/cannot replace conventional manufacturing - problems with distortions, variability in micro-structure and consequences, etc.
· Comparison of additive and conventional micro-structures and their impact on macro-mechanical properties: strength, fragility, fatigue, impact resistance, etc.
· Use of additive manufacturing for meta-materials (auxetic and other) for the purposes of "energy absorption or distribution" and "mechanical strength with low weight".
· Additive manufacturing process certification and/or serial production of components - competitiveness in terms of both function and price.
· Design of components for Additive Manufacturing - material only there "where needed" and the related development of software (e.g. topology optimization).
· Future of AM - visions and expectations.
· Describe and introduce FIESC - SENAI focus in AM.
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.
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| 15. 6. 2017, 10:00 |
Quasibrittle Failure Probability and Scaling
Prof. Zdeněk P. Bažant, Northwestern University, Evanston, Illinois, USA
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The size effect on structural strength and its probability distribution function (pdf) is a complex problem for quasibrittle materials because their failure behavior transits from quasi-plastic at small sizes to brittle at large sizes. These are heterogeneous materials with brittle constituents in which the size of inhomogeneity, or representative volume element (RVE), is not negligible compared to the structure size. Aside from concrete, the archetypical example, they include fiber composites, coarse-grained ceramics, rocks, sea ice, snow slabs, wood, bone, foam, stiff soil, dry snow,ccarton, etc., and on the micro- or nano-scale, all brittle materials become quasibritle. Since the break probability is known exactly only for interatomic bonds (being equal to frequency), Kramer’s rule of transition rate theory is applied to nano-crack jumps. Based on proving the rules of multiscale transition of tail probabilities of break to material scale, the probability distribution function (pdf) of strength of one macro-scale representative volume element (RVE) is shown to have a Weibullian tail, calibrated to reach to probability circa 0.001, the rest being Gaussian. On the structure scale, only Type 1 failure is considered, i.e., the structure fails as soon as the first RVE fails. Hence the weakest-link model applies on the structure scale. But, crucially, the number of links is finite, because of non-negligible RVE. For increasing structure size, the Weibullian portion gradually spreads into the Gaussian core. Only in the infinite size limit the distribution becomes purely Weibull, but, importantly, with a zero threshold. Based on an atomistic derivation of the power law for subcritical macro-crack growth, a similar Gauss-Weibull transition is shown to apply to structure lifetime. The theory is then extended to the size dependence of Paris law and Basquin law for fatigue fracture, to statistics of fatigue lifetime, and to residual strength after a period of preload. The theory is shown to match the existing experimental results on the monotonic strength, residual strength after preload, static and fatigue crack growth rates, and static and fatigue lifetimes, including their distributions and size effects on the distributions. There are three essential consequences: 1) The safety factors must depend on structure size and shape; 2) To predict the pdf of strength, the size effect tests of mean strength suffice; 3) To predict the static and fatigue lifetimes, it suffices to add tests of initial subcritical crack growth rate. An interesting mathematical analogy predicting the lifetime of nano-scale high-k dielectrics is also pointed out. Finally, a new “fishnet” statistics for strength of biomimetic nacre-like lamellar structures, modelled as a square fishnet pulled diagonally, is presented. This simple model differs from the weakest-link model as well as the fiber bundle model. The pdf is found again to transit from Gaussian to Weibullian, but in a different way.
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| 30. 5. 2017, 15:00 |
ISG-Israel Smart Grid consortium and Large-Scale Power System Dynamics
Prof. Yuval Beck, Head of Power Engineering, Faculty of Engineering, Holon Institute of Technology, Israel
Prof. Yoash Levron, Professor of Electrical Engineering, Faculty of Electrical Engineering, Technion – Israel Institute of Technology
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A Smart Grid demonstrator was implemented within the framework of the "Israel Smart Grid
– ISG" Magnet project. The goal of the project was to implement and develop technologies for
optimizing and controlling Smart Grids. The main achievement of the project is its operation
system which is hierarchical in nature. Namely, the control and commands are not centralized
but rather distributed from top levels downwards. Every such control level can also be selfcontained.
The project consists of a demonstration of rout of electric power that is delivered to
a modern "Procumer" (Producer and Consumer), precisely upon its request, with minimum
power failures, energy optimization and minimal electricity costs. The demonstrator is
constructed in various sites and controlled by a virtual network. The virtual network consists
of controllable loads and some generators. Some of the actual controllable loads are motors,
air conditioning chiller, air treatment units and others. The loads are controlled by an
Intelligent Home Gateway Unit (IHGU) which operates in accordance to the contract between
the grid and the prosumer or consumer. The system also controls, via web services, two
remote sites of the Israel Electric Company – IEC, consisting of a virtual neighborhood.
Large-scale dynamics models of power systems are mostly based on time-varying phasors.
However, with increasing integration of distributed and renewable sources into existing power
grids, the assumption of time-varying phasors (or quasi-static models) becomes less accurate,
and may even lead to misleading conclusions regarding the system dynamics and stability.
During the lecture I will briefly review and compare several types of dynamic models,
describe several paradoxes that result from misuse of these models, and describe our group's
approach to this problem.
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| 3. 5. 2017, 10:00 |
Nestandardní tlumené oscilátory
Doc. RNDr. Dalibor Pražák, Ph.D, Katedra matematické analýzy, Matematicko-fyzikální fakulta, Univerzita Karlovy v Praze
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Tlumené oscilátory typu x'' + a(x)x' + b(x) = f(t) patří k elementárním problémům v mechanice. Pro přiměřeně hladké funkce a(.), b(.), např. C1 nebo Lipschitzovské, existuje rozsáhlá a klasická matematická teorie. Tzv. nestandardní analýza (NSA) je velmi silný a abstraktní logický rámec, který umožňuje vložit libovolnou matematickou teorii do rozšířeného univerza, které typicky obsahuje nestandardní („ideální”) prvky. Nejjednodušším a nejznámějším příkladem jsou nekonečně malá a velká čísla - čísla, s nimiž se moderní analýza před cca 150 lety možná k oboustranné škodě rozešla.
Pokusíme se ukázat, že určité nestandardní volby funkcí a(.), b(.) přirozeně vedou k popisu „nestandardních” mechanických jevů: Coulombovo tření, neroztažitelná struna, či, obecněji, náraz tělesa na pevnou stěnu. Díky jazyku NSA zároveň zůstaneme v rámci klasické teorie diferenciálních rovnic. |
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| 5. 4. 2017, 10:00 |
Implozivní magnetokumulativní generátor pro účinnou přeměnu energie
Ing. Jiří Šonský, Ph.D., Ústav termomechaniky AV ČR, v. v. i.
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Historie magnetohydrodynamických generátorů sahá až do roku 1832, kdy Michael Faraday začal s prvními experimenty. Magnetokumulativní generátory byly vyvinuty Andrejem Sacharovem již na začátku padesátých let minulého století, ale stále nejsou využívány v civilní energetice a zůstávají na experimentální, navíc často vojenské úrovni vývoje. Proto jsme vyvinuli nový zdroj termického plazmatu pro magnetohydrodynamické nebo magnetokumulativní generátory vhodné pro obecné použití v energetice. Plazma je vytvořeno z hořlavé směsi implozí – tedy sférickou kompresí konvergentní detonační vlnou. Konvergentní detonační vlna je spuštěna přechodem deflagrace do detonace po zapálení elektrickou jiskrou v detonační trubici. Konvergentní polyedrální tvar detonační vlny je vytvarován velkým počtem větvících se zátravek ústících do hemisférické spalovací komory. Vzniklé plazma vytryskne vysokou rychlostí tryskou ve středu zařízení a je sledováno vysokorychlostní kamerou. Postup detonační vlny je také sledován ionizačními sondami. Konstrukce implozivních zdrojů plazmatu a možnosti extrakce elektrické energie z kinetické energie plazmatu působením na počáteční magnetické pole bude v této přednášce také probrána
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| 1. 3. 2017, 10:00 |
Mezní vrstva atmosféry: vlastnosti a metody výzkumu v kontextu mechaniky kontinua
prof. RNDr. Zbyněk Jaňour, DrSc., Ústav termomechaniky AV ČR, v.v.i.
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Převážná část tekutin na zemském povrchu se nachází v atmosféře a oceánech. Těmito tekutinami se zabývá tzv. geofyzikální mechanika tekutin. Při jejím pohybu, v jisté analogii s klasickou teorií mechaniky tekutin, je její oblast přiléhající zemskému povrchu označována jako Mezní vrstva atmosféry. Její vlastnosti, metody výzkumu, s přihlédnutím jejich nedostatků, budou naznačeny v následujících bodech.
- Úvod: zavedení pojmu a důvody jejího sledování;
- Základní vlastnosti. (Pohybové rovnice v rámci aproximace mechaniky kontinua, Proudění v rotující soustavě souřadné, Teplotní zvrstvení, Turbulence a determinismus)
- Metody výzkumu (Experimentální, Numerické)
- Případy řešené v Laboratoři aerodynamiky prostředí;
- Nové problémy k řešení: Verifikace a validace matematických modelů, Problém mnoha měřítek;
- Závěr: možnosti aplikace získaných poznatků.
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| 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
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Thermal plasmas have been expected as a promising tool for mass-production of nanopowders [1] because thermal plasmas offer a distinctive thermal-fluid 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 micro-second scales. Moreover, the plasma fringe is fluid-dynamically 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.
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