Research on synthetic jets

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.

Modelling of yield surface distortion in the finite strain range

10. 11. 2017, 10:00

Cellular structures and materials – fabrication, properties characterisation and applications

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 quasi-static 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 quasi-static 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

Four weeks ago Nobel Prize for Physics had been awarded to three leading scientists from LIGO-VIRGO 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.
The crucial role of the three Nobel Prize Laureates will be emphasized and all five signals of gravitational waves so far recorded by LIGO will be briefly described.. Particular attention will be paid to the very recent one, announced on October 16, which originated from the collision of two neutron stars and which has also its optical counterpart as Gamma Ray Burst, observed by two space-based telescopes. Finally, future observatories, that would significantly extend the capabilities of LIGO-VIRGO, will be briefly discussed.
 

2. 10. 2017, 11:00

Internal Variables associated with Microstructure

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 one-dimensional 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 Love-Rayleigh equation for rods accounting for lateral inertia, the Maxwell-Rayleigh 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

in collaboration with Mart Ratas

In case of 1D wave propagation the discrete spectral analysis is very helpful method in order to analyze the space-time 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 pseudo-spectral 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

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 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.
 

17. 8. 2017, 10:00

Multi-Scale Structural Gradients Optimize  the Bio-Mechanical Functionality of the Spider Fang

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.

26. 6. 2017, 10:00

Modelling extreme deformation and dynamic behaviour of materials using mesh-less methods

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.

19. 6. 2017, 10:00

Additive Manufacturing of metals: Past, today and tomorrow

 
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.
 

15. 6. 2017, 10:00

Quasibrittle Failure Probability and Scaling

 
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.
 

ISG-Israel Smart Grid consortium and Large-Scale Power System Dynamics

3. 5. 2017, 10:00

Non-standard damped oscillators

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. Non-standard 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 non-standard ("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 "non-standard" mechanical elements: damper with Coulomb's friction, inextensible string, or more generally, collision of a moving mass with a wall.
In our talk, we will see how these situations can be modelled within the framework of NSA. We show that interesting dynamics can occur and even more, new interesting questions can be asked.
 

5. 4. 2017, 10:00

Implosive magnetocumulative generator for effective energy conversion

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 hi-speed 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

 
The bulk of the fluid on the Earth's surface can be found in the atmosphere and the oceans. Geophysical Fluid mechanics investigate it. The flow inside the area near the Earth's surface is called Atmospheric Boundary Layer in a certain analogy with the classical theory of fluid mechanics. Its properties, methods of research, taking into account their shortcomings; will be discussed in the following paragraphs.
1) Introduction: the introduction of the concept of and the reasons for its monitoring;
2) The basic characteristics (Equations of motion in continuum mechanics approximation, Flow in a rotating coordinate system, The temperature stratification, Turbulence and determinism)
3) Modelling (experimental methods, numerical methods)
4) Tasks solved in the Laboratory of Environmental Aerodynamics;
5) New problems to solve:
Verification and validation of mathematical models,
Many scales problem;
6) Conclusion: applications
 

14. 2. 2017, 10:00

Modelling of complex processes in nanopowder fabrication using thermal plasma flows

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.

1. 2. 2017, 10:00

Highlights of plasma spraying in the life of one researcher

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

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.