Articles (NTIS)
Permanent URI for this collection
Browse
Recent Submissions
Item Cubic graphs with colouring defect 3(2024) Nedela, Roman; Karabáš, Ján; Máčajová, Edita; Škoviera, MartinThe colouring defect of a cubic graph is the smallest number of edges left uncovered by any set of three perfect matchings. While 3-edge-colourable graphs have defect 0, those that cannot be 3-edge-coloured (that is, snarks) are known to have defect at least 3. In this paper we focus on the structure and properties of snarks with defect 3. For such snarks we develop a theory of reductions similar to standard reductions of short cycles and small cuts in general snarks. We prove that every snark with defect 3 can be reduced to a snark with defect 3 which is either nontrivial (cyclically 4-edge-connected and of girth at least 5) or to one that arises from a nontrivial snark of defect greater than 3 by inflating a vertex lying on a suitable 5-cycle to a triangle. The proofs rely on a detailed analysis of Fano flows associated with triples of perfect matchings leaving exactly three uncovered edges. In the final part of the paper we discuss application of our results to the conjectures of Berge and Fulkerson, which provide the main motivation for our research.
Item Homogenization of fluid saturated double porosity media with a new type of the contrast in the Biot mesoscopic model(2024) Rohan, Eduard; Nguyen, Vu-Hieu; Naili, SalahThis paper presents a mathematical model of a double poro-elastic medium derived by the homogenization of a two-component periodically heterogeneous Biot continuum characterized by strong heterogeneities in the permeability and poroelastic coefficients of mesoscopic structure. A new scaling of the mesoscopic material parameters with respect to the small parameter which is involved in the asymptotic analysis is introduced to capture this high contrast quality of the mesoscopic model. It is shown that such a scaling ansatz of the mesoscopic material parameters can be justified using different micromodels associated with the two mesoscopic components. While the “matrix” is a little permeable stiff phase, the “conductive channels” are made of a very soft fibrous structure equivalently represented by an network of helical springs. The unfolding method of the homogenization is employed to derive the macroscopic model involving the frequency-dependent effective parameters. Semipermeable interfaces between hard dual porosity and soft primary porosity are considered. The wave dispersion of two shear wave modes S1, S2, and two pressure wave modes P 1, P 2, is illustrated in an numerical example and validated, in a part, using the reference dispersion analysis based on the Bloch wave decomposition.
Item Implementace a testování softwarové knihovny pro výpočet efektů vzdálené zóny u sférických integrálních transformací(2024) Belinger, Jiří; Šprlák, Michal; Pitoňák, Martin; Trnka, Petr; Novák, PavelIntegrální transformace jsou užitečným matematickým aparátem a základem pro formulaci odhadů veličin tíhového pole včetně šíření chyb. Jedním z předpokladů integrálních transformací je globální datové pokrytí. Dostupnost pozemních měření je však obecně limitována. V praxi se globální integrál rozkládá na dvě oblasti – blízké a vzdálené zóny. Nezanedbatelný příspěvek dat ve vzdálené zóně vyžaduje přesné vyhodnocení. Za tímto účelem je v prostředí MATLAB vytvářena knihovna pro výpočet efektů vzdálené zóny u integrálních transformací pro derivace tíhového potenciálu až do třetího řádu. Příspěvek popisuje implementaci teoretických odvození efektů vzdálených zón a numerické testování knihoven.Item Identifying internal resonance regimes in free-vibrating systems with multiple autoparametric couplings(2024) Dyk, Štěpán; Bulín, Radek; Rendl, Jan; Smolík, LubošThis paper introduces a method for identifying internal resonance regimes in free-vibrating systems with multiple nonlinear couplings, illustrated using a chain of two Rott’s pendula. The method based on decomposing Hamiltonian and subsequent treatment of the coupling energies allows for a comprehensive understanding of autoparametric system behaviour. A novel signal-processing-based coupling energy evaluation significantly enhances the effectiveness of the method, revealing internal resonances with slow energy exchange. In the theoretical part, a general step-by-step procedure applicable to any conservative coupled oscillators is introduced. We demonstrate that concerning such free-vibrating systems, the proposed method effectively uncovers internal resonances influenced by system parameters, initial conditions, and the initial energy of the system. Through an illustrative example of the system with multiple quadratic couplings, we also show that slow energy exchange occurs for sum and difference internal resonances.Item Enhancement of hole mobility in high-rate reactively sputtered Cu2O thin films induced by laser thermal annealing(2024) Rezek, Jiří; Kučera, Martin; Kozák, Tomáš; Čerstvý, Radomír; Franc, Aleš; Baroch, PavelIn presented work, a reactive high-power impulse magnetron sputtering (r-HiPIMS) was used for high-rate deposition (≈ 170 nm/min) of Cu2O films. Films were deposited on a standard soda-lime glass (SLG) substrate at a temperature of 190 °C. As-deposited films exhibit poor hole mobility in the orders of ≈ 1 cm2/Vs. We have systematically studied the effect of laser thermal annealing (LTA) procedure performed using high-power infrared laser under different laser parameters (number of pulses, length of the pulse). We have found, LTA procedure could significantly enhance the hole mobility (up to 24 cm2/Vs in our case). We have also fitted the results of a temperature-dependent Hall measurement to clarify the mechanism of the reported increase in hole mobility. Moreover, we have discussed the effect of the LTA procedure on microstructure (crystallinity, surface morphology) and on the value of optical band gap.Item On a Hamiltonian elliptic system with concave and convex nonlinearities(2023) Agudelo Rico, Oscar Iván; Ruf, Bernhard; Veléz, CarlosIn this work we study a Hamiltonian elliptic system of equations with Dirichlet boundary condition and with non-linearities that are concave near the origin and are convex and superlinear at infinity. The concavity of the non-linearities depends on non-negative parameters lambda and mu and we provide regions for the pairs (lambda, mu) guaranteeing existence and non-existence of nonnegative solutions. This work is inspired by the seminal work for the single equation done by Ambrosetti, Brezis and Cerami in 1994.Item Homogenized model of peristaltic deformation driven flows in piezoelectric porous media(2024) Rohan, Eduard; Lukeš, VladimírThe paper presents a new type of weakly nonlinear two-scale model of controllable periodic porous piezoelectric structures saturated by Newtonian fluids. The flow is propelled by peristaltic deformation of microchannels which is induced due to piezoelectric segments embedded in the microstructure and locally actuated by voltage waves. The homogenization is employed to derive a macroscopic model of the poroelastic medium with effective parameters modified by piezoelectric properties of the skeleton. To capture the peristaltic pumping, the nonlinearity associated with deforming configuration must be respected. In the macroscopic model, this nonlinearity is introduced through homogenized coefficients depending on the deforming micro-configurations. For this, linear expansions based on the sensitivity analysis of the homogenized coefficients with respect to deformation induced by the macroscopic quantities are employed. This enables to avoid the two-scale tight coupling of the macro- and microproblems otherwise needed in nonlinear problems. The derived reduced-order model is implemented and verified using direct numerical simulations of the periodic heterogeneous medium. Numerical results demonstrate the peristaltic driven fluid propulsion in response to the electric actuation and the efficiency of the proposed treatment of the nonlinearity. The paper shows new perspectives in homogenization-based computationally efficient modelling of weakly nonlinear problems where continuum microstructures are perturbed by coupled fields.Item Flexible hard nanocoatings with high thermal stability(2024) Musil, Jindřich; Kos, Šimon; Baroch, PavelOur investigation concentrated on two basic challenges in the field of hard protective coatings prepared by magnetron sputtering: (1) Toughness and resistance to cracking (TRC) and (2) Flexible Hard Coatings with high thermal stability (TS). While results of the investigation of TRC were already comprehensively published in a chapter of the book Thin Films and Coatings, Toughening and Toughness Characterization, results of the investigation of TS have not been comprehensively published yet. These results are described in detail in this new article. The choice and presentation of the material in this article reflect the interests and perspectives of the principal author.The paper reports on the enhanced hardness and thermal stability of nanocomposite coatings, and the formation of the X-ray amorphous coatings with thermal stability and oxidation resistance above 1000 °C and of flexible hard coatings. Reported results can be used in the development of flexible ceramic coatings, in the surface strengthening of brittle materials, and in the prevention of (i) cracking of the functional coatings and (ii) the crack formation on the surface of bent materials. It is shown that the energy delivered into the flexible hard coatings during their growth plays a key role in their formation. Special attention is devoted to the formation of coatings with unique, fully reproducible properties and to new technology based on highly non-equilibrium processes at the atomic level. Special attention is also devoted to the speed of cooling of hard nanocoatings and its effect on their thermal stability at high temperatures reaching up to 2000 °C. The key role of interdisciplinary insights in the development of new advanced hard coatings is also discussed. In conclusion trends in the next development of new advanced hard nanocoatings are indicated.Item Gamma-flash generation in multi-petawatt laser–matter interactions(2023) Hadjisolomou, Prokopis; Jeong, Tae Moon; Kolenatý, David; Macleod, Alexander J.; Olšovcová, Veronika; Versaci, Roberto; Ridgers, Christopher Paul; Bulanov, Sergei V.The progressive development of high-power lasers over the last several decades enables the study of γ-photon generation when an intense laser beam interacts with matter, mainly via inverse Compton scattering at the high-intensity limit. γ-ray flashes are a phenomenon of broad interest, drawing the attention of researchers working on topics ranging from cosmological scales to elementary particle scales. Over the last few years, a plethora of studies have predicted extremely high laser energy to γ-photon energy conversion using various target and/or laser field configurations. The aim of this article is to discuss several recently proposed γ-ray flash generation schemes, as a guide for upcoming γ-photon related experiments and for further evolution of the presently available theoretical schemes.Item Mechanical interaction of bowed hexagonal fuel assemblies in PWR core(2023) Dyk, Štěpán; Zeman, Vladimír; Hlaváč, ZdeněkDue to the lateral and axial forces or irradiation growth, nuclear fuel assemblies (FAs) are prone to bowing during reactor operation. In some cases, FAs can even mutually contact. The paper focuses on the modelling of the bowed hexagonal FAs in mutual interaction considering complex operation conditions given by coolant pressure pulsations. The multi-stage modelling is used to describe the behaviour of the chosen FA in the reactor core. A possible contact between FA and reactor core–shell is also taken into account. All the analyses are performed with respect to the fuel burn-up that significantly influences FAs dynamic behaviour.Item Vibration monitoring of axial fan blades(2023) Jakl, Jan; Liška, JindřichThis paper deals with the blade tip timing (BTT) method of blade vibration measurement used for condition monitoringof variable pitch axial fan blades. The efficiency of axial fans with variable pitch is controlled by adjustment of the bladeangle. BTT analysis may lead to incorrect results when only one BTT sensor is used for blade time of arrival (TOA)measurement without considering the current blade angle and the use of at least two sensors. The method of bladevibration monitoring based on multiple BTT sensors is described in this paper. The suitability of the designed method isverified by a case study.Item MuTr: Multi-Stage Transformer for Hand Pose Estimation from Full-Scene Depth Image(2023) Kanis, Jakub; Gruber, Ivan; Krňoul, Zdeněk; Boháček, Matyáš; Straka, Jakub; Hrúz, MarekThis work presents a novel transformer-based method for hand pose estimation—DePOTR. We test the DePOTR method on four benchmark datasets, where DePOTR outperforms other transformer-based methods while achieving results on par with other state-of-the-art methods. To further demonstrate the strength of DePOTR, we propose a novel multi-stage approach from full-scene depth image—MuTr. MuTr removes the necessity of having two different models in the hand pose estimation pipeline—one for hand localization and one for pose estimation—while maintaining promising results. To the best of our knowledge, this is the first successful attempt to use the same model architecture in standard and simultaneously in full-scene image setup while achieving competitive results in both of them. On the NYU dataset, DePOTR and MuTr reach precision equal to 7.85 mm and 8.71 mm, respectively.Item Robustness and sensitivity analyses of rough Volterra stochastic volatility models(2023) Matas, Jan; Pospíšil, JanIn this paper, we analyze the robustness and sensitivity of various continuous-time rough Volterra stochastic volatility models in relation to the process of market calibration. Model robustness is examined from two perspectives: the sensitivity of option price estimates and the sensitivity of parameter estimates to changes in the option data structure. The following sensitivity analysis consists of statistical tests to determine whether a given studied model is sensitive to changes in the option data structure based on the distribution of parameter estimates. Empirical study is performed on a data set consisting of Apple Inc. equity options traded on four different days in April and May 2015. In particular, the results for RFSV, rBergomi and $\alpha$RFSV models are provided and compared to the results for Heston, Bates, and AFSVJD models.Item World Marrow Donor Association guidelines for the reporting of novel HLA alleles(2023) Hofmann, Jan A; Bochtler, Werner; Robinson, James; Sauter, Jürgen; Askar, Medhat; Houdová, Lucie; Melchers, Mark; Schmidt, Alexander H; Spierings, Eric; Urban, Christine; Venter, Alicia; Maiers, Martin; Marsh, Steven G E; Eberhard, Hans-PeterThe guidelines for the implementation and reporting of HLA nomenclature for the World Marrow Donor Association have served as a reliable standard for communication of HLA data in the hematopoietic cell transplantation process. Wider use of next-generation sequencing made a special provision of the guidelines increasingly pertinent: how to communicate novel HLA alleles. Novel alleles need to be recognized by the WHO Nomenclature Committee for Factors of the HLA system to obtain official allele designations. Until then they have to be handled according to the specific rules. Leaving the actual rules basically unchanged we give some advice on how to communicate novel alleles to best facilitate the search process for cases where novel alleles are identified on donor or patient side.Item Automated Drone Battery Management System-Droneport: Technical Overview(2023) Bláha, Lukáš; Severa, Ondřej; Goubej, Martin; Myslivec, Tomáš; Reitinger, JanThe popularity of using vertical take-off and landing unmanned aerial systems continues to rise. Although the use of these devices seems to be almost limitless, the main drawback is still the battery capacity and the need to replace or recharge it several times per hour. This article provides a technical overview of the development of an experimental mechatronic system for automatic drone battery management called Droneport. It was developed as a system with a landing platform, automatic battery exchange and recharging outside the drone, allowing a quick return to the mission. The first part presents its mechanical design, installed instrumentation and software environment. The next part is devoted to the description of the individual hardware components, highlighting the specific problems that had to be solved to optimize size, weight and robustness requirements. The final section summarizes our observations regarding the contribution of this tool to the autonomy of drones or UAVs in general.Item Half-arc-transitive graphs of arbitrarily large girth(2024) Nedela, Roman; Širáň, JozefWe show that for any k ≥ 2 and g ≥ 3 there are infinitely many finite 2k-valent halfarc-transitive graphs of girth g with cyclic vertex-stabiliser of order k.Item Fluid Flow Modelling Using Physics-Informed Convolutional Neural Network in Parametrised Domains(2023) Bublík, Ondřej; Heidler, Václav; Pecka, Aleš; Vimmr, JanWe design and implement a physics-informed convolutional neural network (CNN) to solve fluid flow problems on a parametrised domain. The goal is to compare the effectiveness of training based solely on CFD-generated training data with purely physics-informed training and training based on a combination of both. We consider the problem of incompressible fluid flow in a convergent-divergent channel with variable wall shape. A speciality of the designed neural network is the incorporation of the boundary condition directly in the CNN. A physics-informed CNN that uses a non-Cartesian mesh poses a challenge when evaluating partial derivatives. We propose a gradient layer that pproximates the first and second partial derivatives by finite differences using Lagrange interpolation. Our analysis shows that the convergence of purely physics-informed training is slow. However, using a small training dataset in combination with physics-informed training can achieve results comparable to physics-uninformed training with a considerably larger training dataset.Item T5G2P: Text-to-Text Transfer Transformer Based Grapheme-to-Phoneme Conversion(2024) Řezáčková, Markéta; Tihelka, Daniel; Matoušek, JindřichThe present paper explores the use of several deep neural network architectures to carry out a grapheme-to-phoneme (G2P) conversion, aiming to find a universal and language-independent approach to the task. The models explored are trained on whole sentences in order to automatically capture cross-word context (such as voicedness assimilation) if it exists in the given language. Four different languages, English, Czech, Russian, and German, were chosen due to their different nature and requirements for the G2P task. Ultimately, the Text-to-Text Transfer Transformer (T5) based model achieved very high conversion accuracy on all the tested languages. Also, it exceeded the accuracy reached by a similar system, when trained on a public LibriSpeech database.Item On unipolar and bipolar HiPIMS pulse configurations to enhance energy flux to insulating surfaces(2025) Farahani, Mina; Kozák, Tomáš; Pajdarová, Andrea Dagmar; Tölg, Tomáš; Čapek, JiříThis study explores the effect of various HiPIMS pulse configurations (unipolar, bipolar, chopped unipolar, and chopped bipolar) to increase energy flux to an insulated surface (e.g., substrate or growing film). The chopped bipolar HiPIMS configuration, featuring several short positive pulses replacing a single long positive pulse, is introduced, and the total energy fluxes are subsequently measured using a passive thermal probe. Moreover, the effect of the probe's capacitance with respect to the ground is systematically investigated by connecting an external capacitor. For surface capacitance in tens of nF, chopping the positive pulse in bipolar HiPIMS effectively increases the energy delivered to the film by discharging the surface in the off-times.Item Angle Combinations in Tilings of the Sphere by Angle Congruent Pentagons(2025) Luk, Hoi Ping; Yan, MinWe develop a systematic method for deriving the angle combinations at all vertices in an edge-to-edge tiling of the sphere by pentagons with the same five angles. The method provides an effective and necessary tool in solving many tiling problems about pentagonal tilings of the sphere. More significantly, we determine all the angle combinations that allow free and continuous choices of two angle values.
