CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)
Not a member yet
4735 research outputs found
Sort by
Mass growth of massive quiescent galaxies in the Illustris TNG simulation
Full text linkInteractions between galaxies could potentially lead to increased rates of star formation within these galaxies, potentially leading to the occurrence of multiple core-collapse supernovae and the ejection of hot gas. Numerical simulations are crucial for understanding the formation and evolution of galaxies. Using the IllustrisTNG cosmological simulation, we show how the massive quiescent galaxies evolve in cosmic times. We use the merger tree from the TNG300-1 simulation to demonstrate the impact of dwarf galaxies on the mass augmentation of these galaxies during the early Universe. We made a galaxy sample with restrictions on the specific star formation rate log(sSFR) and mass (M). Most of the mergers have a <1:1 000 ratio to the host galaxy and 5–25% of the falling material is connected to the merger events. These results show that dwarf galaxies could play a significant role in the growth of these massive galaxies
The spatial distribution of GRBs
Full text linkWe analysed the different aspects of the spatial distribution of 542 Gamma-Ray Bursts with precisely determined positions and spectroscopic redshifts. The data were divided according to the origin of the redshift (afterglow or host galaxy). The yearly rate of afterglow and host-based redshift observations are different, with only a few host observations in the recent years. Since the launch of the Swift, the rate of afterglow observations fall exponentially by 50 % in 15 years, potentially affecting all planned GRB missions. We also analysed the rest-frame T90 values from the Swift BAT and FERMI GBM catalogues with the redshift data. The host- and afterglow-based points are separated in the redshift range due to observational effects, but no direct distinction could be made between the rest-frame T90 values. The correlation analysis between the GRB redshift and sky position shows that the GRB distribution could be factorised into a separate sky and radial components
Review: the landscape of gravitational wave astronomy
Full text linkThe direct detection of gravitational waves from a binary black hole merger has opened a new window in observational astronomy. The first three observing runs of the LIGO/Virgo groundbased interferometers have produced a broad range of scientific results, including the first observationsof a binary neutron star merger and a neutron star-black hole merger. The observations include some exceptional events and other mergers reported in the GWTC-1, GWTC-2, GWTC-2.1, GWTC-3 catalogues, that have allowed tests of general relativity and studies of black hole and neutron star populations. The paper is a concise review of ground-based gravitational wave astronomy and related multi-messenger observations over the electromagnetic spectrum and the neutrino domain. Since the spectrum of gravitational waves extends over a broad frequency range, other techniques for gravitational wave detection outside the sensitivity band of ground-based interferometers will also be discussed
Experimental studies of microarcs between copper electrodes in air
Full text linkLow direct current arcs generated between Cu electrodes in atmospheric pressure air are investigated in relation to low-voltage switching. Electrical and optical measurements and high-speed imaging give insight into the dynamics of the arc. Side-on spectroscopy with a grating spectrometer and suitable optical imaging delivers spatially resolved spectral emission coefficients of three emission lines of the Cu atom. The experimental findings are compared with results from modelling
Global sensitivity analysis of polymer ablation modeling for magnetohydrodynamic fault arc models
Full text linkMagnetohydrodynamic fault arc model calibration presents a key challenge. The current literature lacks efficient approaches to calibrate and validate these models. A global sensitivity analysis of the ablation model is conducted to reduce the number of calibration parameters, using the Elementary Effect and Morris method. By analyzing sensitivities of calibration parameters, combined with the model outputs\u27 standard deviation uncertainty quantification is achieved. The results help reduce uncertain parameters and improve calibration efficiency
Experimental characterization of pre-strike arc duration of a MV-LBS using N2/CO2: influence of closing speed, pressure and mixing ratio
Full text linkShort-circuit current making in a medium voltage load-break switch using N2/CO2 is expected to result in longer pre-strike arcs compared to SF6, due to lower dielectric strength, leading to increased contact degradation. Measures to limit impact of pre-strike arc on the contact system are needed. This paper aims to experimentally investigate the influence of closing speed, filling pressure and mixing ratio on pre-strike arc duration towards increasing contact system life time
Numerical characterization of arc-induced turbulence in the vicinity of current zero
Full text linkThis work presents a numerical investigation of the final physical mechanism responsible for the thermal interruption of the plasma arc in high-voltage circuit breakers, i.e. turbulence, and its interaction with the arc itself in the vicinity of current zero (CZ). In this context, an experiment in which an arc is ignited between two stationary electrode rods, and it is maintained steadily inside a Teflon de Laval nozzle by applying a 2 kA constant current is reproduced. The results show that the three-dimensional instabilities observed experimentally to develop in the arc boundary layer around CZ are only a product of the interaction between the hot arc and its surrounding cold flow, and that no magnetic effects nor magnetic-related instabilities are required for turbulence to develop and grow
Multi-temperature thermophysical properties and quenching characteristics of arc plasmas in SF6 and its alternative
Full text linkGaseous switching arc plasmas near current zero typically deviate from LTE and are often modeled using a two-temperature approach. However, in non-equilibrium plasmas, different energy states may follow separate Boltzmann distributions, each with distinct temperatures. This study calculates the thermophysical properties of SF6 and C4F7N arc plasmas by explicitly considering translational, rotational, vibrational, and electronic excitation temperatures. The results reveal that compared to the conventional two-temperature model (distinguishing heavy species and electrons), the multi-temperature framework leads to measurable changes in plasma properties. The C4F7N mixture is more sensitive to multi-temperature effects than SF6. Additionally, the two-temperature simulation model may slightly underestimate the plasma’s non-equilibrium degree.
Impact of process parameters on the mechanical and surface properties of AA6082
Full text linkThis study investigates the effects of chemical composition, heat treatment, and processing parameters on the mechanical properties, microstructure, and surface quality of AA6082 aluminium alloy. Three castings with varying Mg and Si content were subjected to homogenisation at 580 °C for 8 and 10 hours, followed by extrusion and artificial ageing at 180 °C for 6, 8, and 10 hours, yielding 18 samples. Tensile testing, microstructural analysis, SEM examination, and anodic oxidation (18 V, 26–32 minutes) were conducted to assess performance. Results revealed that the highest tensile strength (361MPa) and yield strength (332 MPa) were achieved with 8-hour homogenisation and ageing (B2 sample), attributed to optimal β′′ precipitate formation, finer grain sizes, and a favourable composition (Mg: 0.92 %, Si: 1.09 %). The surface quality and hardness were enhanced by water shock and nitrogen gas cooling during extrusion, while the 10-hour homogenisation reduced porosity but increased the risk of overageing. SEM analysis confirmed ductile fracture in high-strength samples, and anodising produced thicker (11.7 μm), glossier coatings with longer processing times. These findings highlight the critical interplay of heat treatment duration, cooling strategies, and surface finishing in optimising AA6082 for industrial applications, with implications for process design and alloy development
Comparison of cavitation erosion of NiCrBSi and AISI 316L coatings deposited by powder plasma transferred Arc welding
Full text linkThis study carefully compares the effect of microstructure and hardness on the cavitation wear resistance of PPTA (Powder Plasma Transferred Arc) deposited coatings. Deposits were made on a substrate of S235JR structural steel. Two types of feedstock powder were used: material with the chemical composition of AISI 316L stainless steel, and a nickel-based, self-fluxing powder type NiCrBSi. This study involved conducting cavitation erosion tests on a vibratory test rig in accordance with the ASTM G32 standard, using the stationary specimen method. Metallographic investigations confirmed the presence of austenitic dendrites and delta ferrite precipitations in the microstructure of AISI 316L and the presence of hard carbides and borides within the nickel-based matrix of NiCrBSi hardfacing. It can be seen that the AISI 316L coating, having a much lower hardness in the range of 250–280 HV1, achieved four times poorer cavitation erosion resistance compared to the NiCrBSi coating, which has a hardness in the range of 820–890 HV1. Following the erosion testing, the AISI 316L stainless steel exhibited a mean depth of erosion of MDE6h = 28.8 μm, whereas the NiCrBSi hardfacing exhibited a mean depth of erosion of MDE6h = 7.1 μm. Moreover, NiCrBSi hardfacing exhibits a higher cavitation erosion resistance than the stainless steel coating, with erosion rates of 2.59 mgh−1 (1.60 μmh−1) and 8.11 mgh−1 (5.28 μmh−1), respectively. In the case of different types of overlay material, such as stainless steel and NiCrBSi coatings, the higher hardness and fine, hard particle-rich microstructure improves the cavitation erosion resistance of PPTA overlays