Institute of Theoretical and Applied Mechanics

Bacterial cellulose (BC) is a promising biomaterial for medical and biotechnological applications. However, microbial contaminants and their metabolic residues remain a critical limitation for its clinical use. Many of the BC purity tests are labor-intensive and time-consuming. This study investigates the feasibility of using laser-induced fluorescence (LIF) spectroscopy for monitoring microbial contamination in BC. BC samples were obtained from a Medusomyces gisevii consortium and subjected to various purification protocols (alkaline, detergent and oxidative treatments). LIF spectra were recorded across 220-290nm excitation wavelengths and analyzed chemometrically. For better interpretation of the results, the same samples were examined by laser scanning confocal microscopy (LSM). The results reveal that both native and treated BC samples exhibit fluorescence features associated with tryptophan and tyrosine, indicative of microbial residues. Treatment with NaOH effectively reduced tryptophan-associated signals, while hydrogen peroxide diminished tyrosine-related fluorescence. None of the purification strategies completely eliminated these signals. A good correlation between the LIF and the more labor-consuming LSM data was observed. LIF showed the capability of rapid and reliable differentiation between treatment variants and provided spectral fingerprints linked to residual contamination. Future work may focus on standardizing LIF-based diagnostic protocols and integrating them into biotechnological workflows for contamination monitoring.

The processing of experimental dependences for pitching vs. time curve for free oscillations of a cone with the attached hemisphere rear part gives the pitching moment coefficients, equivalent coefficients for pitch damping, spectral coefficients for pitching angle and pitching moment coefficient for self-induced oscillations flow mode. It was shown that a dependency of pitch damping vs. oscillation amplitude presents a hyperbolic behavior. The frequency spectrum for pitching and pitching moment coefficient demonstrates a domination of one frequency. There is no hysteresis in a dependency of the pitching moment on the attack angle for self-induced oscillations.

The study is aimed at increasing the efficiency of the direct numerical solution of the Boltzmann equation. An approach is proposed to speed up the collision integral computation by means of taking into account only those nodes of the velocity grid that make significant contributions to calculating the distribution function moment of a prescribed order. The algorithm retains the conservatism of the numerical scheme used to calculate the collision integral. The algorithm is tested on the Bobylev–Krook–Wu analytical solution. The tests show that more than a 20-fold speedup of the collision integral computation can be provided with acceptable solution accuracy being retained. A problem of the plane shock wave structure is solved; the numerical solutions obtained with and without the node selection algorithm are found to be in good agreement. The most optimal parameters of the node selection algorithm are determined. The problem of decay of a discontinuity in a microchannel is simulated; the results are found to agree well with the data obtained by the direct simulation Monte Carlo method. The algorithm provides a speedup by a factor of 21.75.

In this work, an in-situ study of phase transitions in low-carbon steel is presented. The phase changes were monitored by the transmission energy dispersive X-ray diffraction technique during the heating, annealing and quenching cycle of the sample under standard laboratory conditions. During energy dispersive X-ray diffraction, the sample volume was transmitted with a pencil beam generated by a standard polychromatic X-ray tube without any spectral filtering. Two-dimensional polychromatic diffraction images were acquired by a Timepix3 pixelated detector. This detector is capable of achieving a throughput of up to 38Mhits/s in continuous stream mode using USB 3.0 interface. For each detected photon, its position is known with an accuracy of 55µm in the detector plane and its energy with a resolution of 4keV at 60keV. The recorded polychromatic data is then recomputed to get the equivalent monochromatic XRD pattern that would be produced using a monochromatic X-ray source. Thanks to the 90kV voltage potential of the X-ray tube, the polychromatic pencil beam was able to pass through a highly attenuating sample made of 1.5 thick steel sheet. In addition, by utilizing the entire X-ray spectrum, the pencil beam has a sufficiently high brilliance to obtain XRD patterns rapidly enough to investigate relatively fast processes with temporal resolution of 10s. It made to possible to analyze the phase transitions in a polycrystalline sample during its temperature treatment under standard laboratory conditions.

A fiber laser and post heat treatment have been used to produce high-strength laser welded joints of Al–Cu–Li alloy. The structure–phase state of the weld seams has been studied by synchrotron X-ray diffraction in transmission geometry, and data have been obtained on the structural and phase compositions of the weld seams as three-dimensional objects. We have studied the effect of heat treatment on the fatigue resistance of laser welded joints of aluminum–lithium alloy of the Al–Cu–Li system and performed static mechanical tests of the laser welded joints at a normal, an elevated, and a reduced temperature in order to assess the strength and deformation properties of the Al–Cu–Li system. Welded joints of the Al–Cu–Li system have been dynamically tested in impact toughness by the Charpy method.

Pressure fluctuations on a semi-infinite flat obstacle impinged by a supersonic underexpanded jet are studied. The nozzle exit Mach number is $${{\text{M}}_a} = 1$$ , the jet off-design parameter $$n = 2.1$$ , and the distance from the nozzle exit to the obstacle $$h{\text{/}}{D_a} = 2{-} 15$$ ( $${D_a}$$ is the nozzle exit diameter). It is shown that tangential injection of six microjets can lead to a significant reduction in pressure fluctuations on the obstacle. The mass flow rate through all microjets is 0.3% of the mass flow rate through the main jet. A self-oscillating mass-flow-rate regime of jet–obstacle interaction is found to occur at $$h{\text{/}}{D_a} < 3$$ , a self-oscillating regime with acoustic feedback at $$3 < h{\text{/}}{D_a} < 8$$ , and a turbulent aperiodic interaction regime at $$h{\text{/}}{D_a} > 8$$ .

The paper presents results of experimental investigation on the stability of the supersonic boundary layer in relation to natural disturbances of the first vorticity mode. The surface of the flat plate model was equipped with grooves (slots, rectangular elongated cavities) of small depth and various angles of their orientation 0 and 60°. Wind tunnel experiments have been performed at Mach number 2. It was found that with decrease of orientation angle from 60° to 0 the maximum spatial amplification rate of disturbances is also decreased. For zero angle this growth rate becomes smaller in comparison with a smooth plate. The obtained results show that the first mode disturbances in supersonic boundary layer can be stabilized by streamwise grooves of a small depth. However, presence of surface grooves of the same depth with orientation angle 60° leads to noticeable flow destabilization.