Home – Home – Jornals – Thermophysics and Aeromechanics 2025 number 4

The results of direct numerical simulation (DNS) of the influence of a horizontal surface temperature gradient on the dynamics of a thermal convective flux are presented. A series of simulations was performed at moderate Rayleigh numbers (10⁵ - 10⁸). The computational domain was rectangular and its two horizontal walls had a specified average temperature difference, leading to the formation of a free-convective flow. The temperature on the lower wall was nonuniformly distributed along one of the horizontal axes. Convective cells formed due to the vertical density gradient were subject to a weak horizontal temperature gradient, which led to their deformation and transfer toward the central axis of the computational domain. As a result, intense coherent horizontal oscillations in the position of the central upflow at a low frequency were observed. This frequency depended weakly on the Rayleigh number, and the oscillation amplitude increased with its increase. This low-frequency oscillation effect is not evident in two-dimensional modeling and arises from the interaction of adjacent three-dimensional convective cells entering the central upflow. The authors propose an asymptotic estimate for the period of these oscillations. It is shown that the oscillations do not disappear with increasing Rayleigh number, suggesting their influence on heat and mass transfer in natural flows.

The main goal of the study is an experimental investigation of the mechanism of weakly nonlinear interaction of low-amplitude unsteady modes of crossflow instability in the boundary layer on a 35-degree swept wing with steady vortices of crossflow instability. The dominating factor in the flow under study is crossflow instability, while the Tollmien-Schlichting instability is suppressed by the favorable pressure gradient. High-amplitude steady distur-bances (up to 20% at the end of the measurement region) are excited by a surface roughness element. Controlled low-amplitude unsteady disturbances are generated in the boundary layer by a source of disturbances located upstream from the roughness element. Their amplitude in the main interaction region does not exceed 1%. The source excites quasi-two-dimensional (spanwise-uniform) waves at low frequencies corresponding to the primary crossflow instability region. The results of hot-wire measurements show that the characteristics of the mean flow, as well as of steady and unsteady disturbances are independent of the disturbance amplitude. However, the evolution of unsteady disturbances is strongly affected by the presence of vortices. The excited quasi-two-dimensional instability waves rapidly decay in the downstream direction, while the forming and growing (in a certain range of transverse wave numbers) steady vortices transform two-dimensional waves to essentially three-dimensional waves with the transverse wave spectrum corresponding to the most rapidly growing crossflow instability modes. This transformation does not occur locally, in the near field of the surface roughness, but is distributed in the streamwise direction. The amplitudes of steady disturbances grow almost exponentially, with the growth rate depending on the transverse wave number in a manner typical for crossflow instability modes. The growth of the amplitudes of unsteady modes exhibits a more complicated, sometimes nonmonotonic character owing to their nonlinear interaction. It is found that the amplitudes of unsteady disturbances of all frequencies in the plane normal to the flow and the wall are strongly localized in regions of high values of the mean flow velocity gradient over the model span. An essentially three-dimensional physical mechanism of weakly nonlinear transformation of quasi-two-dimensional wave disturbances to three-dimensional waves by high-amplitude steady instability vortices is proposed, which is similar to the lift-up effect used previously to explain the growth of streaky structures in two-dimensional boundary layers.

The experimental results on the swirling flow structure behind a vane swirler in a smooth tube at an axial flow Reynolds number varying within Re = 240 - 1640 are presented. A change in the degree of flow swirl along the tube length is analyzed as a function of the Reynolds number. The main patterns in the evolution of the profiles of longitudinal and circumferential velocity components and the distribution of the mean square pulsations of the longitudinal velocity component with increasing distance from the swirler are determined. It is shown that at Re = 1640, flow instability develops in the tube, which is a consequence of the formation of a reverse flow region on the tube axis in the immediate vicinity of the swirler. The influence of flow swirl on the appearance of signs of a local laminar-tur-bulent transition in vicinity of the tube axis and near its wall is analyzed (a sharp increase in the mean square velocity pulsations with increasing Reynolds number and the appearance of intermittency in the flow velocity oscillograms). It has been established that in the near-wall region, local turbulence of the flow is caused by the interaction between the swirler vane wakes and the wall.

The unfluence of the mass flow rate ratio of the input flows of distilled water on the efficiency of liquid mixing within a T-shaped microchannel was studied using numerical and experimental modeling. The model was verified by comparing the calculated results with experimental data. High-resolution digital tracer visualization was used to measure the flow velocity characteristics. Computer modeling was used to obtain and describe the flow structure and the dynamics of flow pattern changes at unequal mass flow rates. The possibility of increasing the efficiency of liquid mixing by varying the mass flow rate ratio is demonstrated.

Methodical issues associated with investigations of the laminar-turbulent transition in subsonic boundary layers with the use of video filming in the infrared range are considered. A method of estimating the wall friction based on the analysis of the temperature behavior of the heated model surface is proposed. An improved method of finding the lines of transition beginning and end is proposed.

The paper considers a possibility of controlling the structure of a fluoropolymer coating by diluting the precursor gas with an inert gas during synthesis by the Hot Wire Chemical Vapor Deposition method (Hot Wire CVD). Dilution of the precursor gas significantly affects the morphology of the formed coating and alters the coating wettability and the growth rate.

The dynamics of a slug flow in immiscible liquids remains incompletely understood due to the complex effects of channel geometry and the properties of the working fluids. The presence of surfactants in the system further complicates the problem. This work deals with the effect of an water solution of Tween 20 surfactant on the dynamics of a slug flow in a curved microchannel. Situations with surfactant concentrations below and above the critical concentration of micelle formation are considered. Regime maps are plotted, and three regimes of water phase dispersion are identified. It is shown that increasing the surfactant concentration leads to stabilization of slug interfaces in straight sections, while the probability of slug disintegration in curved sections increases due to reduced interfacial tension and intense mixing of the surfactant within the slugs. The decisive influence of the surfactant concentration relative to the critical concentration of micelle formation on the dynamics of microdroplet detachment processes in straight and curved sections of the channel is revealed.

The paper describes experimental results on the action of weak shock waves on the development of controlled disturbances in a supersonic boundary layer on a flat plate at the Mach number of 2.0, which are obtained by measurements performed by a constant-temperature hot-wire anemometer. A two-dimensional surface roughness element 150×7×0.13 mm in size, mounted on the side wall of the test section of the T-325 wind tunnel based at ITAM SB RAS, generated a pair of weak shock waves in the free stream. Controlled disturbances are inserted into the flow by a high-frequency glow discharge in a chamber inside the model. Owing to interaction of the pair of weak shock waves with the leading edge of the flat plate, a steady wake is formed in the boundary layer, where the evolution of controlled disturbances occurs. A weakly nonlinear regime of evolution of controlled disturbances is studied under the conditions of a homogeneous boundary layer on the flat plate and a boundary layer distorted by the wake. The wave characteristics of disturbances are analyzed. A new method of estimating the dispersion relation is proposed. The experimental data are compared with the numerical results. Numerical simulations are performed based on the liner stability theory under the conditions of a homogeneous boundary layer.

Investigation results on combustion of methane and ammonia diluted with hydrogen are presented. The effect of hydrogen addition on the physicochemical properties, flame flashback conditions, and boundaries of stable combustion is determined over a wide range of fuel-air equivalence ratios (from lean to rich mixtures).

The paper describes the results of an experimental study of a supersonic transverse flow around a cylindrical body. The complicated flow structure being formed is visualized by means of shadowgraphy and by seeding the flow with particles. The experiments are performed to study the classical transverse flow around the cylinder and the flow with ejection of a gas jet from the cylinder surface. Several locations of the hole over a circumference in the cylinder midsection are considered (the aspect ratio of the cylinder is λ = 3.2). The influence of the hole location on the flow structure formed due to supersonic interaction of the incident flow and the jet is described. The study yields new experimental data on the flow structure formed due to supersonic interaction of the incident flow and the jet for different locations of the hole. The flow patterns are provided and analyzed.

The paper describes a method for the synthesis of titanium carbide films using magnetron sputtering in a small-anode discharge. As a result, the synthesized films exhibit a density lower than that for the films produced by the stan-dard method of magnetron sputtering. The generate coating were subjected to annealing in oxygen-bearing or nitrogen atmosphere. The resulting films were studied using several characteristics and methods: volt-ampere characteristics of the magnetron discharge and the small anode, images from scanning electron microscopy, x-ray phase analysis, resistometry and spectral measurements for optical transparency. Our study demonstrated that the samples synthesized in a setup with an anode are thicker than those synthesized without anode. They also exhibit a smaller size of crystallites, lower transparency, forbidden band and a higher resistance. Taking into account the unchanged regime of magnetron operation, this indicates a reduced film density.

The paper discusses computational and experimental methods for evaluation the thermo-erosion characteristics of launch escape propulsion subsystems (LEPS) equipped with a thermal protection coating under natural operating conditions. Combustion mixture flow was simulated using a commercial CFD software. Based on the obtained data, heat flux distribution across the LEPS surface was determined. Firing tests conditions of thermal protection materials are also described. A gas-dynamic tunnel was used to reproduce the natural operating conditions. Quantitative characteristics of thermo-erosion resistance for materials were calculated based on the experimental results with corrections for the period of non-stationary coating heating.

It seems to be very promising to study the fundamental processes occurring during the mechanochemical production and subsequent combustion of composite powder fuels consisting of coal and waste of the wood, pulp and paper, and agricultural industries. The mechanisms of combustion of composite fuel particles made of lignocellulose and coal remain understudied. This paper examines the processes of ignition and flame combustion of composite fuel samples and mixtures obtained from sawdust and anthracite.

Methods of numerical simulation were applied to studying the interaction of high-temperature air jets arranged in a mixing chamber of a triple-jet plasma-chemical direct-flow reactor. Computations produce the 3D distributions for velocity and temperature in the mixing chamber. Simulation indicates the flow circulation zones and the spatial temperature distributions on the inner walls of the reactor. The study offers the data on the heat flux for inner walls depending on the existence/absence of a thermal protection coating on the inner walls. This approach can be used for numerical simulation for multiple-jet reactors with different designs and purpose.

The density and thermal expansion of a liquid rubidium-lead alloy containing 50 at. % Pb were investigated for the first time by the gamma-ray attenuation technique in the temperature range from the liquidus to ~1000 K. It was found that the molar volume of the Rb₅₀Pb₅₀ melt is 30 % less than the molar volume of an ideal solution of the same composition. The temperature dependence of the melt density is highly nonlinear, therefore the volumetric coefficient of thermal expansion decreases by more than 1.5 times with an increase in temperature from 864 to 1010 K. Based on modern concepts of the structure of liquid metal systems with partially ionic nature of interatomic interaction, the behavior patterns of the thermal properties of a liquid alloy are briefly analyzed.