Home – Home – Jornals – Thermophysics and Aeromechanics 2025 number 5

An experimental study of the primary and secondary unsteady instabilities of the boundary layer of a swept wing was conducted under conditions of dominant primary crossflow instability in the presence of a localized three-dimensional roughness element on the streamlined surface. Measurements were performed using a hot-wire anemometer in a low-turbulence wind tunnel at low subsonic free-stream velocity under conditions of uncontrolled ("natural") unsteady disturbances. Four types of amplified unsteady boundary layer disturbances were detected and studied in detail: low-frequency disturbances associated with primary flow instability, mid-frequency disturbances (type III secondary instability), and high-frequency disturbances (types I and II secondary instability). The properties of these disturbances were studied in detail, and an analysis of the position and shape of their localization regions in the plane normal to the wall and flow was performed relative to the layers of strong shear of the longitudinal component of the mean flow velocity along the spanwise and normal to the wall. The complex nature of the secondary disturbances is demonstrated, which is difficult to explain by the simplified concepts of the existence of z- and y-modes of secondary instability considered in previous studies.

Based on a system of gas dynamics equations for a mixture of vibrationally excited chemically reacting molecular gases, the effect of a carbon dioxide additive on the stability of a hypersonic boundary layer of neutral nitrogen on a plate was studied. Calculations were performed for five mixture composition variants. Steady-state flow parameters were calculated using a locally self-similar approximation of the boundary layer equations. Within the framework of linear stability theory, the dependences of the critical Reynolds numbers Re_δc and the laminar-turbulent transition Reynolds numbers Re_xT* on the molar concentration of the additive were obtained. In particular, for a 50% mixture, the relative increase in both criteria compared to the corresponding values for a mixture of perfect gases is approximately 53%. Moreover, the contribution of CO₂ dissociation to the shift of the laminar-turbulent transition zone is twice as great as the contribution of vibrational mode relaxation. It is shown that the obtained dependence ofRe_xT* on the molar concentration of the additive correlates with the corresponding experimental results of Professor H. Hornung's group.

The article presents the results of hot-wire measurements of the free-flow and boundary layer of a flat plate with a blunt leading edge, in the presence of an N-wave disturbance source. Based on the measurements, estimates of the cross-correlation characteristics were obtained. Based on the distribution of the mutual phases, conclusions were drawn regarding the presence of diffraction phenomena of free-flow pulsations. It is shown that, in contrast to the case with a sharp leading edge, in the case of a blunt leading edge it is not possible to reliably determine the presence of diffraction phenomena.

The flow pattern in the planar turbulent far wake past a body in a passively stratified medium is investigated by applying a mathematical model including the following features: Rodi’s algebraic approximations for Reynolds stress, the differential equation for transfer of a deficit in the mean velocity longitudinal component, the balance of the turbulence energy and its dissipation rate, fluid density deficit, vertical component of the mass flux vector, and the equation for dispersion in the turbulent fluctuations of density. The far-wake approximation is used in this problem. The paper discusses an issue of the local-equilibrium truncation taken for the two last equations. We compare the solutions of self-similar degeneration of the density field characteristics for a case of a classical turbulent wake behind a towed cylinder and for a case of momentumless turbulent wake. The results of numerical experiments were a foundation for interpreting the observed high error while using the locally-equilibrium truncation equation for transfer of the density fluctuations dispersion.

An experimental study was performed for a flow in a channel with an intermittent pattern of roughness. The influence of the relative obstacle height (ranging from 2 to 10 % of the channel height) on the turbulence characteristics and flow structuring in the channel nearwall zone was studied. The research was focused on the case of obstacles with a low height. We found that the obstacle height below 5.5 % induces a drastic change in pulsation and spatial flow characteristics; this alters the flow pattern in the separation zone and the vortex generation mechanisms. The across size of vortex behind the obstacle with a relative height of 2 % is twice as big as the obstacle height. The characteristic peak in spectra that corresponds to the vortex shedding frequency with fully developed roughness tends to degenerate with a decrease in the obstacle height. Here the key factor for vortex generation is due to vertical swing motion of the boundary for separation zone.

Methods of 3D numerical simulation were applied for evaluating the flow dynamics in the internal electrode of a plasmatron with a two-chamber design while a “cold” blowout through the electrode. This study presents spatial distributions for the gas flow velocity, turbulence kinetic energy and the pressure inside the two-chamber electrode equipped with two swirlers (they provide tangential input for plasma-forming air flow). Variations in the gas velocity for transversal cross-sections of the arc chamber are in agreement with previously published data for the case of “cold” smoke-based blowout of the two-chamber plasmatron equipped with a cylindrical electrode. It was established that the cylinder-conical shape of the internal electrode might result in a shorter initial laminar column in the electric arc; this happens due to expansion of the turbulent interval at a fixed length of the arc. Experiments in a plasmatron with an inter-electrode insert demonstrated that the cylinder-and-cone shape of the electrode causes a higher (by 30-35 %) voltage on the plasmatron as compared with the cylinder-only shape of the electrode. This is a qualitative confirmation for calculated results about a higher kinetic turbulence energy and a wider turbulent zone of the electric arc occurring in a two-chamber design plasmatron.

This paper considers a relation between the emissivity of metals from the V period of the Periodic System while melting process with thermophysical properties: surface tension coefficient (γ) and specific heat capacity (C_p). Taking the own and available experimental data and using the theoretical calculation within Foot’s approximation, we elucidated the laws of emissivity as a function of electronic stricture and interatomic interaction. It was demonstrated that a correlation between the emissivity and γ and C_p has a complicated behavior and this is dictated by general trends (emissivity increases with γ) and by a specific electron configuration for given elements. The results underline the importance of considering the periodicity and electronic configuration in forecasting the thermophysical properties of metals in the phase transition state.

An experimental study of polydisperse pulp hydrodynamics is relevant for the tasks of aluminum production. The diagnostics method of matrix electrical impedance has been developed based on the synchronous recording of the spatial distribution of parameters of the complex conductivity and dielectric permittivity of the medium at the nodes of a wire mesh. This method allows for the study of three-dimensional flows of a multiphase model. The core of the measuring system is a matrix mesh electrical impedance sensor consisting of coordinate-linked sets of conductors whose spatial intersections form the measuring nodes of the system. The tracer is a liquid with a complex electrical conductivity different from that of the main flow. During the measurements, impedances are dynamically recorded at each three-dimensional spatial node of the mesh, determining the parameters of multiphase flows. The study experimentally demonstrated the feasibility of measuring impurity propagation during mixing in liquids with a dynamic range extended to 90 dB.

The results of an experimental study of the influence of tabs (teeth) of various shapes on the intensity of turbulent heat transfer in the separation zone in the near wake of a backward-facing step are presented. In this stagnation region, a significant decrease in heat transfer is observed, which negatively affects the thermal-hydraulic efficiency. Five tab shapes were studied in the experiments: square, triangular, cylindrical, semi-cylindrical, and semi-conical ones. The tabs were installed at the step edge immediately before flow separation. Measurements were conducted at Reynolds number Re_Н = 4000 and varying tab pitch. It was found that square tabs with a pitch of 2.5 calibers (step height) have the greatest effect on flow and heat transfer, but at the same time, they cause the greatest hydraulic resistance, therefore, their thermal-hydraulic efficiency is lowest. Semi-conical tabs installed with a pitch of five calibers showed the best results.

Experimental results on the diffusion combustion of a hydrogen microjet escaping from a sphere enveloped in an air flow are presented. Two limiting cases of the outflow of a burning hydrogen microjet are considered: when the jet is located at the flow divergence point on the sphere and at the opposite point, at the aft end of the sphere, where different separated flow structures are realized. Three characteristic hydrogen microjet flow velocity regimes were studied, allowing for the creation of various burning jet structures at zero incident flow velocity. The changes in the jet topology and structure were demonstrated. Flow visualization was performed using a thermal imager. A comparison was made with results obtained using a shadowgraph.

The dynamics of dispersed phase propagation during injection of a gas-droplet wall jet into a co-current turbulent heated airflow is numerically simulated with variations in the droplet mass concentration at the inlet cross-section and their initial diameter. The solution is based on a system of axisymmetric Reynolds-averaged Navier-Stokes (RANS) equations, taking into account the two-phase character of the flow. The Eulerian approach is primarily used to describe the aerodynamics and heat and mass transfer in the gas and dispersed phases. The Lagrangian and full Lagrangian approaches are used in the study for additional verification of the developed mathematical model. A significant effect of the liquid mass concentration on the particle concentration profiles across the channel cross-section is demonstrated. The results obtained using the Eulerian and Lagrangian descriptions are compared. The applicability of both approaches for describing the dynamics and heat transfer of a two-phase wall jet is demonstrated (the difference between the two approaches does not exceed 15%).

A numerical study of the flow in a tangential vortex chamber using unsteady full and Reynolds-averaged Navier-Stokes equations is presented. The spatial flow of a viscous incompressible gas (air) is considered at Reynolds number Re = 3.4·10⁴. The distributions of the axial and circumferential velocity components, as well as their pulsations, are analyzed. It is established that the solution of the full Navier-Stokes equations provides distribution of parameters in a swirling flow close to the results of experimental measurements, whereas turbulence models based on the Boussinesq hypothesis do not allow numerically reproduction of the dynamics of a confined swirling flow.

Ground-based simulation of processes around space vehicles in open space is a topical issue for rocket industry. Some problems can be covered by middle-size vacuum-based gas-dynamic setups; they provide model flow regimes for imitation of natural regimes befitting from low financial and material costs. The updates for the laboratory vacuum setup LAMPUS-2 based in Novosibirsk State University and improvements in gas dynamic diagnostic tools enabled a wide range of study for supersonic jets emerging from a nozzle cluster. This paper presents the measurement results performed for binary cluster jets using photo visualization method and scanning of argon flows for different ratio of stagnation pressure to the ambient pressure. The process of condensation has a significant influence on the dynamics of interacting jets.

The three-dimensional structure of the flow in narrow gaps between fuel assembly (FA) rods has a decisive influence on the heat and mass transfer coefficients, which, in turn, determine the FA performance. In this work, instantaneous three-dimensional velocity fields were measured for the first time in a model of a FA peripheral cell using the Tomographic PIV method with high temporal resolution. The experiments were performed at Reynolds number Re = 2100. The study, conducted using the TR-Tomo PIV method, allowed characterization of the three-dimensional structure and dynamics of the flow in a narrow gap of a FA peripheral cell. Based on the obtained data, the components of the vorticity vector in the flow volume were calculated. It is shown that the key mechanisms, responsible for generation of extreme values of longitudinal vorticity, are transverse flow oscillations. These oscillations are quasi-periodic in nature (Sh = 0.091) and alternate with stabilization phases, which are characterized by a straight-through flow.

A newly discovered phenomenon-the sudden dispersion of starch grains upon a sharp decrease in ambient pressure-was experimentally studied. The aim of the study was to determine the mechanism of this phenomenon. This effect was observed for various types of starch: potato, corn, and rice. It was established that starch moisture content is the primary parameter determining the occurrence of this phenomenon. The experimental results showed that grain dispersion occurs due to the reactive force generated by evaporation and release of water from the starch grains. The release is caused by an increase in water vapor pressure within the starch grains due to a phase transition of water upon a decrease in external pressure. The data obtained may be useful for further research in materials science and biotechnology, as well as for the development of new methods for studying the properties of starch materials.

The development of optoelectronic methods for measuring through a multiphase barrier in the study of a dispersed component in experiments on modeling the natural and technical hydrodynamic systems is a pressing issue. The difficulties of direct measurements under hurricane conditions necessitate laboratory measurements. Spray formation was studied in detail during the laboratory experiments in the high-speed wind-wave channel (WWC) at the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS). A modern, non-contact optical method for diagnosing two-phase flows-laser Doppler anemometry (LDA)-was used to measure the kinematic characteristics of the aerosol flow. Experiments were carried out to measure the aerosol droplet aerodynamics in WWC of IAP RAS. It was demonstrated that the measuring system enables to measure all three components of the aerodynamic flow velocity through a multiphase barrier at wind speeds from 10 to 40 m/s over a wide range of conditions. The results of measurements were compared with the readings of the Pitot tube, and good agreement and reproducibility of the experimental results were shown.