Home – Home – Jornals – Thermophysics and Aeromechanics 2020 number 1

This review is devoted to the analysis of the history and current trends in the development of the velocimetry method based on particle images for an aerodynamic experiment. The authors consider the basics of the method, various implementations, former and current status of equipment. Special attention is paid to the methods of data processing and evaluation of various physical values in the flow from the measured velocity fields. The paper briefly analyzes some optical methods that can be used together with velocimetry based on particle images and are implemented using similar equipment. The main focus of the review is set on the works that demonstrate the potential and the current level of the anemometry method based on particle images in the context of an aerodynamic experiment.

The present work is devoted to the experimental study of a high-speed jet exhausting from a model dual-stream jet nozzle which was performed using non-contact (shadow visualization) and probing (Pitot pressure tube) methods for measuring gas-dynamic parameters. Azimuthal non-uniformity of the pressure distribution, whose magnitude in the external duct is much higher than that in the internal one, is revealed. The cause for the formation of the 3D flow structure is related to the supporting pylons installed inside the nozzle contours and with the formation of a transonic flow mode in the external duct.

Stability of the laminar boundary layer on the surface of a hypersonic nozzle for the Mach number M = 6 of the Transit-M wind tunnel is calculated. The laminar boundary-layer profiles are obtained by solving the Navier-Stokes equations numerically within the framework of the Ansys Fluent software. N -factors of the Goertler vortices and of the first and second Mack modes are obtained in the approximation of the linear stability theory. It is demonstrated that the Goertler vortices are the most unstable disturbances for the nozzle under consideration. Empirical dependences of the local Reynolds number of the laminar-turbulent transition on the N -factor and unit Reynolds number are determined.

This work investigates the turbulent flow in a channel roughened by seven ribs of rectangular cross section disposed transversely. The flow configurations of identical ribs from the first one generate a large eddy spreading along the top of the two first ribs, trapping the flow of the first cavity. The widening of the first rib may solve this problem. Therefore, this flow configuration might be required in building structure applications necessitating regular structures from the first cavity. Streamlines contours indicates that the first rib behaves as a forward facing step when L₁ > 5h (L₁ is the first rib width), regular structures of the flow occurs from the first cavity. The effect of wider first rib is highlighted by friction and pressure coefficients profiles and those of the turbulent kinetic energy. Its effect also appears in the Darcy friction factor. The viscous and pressure forces applied on the first rib and the 5th pitch roughness indicate that the pressure force is dominant. Darcy friction factor characterizing the flow and pressure drag coefficient evaluated at the 5th pitch roughness remains independent of Reynolds number, while the drag force applied on the first rib increases when Reynolds number augments.

Results on flight dynamics of an axisymmetric vehicle with a control action in the form of gas jet ejection are reported. The differences in the flight trajectories with and without allowance for the pressure redistribution over the vehicle surface due to transverse gas jet ejection are considered and analyzed. Targeted software is developed on the basis of the equations of motion of a material point in space and rotation of the flying vehicle around its center of mass. The influence of the pressure redistribution over the axisymmetric vehicle surface owing to ejection of a lateral transverse gas jet on the flight trajectory and on the rate of vehicle deviation in terms of the yaw angle is demonstrated.

Deformations of a compliant coating in a turbulent flow, measured previously and calculated in this work, are compared. The calculated spectral density of coating deformations at low frequencies (25-250 Hz) turned out to be almost two orders of magnitude higher than the measured one, and the rms value of the measured deformation was seven times lower than the calculated one. The transitional regime for stabilizing the forced coating oscillations under the action of a pressure wave is calculated. It is shown that the coating almost always works in the transition regime, without reaching the maximum deformation amplitudes that are characteristic of the steady state. It is concluded that it is necessary to use more complex boundary conditions that take into account the non-stationary character of the process because the amplitude of coating deformation varies in a complex way during the lifetime of organized structures moving in a turbulent boundary layer.

A method of direct numerical simulation and a method of solving the boundary-layer equations were applied to parameters of a supersonic boundary layer for a flow past a flat plate (Mach number M = 2) for the case of a plate coated with a sublimation material. The sublimating material is naphthalene (C₁₀ H₈). Comparison of results from these two approaches ¾ numerical simulation and solution of a boundary layer under the assumption on the local self-similarity ¾ demonstrated a satisfactory agreement between them. Calculations demonstrated that a higher surface temperature produces a higher mass rate of evaporation. Meanwhile, the total heat flux to the solid wall decreases and the wall temperature is lower than for the case of zero sublimation. Since the molecular mass of naphthalene is by several times higher than the molecular mass of air and due to evaporation-induced wall cooling, we observe a higher density of the mixture of air with the sublimating substance vapor near the wall. This may facilitate a higher stability of the supersonic boundary layer and delays the flow transition to the turbulent state.

The paper analyzes the complex topology of swirling flows generated in the cylinder by its rotating end face. Using the flow visualization for different parameters of swirl of the upper end of the cylinder, the general laws of the evolution of the region with a counter flow (bubble-like vortex decay) are shown regardless of the contact of the studied vortex flow with various liquids or gas at the free end. The research has found for the first time that the scenario for the appearance of the bubble-like breakdown region depends weakly on the properties of the medium that restricts the circulation of the working fluid, but differs significantly from the dynamics of vortex flows limited by the “solid” second wall - the fixed end of the cylinder. For example, during the axial vortex breakdown, the modes of stationary vortex motion with the appearance of the recirculation zone contact with the interface surface of two media have been revealed, which is not typical for closed flows. The results obtained are of interest for further development of vortex devices and reactors that provide complex vortex motion of ingredients for mass transfer intensification, both in terms of optimizing the operation of existing setups and for designing new devices.

The study is about developing a gasdynamic model of motion of multispeed continua for the case of nozzles with complex geometry. These nozzles imitate the shape of a two-stage confusor-shaped dust cleaner. The paper studied the influence of force interaction between the gas phase and solid phase in the flow duct of the nozzle; this effect was studied for a wide range of input flow parameters. A special focus was on the regimes of gas-solid flows through curved channels for the case of high loads of solids at the nozzle inlet. A mathematical model was developed that takes into account the impact from the wall-bounced particles upon the distributions of gas phase and solid phase for the entire region. It was shown that account for particle bouncing produces an extremum in solids concentration distribution in the flow duct of the separator.

The heat transfer from a single sphere suspended in a cylindrical channel with finely dispersed air-water (aerosol) flow is experimentally studied. Under stationary heating conditions, the values of the heat-transfer coefficients are obtained as dependent on the Reynolds number and the water mist rate. A physical model of heat transfer from the sphere surface with water mist-air flow is proposed, which allows evaluating the processes proceeding at the various stages of droplet evaporation in the flow near a heated surface and formation of a water film on the surface itself. The relative mass of the droplet moisture deposited on the sphere surface is estimated depending on the water mist rate and Reynolds number of the flow. A criterial equation is obtained that generalizes the experimental data in the form of the dependence of Nusselt number on the regime parameters of Reynolds, Weber, and the water-to-steam phase transition parameter is obtained.

The dynamics of evaporation from the confined surface of a horizontal layer of liquid (ethanol) under the action of a gas (air) flow has been experimentally studied. The influence of the longitudinal dimension of the interface (0.01-0.03 m) on the specific mass rate of evaporation was studied. It was found that the specific mass evaporation rate decreases with an increase in the interface length due to a decrease in the vapor concentration gradient in the boundary layer.

In the framework of the thermal energy scheme, the growth of a vapor bubble in a uniformly superheated liquid is simulated numerically. The results of numerical calculations are in good agreement with the solution of [3] in a wide range of Jacob numbers. The account for the interfacial surface permeability at high values of Stefan number shows a good match with the results of numerical calculations.

The paper offers a numerical simulation of the heat state for a two-layer semitransparent medium that imitates a hypothetical snow cover upon an opaque semi-infinite substrate; the layers of cover have different absorption and scattering coefficients. Computations were performed for key parameters typical of winter season. It was shown that, depending on the optical thickness of every layer and proportions of absorption to scattering in the incident radiation, we observe more intense heating of near-surface or deeper zones in the snow-ice medium. The radiation part of problem was solved using a modified average-flux method used to take into account the dependency of optical properties on the wavelength of incident radiation, scattering, and reflectance at the layers boundaries.

The laser-pulse method was used to measure the thermal diffusivity of La_98.8Fe_1.2 alloy in the temperature range of 293-1623 K of the solid and liquid states, including the regions of phase transformations. The measurement uncertainties of the heat transfer coefficients were ± (3-6) %. Approximating equations and tables of reference data on thermal conductivity and thermal diffusivity of La_98.8Fe_1.2 alloy have been obtained for scientific and practical use. A comparison of measurement results with known literature data on thermal diffusivity of pure lanthanum has been carried out.

The effect of mechanically activated coal grinding on the time of their ignition in a vertical tubular reactor is studied experimentally and numerically. The experiments showed that coal, ground by an activator mill (disintegrator mill), ignites faster due to the effect of mechanical activation. According to numerical calculation of the processes of ignition and combustion of mechanically activated pulverized coal fuel, it is clear that the use of non-stationary methods for turbulence simulation and a model of the volatile yield, taking into account the structure of coal matter, allow an estimate of the contribution of mechanically activated grinding to the intensity of devolatilization.

This work deals with the experimental study of transient flow regimes occurring in the laboratory model of a hydraulic turbine draft tube at a change in the control parameters of setup. The transition from the partial load regime, when a precessing vortex core is formed, to the best efficiency point without a core is considered. To set accurately the speed of runner rotation and the flow rate of the working medium, the setup was controlled by special software. To ensure accurate multiple reproduction of a given transition cycle between two regimes, the control program has been upgraded. Using a specially developed procedure of phase averaging of a laser Doppler anemometer (LDA) signal recorded during multiple repetitions of transient regimes, the velocity profiles were obtained for different phases of changing the setup operating conditions.

The problem is considered on a periodical of time motion of a viscous liquid bordering on a solid body and a gas. The liquid is affected by oscillatory influences which have no predominant direction in space. The new hydro-mechanical effect is revealed which consists in that the free parts of the hydro-mechanical system - liquid layers - perform unidirectional stationary rotational motion with the background of oscillations.