Vol 23, No 6 (2016)

The flow past an axisymmetric body with laminar boundary-layer separation in a low-velocity air stream has been studied. The hot-wire technique was employed to identify the variation of velocity field induced by a local stationary perturbation of separation region at the stern of the experimental model. A large-scale influence upon the near-wall flow due to a cylinder roughness element provided on the model surface was observed. The obtained data substantiate the possibility of controlling the laminar boundary-layer separation on an axisymmetric body using a local external forcing.

The work studies the flow characteristics in a channel with periodic hills on the basis of three algorithms for calculating the flow velocity fields through the images: Particle Image Velocimetry, Particle Tracking Velocimetry, and Pyramid Correlation. Descriptions of algorithms, detailed information about the experiment and parameters of the received data processing, as well as the results of calculations of instantaneous velocity fields at selected time points obtained by corresponding methods are provided. In addition, the presented techniques are compared on the basis of experimental data.

We present the results of flow visualization and velocity measurements on a hexagonal structured surface. Several configurations with concave and convex hexagonal structures are investigated. Each hexagonal structure is 2.7 mm deep and 33 mm wide (width between flats) and has a height to diameter ratio of 0.05 based on equivalent diameter. Considered are flow velocities 19 m/s, 24 m/s, and 27 m/s. The flow bifurcates on the leading edge of the concave configuration into two counter rotating vortices and propagates further in streamwise direction. The circulating regions are identified by the peaks in r.m.s. velocity curves. In case of concave configuration, the flow splits up into counter rotating vortical structures in a vertical plane parallel to the flow. The lower vortex rotating in the opposite direction of the flow cause the oil film fringes to drift upstream. Complex circulating regions similar to the arrangement of slices in an orange can be observed on the trailing edge of the concave hexagonal structure.

Numerical modeling of heat exchange at a laminar stationary and pulsatile flow in rectangular channels with different aspect ratios of side lengths γ has been carried out by a finite difference method for two boundary conditions: a constant wall temperature and a constant heat flux density on the wall. For the boundary condition of the first kind, the similarity of distributions of the heat flux density and shear stress on the walls over the channel perimeter has been established. The reasons for a nonmonotonous dependence of the initial thermal interval length on γ are discussed. For the boundary condition of the second kind, the difference of the Nusselt number averaged over the perimeter at γ → 0 from its value for a flow in a flat channel has been explained. An increase in the Nusselt number averaged over the perimeter and the period of oscillations has been revealed for a pulsatile flow in the quasi-stationary regime at large amplitudes of the oscillations of the velocity averaged over the cross section.

The regularities of interaction of drops of ultra-high vacuum liquid working media of space drop emitters with surfaces of trapping devices were considered. Their comparison with the characteristics of the interaction of drops of distilled water was performed. The achievability of trapping regimes without secondary drop formation in space was justified.

The paper studies ignition of fine particles, i.e., irreversible growth of particle temperature from an exothermal heterogeneous reaction, with the rate approximated with the Arrhenius law. The particles are suspended in gas with fluctuating temperature, and heat transfer from the particle surface occurs according to the Newtonian law. The equations take into account the temporal structure of gas temperature fluctuations. Modern methods of functional analysis were applied for deriving a closed equation for the probability density function for the particle temperature distribution. The gas temperature fluctuations lessen the threshold for the particle ignition in the hot gas as compared with the deterministic variant. The equations for probability density function produce a closed system of conjugate equations for the average temperature and dispersion of particle temperature fluctuations. The results of simulation illustrate the phenomenon of self-speeding drift of particle temperature towards the temperature of ignition startup.

The structure and tribological properties of coatings made of PN85YU15 powder were studied. The coatings were deposited on the mild steel blanks by the technology of air-plasma spraying using a unit of annular input and gas-dynamic powder focusing. Efficiency of heating and acceleration of powder particles was studied preliminarily. Measurement results on temperature and velocity distributions of particles at a certain spraying distance by the method of spectral pyrometry and time-of-flight method are presented. The effect of plasmatorch arc current and amount of propane-butane in the plasma flow on the structure and properties of coatings is analyzed in this paper. It is determined that the phase composition of coatings and initial powder is the same: the main phase is Ni₃Al compound; moreover, the structure contains Ni₅Al₃ phase. It is shown that an increase in the amount of propane-butane increases coatings porosity. The densest coatings (5.77%) were obtained at the plasmatorch arc current of 200 A with the reduced amount of propane-butane. The coatings obtained at the minimal arc current of 100 A with an increased amount of propane-butane are characterized by maximal porosity (20.38%). The results of tribological testing of the coatings under the conditions of sliding friction with a lubricant by the disc-plane scheme are presented. From the standpoint of obtaining the densest coatings with high performance, the optimal regimes of plasma spraying of PN85YU15 powder are the current from 140 A to 200 and using the air and propane-butane mixture only as the shielding gas (anode curtain).

The work presents the results of numerical modeling of a supersonic flow around a blunted cone with an isolated cylindrical roughness on the forebody surface in the three-dimensional formulation. The roughness element is shown to distort the mean flow and to give rise to small-amplitude disturbances with distinguished spectral peaks in the boundary layer.

Using the method of molecular dynamics, the simulation of folding of an α-helical protein from the unfolded to compact and functional (native) state is performed. The protein folding is interpreted as a stationary motion of a compressible “folding fluid”. It is shown that the densities of folding fluxes obey the same similarity relations as the velocities of an incompressible fluid in the Kolmogorov’s turbulence theory, except that instead of the rate of change of kinetic energy per mass unit, the rate of change of flux variance per volume unit plays the role of the key parameter.