Том 53, № 3 (2018)

The possibility of controlling instability waves in the mixing layer of a subsonic unexcited jet is studied. These waves can be noise sources in both free jets and jets as parts of configurations. In the study the method of experimental diagnostics of the instability waves in the near field of a jet using an azimuthal multimicrophone array is realized. The data on the near field fluctuations are used for testing the control strategy proposed by the authors. The strategy consists in narrowband sliding filtration of the original signal and the formation of a narrowband controlling action on the basis of the linear principle of signal superposition. The results of the study represent the next step toward the realization of an active control system suppressing natural instability waves in turbulent jets.

The liquid viscous film falling down a vertical wall with sinusoidal relief is considered. The linear stability of steady-state flow with respect to time-periodic disturbances is studied using the Floquet theory. It is shown that in the case of applying corrugations the variation in the disturbance growth rate is proportional to the second power of their undulations. Depending on the relief parameters there exist two possibilities: the instability domain can expand or certain disturbances can be stabilized. The growth rates are obtained numerically and analytically in the approximation of low-amplitude corrugations. The development of waves from small disturbances is simulated within the framework of nonlinear equations and the formation of structures whose wavelength is significantly greater than the space relief period is found out.

Viscous gas flow past a finite-length plate in the strong interaction regime is investigated. The expansions of the flow functions in the vicinity of the leading edge are derived and the boundary value problems for the viscous and inviscid flow regions are formulated and jointly solved. The effect of the adiabatic exponent and the temperature factor on the flow parameters in these regions and the eigenvalue determining the intensity of the upstream disturbance transfer is studied. It is shown that in the non-self-similar case a transitional layer arises at the outer edge of the boundary layer.

The supersonic (M_∞ = 4.85) flow past a cylinder with a forward insertmade of a highlyporous cellular material is numerically modeled within the framework of the Reynolds-averaged Navier–Stokes equations. The air flow in the gas-permeable insert is described on the basis of a skeleton model of a highly-porous medium, whose determining parameters are the porosity coefficient (95%) and the pore dimensions (1 mm) of the actual cellular material. The aerodynamic drag coefficients of the model with different lengths of the porous forward insert are calculated on the unit Reynolds number range from 6.9 × 10⁵ to 13.8 × 10⁶ m⁻¹. They are in agreement with the available experimental data, which indicates the adequacy of the proposed skeleton model in describing the actual properties of highly-porous materials.

The results of calculations of water vapor outflow from a source, in which the pressure and temperature are maintained constant, into a vacuum through an orifice in the infinitely thin wall are presented. The calculations are carried out using the direct simulation Monte Carlo method when the processes of water cluster formation/decay in the flow field are taken into account. The vapor flow regimes transient with respect to the Knudsen number are considered. The effect of the cluster formation processes on the parameters of flow which becomes significant for the mole cluster fraction above 4%is analyzed. The effect of “freezing” the mole cluster concentrations with increase in the distance from the source is demonstrated. The influence of the probabilities of monomer and dimer association during pair collisions, used in the model, on themole dimer fraction is investigated. The results obtained are compared with the available experimental data.

Junction flows are subject to an intense adverse pressure gradient and three-dimensional separation when encountering a wall-mounted obstacle. A dynamically rich horseshoe vortex system is formed in this region. In this study the junction flow at the interaction of a wing and a flat plate is investigated. The numerical modelling is carried out using the three-dimensional large eddy simulation (LES) approach at the Reynolds number Re = 1.15×10⁵ based on the wing’s maximum thickness T and the free stream velocity U_ref. The comparison with the experimental results shows that the numerical simulations fairly accurately reproduce the phenomenon under study. The dynamic mode decomposition (DMD) of the resolved flow field is employed to obtain the coherent dynamics of the flow. To clearly demonstrate the oscillation characteristics and the horseshoe vortex structures of junction flow the velocity field in the plane of symmetry is decomposed with eduction of two dominant DMDmodes. These two DMDmodes are reconstituted and developed, together with the mean flow mode to explain the latent dynamics. Mode 1 reveals the merging of the horseshoe vortices and mode 2 is responsible for the process of fission and stretching.