A numerical study was performed to investigate the effect of vibrational excitation and relaxation of sulfur hexafluoride (SF6) molecules on the gasdynamic structure and stability of high-speed microjets exhausting from axisymmetric sonic micronozzles with a diameter of 10–110 µm. Direct numerical simulation of gas flow was based on solving three-dimensional unsteady Navier–Stokes equations using a two-temperature model of relaxation flows. The studies were conducted over a wide range of Reynolds numbers determined by the diameter of the micronozzles. The vibrational relaxation of gas molecules was found to influence the modal composition and spectral characteristics of perturbations, as well as the length of the laminar region of microjets.
An Erratum to this paper has been published: https://doi.org/10.1134/S002189442602001X
This paper presents the results of a numerical simulation and stability analysis of the flow in a boundary layer on a plate with a surface microrelief. The microrelief consists of long slots (recesses) oriented at various angles relative to the incoming flow at a Mach number M = 2. The slots under consideration have a depth-based Reynolds number Reh ≈ 1000 and a width that is small compared to the instability wavelength. Numerical simulation is used to determine the specific features of the flow past a plate with inclined slots and to study the evolution of time-localized disturbances. A stability analysis of the flow is performed within the framework of linear theory, based on averaged profiles of the boundary layer parameters. The computational results show that the presence of inclined slots induces a crossflow in the boundary layer, which leads to an increase in the disturbance growth rates. The obtained data are in good agreement with experimental observations and explain the destabilizing effect of inclined slots on the boundary layer.
One-dimensional unsteady flows of an incompressible non-Newtonian viscoelastic fluid between parallel plates are considered using the Johnson–Segalman model with multiple relaxation times. A distinctive feature of the model under consideration is its hyperbolicity over a wide range of flow parameters. A general model of this type with n relaxation times (modes) is obtained, and a change of variables is made, allowing the model equations to be written in a conservative (divergent) form. A series of calculations of unsteady flows in various regimes is performed, showing the occurrence of the shear banding effect as the mean flow velocity increases. The wall shear stress versus shear rate relation and the flow rate versus pressure gradient relation were obtained for steady plane Couette and Poiseuille flows, respectively. The resulting curves were validated by comparison with several sets of experimental data. The structure of steady solutions exhibiting shear banding, obtained as a numerical limit of unsteady solutions, is investigated. A selection rule is formulated for steady-state solutions that are asymptotically attained in unsteady numerical simulations. The hysteresis phenomenon under cyclic changes in flow velocity is analyzed.
An experimental study of hydraulic fracturing in thick-walled concrete cylinders with a central hole is conducted using a high-pressure fluid test rig. The cylinders are made of sand-based concrete using GTs 50 alumina cement. Estimates of the ultimate pressure obtained from local and nonlocal failure criteria are compared with the experimental data. It is shown that satisfactory agreement between the calculated ultimate loads and the experimental failure data under a nonuniform stress field is achieved using nonlocal failure criteria.
