ИСТИНА

Three-dimensional instability of two-dimensional flow subregions in the wake is considered. A classical example of the transition to three-dimensionality in the wake behind a circular cylinder includes two stages of stability losses with the increase in the Reynolds number Re: modes A and B. These modes can be well predicted by global stability analysis of the two-dimensional periodic von Kármán vortex street. The studies of these modes allow to suggest that the reasons of transitions A and B can be localized and described as instability of particular flow subregions, such as vortex cores and braid shear layers. The aim of the present work is to clarify the physical mechanisms leading to the appearance of three-dimensionality in the local parts of the flow. The local description of the flow is given based on the approach [Aleksyuk, Shkadov, 2018] (Fig. 1): the development of small longitudinal perturbations of vorticity and velocity is described by a closed system of equations with a clear meaning of each term (mechanism, such as vorticity diffusion, stretching and tilting of vortex lines), affecting their growth and decay. The base flow is characterized by vorticity, the positive eigenvalue of the strain rate tensor and its direction. Base and perturbed flows for this analysis are obtained using the direct numerical solution of the Navier-Stokes equations. The results of the instability analysis of idealized flows in vortex cores and in braid shear layers are compared to the real flow data from numerical simulations. Viability of such idealized models is discussed. The research is carried out using the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University. This work is supported by the grants of the Russian Foundation for Basic Research No. 18-01-00762 and 18-51-00006.