Evolution of Perturbations in Submerged Jets

The investigations performed in the Institute of Mechanics of Moscow State University on the evolution of velocity perturbations that propagate in a submerged incompressible jet with an extended laminar section are reviewed. The delay of laminar-turbulent transition is ensured by a developed unique jet-forming device. The first part of the study is devoted to the modal mechanism of perturbation growth in jet flow. Thin rings oscillating at various frequencies were placed in the jet at a small distance from the orifice to amplify the growing jet eigenmodes. A good agreement of the experimental and theoretically predicted wavelengths, the radial distributions of velocity fluctuations, and the growth rate of eigenmodes is demonstrated. The second part of the review considers a non-modal (algebraic) mechanism of perturbation growth. Special wave-like structures (deflectors) that excite a “roller-like” transverse motion were introduced in the jet. The specific features of turbulence transition caused by such steady-state perturbation are considered. Based on the experimental results, a non-modal growth mechanism for introduced perturbations, similar to the “lift-up” mechanism in near-wall flows, is identified. The development of disturbances qualitatively corresponds to the obtained theoretical optimal disturbances of the flow under consideration. The third part of the paper presents the results of computational study of the algebraic growth of perturbations in round submerged jets with the Michalke velocity profiles. A parametric analysis of the optimal growth of spatial disturbances was carried out.