Experimental Study of the Nonmodal Perturbation Growth Mechanism in a Laminar Submerged Jet

It is known that, in wall-bounded flows, along with the growth of instability modes, an important role is played by the nonmodal (algebraic) mechanism of linear growth. In unbounded flows, including submerged jets, the nonmodal growth mechanism has been theoretically studied only in the last decade; this mechanism has not yet been identified in experiments. This paper describes experiments on excitation of the nonmodal "lift-up" growth mechanism. Special wavy structures (deflectors) are introduced into a laminar submerged jet of circular cross section, which excites a roller-like transverse motion. The data obtained make it possible to identify unambiguously the nonmodal lift-up growth of the introduced perturbations. The development of perturbations in the experiment qualitatively agrees with the theoretically calculated optimal perturbations. The specificities of the transition to turbulence caused by nonmodal growth are considered.