Experimental study of perturbation growth in a round laminar jet

Abstract

AbstractJet flows have many applications in various technical processes such as temperature control, mixing, spraying and so forth. Along with wide usage in technology and manufacturing, jet flows are the object of fundamental scientific interest. In particular, a proper understanding of laminar‐turbulent transition in jets is highly important. In this experimental work, we study round laminar submerged jet flow of air and the evolution of controlled perturbations. First, the modal perturbation growth mechanism was under consideration. Thin metal rings were put into the jet at a small distance from the orifice to amplify unstable eigen modes of the jet by oscillating at different frequencies. Experimental and theoretically predicted wavelengths, radial distributions of velocity fluctuations and amplification curves of modal perturbations were compared and a good correspondence was found. Second, the non‐modal perturbation growth mechanism was under consideration. To excite algebraic perturbation growth, we put special wavy structures (deflectors) into the jet, which provide a roller‐like transverse motion in the perturbed jet. The features of the transition to turbulence caused by this non‐modal growth were considered. Based on obtained experimental results, we definitely identify the non‐modal ‘lift‐up’ growth mechanism of introduced disturbances. The development of perturbations qualitatively corresponds to the theoretically calculated optimal perturbations.