Longitudinal structures in plane jets

The physics of mixing in jets is of considerableinterest from the standpoint of both fundamental sci-ence and practical application. The intensity and homo-geneity of mixing significantly affect the combustionefficiency, heat transfer coefficient, formation of waste,and jet noise (see, e.g., [1]).Laminar jets have inflectional mean velocity pro-files. This property leads to the formation of Kelvin–Helmholtz vortices, which represent the main instabil-ity of such shear layers. The initial stage in the develop-ment of these vortices is usually well described by lin-ear stability theory (see, e.g., review [2]). The next,nonlinear stage is characterized by amplitude saturationand vortex pairing due to the resonance of disturbanceswith subharmonics and superharmonics. Further devel-opment of nonlinear structures is often accompanied bythe occurrence of longitudinal, or streamwise, vorticalstructures. Their formation is usually attributed to theso-called secondary three-dimensional instability of theKelvin–Helmholtz vortices [3, 4]. Experiments showthat the dynamics of these structures play an importantrole in the mixing process in the far wake of a jet [5].Other longitudinal disturbances that can often bedeveloped in shear layers are formed downstream ofnozzle surface irregularities [6]. These are regions ofquasi-stationary three-dimensional deformations,mainly of the longitudinal velocity in the shear flow,and have a characteristic form of “streaks” in the visu-alization pictures [7]. Their occurrence is not due to thesecondary instability of the Kelvin–Helmholtz vortices.These structures arise as a result of stabilization effectsupon the development of compact three-dimensionaldisturbances of the normal velocity component evenwith small amplitude [7]. These longitudinal structuresintensely interact with other flow disturbances, e.g.,instability waves. This interaction usually acceleratesflow turbulization [8]. This feature of the streaky struc-tures makes them a promising means for improving themixing and control over the flow in jets.There is an extensive literature on the investigationof longitudinal structures in wall flows (see, e.g., theoverview in [7]). However, investigations into the phys-ics of similar structures in jets began only recently. Suc-cessful excitation and observation of natural longitudi-nal structures and their interaction with Kelvin–Helm-holtz vortices in circular jets [9] offer the possibility ofstudying controlled longitudinal structures in planejets. To this end, a simple but efficient smoke visualiza-tion technique using a pulsed laser sheet synchronizedwith the Kelvin–Helmholtz vortex shedding wasapplied in this study.The plane jet generator and visualization schematicsare presented in Fig. 1. The flow was produced by aDISA wind tunnel that is designed for the calibration ofsensors of a hot-wire anemometer and ensures a stableair flow rate. Smoke was supplied to the test sectionthrough a tube connected to a smoke generator. The