THREE-DIMENSIONAL VORTICITY DYNAMICS IN COFLOWING JETS SUBJECTED TO AXIAL AND AZIMUTHAL FORCING

Introduction Using TiOi flow visualizations and particle image velocimetry (PIV), we examine the vorticity dynamics in the near field of both laminar and turbulent coflowing jets subjected to the single or combined effect of axial and azimuthal forcing. The static azimuthal perturbation consists of a gradual corrugation of a varying number of waves per circumference introduced at the nozzle exit. Through a set of well-controlled experiments, we present evidence of the existence of several three-dimensional vorticity modes, all of which involve the formation of a symmetric pattern of closed vortex loops. Depending on the relative strength of the coherent vortex rings forming in the jet and the size of their cores, we show that, through a process of amplification of the vortex ring instability followed by reconnection, vortex loops are formed, resulting in masses of fluid being ejected sideways. The observed formation of the various three-dimensional vortex patterns is interpreted in terms of the global induction of two arrays of coaxial vortex rings of opposite sign and unequal strength. This proposed mechanism is shown to be consistent with phase-locked, threedimensional vorticity reconstructions performed with PIV techniques. Copyright ©1997 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. The modification of the entrainment and mixing properties of an axisymmetric turbulent jet is a problem of fundamental importance to many technological applications. Thus, it is not surprising that after many decades of research on this subject, we are still in search of new methods for controlling or enhancing the mixing characteristics of jets. Among the several mechanisms identified as potential candidates for enhancing the spreading of a jet, modifying the vorticity dynamics of the near field appears to be one of the most promising. Examples of the many attempts to achieve this include, but are not limited to, the use of tabbed nozzles,' lobed nozzles," and nozzles with noncircular geometries, such as square and elliptic nozzles, and many others. In our early studies performed with laminar jets subjected to both axial and azimuthal forcing, we found that the dynamics of the vorticity in the near field of the jet may lead to different modes of induction which result in drastic changes in the entrainment and mixing characteristics of these jets. These various modes were shown to result from the amplification of the vortex ring instabilities, which caused the formation and pinching-off of vortex loops that, under their own induction, are propelled sideways, thus modifying the local spreading of the jet. In this paper, we present a more detailed American Institute of Aeronautics and Astronautics Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. description of these distinct modes and extend the study to high Reynolds number turbulent jets. In order to analyze the most general case involving forced jets, we have conducted our experiments with laminar as well as turbulent coflowing jets (the case of a jet discharging into a stagnant medium being a particular case). In both laminar and turbulent cases, the coflowing jets were subjected to the combined effect of a pair of orthogonally oriented perturbations. The first one (hereafter referred to as the primary or axial perturbation) consists of a single wave, sinusoidal perturbation in the axial velocity component of the jet at the nozzle's exit section. The use of this primary perturbation was intended to lock to a given value (at least in the near region of the jet) the frequency and thus the wavelength of the jet's vortex rings. The second one (hereafter referred to as the secondary or azimuthal perturbation) consists of a wavy, sinusoidal radial displacement of the cylindrical vorticity layer emanating from the nozzle. This was achieved by means of a corrugated nozzle exit, where the cylindrical vorticity layer emanating from it is periodically displaced radially in and out along the jet's circumference. Early experimental and numerical work on the effects of the azimuthal perturbation' has shown that the growth and amplification of this secondary perturbation results in the formation of counter-rotating pairs of streamwise vortices which further interact with the vortex rings, locking their azimuthal instability mode to a predetermined number of waves along the circumference. Thus, the intention of the secondary perturbation was to lock the ring's instability to a given azimuthal wavelength (i.e., number of waves along its circumference). The work is, in this respect, an extension to the axisymmetric configuration of similar studies conducted for free shear layers and wakes in the planar configuration," where we analyzed the combined growth of streamwise and spanwise perturbations. Although our investigation has the general objective of obtaining a clearer understanding of the vorticity dynamics of the near region of coflowing jets under a wide range of axial and azimuthal forcing conditions, this paper has a less broad scope. Rather, it is devoted to demonstrating that the structure of the near region of the jet can be altered considerably as a result of the growth and nonlinear interactions between the vortical structures formed through the jet's instabilities. Without intending to provide an exhaustive mapping of all possible vorticity modes, our aim is to show the existence, in both laminar and turbulent jets, of several modes of three-dimensional vortical patterns resulting from the global induction of the jet's vorticity when subjected to a combined axial and azimuthal forcing. For the particular case of coflowing jets, we will show that, depending on: the jet's Reynolds number, the relative velocities between the two streams, and the wavelengths of the primary and secondary perturbations, the global induction of the vortex rings and tubes results in a large variety of three-dimensional vortex patterns which lead to modified spreading and mixing characteristics of the jet. In the first part of this paper we will present a study conducted with laminar air jets (Re = 1000) in which the wavelengths of both the primary and secondary perturbations were systematically varied. The second part will be devoted to showing that the mechanism responsible for these ejections is also taking place in turbulent jets. In this higher Reynolds number case (up to Re = 9000), we will discuss a detailed reconstruction of the three-dimensional vorticity field performed using phase-locked particle image velocimetry (PIV) measurements. Although we will present only a limited set of measurements in the turbulent case (Re = 1500), we will demonstrate the feasibility of this methodology for the purpose of quantifying the evolution of the vorticity field of turbulent jets and isolating the mechanism responsible for the formation of the various entrainment modes. American Institute of Aeronautics and Astronautics Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.