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We present a new method for controlling vortex breakdown (VB) via addition of co- or counter-rotation near the axis. Co-rotation is adequate to totally suppress VB, whereas counter-rotation increases the number and size of VB “bubbles” and makes the flow unsteady. We study these effects in a closed cylindrical container, in which a rotating end disc drives the base flow; an independently rotating central rod (with rod radius≪disk radius) is employed to control VB. This flow, being free of ambient disturbances, is well suited for understanding both the VB mechanism and its control; the present work appears to be the first to study VB control. We develop and explain our control strategy using flow visualization and simple analytical reasoning. Our results suggest that an additional co- or counter-rotation, applied near the vortex axis, can be effective in suppressing or enhancing VB in practical flows.

Elliptic jets have decided advantages for technological applications over circular jets; this paper explores further advantages achieved by jet forcing due to self-excitation. Using hot-wire measurements and flow visualization, we have studied an elliptic whistler (i.e. self-excited) air jet of 2:1 aspect ratio which, in contrast to an elliptic jet issuing from a contoured nozzle, displays no axis switching, but significantly increased spread in the major-axis plane. Its near-field mass entrainment is considerably higher (by as much as 70%) than that of a non-whistling jet. Flow visualization reveals unexpected dynamics of the elliptic vortical structures in the whistler jet compared to that in the non-whistling jet. Vortices rolled up from the lip of the elliptic pipe impinge onto the collar, producing secondary vortices; interaction of these two opposite-signed vortices is shown to cause the different behaviour of the whistler jet.

The dynamics of the preferred mode structure in the near field of an elliptic jet have been investigated using hot-wire measurements. A 2:1 aspect ratio jet with an initially turbulent boundary layer and a constant momentum thickness all around the nozzle exit perimeter was used for this study. Measurements were made in air at a Reynolds number ReDe (≡ UeDe/v) = 3.5 × 104. Controlled longitudinal excitation at the preferred mode frequency (StDe ≡ fDe/Ue = 0.4) induced periodic formation of structures, allowing phase-locked measurements with a local trigger hot wire. The dynamics of the organized structure are examined from educed fields of coherent vorticity and incoherent turbulence in the major and minor symmetry planes at five successive phases of evolution, and are also compared with corresponding data for a circular jet. Unlike in a circular jet, azimuthally fixed streamwise vortices (ribs) form without the aid of azimuthal forcing. The three-dimensional deformation of elliptic vortical structures and the rib formation mechanism have also been studied through direct numerical simulation. Differential self-induced motions due to non-uniform azimuthal curvature and the azimuthally fixed ribs produce greater mass entrainment in the elliptic jet than in a circular jet. The turbulence production mechanism, entrainment and mixing enhancement, and time-average measures and their modification by excitation are also discussed in terms of coherent structure dynamics and the rib-roll interaction. Various phase-dependent and time-average turbulence measures documented in this paper should serve as target data for validation of numerical simulations and turbulence modelling, and for design and control purposes in technological applications. Further details are given by Husain (1984).

The dynamics of coherent structure interactions, in particular the jet column mode of vortex pairing, in the near field of an elliptic jet have been investigated using hotwire measurements and flow visualization. A 2:1 aspect-ratio jet with an initially laminar boundary layer and aconstantmomentum thickness all around the nozzle exit perimeter is used for this study. While detailed hot-wire measurements were made in air at a Reynolds numberReDe(≡UeDe/ν) = 3.2 × 104, flow visualization was performed in water at a lowerReDe= 1.7 × 104; hereUeis the exit speed andDeis the equivalent diameter of the nozzle exit cross-section. Excitation at the stable pairing mode induced successive pairings to occur periodically at the same location, allowing phase-locked measurements using a local trigger sensor. Coherent structures were educed at different phases of pairing in the planes of both the major and minor axes. These are compared with corresponding data in a circular jet, educed similarly.Pairing interactions are found to be quite different from those in a circular jet. Owing to non-planar and non-uniform self-induction of elliptical vortical structures and the consequent effect on mutual induction, pairing of elliptic vortices in the jet column does not occur uniformly around the entire perimeter, unlike in a circular jet. Merger occurs only in the initial major-axis plane through an entanglement process, while in the initial minor-axis plane, the trailing vortex rushes through the leading vortex without pairing and then breaks down violently. These motions produce considerably greater entrainment and mixing than in circular or plane jets. From distributions of dynamical properties over the extent of coherent structures, the production mechanism is explained in terms of the longitudinal vortices (orribs) connecting the elliptic structures. Time-average measures and their modification by controlled excitation are also discussed in terms of coherent structure dynamics. A significant space in this paper is devoted to documenting phase-dependent and time-average flow measures; these new results should serve as target data for numerical simulations. Further details are given in Husain (1984).

The subharmonic resonance phenomenon in a free shear layer is studied experimentally and numerically. Excitation using both fundamental and subharmonic at an initial phase difference (ϕin shows stable pairing for a wide range of ϕin. However, for a narrow range of ϕin, either ‘shredding’ occurs or pairing is temporarily suppressed and occurs downstream without periodicity. Under detuned excitation, which is more representative of feedback-driven subharmonic growth, amplitude and phase modulations produce multiple sideband frequencies reflecting variations in the pairing location and occasional nonpairings. In direct numerical simulations (DNS) of a temporal shear layer, we uncover and analyze a new mechanism of transition, based on excitation of the ‘bulging’ instability by pairing of spanwise vortices (‘rolls’) with out-of-phase spanwise undulations. This 3D pairing generates strong internal core dynamics, consisting of core size oscillation driven by oscillating cells of spanwise flow within the rolls. Core dynamics amplify due to instability, can grow alongside streamwise vortices (‘ribs’), and eventually initiate mixing transition at a lower initial 3D disturbance level than that required for transition by ribroll interaction alone. We emphasize that the limitations of traditional perturbation analysis in understanding of the nonlinear and 3D aspects of instability and transition need to be overcome by new approaches such as vortex dynamics and topology.

A phenomenon of spontaneous separation of components in an initially uniform fluid mixture is found experimentally. A qualitative explanation of the effect is proposed in terms of nonparallel streamlines in the medium.

This paper summarizes experimental studies of incompressible elliptic jets of different aspect ratios and initial conditions, and effects of excitations at selected frequencies and amplitudes. Elliptic jets are quite different from the extensively studied plane and circular jets - owing mainly to the fact that the azimuthal curvature variation of a vortical structure causes its non-uniform self-induction and hence complex three-dimensional deformation. Such deformation, combined with properly selected excitation can substantially alter entrainment and other turbulence phenomena, thus suggesting preference for the elliptic shape in many jet applications. The dominance of coherent structures in the jet far field is evident from the finding that switching over of the cross-section shape continues at least up to 100 equivalent diameters De. The locations and the number of switchovers are strongly dependent on the initial condition, on the aspect ratio, and, when excited, on the Strouhal number and the excitation level. We studied jets with constant exit momentum thickness θe, all around the perimeter, thus separating the effects of azimuthal variations of θe, (typical of elliptic jets) and of the shear-layer curvature. Also investigated are the instability characteristics, and enhanced entrainment caused by bifurcation as well as pairing of vortical structures. We discuss shear-layer and jet- column domains, and find the latter to be characterized by two modes : the preferred mode and the stable pairing mode - similar to those found in circular jets -both modes scaling on the newly-defined lengthscale De. The paper documents some time- average measurements and their comparison with those in circular and plane jets.

Recent studies of free shear flows are reviewed. Included are experimental studies of: excited and unexcited circular and elliptic jets, plane wake and mixing layers, and effects of excitation on augmentation and reduction of jet turbulence and jet noise. It is shown that proper excitation can produce both large increases and large decreases in turbulence and Reynolds stress level, mixing and noise generation, suggesting promising technological applications of self-excited jets such as whistler jets. It is also argued that 'ribs' or longitudinal vortices play an important role in production and mixing in free, and perhaps all, turbulent shear flows. The 'cut-and-connect' interaction of vortical structures is proposed as a key mechanism for coherent structure breakdown, mixing, turbulence production, and aerodynamic noise generation. A simple analysis of the viscous vorticity equation using symmetry of configurations of vortical structures before and after the interaction gives realistic predictions.

Turbulence management can be accomplished via manipulation of large-scale coherent structures (CS) by controlling their generation, growth and interactions. By turbulence management, we mean either enhancement or suppression of turbulence and related phenomena. Since accumulated evidence so far suggests that turbulent shear flows are characterized by CS, it is evident that turbulence management can be achieved in all turbulent shear flows. Obviously, such turbulence management can yield profound potential benefits in controlling turbulence phenomena, e.g., enhancement of mixing, heat transfer and chemical reaction (including combustion), and reduction of drag and aerodynamic noise.

The initiation, growth, interaction, breakdown and regeneration of coherent structures in turbulent shear flows are the manifestations of a hierarchy of flow instability mechanisms. After the initial roll up of the shed vortex sheet into discrete vortices, the most common interaction that a shear layer undergoes is pairing of two neighboring vortices. The pairing process, i.e. the growth of the subharmonic, is a consequence of what has come to be known as subharmonic resonance -- the nonlinear interaction between a wave of frequency f and a subharmonic wave of frequency f/2, producing an f/2 component which is capable of reinforcing the subharmonic. Such reinforcement depends on excitation parameters such as the phase between the fundamental and the subharmonic, and the critical fundamental amplitude[1, 2].