Since the review of periodic flow phenomena by Berger & Wille (1972) in this journal, over twenty years ago, there has been a surge of activity regarding bluff body wakes. Many of the questions regarding wake vortex dynamics from the earlier review have now been answered in the literature, and perhaps an essential key to our new understandings (and indeed to new questions) has been the recent focus, over the past eight years, on the three-dimensional aspects of nominally two-dimensional wake flows. New techniques in experiment, using laser-induced fluorescence and PIV (Particle-Image-Velocimetry), are vigorously being applied to wakes, but interestingly, several of the new discoveries have come from careful use of classical methods. There is no question that strides forward in understanding of the wake problem are being made possible by ongoing three- dimensional direct numerical simulations, as well as by the surprisingly successful use of analytical modeling in these flows, and by secondary stability analyses. These new developments, and the discoveries of several new phenomena in wakes, are presented in this review.

Vortex Dynamics in the Cylinder Wake

Fluctuating lift on a circular cylinder: review and new measurements

Results are reported from a highly accurate, global numerical stability analysis of the periodic wake of a circular cylinder for Reynolds numbers between 140 and 300 The analysis shows that the two-dimensional wake becomes (absolutely) linearly unstable to three-dimensional perturbations at a critical Reynolds number of 1885±10 The critical spanwise wavelength is 396 ± 002 diameters and the critical Floquet mode corresponds to a ‘Mode A’ instability At Reynolds number 259 the two-dimensional wake becomes linearly unstable to a second branch of modes with wavelength 0822 diameters at onset Stability spectra and corresponding neutral stability curves are presented for Reynolds numbers up to 300

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Three-dimensional Floquet stability analysis of the wake of a circular cylinder

It is now well-known that the wake transition regime for a circular cylinder involves two modes of small-scale three-dimensional instability (modes “A” and “B”; Williamson, 1988), depending on the regime of Reynolds number (Re), although almost no understanding of the physical origins of these instabilities, or indeed their effects on near wake formation, have hitherto been made clear. There is now some strong interest in this problem, coming not only from experiment, but also from Direct Numerical Simulation, where, in some cases, these modes A and B have been found clearly (Thompson & Hourigan, 1996; Zhang et al., 1995; Henderson, 1995; Mittal & Balachandar, 1996). Much of the recent surge of activity concerning the wake transition and development of turbulence in wakes has been addressed comprehensively in a review paper. Williamson (1996a).

Three-dimensional wake transition

We present simulation results of vortex-induced vibrations of an infinitely long flexible cylinder at Reynolds number Re = 1000, corresponding to a ‘young’ turbulent wake (i.e. exhibiting a small inertial subrange). The simulations are based on a new class of spectral methods suitable for unstructured and hybrid grids. To obtain different responses of the coupled flow–structure system we vary the structure's bending stiffness to model the behaviour of a vibrating inflexible (rigid) cylinder, a cable, and a beam. We have found that unlike the laminar flow previously studied, the amplitude of the cross-flow oscillation is about one diameter for the cable and the beam, close to experimental measurements, but is lower for the rigid cylinder. We have also found that for the latter case the flow response corresponds to parallel shedding, but for the beam and cable with free endpoints a mixed response consisting of oblique and parallel shedding is obtained, caused by the modulated travelling wave motion of the structure. This mixed shedding pattern which alternates periodically along the span can be directly related to periodic spatial variation of the lift force. In the case of structures with pinned endpoints a standing wave response is obtained for the cylinder; lace-like flow structures are observed similar to the ones seen in the laminar regime. Examination of the frequency spectra in the near wake shows that at Re = 1000 all cases follow a −5/3 law in the inertial range, which extends about half a decade in wavenumber. However, these spectra are different in all three cases both in low and high frequencies, with the exception of the beam and cable, for which the high-frequency portion is identical despite the differences in the displacement time history and the large-scale features of the corresponding flow.

Dynamics and flow structures in the turbulent wake of rigid and flexible cylinders subject to vortex-induced vibrations

Recently there has been a surge of new interest in three-dimensional wake patterns In the present work, we have devised a method to control the spanwise end conditions and wake patterns using “end suction”, which is both continuously-variable and admits transient control Classical steady-state patterns, such as parallel or oblique shedding or the “chevron” patterns are simply induced The wake, at a given Reynolds number, is receptive to a continuous range of oblique shedding angles (θ), rather than to discrete angles, and there is excellent agreement with the “cos θ” formula for oblique-shedding frequencies We show that the laminar shedding regime exists up to Reynolds numbers (Re) of 205, and that the immense disparity among reported critical Re for wake transition (Re = 140–190) can be explained in terms of spanwise end contamination Our transient experiments have resulted in the discovery of new phenomena such as “phase shocks” and “phase expansions”, which can be explained in terms of a simple model assuming constant normal wavelength of the wake pattern Peter Monkewitz (Lausanne) also predicts such transient phenomena from a Guinzburg-Landau model for the wake

Control of three-dimensional phase dynamics in a cylinder wake

Recently there has been a new surge of interest in three-dimensional wake patterns, from both an experimental and analytical standpoint. One of the central discoveries is that the patterns of vortex shedding are dependent on the specific end conditions of a long cylinder span. However, a number of outstanding questions have remained unanswered, in part because techniques had not existed to control such patterns in a continuous fashion and from outside a test facility. In the present work, we have devised a method to control the end conditions of a cylinder span by non-mechanical and continuously-variable means, namely by the use of end suction. The technique allows a continuous variation of end conditions and admits transient or impulsive control. With the method, the classical steady-state patterns, such as parallel or oblique shedding or the ‘chevron’ patterns are simply induced. These experiments demonstrate that the wake, at a given Reynolds number, is receptive to a continuous (but limited) range of oblique shedding angles (θ), rather than to discrete angles. There is excellent agreement in these results with the ‘cos θ’ formula for collapsing oblique-shedding frequencies onto a single ‘universal’ frequency curve. The use of suction has avoided the grossly unsteady motions at the ends of the cylinder span brought about by the wakes of mechanical end manipulators, and we show that the laminar shedding regime exists up to Reynolds numbers (Re) of 205. The surprisingly large disparity among reported measurements of criticalRe for wake transition (Re=140−200), over the last forty years, can now be explained in terms of spanwise end contamination.

G. D. Miller

Papers

Control of three-dimensional phase dynamics in a cylinder wake

Three-dimensional phase dynamics in a cylinder wake