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

American Society of Mechanical Engineers

Two fundamental characteristics of the low-Reynolds-number cylinder wake, which have involved considerable debate, are first the existence of discontinuities in the Strouhal-Reynolds number relationship, and secondly the phenomenon of oblique vortex shedding. The present paper shows that both of these characteristics of the wake are directly related to each other, and that both are influenced by the boundary conditions at the ends of the cylinder, even for spans of hundreds of diameters in length. It is found that a Strouhal discontinuity exists, which is not due to any of the previously proposed mechanisms, but instead is caused by a transition from one oblique shedding mode to another oblique mode. This transition is explained by a change from one mode where the central flow over the span matches the end boundary conditions to one where the central flow is unable to match the end conditions. In the latter case, quasi-periodic spectra of the velocity fluctuations appear; these are due to the presence of spanwise cells of different frequency. During periods when vortices in neighbouring cells move out of phase with each other, ‘vortex dislocations’ are observed, and are associated with rather complex vortex linking between the cells. However, by manipulating the end boundary conditions, parallel shedding can be induced, which then results in a completely continuous Strouhal curve. It is also universal in the sense that the oblique-shedding Strouhal data (S_θ) can be collapsed onto the parallel-shedding Strouhal curve (S_0) by the transformation, S_0 = S_θ/cosθ, where θ is the angle of oblique shedding. Close agreement between measurements in two distinctly different facilities confirms the continuous and universal nature of this Strouhal curve. It is believed that the case of parallel shedding represents truly two-dimensional shedding, and a comparison of Strouhal frequency data is made with several two-dimensional numerical simulations, yielding a large disparity which is not clearly understood. The oblique and parallel modes of vortex shedding are both intrinsic to the flow over a cylinder, and are simply solutions to different problems, because the boundary conditions are different in each case.

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Oblique and Parallel Modes of Vortex Shedding in the Wake of a Circular Cylinder at Low Reynolds Numbers

Fluctuating lift on a circular cylinder: review and new measurements

The dynamic characteristics of the pressure and velocity fields of the unsteady incompressible laminar wake behind a circular cylinder are investigated numerically and analysed physically. The governing equations, written in a velocity—pressure formulation and in conservative form, are solved by a predictor—corrector pressure method, a finite-volume second-order-accurate scheme and an alternating-direction-implicit (ADI) procedure. The initiation mechanism for vortex shedding and the evaluation of the unsteady body forces are presented for Reynolds-number values of 100, 200 and 1000.The vortex shedding is generated by a physical perturbation imposed numerically for a short time. The flow transition becomes periodic after a transient time interval. The frequency of the drag and lift oscillations agree well with the experimental data.The study of the interactions of the unsteady pressure and velocity fields shows the phase relations between the pressure and velocity, and the influence of different factors: the strongly rotational viscous region, the convection of the eddies and the almost inviscid flow.The interactions among the different scales of structures in the near wake are also studied, and in particular the time-dependent evolution of the secondary eddies in relation to the fully developed primary ones is analysed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112086003014

Numerical study and physical analysis of the pressure and velocity fields in the near wake of a circular cylinder

This paper mainly describes mechanisms of turbulence in the near wake of a circular cylinder through interaction of vertical motions of different scales.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112087002039

Nonlinear interaction and the transition to turbulence in the wake of a circular cylinder

A direct numerical simulation of the viscous incompressible flow around a circular cylinder is carried out in the Reynolds number range 2000–10 000 by solving the two‐dimensional time‐dependent Navier–Stokes equations, using a pressure–velocity finite‐volume method. Apart from the vortex shedding phenomenon, it is shown that transition waves develop in the separated shear layers and lead to mixing layer eddies. The ratio of the computed transition wave frequency over Strouhal number is in good agreement with experimental results. This allows the supposition that the instability leading to mixing layer eddies has a predominant two‐dimensional origin and is predicted by the Navier–Stokes equations.

Prediction of large‐scale transition features in the wake of a circular cylinder

Some experimental results of intermittency in the near wake (X/D=2 and 4) of a circular cylinder at a Reynolds number of 52 300 are presented Both conventional and phase‐averaged properties of the intermittency are given From the conditional analysis of turbulent/nonturbulent events, some features of the phase control of the intermittency by the periodic motion due to the vortex shedding are emphasized

Conditional analysis of intermittency in the near wake of a circular cylinder

Some properties of the coherent structures in the turbulent wake behind a circular cylinder (Re = 52300) are analyzed in two downstream sections (X/D = 2 ; X/D = k) using a phase averaging technique for the velocity fluctuations and the intermittency signal. A comparison with the Von Karman flow (Re = 100) given by a direct numerical solution of Navier Stokes equations provides some practical information about the relevance of the double decomposition taking into consideration the two dimensional deterministic structures and the “fine scale” three dimensional phase random motions.

H. Ha Minh

Papers

Numerical study and physical analysis of the pressure and velocity fields in the near wake of a circular cylinder

Nonlinear interaction and the transition to turbulence in the wake of a circular cylinder

Prediction of large‐scale transition features in the wake of a circular cylinder

Conditional analysis of intermittency in the near wake of a circular cylinder

Some Characteristics of the Unsteady Wake Flow Past a Circular Cylinder