Wake transition of two-dimensional cylinders and axisymmetric bluff bodies

This article presents a short review of the three-dimensional transition of wakes from two-dimensional bodies, such as cylinders of various cross-sectional shape, and axisymmetric tori or rings. The nature and sequence of instabilities are compared and contrasted, especially with reference to the base case of the circular cylinder wake. The latter has been the subject of intense interest and scrutiny for well over a century, and has implicitly assumed the role of providing the generic transition scenario for turbulent wake flow. For elongated cylinders with streamlined leading edges, the analogues of the instability modes for a circular cylinder become unstable in the reverse order, which may have implications for the route to wake turbulence for such bodies. As well, the analogue of mode B has a significantly increased relative spanwise wavelength and appears to have a different near-wake structure. At the other extreme, for a normal flat plate, the wake first becomes unstable to a nonperiodic mode that appears distinct from either of the dominant circular cylinder wake modes. For tori, which have a local geometry approaching a two-dimensional circular cylinder for high aspect ratios (ARs), the sequence of transitions with increasing Reynolds number is a strong function of AR. For intermediate ARs, the first occurring wake instability mode is a subharmonic mode. Possible underlying physical mechanisms leading to some of these instabilities are also examined. In particular, support is provided for the role of idealized physical instability mechanisms in controlling wavelength selection and amplification for the dominant wake instability modes. The results presented in this article focus on relevant research undertaken by the Monash group but draws in results from many other international groups.