Evolution of mushroom-type structures behind a heated cylinder

Abstract

The three-dimensional transition in the wake flow behind a heated cylinder occurs at a much lower Reynolds number than for the unheated case. The three-dimensional transition is initialized in the near-wake by the formation of Λ-shaped structures and manifests itself in the far-wake as escaping mushroom-type structures from the upper vortices. In this study, both experimental and numerical techniques are used to investigate the origin and development of these mushroom-type structures. The formation of the mushroom-type structures is associated with the occurrence of Λ-shaped vortices in the near-wake. Hot fluid between the legs and the head of the Λ-shaped structure is lifted up. This lift-up process together with the action of buoyancy pulls out hot fluid from the upper vortex cores, resulting in a mushroom-type structure, which is comprised of a so-called stem and cap. Hot fluid is continuously transported through the stem to the advancing front of the mushroom-type structure. Finally, a pinch-off phenomenon is observed of the cap, ending up as a buoyant vortex ring. An analytical model is presented for the pinch-off process.

Flow control via instabilities, vortices and steady structures under the action of external microwave energy release

“…This type of instability is connected with the shear layer and has been widely investigated (see survey in Fridman 33 ). Lines of vortices in the wakes of controlled cylinders were obtained in Ren et al 34 and Bergmann et al 35 The classic scheme of the Kelvin-Helmholtz instability generation described in Fridman 33 for incompressible flow is observed in the calculations (in section 'Numerical analysis of the vortices generated as the result of Kelvin-Helmholtz instability' ¼ 0.5). The mechanism if this instability was described in Azarova 24 (Figure 17).…”