The 3D transition for a heated-cylinder flow occurs at a much lower Reynolds number than for the unheated case. The 3D-transition is initialized in the near-wake by the formation of Λ-shaped structures and manifests itself in the far-wake as escaping buoyant plumes from the upper vortex row. In this study, both experimental and numerical techniques are used to investigate the origin and development of these buoyant plumes.
The formation of the buoyant plumes 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 buoyant plume, which comprises of a so-called stem and cap. Then, a vortex ring is formed at the advancing front, where a region of positive vorticity symbolizes the formation of the mushroom-type structure. Additionally, the strength of the positive vorticity increases with time due to baroclinic vorticity production. In the end, a pinch-off of the vortex ring occurs when the cap is separated from the stem. The vortex ring accelerates after the pinch-off, which is most likely due to the sudden change of the volume increase of the cap. An analytical model is presented of this pinch-off process.