Three-Dimensional Numerical Analysis of Fluid Flow and Heat Transfer in Wake with Positive Buoyancy (2nd Report, 3D Structure of Isothermal Wake)

Abstract

The isothermal wake from a circular cylinder at the Reynolds number Re=300 was analyzed by a DNS computation, computed results were discussed, and features of 3 D motion were elucidated as follows : Qualitative features of the vorticities ωx, ωy with the upward direction and the horizontal directions and the velocity ω in the cylinder axis direction were revealed by an analogy of the vorticity ωz in the cylinder axis direction. The vortex dislocation was visualized in the farwake streaklines, had a band spectrum of time trace of the upward velocity, and was found to be resulted from the ω motion. A mechanism of the vortex dislocation was made clear by the ω motion which was different between the nearwake with fp=fwake and farwake with fp=2fwake, here fp is the dominant frequency and fwake is the wake frequency. The wave lengths of ωx and ωy were 1, i. e. the B mode oscillation, in the nearwake, and 2, i. e. the C mode oscillation, in the farwake. The 3D oscillation in the isothermal Karman vortex street at Re=300 was therefore composed of the B mode with the wave length 1 in the nearwake and the C* mode with both the length 2 and the vortex dislocation in the farwake.