Three-Dimensional Numerical Analysis of Fluid Flow and Heat Transfer in Wake with Positive Buoyancy (3rd Report, Positive Buoyancy Effect on 3D Structure)

Abstract

The wake with positive buoyancy at the Reynolds number 300 and the Richardson number 0.3 of air was analyzed by the DNS computation, and revealed as follows : Positive buoyancy effects are that one is Effect I, that is, a decrease of the axial direction vorticity|ωz|, and the other is Effect II, that is, a decrease of the 3D motion, i. e. the vorticities |ωx|, |ωy| with the upward and horizontal directions, and the velocity |w| with the axial direction. Effect II is more dominant than Effect I.That is, the 3D motion moving right and left decreases remarkably, but the suppression of vortex street becomes weak. A space effect of Effect II is that the vortex dislocation becomes vague and a dislocation location becomes time-dependent. A time effect of Effect II is that the frequency of the 3D motion increases, the amplitude of the motion decreases, and the formation region length decreases. A space and time effect of Effect II is that the oscillation mode becomes the B mode with 1 of the wave length Λ, and 2fwake and 5fwake of the dominant frequency fp of the 3D motion in the nearwake, and the A* mode with 3 of Λ, 2fwake of fp, and the weak vortex-dislocation in the farwake, here fwake is the wake frequency.