Numerical Computation for Buoyancy Effect on Three-Dimensionality and Vortex Dislocation in Heated Wake with Vertical Mainstream

Abstract

For a heated circular cylinder wake with a vertical mainstream of air at Reynolds number 300 and Richardson number 0.3, the time-dependent three-dimensionality leading to the vortex dislocation is analyzed by direct numerical simulation (DNS). Time-series images of 3-D streaklines are obtained. The computed results are compared with those in the isothermal wake. The three-dimensionality and vortex dislocation in the heated wake are discussed, and the positive buoyancy effects are revealed. Buoyancy effects are as follows: (1) an increase of the upward velocity u in the whole wake (Effect I); (2) activation of three-dimensionality in the nearwake (Effect II); (3) suppression of the Karman vortex street in the farwake (Effect III); (4) suppression of the three-dimensionality and vortex dislocation in the farwake (Effect IV). Effect IV is more dominant than Effect III. Because of buoyancy, the vortex dislocation is more suppressed than the Karman vortex street. Because of Effect II, the nearwake dominant frequency f p of spanwise velocity w is increased to 2f wake and 5f wake . Because of Effect IV, Λ d and Λ v in the farwake are increased to Λ d fallingdotseqΛ v = 3 (Mode-A*). Here f wake is the wake frequency, and Λ v and Λ d are the spanwise wavelengths of ω x , ω y , and vortex dislocation normalized by the cylinder diameter.