Direct Numerical Simulation of Air Heated Cylinder Wake in Transitional State. Part II: Visualization and Mechanism of Unified 3-D Streaklines, Vortex Dislocation, and Turbulence Wreck

Abstract

The variation of vortex dislocations (VD) with cylinder heating, and turbulence wrecks (TW) or imaginary VD, in isothermal and heated wakes have not been elucidated at all. Therefore unified 3-D streaklines, VD, TW, and their variations with cylinder heating are here first visualized and elucidated in air isothermal and heated wakes with suppressed strong turbulence anisotropy (STA) [1] in the transitional state at Reynolds number Re = 300 and Richardson number Ri = 0.3. A difference between VD and TW is shown in the item 2 below.

1.	With cylinder heating, the three-dimensionality which results from the spanwise discontinuity in the vortex spiral (SP) is suppressed, but two-dimensionality and laminarization in the SP are enhanced.
2.	There are two kinds of spanwise discontinuity in the streakline SP as follows: a. One is VD, which arises in the unstable region with turbulence in w. The discontinuity in the region with turbulence in w is VD. b. The other is TW, which arises in the stable region of laminar w. The discontinuity in the region of laminar w is TW.
3.	Methods for determining VD or TW by direct numerical simulation (DNS) are here first obtained as follows: Whether the discontinuity in streaklines is VD or TW can be resolved by employing the present DNS visualized streaklines and the thermal flow states [1, 2], i.e., turbulent, transitional, or laminar, in w, and by determination according to the above item 2.
4.	VDs arise due to STA, leading to the transitional state and instability in isothermal and heated wakes. The discontinuity in VDs is first found to be suppressed by cylinder heating. VDs suppressed by cylinder heating are here first visualized.
5.	VD arrangement in the transitional region in w is certainly carried into the laminar region, and becomes TW. In the laminar region, the density of streakline particles is decreased strongly in the isothermal wake, but the discontinuity in TW is decreased in the heated wake.