The Dynamics of Cylinder-Wake/boundary-layer Interaction Revealed by Turbulent Transports

The flow past a cylinder near a plane wall for small gap ratios (G/D=0.1, 0.3, and 0.9) and fixed ReD = 1000 is numerically studied. The fundamental flow features are characterized by the instantaneous and mean fields. Then, the dynamics of cylinder-wake/boundary-layer interaction are revealed by the turbulent momentum transport and kinetic energy production. The turbulent fluctuations caused by the secondary vortex (SV) (at G/D=0.3, 0.9) and the novel tertiary vortex (TV) (at G/D=0.9) can be observed in the distributions of Reynolds stresses. For G/D=0.1 and G/D=0.3, the wake/boundary-layer interaction is dominated by ejection and sweep events, which are related to the generation of the hairpin vortex. These two bursting events lead to the momentum transport between the high- and low-speed sides. For G/D=0.9, the ejection event is not found in the interaction region because the head of the hairpin vortex is entrained into the wake. The upper roller (RU) helps to transport high-momentum fluid toward the wall in this case, although it does not take part in the interaction directly. The shedding of RU, the lower roller (RL), SV (at G/D=0.3 and 0.9), and KH (Kelvin–Helmholtz) vortex (at G/D=0.1) and the generation of the hairpin vortex are crucial to turbulent kinetic energy (TKE) production. The RU, KH vortex, and SV transfer ⟨u′u′⟩ out to ⟨v′v′⟩ and ⟨w′w′⟩ resulting redistribution of the TKE. While RL, surviving for a shorter time, transfers ⟨v′v′⟩ out to ⟨u′u′⟩ and ⟨w′w′⟩, helping explain why it disappears quickly, TV only transfers out ⟨v′v′⟩ out to ⟨u′u′⟩, and its TKE comes from other terms rather than the production term. The redistribution of TKE due to the generation of the hairpin vortex can result in the slower growth rate of the secondary disturbance growth stage, promoting the wall boundary layer transition.