Identification of distinct flow behaviours around twin rough cylinders at low wind incidence

Wind tunnel experiments are performed on twin rough cylinders with a centre-to-centre spacing ratio L/D=1.2. The unsteady flow field is measured at two different Reynolds numbers, Re = 45 k and 275 k, corresponding to the sub- and post-critical flow regimes, respectively. The wind incidence α is varied from 0°to 10°. The detailed analysis of the lift coefficients as a function of the wind incidence reveals the existence of three flow behaviours in both flow regimes. Bi-stabilities are observed and occur within different ranges of wind incidence depending on the flow regime. A methodology is proposed to decouple the modes which intermittently occur in case of bi-stability, allowing to identify the flow behaviour of each mode. In the sub-critical flow regime, the flow behaviour at very low wind incidences (α=0°–2°) corresponds to an alternate re-attachment on both sides of the rear cylinder. In the post-critical flow regime, the flow behaviour is bi-stable because of the intermittent re-attachment of the shear layers. For larger wind incidences (α= 4°–6°), the lower shear layer intermittently re-attaches steadily or alternately onto the rear cylinder in the sub-critical flow regime, evidencing a bi-stability. On the other hand, it always re-attaches steadily onto the rear cylinder in the post-critical flow regime. A gap flow is observed for the largest wind incidences (α=8°–10°). It is found to be stronger in the sub- than in the post-critical flow regime.