Flow around twin rough cylinders at low wind incidence: bi-stability and gap flow

Wind tunnel experiments are performed on twin rough cylinders with a centre-to-centre spacing ratio L/D = 1.2 at two different Reynolds numbers, Re = 45k and 275k, corresponding to the sub- and post-critical flow regimes, respectively. The wind incidence is varied from 0° to 10°. A strong gap flow is observed for the largest incidences in both flow regimes and appears at the same wind incidence in the sub- or post-critical flow regimes. The frequency analysis of the lift forces reveals the existence of bi-stabilities before the appearance of the gap flow.