This paper presents an experimental investigation on the flow around a circular cylinder
controlled by a pair of rods placed close to the separated shear layers from the leeward
face of the cylinder. Past research of Cicolin et al. (2021) has shown that one control rod,
placed at specific positions, induces a late flow separation on the main cylinder’s surface
in a typical occurrence of the Coandă effect. As a result the control rod reduced the mean
drag acting on the cylinder but also produced a net mean lift force. We now present
an extension of the aforementioned study to consideration of a pair of symmetrically
arranged control rods, aimed at eliminating the mean lift force whilst maintaining a drag
reduction. The experiments were carried out at a Reynolds number Re = 20, 000 based
on the diameter of the main cylinder D, with the diameter of the control rods being ten
times smaller than that of the cylinder. The centre-to-centre distances between each
control rod and the cylinder was 0.7D, and the angular position of the rods varied from
θ = 120◦
to 125◦
, where θ is measured from the front stagnation point. Time-resolved
PIV velocity fields and hydrodynamic forces were measured for the different setups.
Results show that the mean flow is asymmetric in spite of the symmetry of the geometric
model and position of the rods. The pair of control rods induces a bistable flow, induced
by the imbalance of two colliding jets in the near wake. The dynamics were found to
be random, with the average switching time between stable states depending on the
position of the rods. The mean drag force was reduced by up to 15%, and the mean lift
force was reduced by 80% compared to the cases with a single control rod. It was also
observed that the control rod can induce two different types of flow separation from
the main cylinder, one in which the flow separates from the main cylinder only once
and one in which a small separation bubble forms in proximity to the control rod before
reattaching and then permanently separating. These distinct features play an important
role in the dynamics of the bistability and hint at a possible extension of the total drag
reduction to 30% if the bistability is suppressed.