Bistability in the wake of a circular cylinder with passive control using two leeward rods

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 Coanda 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 & theta; = 120 degrees to 125 degrees, where & theta; 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. & COPY; 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).